OCS Study 

•

MMS 2003-054 

A Methodology for Investigation of 

Natural Hydrocarbon Gas Seepage in 

the Northern Santa Barbara Channel 

Final Technical Summary 

Final Study Report 

U.S. Department of the Interior 

Minerals Management Service

Pacific OCS Region 

OCS Study 

MMS 2003-054 

A Methodology for Investigation of 

Natural Hydrocarbon Gas Seepage in 

the Northern Santa Barbara Channel 

Final Technical Summary 

Final Study Report 

Authors 

Bruce P. Luyendyk 

Libe Washburn 

Sanjoy Banerjee 

Jordan F. Clark 

Principal Investigators 

and 

Derek C. Quigley 

Prepared under MMS Cooperative 

Agreement No. 14-35-0001-30758 

by 

Coastal Marine Institute 

Marine Science Institute 

University of California 

Santa Barbara, CA 93106 

U.S. Department of the Interior 

Minerals Management Service Camarillo

Pacific OCS Region March 2003
Disclaimer 

This report has been reviewed by the Pacific Outer Continental Shelf Region, Minerals Management 

Service, U.S. Department of the Interior and approved for publication. The opinions, findings, 

conclusions, or recommendations in this report are those of the authors, and do not necessarily reflect 

the views and policies of the Minerals Management Service. Mention of trade names or commercial 

products does not constitute an endorsement or recommendation for use. This report has not been 

edited for conformity with Minerals Management Service editorial standards. 

Availability of Report

Extra copies of the report may be obtained from: 

U.S. Dept. of the Interior 

Minerals Management Service 

Pacific OCS Region 

770 Paseo Camarillo 

Camarillo, CA 93010 

Phone: 805-389-7621 

A PDF file of this report is available at: 

http://www.coastalresearchcenter.ucsb.edu/CMI/ 

Suggested Citation 

The suggested citation for this report is: 

Luyendyk, Bruce P., Washburn, Libe, Banerjee, Sanjoy, Clark, Jordan F., and Derek C. Quigley. A 

Methodology for Investigation of Natural Hydrocarbon Gas Seepage in the Northern Santa Barbara 

Channel. MMS OCS Study 2003-054. Coastal Research Center, Marine Science Institute, University 

of California, Santa Barbara, California. MMS Cooperative Agreement Number 14-35-0001-30758. 

66 pages. 
Table of Contents 

FINAL TECHNICAL SUMMARY 1 

FINAL STUDY REPORT: Quantifying Spatial and Temporal 

Variations in the Distribution of Natural Marine 

Hydrocarbon Seeps in the Santa Barbara Channel, California. 

M.S. Thesis by Derek C. Quigley. 9 

 Title Page 9 

 Abstract 10 

 1.0 Introduction 11 

 2.0 Environment of Natural Hydrocarbon Seeps 13 

 3.0 Mechanisms of Hydrocarbon Migration 

 and Seepage 15 

4.0 Time Variation in Emission and Distribution 

 of Seepage 17 

5.1 Natural Hydrocarbon Seepage at Coal Oil Point, 

 California 19 

5.2 Previous Estimates of Seepage Flow Rates at 

 Coal Oil Point 24 

 6.1 Oceanographic Surveys 24 

 6.2 Method of Mapping Seep Distribution 25 

 7.0 Seepage Distribution Maps 28 

 8.1 Quantifying Seepage Emission Rates – 

 Theoretical Considerations 35 

 8.2 Empirical Calibration of Seepage Rates 38 

 8.3 Emission Estimates 43 

 9.1 Discussion 48 

 9.2 Seep Methane Flux 53 

 9.3 Seepage of Liquid Petroleum 55 

 9.4 Seep Flux of ROG’s 56 

 10.0 Conclusions 57 

 Appendix 1 58 

 Appendix 2 59 

 References 60 

 Acknowledgements 66 
List of Tables

Table 1. Analysis of Natural Seep Gases Collected Near the Arco/Mobil Seep Tents 12 

Table 2. Summary of Emission Estimates for the Coal Oil Point Area 44 

List of Figures 

Figure 1. Location Map of the Study Area 20 

Figure 2. Compilation of Seep Distribution from 1946-1976 21 

Figure 3. Submarine Geologic Structure Offshore Coal Oil Point 22 

Figure 4. Normalized Intensity Profile for Water Column 26 

Figure 5. Distribution of Seepage (August 1996) 27 

Figure 6. Comparison of 1973 and July 1995 Surveys 29 

Figure 7. Comparison of 1973 and July 1995 Data 30 

Figure 8. Distribution of Seepage (September 1995 and July 1995) 31 

Figure 9. Comparison of 50 kHz Relative Intensity Profiles 33 

Figure 10. Distribution of Seepage (Nov. 1994, Dec. 1994, Aug. 1995, Nov. 1995) 34 

Figure 11. Comparison of Distribution from all Surveys 1994-1996 35 

Figure 12. Calculated Theoretical Scattering and Distribution of Bubble Radii 37 

Figure 13. Empirical Calibration for 3.5 kHz and 50 kHz Data 39 

Figure 14. Sample Intensity Profile Calculated from a Plume (November 1996) 40 

Figure 15. Calibrated Map of Flow Rate for August 1996 42 

Figure 16. Calibrated Map of Flow Rate for July and September 1995 42 

Figure 17. Calibrated Map of Flow Rate for 50 kHz Survey 43 

Figure 18. Comparison of Gas Emission for 1973 and July 1995 Data 46 

Figure 19. Monthly Averages of Production Volumes of Seep Gas at the 

 Seep Tents and Annual Oil Production from Platform Holly 47 

Figure 20. Contour Plot of Thickness of Quaternary Sediment Overburden, Geologic 

 Structure, and Aug. 1996 Seep Distribution Offshore Coal Oil Point 48 

Figure 21. Stylized Interpretation of Subsurface Geologic Structure Offshore 

 of Coal Oil Point 49 

Figure 22. Reservoir Pressure Values in the Monterey Formation Reservoir 51 

Figure 23. Distribution of Emission Rates per Grid Node for Each Dataset and 

 log-log Plot of Emission vs. Cumulative Percent 55 

 

 

 
A Methodology for Investigation of Natural Hydrocarbon Gas Seepage 

1

FINAL TECHNICAL SUMMARY 

STUDY TITLE: A Methodology for Investigation of Natural Hydrocarbon Gas Seepage 

in the Northern Santa Barbara Channel 

REPORT TITLE: A Methodology for Investigation of Natural Hydrocarbon Gas Seepage 

in the Northern Santa Barbara Channel 

CONTRACT NUMBER: 14-35-0001-30758 

SPONSORING OCS REGION: Pacific 

APPLICABLE PLANNING AREA(S): Southern and Central California 

FISCAL YEAR(S) OF PROJECT FUNDING: FY 96 

COMPLETION DATE OF REPORT: October, 1997 

COSTS: FY 96 - $88,607 

CUMULATIVE PROJECT COST: $88, 607 

PROJECT MANAGERS: Bruce P. Luyendyk1

, Libe Washburn2, Sanjoy Banerjee3, 

Jordan F. Clark4 

AFFILIATION: 1Director, Institute for Crustal Studies, UCSB, and Professor, Geological 

Sciences; 2Associate Professor, Geography, UCSB; 3Professor, Chemical and Nuclear 

Engineering, UCSB; 4

Assistant Professor, Geological Sciences, UCSB 

ADDRESS: University of California, Santa Barbara, CA 93106 

PRINCIPAL INVESTIGATORS: Bruce P. Luyendyk, Libe Washburn, and Sanjoy 

Banerjee 

KEY WORDS: Santa Barbara Channel; hydrocarbons; marine seeps; methane, smog, air 

pollution 

BACKGROUND: The largest marine hydrocarbon seeps in the United States are located 

near Coal Oil Point in the Santa Barbara Channel (Figures 1 and 2). This investigation 

developed methods to quantify natural hydrocarbon seepage rates using geophysical and 

geochemical data. The La Goleta seep field in the Coal Oil Point area was selected as the 

test area because of its extreme activity. Methods to determine the amount of hydrocarbon 

gas being released to the atmosphere were investigated. First, the release rate can be 

estimated by analyzing high-resolution sonar data, because the amount of acoustic 
Final Technical Summary – Luyendyk et al. 

2

backscatter from the gas bubbles in the water column is proportional to the amount of gas 

rising through the water column. Secondly, the dissolved concentrations of hydrocarbons 

in the ocean are proportional to the natural hydrocarbon seepage rate. Hence, their 

inventories in the water column can used as a method of quantifying natural hydrocarbon 

gas seepage rates. 

OBJECTIVES: Our project objectives included developing methods to quantify the 

amount of hydrocarbon emissions resulting from natural gas seepage in the northern Santa 

Barbara Channel in 1994-1996 and establishing a baseline for future comparisons. This 

research would improve the air pollution emission inventory of the Santa Barbara and 

Ventura County Air Pollution Control Districts. In particular, it would help to quantify the 

amount of reactive organic gases contributed to the atmosphere by the natural seeps, and 

would allow documentation of future changes in the natural seepage rates. 

There are five major concerns about marine seeps as natural systems, and we addressed 

some aspects of these in this project: 

(1) What are the seeps emitting (and which seeps are emitting what)? 

(2) What is the spatial distribution of oil and gas seeps in the Santa Barbara Channel? 

(3) What is the natural temporal variation in seep distribution? 

(4) What is the natural total emission rate, and what is its variation? 

(5) What is the ultimate fate of seep emissions? 

DESCRIPTION:

Specific Approach 

Our research analyzed geophysical, oceanographic and geochemical data to map seeps and 

estimate seepage rates in the northern Santa Barbara Channel. The approach included 

mapping and characterizing seep areas by acoustic methods, building a scalar turbulent 

transport model from measured hydrocarbon concentration and current velocity data, and 

employing geochemical tracers to quantify emission rates, gas exchange rates, and 

dispersion. 

The area surveyed is the Coal Oil Point seep field over the South Ellwood anticline. The 

western end of this oil-bearing anticline was documented as an area of natural hydrocarbon 

seepage in the 1970's and early 1980's and has been producing oil and gas from offshore 

platform Holly for nearly three decades. 

The hydrocarbon seepage rises though the water column from sea floor vents above subsea floor hydrocarbon accumulations. Free hydrocarbon gases (bubbles), can be imaged by 

acoustical methods. Dissolved hydrocarbon gases are eventually liberated to the 

atmosphere down current from the seeps. Geochemical and oceanographic sampling and 

modeling are required to calculate the total hydrocarbon input to the air from this dissolved 

component. 
A Methodology for Investigation of Natural Hydrocarbon Gas Seepage 

3

Acoustic surveys 

Geophysical systems used include 50 kHz, 12 kHz and 3.5 kHz echo sounding sonars to 

image seep bubble trains in the water column, 3.5 kHz, Uniboom, and one kilo Joule 

sparker seismic reflection equipment to image the sub bottom, and 325 kHz side scan sonar 

to map structure on the sea floor. Emission volumes for individual seeps were quantified 

from calibrated sonar data. A weighted hose delivered compressed air at a controlled rate 

from a survey ship to the sea floor. Discharge generated by this was imaged by the 3.5 and 

12 kHz systems. 

Side-scan sonar and sub bottom reflection systems were used to map the sea floor geology 

at the location of the seep vents to determine what is controlling the location of the seeps. 

Dissolved hydrocarbon plume measurement and modeling 

Oceanographic observations focused on the spatial structure of the dissolved hydrocarbon 

plume and the ambient ocean conditions which affect its dispersal. To understand the 

spatial structure of the plume, a ship-board gas chromatograph, configured as a "sniffer", 

was operated to determine the concentration and composition of hydrocarbons over the 

seep field. A submerged hose pulled sea water onto the research vessel where it was 

analyzed continuously (one complete analysis every 10 minutes) as the survey progressed. 

The "sniffer" data was acquired both near the sea floor and in the overlying water column. 

Several vertical sniffer profiles were made to resolve the vertical extent of the hydrocarbon 

plume. 

The primary physical oceanographic measurements included vertical profiles and twodimensional sections of water properties in and around the seep field. The detailed spatial 

structure of the temperature, salinity, and density fields in vicinity of the seep field was 

observed using a "towyo" technique in which a platform carrying a second CTD was 

winched between the surface and the bottom as the research vessel steams at 4-6 kts. 

Time series of current velocity near the seeps was obtained from vector-measuring current 

meters located off the coast of Goleta, CA. 

Computer modeling of dissolved hydrocarbon transport 

The dynamics of the hydrocarbon plume and seepage rates from the sea floor can be 

inferred from sniffer data by building a scalar turbulent transport model for the area around 

the seeps. Using the dissolved hydrocarbon concentrations, gas solubilities, and ocean 

conditions (current velocity, sea water temperature, salinity, and density) as input 

parameters, a dissolved hydrocarbon concentration scalar field can be calculated (e.g., 

Radlinski and Leyk, 1995). 

Tracer Release Experiments 

Tracer gas release experiments were used to help calibrate both the turbulent transport 

model and the geophysical techniques used for determining the amount of hydrocarbon gas 

that is released into the atmosphere across the air-ocean interface. The dual tracer or 3

He - 

SF6 method, has been developed which can quantify gas transfers velocities over periods 

of days in coastal areas (Watson et al., 1991; Wanninkhof et al., 1993; Clark et al. 1994). 
Final Technical Summary – Luyendyk et al. 

4

Offshore Surveys 

This MMS project was conducted in conjunction with three related projects funded by the 

University of California Energy Institute (UCEI) and awarded to Luyendyk, Washburn, and 

Trial. This agency, along with Mobil Corp., supported a total of six surveys prior to start 

of MMS funding. These surveys employed 50 kHz and 3.5 kHz sonars, and included 

sniffer experiments. 

During the second week of August, 1996 we conducted a survey with MMS funding that 

included 2 days of 3.5 kHz sonar profiling along with 3 days of oceanographic sampling. 

We had two objectives: 1) to track dissolved hydrocarbons emitted from the seeps; and 2), 

repeat sonar measurements of seep intensity to study time-variations in seepage. An onbottom SF6 tracer source was lowered to the sea floor in the seep field at the start of the 

survey. An acoustic Doppler current profiler was also installed on the seafloor to measure 

currents during the survey. The emission of the SF6 tracer and ambient hydrocarbon 

concentrations were measured and mapped with an on-board Gas Chromatograph and 

tracked for 3 days. Vertical sections of Conductivity-Temperature-Depth were obtained at 

the end of the tracking plume period. The sonar survey duplicated tracks we made on 

earlier surveys in 1995. 

A sonar experiment was conducted in November to calibrate the 3.5 kHz system utilized 

for imaging gas seep bubble plumes. We sought to establish an empirical relation for a 

variety of bubble sizes and gas flux from an artificial source and the amplitude of the sonar 

return. The experiment was undertaken off of San Pedro, California at 120 foot water 

depth. A bubble release device was suspended 5 feet above the seafloor and the bubble 

size and flow rate were varied. Repeated passes over the bubble release device were made 

with the 3.5 kHz sonar towfish. Data were collected using an analog thermal recorder and 

also digitally at 10 kHz. 

Volume flux emission estimates from the calibrated 3.5 kHz data were compared to 

estimates based on a previous calibration of 50 kHz sonar data and determined that the 

total emissions estimated using the two different sonar systems are similar in magnitude. 

We arranged for a side scan sonar survey of the study area that will yield a high resolution 

map-view image of the sea floor. The side scan sonar map will help us understand the 

structural geological characteristics of the hydrocarbon seeps field. 

Our final efforts involve finishing the side scan mapping, refining the relation between 

signal strength and gas flux, mapping the detailed seafloor geology, and preparing two 

manuscripts for publication; one on a quantitative estimate of gas discharge form this seep 

field, and a second on the dissolved gas content of the ocean waters around the seep field. 

SYNOPSIS OF MAJOR FINDINGS or SIGNIFICANT CONCLUSIONS: Our 

analysis of the calibrated sonar data suggest a Coal Oil Point seep field flux of 1.1 x 105 

m3/day for the 3.5 kHz data. Adding the contribution from the seep tents near offshore 

Platform Holly gives a total estimate of 1.2 x 105 m3/day. This compares favorably with 

the estimate from the our calibrated 50 kHz sonar data of 1.5 x 105 +/- 0.2 m3/day (5.3 x 
A Methodology for Investigation of Natural Hydrocarbon Gas Seepage 

5

106 ft3/day). The methane emission rate we determined is 72 +/- 11 metric tonnes/day and 

the non-methane hydrocarbon rate is 30 +/- 4 metric tonnes/day. Reactive organic gas 

(ROG) emissions from the seeps are 19 +/- 3 metric tonnes/day. Our estimated methane 

emission rate for the Coal Oil Point field is five times higher than previous estimates. The 

most intense areas of seepage correspond to structural culminations along anticlinal axes. 

Seep locations are mostly unchanged from those documented in 1946, 1953, and 1973. An 

exception is the seepage field that once existed near offshore oil Platform Holly. A 50% 

reduction in seepage within a one kilometer radius around this offshore platform is 

correlated with reduced reservoir pressure beneath the natural seeps due to oil production. 

The decrease in hydrocarbon seepage rate near Platform Holly suggests that oil production 

here has resulted in an unexpected benefit to the marine and atmosphere environment.

The reactive organic gas emission rates from the Coal Oil Point seeps alone are a large 

source of hydrocarbon air pollution in Santa Barbara County (equal to the emission rate 

from all the vehicle traffic in the County in 1990). The ROG emission rates found in our 

study for Coal Oil Point seep field are three times the seep ROG emission rates estimated 

in the official Santa Barbara County air emission inventory (SBCAPCD, 1994). 

Furthermore, Fischer (1977) estimated that the Coal Oil Point seep field contains only half 

of the marine seeps in Santa Barbara County. The official Air Pollution Control District 

estimate of ROGs in Santa Barbara County is therefore too low by at least a factor of four 

(Saxena and Oliver, 1984). Reaching EPA air quality attainment status in Santa Barbara 

County may require an effective means of containing or remediating the natural seeps. 

Surveys of dissolved hydrocarbons within 15 km the Coal Oil Point seep field identified a 

plume which extends down-current of the seeps. In most samples, ethane and propane 

concentrations correlated well with methane concentration. The maximum and mean 

methane concentrations within the plume were approximately 5200 nmol l-1 and 1300 

nmol l-1, respectively. Concentration distributions were characterized by areas of high 

and low concentrations which did not correspond to different regions of the plume. These 

areas resulted from the complicated geometry of the seep field as well as the chaotic nature 

of mixing within the field area. Mass balance calculations indicate that approximately half 

of the methane (30% - 60%) dissolves during transit through the water column. 

Cynar and Yayanos (1992) showed that a subsurface methane maximum occurs along a 

narrow density horizon throughout most of the Southern California Bight. They argued 

that this maximum was supported by coastal sources including natural hydrocarbon seeps. 

At the time of our survey, the density of the water receiving the influx of methane from the 

Coal Oil Point seeps was similar to the density of the water containing the subsurface 

methane maximum Cynar and Yayanos (1992) observed off shore. Furthermore, the total 

flux of methane into water column from the Coal Oil Point seeps, 2.4 ± 0.6 x 1010 g yr-1, 

is approximately equal to the total flux of dissolved methane to the atmosphere over this 

broad area. These observations strongly support the inference that coastal sources maintain 

the excess methane observed off shore as argued by Cynar and Yayanos (1992) and Ward 

(1992). 
Final Technical Summary – Luyendyk et al. 

6

We also have completed a high resolution isopach map of the Quaternary overburden in the 

Coal Oil Point seeps study area. We found that a great portion of the seep field is not 

coated by a thick layer of pelagic and terrigenous sediment, but rather by less than a meter 

of apparently unconsolidated overburden above siliceous shale of the Sisquoc formation. 

Our computer models of advection of dissolved hydrocarbons agree well with 

observations. These show that the modeling technique is valid and well constrained by the 

data we obtained. The model can therefore be used to predict the dispersion of dissolved 

hydrocarbons in the Channel. 

STUDY RESULTS: 

1. Gas volume discharge has been estimated for the Coal Oil Point seep field. 

2. The concentration of dissolved hydrocarbons from the seep discharge has been mapped 

and quantified. 

3. Seep intensity has decreased around offshore Platform Holly since 1973. 

STUDY PRODUCTS: 

1. Map of gas seep locations 

2. Estimates of seep gas discharge 

3. Map of dissolved hydrocarbon plume 

4. Model of advection of dissolved hydrocarbons away from Coal Oil Point. 

Publications: 

Boles, J.R., Clark, J.F., Leifer, I., and L. Washburn. 2001. Temporal variation in natural 

methane seep rate due to tides, Coal Oil Point area, California. J of Geophys ResOceans 106(C11): 27077-27086. 

Clark, J.F., Washburn, L., Hornafius, J.S., and B.P. Luyendyk. 2000. Dissolved hydrocarbon 

flux from natural marine seeps to the southern California Bight. J of Geophys ResOceans 105(C5): 11509-11522. 

Hornafius, J.S., Quigley, D., and B.P. Luyendyk. 1999. The world's most spectacular marine 

hydrocarbon seeps (Coal Oil Point, Santa Barbara Channel, California): Quantification 

of emissions. J Geophys Res-Oceans 104(C9): 20703-20711. 

Quigley, D.C., Hornafius, J.S., Luyendyk, B.P., Francis, R.D., Clark, J., and L. Washburn. 

1999. Decrease in natural marine hydrocarbon seepage near Coal Oil Point, California, 

associated with offshore oil production. Geology 27(11): 1047-1050. 

Derek C.Quigley. 1997. Quantifying Spatial and Temporal Variations in the Distribution of 

Natural Marine Hydrocarbon Seeps in the Santa Barbara Channel, California. M.S. 

thesis, University of California, Santa Barbara. 95 pp.
A Methodology for Investigation of Natural Hydrocarbon Gas Seepage 

7

Research Presentations:

American Geophysical Union Meeting 

Our research group made four presentations on our seeps project at the Fall ‘96 AGU 

meeting in San Francisco December 18 (list follows). These presentations were directly 

supported by the MMS project, along with additional support from the University of 

California. 

Bartsch, E.C., Gurrola, L.D., Francis, R.D., Quigley, D.C., Hornafius, J.S., and Luyendyk, 

B.P., Structural Control of the Spatial Distribution of Hydrocarbon Seeps in the 

Santa Barbara Channel, California, EOS Trans. Amer. Geophys. Union, vol. 77, 

no. 46, F419. 

Clark, J.F., Washburn, L., Hornafius, J.S., Luyendyk, B.P. and Ho, D.T., Mixing in the 

Vicinity of Natural Marine Hydrocarbon Seeps, Santa Barbara Channel, CA, EOS 

Trans. Amer. Geophys. Union, vol. 77, no. 46, F419. 

Quigley, D.C., Hornafius, J.S., Luyendyk, B.P., Francis, R.D., and Bartsch, E.C., Temporal 

Variation in the Spatial Distribution of Natural Marine Hydrocarbon Seeps in the 

Santa Barbara Channel, California, EOS Trans. Amer. Geophys. Union, vol. 77, 

no. 46, F419. 

Washburn, L., Hornafius, J.S., Luyendyk, B.P., Clark, J.F., Quigley, D.C., and Francis, 

R.D., Dispersal of Hydrocarbon Gas Plumes from Seafloor Seeps in the Northern 

Santa Barbara Channel, CA, EOS Trans. Amer. Geophys. Union, vol. 77, no. 46, 

F419. 

REFERENCES: 

Clark, J. F., R. Wanninkhof, P. Schlosser, and H. J. Simpson. 1994. Gas exchange rates in the 

tidal Hudson river using a dual tracer technique. Tellus, 46B, 274-285. 

Cynar, F. J. and Yayanos, A. A. (1992) The distribution of methane in the upper waters of the 

Southern California Bight: Journal of Geophysical Research, v. 97, p. 11,269-11,285. 

Fischer, P. J., 1977, Natural gas and oil seeps, Santa Barbara basin, California, in California 

Offshore Gas, Oil, and Tar Seeps: State of California, State Lands Commission staff 

report, p. 1-62. 

Radlinski, A. P. and Leyk, Z., 1995. Formation of light-hydrocarbon anomalies in oceanic 

waters. Geology, 23, 265-268. 

Saxena, P., and Oliver, W. R., 1984, Estimation of reactive organic gas emissions from 

offshore oil and gas seeps in the Santa Barbara Channel: Systems Applications, Inc., 

report SYSAPP-84/197 prepared for the Environmental Protection Agency, Region IX, 

San Francisco, California, 25 p. 
Final Technical Summary – Luyendyk et al. 

8

Santa Barbara Air Pollution Control District, 1994, The 1994 Clean Air Plan, Appendix D., 

Table 3.2, Santa Barbara, CA.

Wanninkhof, R, W. Asher, R. Weppernig, H. Chen, P. Schlosser, C. Langdon, and R. 

Sambrotto. 1993. Gas transfer experiment on Georges Bank using two volatile 

deliberate tracers. J. Geophys. Res. 98, 20,237-20,248. 

Ward, B. B. (1992) The subsurface methane maximum in the California Bight. Continental 

Shelf Research, 12, 735-752. 

Watson, A. J., R. C. Upstill-Goddard, and P. S. Liss. 1991. Air-sea exchange in rough and 

stormy sea measured by a dual tracer technique. Nature, 349, 145-147. 
A Methodology for Investigation of Natural Hydrocarbon Gas Seepage 

 9

FINAL STUDY REPORT 

UNIVERSITY OF CALIFORNIA 

Santa Barbara 

Quantifying Spatial and Temporal Variations in the 

Distribution of Natural Marine Hydrocarbon Seeps in the 

Santa Barbara Channel, California 

Thesis submitted in partial satisfaction of the requirements for the degree 

of 

Master of Science 

in 

Geophysics 

by 

Derek C. Quigley 

Committee in charge: 

Professor Bruce Luyendyk, chairperson 

Professor Jim Boles 

Professor Jordan Clark 

September 1997 

Copyright by 

Derek C. Quigley 

1997 
Final Study Report – Luyendyk et al. 

10

ABSTRACT 

Quantifying Spatial and Temporal Variations in the Distribution of Natural Marine 

Hydrocarbon Seeps in the Santa Barbara Channel, California 

Derek C. Quigley 

Prolific natural hydrocarbon seepage offshore of Coal Oil Point in the Santa Barbara Channel, 

California contributes significant quantities of gaseous and liquid hydrocarbons to the local 

environment, and may also represent an important process at the global scale. Insight into rates of 

seepage improves the understanding of local pollution from seeps and global seepage potential. 

Constraining the volume of fluid migration in the crust associated with seepage is also relevant 

since such fluid movement can mark diagenetic processes and affect parameters of structural 

stability causing earthquake swarms. The distribution of gaseous seepage offshore of Coal Oil 

Point was mapped using 3.5 and 50 kHz sonar by imaging rising seep gas bubbles which act as 

acoustic scattering targets within the water column. Examination of a series of mapped seep 

distributions reveals the following: (1) spatial location of seeps is principally controlled by 

structural features of the offshore geology, aligning with the east-west structural trends of 

anticline axes; (2) intensity of seepage emission correlates with structural relief and perhaps 

cross-cutting by faults; (3) both spatial distribution of seepage and volume emission rates vary 

over time; (4) drawing down of reservoir hydrocarbons and the reduction in reservoir pressure 

associated with offshore oil production is one contributing factor decreasing the extent of 

seepage and seep emission volumes; (5) areal extent of seep distribution within 13 km2 of 

platform Holly decreased from 0.9 to 0.4 km2 and volumetric emission declined on the order of 

50,000 m3/day between 1973 and 1995; (6) a current estimate of volumetric flux from seeps for 

the entire Coal Oil Point offshore area suggests emission is on the order of 1-2x105 m3 of gas per 

day and 20,000-30,000 liters of oil. 
A Methodology for Investigation of Natural Hydrocarbon Gas Seepage 

 11

1.0 INTRODUCTION

This thesis is a study of natural marine hydrocarbon seepage offshore of Coal Oil Point in the 

Santa Barbara Channel, California. The object of the study is to quantify seep emissions and 

estimate the flux of hydrocarbons from the seeps. Natural submarine hydrocarbon seepage may 

contribute considerably to global cycling of carbon within the marine environment and is 

certainly a substantial source of pollution within Santa Barbara County. Therefore, this issue is 

an important topic of scientific inquiry. 

At the local level, the prolific natural oil and gas seepage offshore Coal Oil Point represents a 

significant source of pollution. Non-methane hydrocarbons which comprise a portion of the 

gaseous seepage (Table 1) constitute reactive organic gases (ROG’s) which are introduced to the 

Santa Barbara Air Basin. These gases are precursors to smog forming ozone, and represent a 

health risk to Southern California residents. Saxena and Oliver [1984] estimated natural 

hydrocarbon seeps in the Santa Barbara Channel produced 18 to 152 tons of ROG’s per day. By 

comparison, all the on-road vehicle traffic in Santa Barbara County accounted for only 17 tons 

per day of ROG’s in 1990 [Santa Barbara Air Pollution Control District, 1994]. Canister 

hydrocarbon data collected by Killus and Moore [1991] around the South Central Coast air basin 

suggest that geogenic sources of hydrocarbon trace gases (i.e.-natural gas seeps) dominate over 

anthropogenic sources (automobile emissions) in the local atmosphere. In fact, chemical analyses 

of these air grab samples indicated 86% of the non-methane hydrocarbons in the samples 

originated from natural oil and gas seeps. 
Final Study Report – Luyendyk et al. 

12

In addition, the methane in the seep gas (approximately 90% by volume) is of interest because it 

is a greenhouse gas. After carbon dioxide, methane is the most significant contributor to radiative 

forcing [Shine et al., 1990]. Concentration of methane in the atmosphere is currently increasing 

at about 0.9% per year [Watson et al., 1990]. Given the speculation that increased concentrations 

of atmospheric methane released from destabilized methane gas hydrates may have contributed 

to global climate change in the past [MacDonald, 1990; Nisbet, 1990; Kvenholden, 1993], an 

accurate understanding of the sources and sinks of methane is an important scientific issue 

[Cicerone and Oremland, 1988]. 

Venting of methane from submarine seeps has been suggested as a significant, and previously 

overlooked, source of methane in the oceans [Hovland et al., 1993]. Dissolved methane 

concentrations in the upper oceans exceed expected levels based on atmospheric equilibrium 

[Cynar and Yayanos, 1992]. Since submarine methane seepage is hypothesized to effect ocean 

chemistry {Dando and Hovland, 1992], natural seeps may be responsible for this discrepancy. In 

addition, methane leaking from the sub-seabed is depleted in 14C, which could account for the 
A Methodology for Investigation of Natural Hydrocarbon Gas Seepage 

13

shortfall in fossil methane calculated for current atmospheric methane budgets [Lacroix, 1993; 

Hovland et al., 1993]. 

In addition to the flux of methane and other gaseous hydrocarbons from the seeps offshore Coal 

Oil Point, seepage of liquid petroleum can lead to surface oil slicks and tar balls in the vicinity. 

Liquid petroleum (oil and tar) has a greater potential as a marine pollutant due to its long term 

presence as opposed to gaseous components which are either lost to the atmosphere or quickly 

assimilated. Previous authors [Kvenholden and Harbaugh, 1983; Landes, 1973; Wilson et al., 

1974] have addressed the issue of liquid petroleum seepage as a significant source of natural 

pollution within the marine environment. 

Volatile gases evaporating from the surface slicks off Coal Oil Point contribute additional ROG’s 

to the atmosphere. The residual tar is found in abundance on Santa Barbara Channel beaches 

[Welday, 1977]. Hartman and Hammond [1981] claimed based on isotope analysis that nearly all 

of the tar found along the Santa Barbara coastline and 55% of the tar collected in Santa Monica 

Bay, 150 km away, had originated from seeps at Coal Oil Point. Emission of oil and tar from the 

Coal Oil Point seeps is therefore another topic of concern. 

In addition to estimating the volume of seep emissions in the Coal Oil Point area, a second 

important goal of this thesis project is to address the issue of time variation in seep emission 

rates. Previous authors have proposed rates of hydrocarbon leakage vary over time [Wilkinson, 

1972; Fischer and Stevenson, 1973; Fischer, 1977; Kvenholden and Harbaugh, 1983; Clarke and 

Cleverly, 1991; Hovland et al., 1993]. It is important to understand the magnitudes of such 

fluctuations and the mechanisms responsible if one is interested in addressing processes which 

are changing in time such as the increasing concentration of atmospheric methane alluded to 

previously. 

The reasons outlined above demonstrate the importance of understanding emission rates from the 

natural marine hydrocarbon seeps at Coal Oil Point. These seeps are a significant local source of 

marine pollution (oil and tar) and air pollution (ROG’s) in Santa Barbara. Submarine seepage 

may also be important globally as a natural pollution source, affecting ocean chemistry and 

atmospheric quantities of methane. For these reasons, it is important to quantify the emission 

volumes from these natural seeps and examine the time variation in seep emission rates. 

2.0 ENVIRONMENT OF NATURAL HYDROCARBON SEEPS 

Natural hydrocarbon seeps are a well known occurrence throughout the world and historical 

references to seepage date back to earliest recorded history. Asphalt from seeps in the Middle 

East has been used as a building material since 3,000 BC, and burning gas seeps in the Baku 

region were known from several centuries before Christ [Hunt, 1979]. Documented written 

records of seeps include biblical references, and the works of Herodotus and Marco Polo [Wilson 

et al., 1974]. Historically, natural hydrocarbon seeps have provided a useful tool for petroleum 

exploration [Link, 1952; Hunt, 1979; Tedesco, 1995]. 
Final Study Report – Luyendyk et al. 

14

Link [1952] states that the significance of seeps from an exploration standpoint is that they prove 

the presence of source hydrocarbons and suitable structural traps for their accumulation. The 

presence of hydrocarbons is necessary for hydrocarbon seepage, and this requires a sufficient 

thickness of sedimentary rock and source rocks for hydrocarbon generation within the 

sedimentary section. Therefore, hydrocarbon seeps are commonly found throughout the world in 

sedimentary basins containing oil reserves. 

Structures for hydrocarbon accumulation and structural pathways for fluid migration are also 

important factors contributing to the presence of seepage. Conduits along which fluid movement 

could occur include layer contacts, interbedded lenses, or faults, although in the absence of fluid 

pathways in the subsurface the hydrocarbons may move as an amorphous plume [Machel and 

Burton, 1991]. 

Hunt [19791 notes that seeps throughout the world are most numerous at the margins of basins in 

sediments that have been faulted, folded, or eroded, thus creating structures, pathways, and 

exposures conducive to hydrocarbon accumulation, migration, and subsequent leakage. Link 

[1952] categorized the types of structures associated with instances of hydrocarbon seepage as 

follows: (1) seeps emerging from the ends of homoclinal beds where the beds are exposed at the 

surface, (2) seeps associated with beds and formations in which the oil was formed, (3) seeps 

from large oil accumulations bared by erosion or ruptured by faulting or folding, (4) seeps at the 

outcrops of unconformities, and (5) seeps associated with intrusions such as mud volcanoes, 

igneous intrusions, and piercement salt domes. 

Conduits for hydrocarbon seepage are frequently provided by fault induced fracture pathways for 

fluid movement. Hunt [1979] states: “Fissuring and fracturing associated with faulting induces 

secondary permeability and porosity, which favor vertical fluid migration”. Faults may also 

create impermeable seals depending on the nature of faulting and the lithologies of the 

superposed units. Hunt [1979] notes that tensional fractures in hard, brittle rocks favor vertical 

migration whereas faults in clay or shale sections tend to smear clay gouge on the fault plane 

creating a seal. Seals at faults may also result in otherwise permeable units due to hydrodynamic 

effects, or capillary phenomena [Eremenko and Michailov, 1974]. Nevertheless, numerous 

examples [Link, 1952] illustrate that faulting often results in important migrational pathways for 

seepage. 

Given the role of faulting in creating seepage pathways, seismic activity is an important factor in 

the occurrence of seepage. In assessing the seepage potential of the world’s continental margins 

Wilson et al. [1974] defined the highest seepage potential as the margins of sedimentary basins 

located along plate boundaries where earthquake activity was high. In contrast, tectonically stable 

areas such as the Gulf of Mexico exhibit comparatively little seepage despite the presence of 

large oil reserves [Hunt, 1979]. Seismic activity ostensibly augments the available network of 

fracture conduits which serve as fluid migration pathways, thus enhancing seepage. 

Fault fracture meshes form important conduits for large volume fluid flow and can also generate 

earthquake swarms [Sibson, 1996], which are known to occur in the Santa Barbara Channel 

[Sylvester et al., 1970; Sibson, 1996] although in general earthquake swarms are comparatively 
A Methodology for Investigation of Natural Hydrocarbon Gas Seepage 

15

rare within settings of active compressional tectonics. This study of hydrocarbon seepage at Coal 

Oil Point could conceivably be important for the understanding of earthquake hazards in the 

Santa Barbara area. 

The presence of pressurized fluid in reservoirs has previously been linked with seismic activity 

and earthquake swarms [Healy et al., 1968; Gibbs et al., 1973]. Earthquake swarms may result 

from heterogeneity in the distribution of stress concentrations in the crust [Hill, 1977] which are 

affected by fluid distribution [Hubbert and Rubey, 1959]. The Monterey formation, which is the 

origin of the oil and gas leaking at the Coal Oil Point seeps [Reed and Kaplan, 1977; Hartman 

and Hammond, 1981], is an example of interlayered beds of strong competence contrast leading 

to well developed Hill-type meshes [Sibson, 1996]. These are structures conducive to both fluid 

flow (seepage) and earthquake swarm activity. 

3.0 MECHANISMS OF HYDROCARBON MIGRATION AND SEEPAGE 

Seepage in the literature is divided into macroseepage and microseepage. Macroseepage refers to 

a macroeffusion of hydrocarbons in large quantities such as the massive outpouring or ejection of 

fluid in an extrusive volcanic flow. Microseepage implies a less voluminous discharge not visible 

to the naked eye but which can be sensed using enhanced analytical techniques. Sensing of such 

surface geochemical anomalies has been used as an exploration tool for identifying new 

hydrocarbon reservoirs. Clarke and Cleverly [1991] note that conditions for subsurface 

movement of oil and gas preclude the flow of trace quantities, so microseepage may actually 

indicate the presence of hydrocarbon residues in soils as a product of near-surface seep dispersal. 

The Coal Oil Point seeps clearly qualify as an instance of macroseepage. This can involve 

leaking of hydrocarbons either directly from source rocks or from reservoir beds. Possible forms 

of macroseepage summarized by Tedesco [1995] include: (1) mass migration through open 

faults and fractures, (2) megaventing along migrational pathways, (3) flowing from 

overpressurized and breached reservoirs via faults, (4) expelling from currently generating 

hydrocarbon source rocks, and (5) occasional venting from basement penetrating faults. The 

migration of hydrocarbons along these subsurface pathways which ultimately leak to the surface 

leads to seepage. 

Traditionally hydrocarbon migration has been divided into primary and secondary migration. 

Hunt [1979] defines these as follows: primary migration is the movement of oil and gas out of 

source rocks into permeable reservoir rocks while secondary migration is the movement of fluids 

within the permeable rocks of the reservoir which eventually leads to segregation and 

accumulation of oil and gas. The transport mechanisms responsible for hydrocarbon movement 

tend to differ depending on whether one refers to primary or secondary migration, mostly due to 

the difference in permeability between source and reservoir units. Factors other than permeability 

may also come into play, including the quantity of organic matter present and the geochemistry 

of the transported hydrocarbons. Many details regarding hydrocarbon migration are still the topic 

of debate, despite having received considerable attention in the literature [Roberts and Cordell, 

1980; Doligez, 1987; England and Fleet, 1991]. 
Final Study Report – Luyendyk et al. 

16

Seepage in some senses falls outside the traditional definitions of primary or secondary migration 

as presented by Hunt. It refers to post-accumulation migration and leakage of hydrocarbons at the 

surface [Clarke and Cleverly, 1991]. In general, the mechanisms of secondary migration would 

be more applicable to seepage in that leaking typically occurs within the more permeable 

reservoir units, although seepage can also occur directly from source rocks [Link, 1952]. For our 

purposes, we are most concerned with the mechanisms of hydrocarbon migration applicable 

within the context of seepage. 

Primary migration probably occurs as petroleum-phase expulsion of distinct oil globules from 

source rocks [Tissot, 1987], and may occur along a kerogen network of organic matter 

[McAuliffe, 1979]. The role of water may be important for secondary migration, hydraulically 

driving separate phase hydrocarbon transport [England and Fleet, 1991]. It is unlikely that 

hydrocarbon migration involves transport in either aqueous or colloidal solution, but rather the 

hydrocarbons move as a distinct phase from the water which saturates the carrier beds. Migration 

in aqueous solution is unlikely due to the extreme insolubility of petroleum in water. Although 

this solubility increases with temperature and pressure [Price, 1976; 1979], it is also dependent 

on hydrocarbon species, and no fractionation of known petroleum accumulations based on 

solubility has been observed. Similarly, hydrocarbon migration in a colloidal (micellar) solution 

is unlikely since there is no evidence that petroleum accumulations fractionate based on their 

ability to form micelles. 

The forces driving hydrocarbon migration result from disequilibrium in the subsurface and 

transport hydrocarbons in the direction of decreasing free energy [Hunt, 1979]. Flow regimes 

include gravity-, pressure-, and density-driven transport [Tedesco, 1995]. Gravity tends to drive 

fluids downward towards the lowest potential energy. More often, however, pressure is dominant 

and directed upward [Hunt, 1979], counteracting gravity. The pressure arises from compactional 

disequilibrium, thermal expansion, and generation of gas [Tedesco, 1995]. 

Pressure is probably the dominant driving force for primary hydrocarbon migration [Tissot, 

1987], squeezing oil from the source rocks [du Rouchet, 1981]. Buoyancy arising from density 

variations accompanying changes in temperature and salinity can also counteract gravity and is 

the likely driving mechanism for secondary migration [England and Fleet, 1991]. Capillary 

pressure resists buoyancy driven transport, which would oppose the expulsion of oil-phase 

droplets within environments of restricted permeability, leading previous authors to argue against 

petroleum-phase expulsion as a mechanism for primary migration [Cordell, 1972; Price, 1976; 

McAuliffe, 1979; Hunt, 1979]. 

On the other hand, separate phase flow of natural gas, consisting primarily of methane, can 

migrate at permeabilities which would restrict oil-phase migration. Methane could dissolve 

increasing amounts of heavier liquid hydrocarbons as temperature and pressure increase [Hunt, 

1979], possibly allowing gas-phase migration of liquid hydrocarbons in gas solution. 

Disequilibrium in concentration could lead to diffusive transport of gaseous hydrocarbons which 

has been proposed as an effective mechanism for the movement of gas but not for oil [Leythauser 

et al., 1982]. However, the principle objection to diffusion as the primary form of hydrocarbon 

transport is that the rates involved are extremely slow [Hunt, 1979]. Krooss et al. [1992] 
A Methodology for Investigation of Natural Hydrocarbon Gas Seepage

 17

acknowledged that diffusive transport through water-saturated cap rock is likely to be less 

efficient than either free gas-phase flow or transport of dissolved gas in aqueous solution. 

Buoyant transport is typically the principle driving force for vertical migration which results in 

surface seepage since buoyancy of the gas phase dominates the fluid potential gradients in the 

near subsurface driving secondary migration along postaccumulation seepage pathways [Clarke 

and Cleverly, 1991]. Pressure is typically dominant driving primary migration at depth [Tissot, 

1987] but could also be effective in the shallow subsurface. In addition, regional gravity driven 

flow of meteoric recharge waters might affect migration patterns of seepage, deflecting migrating 

hydrocarbons from their subsurface accumulations through advection by groundwater [Machel 

and Burton, 1991]. 

4.0 TIME VARIATION IN EMISSION AND DISTRIBUTION OF 

SEEPAGE

Previous work has alluded to the time variation of seepage rates and seep distribution at Coal Oil 

Point [Allen et al., 1970; Wilkinson, 1972; Mikolaj and Ampaya, 1973; Fischer and Stevenson, 

1973; Fischer, 1977]. Natural hydrocarbon seeps should be viewed as dynamic systems which are 

continually evolving. However, the time scale for seep evolution is not well constrained. There 

may be various processes operating simultaneously at different rates and dependent on a variety 

of environmental factors. 

Fischer [1977] suggested an evolutionary sequence of young, predominantly gaseous seeps 

venting off, gradually becoming increasingly oily, and eventually being preserved as tar mounds. 

Vernon and Slater [1963] observed submarine tar mounds SCUBA diving along the northern 

flank of the Santa Barbara basin and noted some oil and gas emerging from tar mound vents, but 

more commonly issuing from nearby sand or rock fractures. They suggested viscous residual tar 

left by escaping oil eventually sealed the seepage fracture conduits, subsequently altering seep 

distribution. 

The effects of subsea weathering would also cause progressive alteration of submarine seep 

products from oil to tar. The weathering process includes mechanisms summarized by Hunt 

[1979] such as leaching of water solubles, microbial degradation, polymerization or combining 

of some molecules into larger structures, and gelation, the formation of a rigid gel structure. Nonmarine surface seeps also lose volatile hydrocarbons through evaporation and auto-oxidation 

from exposure to air and sunlight. 

Other factors may control the variation in seepage. The subsurface hydrocarbon accumulations 

feeding seeps may dry up, or erosion might expose new accumulations previously sealed within 

the subsurface. Structural changes could open new migration pathways within the seep plumbing 

system. Seismic activity would exert an important influence in opening new meshes of faults and 

fractures which would enhance fluid flow. Fischer and Stevenson [1973] comment on the 

apparent correlation between earthquake activity and seepage at Coal Oil Point. Wilkinson 

[1972] offers anecdotal evidence of an episodic methane gas seep introduced offshore of Malibu 

following a major earthquake centered in the San Fernando Valley in 1971. Wilkinson states the 
Final Study Report – Luyendyk et al. 

18

cause was apparently an earth movement which fractured the Monterey Shale bedrock thinly 

overlain by a layer of unconsolidated sediment. 

Other types of episodic seepage events could accompany migratory earthquake swarms [Sibson, 

1996] or result from sediment degassing to the water column due to storm surges or mud 

diapirism [Hovland, 1993]. Such activity would be ephemeral in nature, comprised of sporadic 

events. On the other hand, changes in seep plumbing resulting from blockage of seepage 

pathways, seismically induced structural changes, exposure by erosion or burial by deposition 

would tend to irreversibly alter seep distribution. This illustrates the extremes over which 

changes in seep distribution can occur, from transitory events to permanent alterations. 

Cyclic variations in seepage may also operate between these two extremes. Seasonal changes or 

tidal cycles have been hypothesized to cause periodic oscillations in seepage rates. Wilkinson 

[1972] states some observers claim flow volumes of oil from Coal Oil Point seeps increase 

during the summer months, although he does not quantify this observation. Possible causes for 

this phenomena would include the decrease in oil viscosity and solubility of methane gas with 

increasing water temperature. Mikolaj and Ampaya [1973] claimed that increased tidal height 

decreased seepage of oil at the nearshore Isla Vista seeps just off Coal Oil Point which they 

attributed to the increase of the hydrostatic head. Hovland et al. [1993] cite reduction in 

hydrostatic pressure associated with ebb tide triggering ebullition of methane from supersaturated 

pore waters. Such intermittent oscillations based on seasonal or tidal cycles could vary seep 

emission volumes in addition to episodic seepage events or permanent changes altering seep 

distribution. 

Anthropogenic effects could also affect seep distribution patterns and emission rates over time. 

Many authors point to the effect of oil production in reducing seepage rates [Fischer and 

Stevenson, 1973; Landes, 1973; Fischer, 1977; Kvenholden and Harbaugh, 1983]. Numerous 

examples throughout the world demonstrate a reduction in natural hydrocarbon seepage 

associated with oil production. The reduction of reservoir pressure associated with production is 

one possible mechanism for causing seepage to dry up. Fischer and Stevenson [1973] 

propounded this hypothesis for the Coal Oil Point seeps. On the other hand, production may 

simply remove leaking hydrocarbons from the subsurface. Landes [1973] offers examples of the 

Kirkuk field in Iraq and Mene Grande in Venezuela where natural seeps have diminished as 

production has drawn down hydrocarbons in the reservoirs. Kvenholden and Harbaugh [1983] 

offer the example of Pitch Lake on Trinidad which once provided a continuous flow of asphalt 

into the Gulf of Paria until mining lowered the lake below its outlet. 

In alluding to the role of man’s activities in reducing hydrocarbon seepage into the marine 

environment worldwide, Kvenholden and Harbaugh [1983] point to the previous examples which 

they state “document the assertion that continued production of petroleum should cause seepage 

rates to decrease in the future”, but they also point out that in some cases “secondary recovery 

methods using increased formation pressures could possibly cause increased rates of oil 

seepage”. 
A Methodology for Investigation of Natural Hydrocarbon Gas Seepage

 19

Wilkinson [1972] relates the sequence of events leading to one such man made oil seep during 

the 1969 oil spill in Santa Barbara: 

“On January 29, 1969, a well being drilled from (Union Oil Company’s) platform “A” 

penetrated a high pressure zone below 3,400 feet. Gas from this zone started flowing up the 

well bore to the surface. The well was closed in and remained temporarily under control, but 

pressure continued to increase within the well until gas-charged drilling fluid finally broke 

through the uncased portion of the hole into the shallow oil sand lying just below the sea 

floor. The gas soon permeated the sand apparently forming fissures through which the now 

gas-saturated oil literally erupted from the sea floor forcefully expelling oil and gas into the 

sea.” 

This example illustrates the potential for anthropogenic effects associated with oil production 

altering rates and distribution of hydrocarbon seepage. In this case gas formed fissures provided 

the fracture pathways and the seepage was driven by a high pressure gas zone, which highlight 

the possibility that similar mechanisms may be responsible for natural seepage. Although the 

immediate effects of this oil spill were severe, with flow rate estimates from 500 to 5,000 barrels 

per day (80,000-800,000 liters/day), long term effects of the spill were negligible and recovery of 

the area complete [Straughan and Abbott, 1971]. 

For the seeps in the Coal Oil Point area, oil production is most likely responsible for a long term 

trend of decreasing seep emissions coinciding with a progressive reduction in the area covered by 

seepage [Fischer and Stevenson, 1973; Fischer, 1977]. One important goal of this work is to 

attempt to corroborate this hypothesis but in a more rigorous way using quantified estimates of 

seep emissions and variations. 

5.1 NATURAL HYDROCARBON SEEPAGE AT COAL OIL POINT, 

CALIFORNIA 

W. P. Blake of the U. S. Geological Survey, an early explorer of the western United States, 

commented on natural marine hydrocarbon seepage in the Santa Barbara Channel in 1853, “I am 

informed ... that the channel between Santa Barbara and the islands is sometimes covered with a 

film of mineral oil.” [Hunt, 1979]. It is quite likely that this reference referred to seepage in the 

vicinity of Coal Oil Point. Early explorers and Spanish missionaries confirm the long 

documented historical record of the prolific seepage at this site. For example, Padre Pedro Font 

wrote in 1776 while near Goleta: “Much tar which the sea throws up is found on the shores ... 

perhaps there are springs of it which flow out into the sea.” [Wilkinson, 1972; Fischer, 1977], 

and Captain Cook’s navigator Vancouver wrote in his journal while anchored off Goleta in 1793: 

“ ... the sea had the appearance of dissolved tar floating on its surface, which covered the ocean 

in all directions within the limits of our view ...“ [Fischer, 1977]. In addition, Native American 

Chumash in the Santa Barbara area were known to utilize seep tar for caulking canoes and 

waterproofing baskets at least 7,000 years ago, well before the arrival of westerners [Wilkinson, 

1972; Fischer, 1977]. 
Final Study Report – Luyendyk et al. 

20

References to the Coal Oil Point seeps are also abundant within the scientific literature [Vernon 

and Slater, 1963; Allen et al., 1970; Wilkinson, 1972; Fischer and Stevenson, 1973; Mikolaj and 

Ampaya, 1973; Wilson et al., 1974; Fischer 1977; Reed and Kaplan, 1977; Kvenholden and 

Harbaugh, 1983; Hovland et al., 1993]. Wilson et al. [1974] note in general the paucity of data 

on offshore seeps throughout the world due to insufficient exploration, but the Coal Oil Point 

seeps are an exception, and probably the most studied in the literature. 

The Coal Oil Point seeps are located along the northern flank of the Santa Barbara basin (Figure 

1). This is a typical environment with high seepage potential [Link 1952; Wilson et al., 1974], 

the margin of a basin with known petroleum accumulations and strike slip faulting and tight 

compressive folding associated with a high incidence of earthquakes. The bedrock of the 

northern shelf is predominantly Miocene age siliceous and diatomaceous shales and silts of the 

Monterey and Sisquoc Formations onlapped at the seaward edge by the Pliocene age Repetto and 

Pico Formations [Vernon and Slater, 1963; Fischer and Stevenson, 1973; Fischer, 1977]. A 

variable thickness veneer of unconsolidated late Quaternary silts, sands, and conglomerates rests 

unconformably atop the bedrock. The fractured Monterey Formation is both source and reservoir 

rock for hydrocarbons [Isaacs and Peterson, 1987] and is the source of the seep oil and gas at 

Coal Oil Point [Reed and Kaplan, 1977; Hartman and Hammond, 1980]. 

Seep trends (Figure 2) align with dominant east-west structural features which parallel the 

tectonic grain of the Western Transverse Ranges. Faulted east-west trending anticlines correlate 

with tar seeps from fractures in Miocene siliceous shales near Point Conception [Vernon and 

Slater, 1963], and elongated seepage structures in the Coal Oil Point area also parallel anticline 
A Methodology for Investigation of Natural Hydrocarbon Gas Seepage

 21

crests (Figure 3) [Fischer and Stevenson, 1973; Fischer, 1977]. Dense networks of crestal 

fractures within the anticline axes are indicated by the absence of seismic reflections [Fischer and 

Stevenson, 1973], and inferred to be seep conduits for vertical fluid migration. Faulting is 

assumed to be an important mechanism for introducing the secondary porosity of these fracture 

networks, and high angle cross faults may serve as additional seep pathways [Fischer, 1977]. 

This could lead to “bright spots” of intense seepage where faults intersect anticline axes, and 

fault splays may cause irregular deviations from the linear seepage trends [Fischer, 1977]. 
Final Study Report – Luyendyk et al. 

22

Thickness of the overburden of unconsolidated Quaternary sediment has also been suggested to 

control the distribution of seepage [Fischer and Stevenson, 1973; Fischer, 1977]. However, since 

sediment cover tends to truncate over structurally controlled topographic highs such as anticlinal 

axes [Fischer, 1977], it is possible that this trend indicates the structural influence governing 

seepage rather than demonstrating a causal effect of its own. 

Figure 2 shows a comprehensive map of seep distributions in the Coal Oil Point area compiled 

using oil company data from 1946-1947 and 1953-1956, and sonar data from 1972-1973 as 

presented by Fischer [1977]. It should be noted that by combining the data sets, the map fails to 

illustrate the erratic variations in seep distribution claimed by Fischer [1977] which he attributed 

to the intermittent nature of seepage, but which could also arise from different coverage of the 

surveys, or navigational errors. However, the map does demonstrate the linear east-west trends of 

seepage which Fischer [1977] described. 

There is no consistent naming convention in the literature regarding the seep areas mapped in 

Figure 2. The entire map view area has been collectively termed the “Coal Oil Point Seeps” 

[Wilson et al., 1974; Kvenholden and Harbaugh, 1983; Hovland et al., 1993], but Fischer [1977] 

referred to the “Coal Oil Point Seeps” as the nearshore trend while Wilkinson [1972] designated 

the “Coal Oil Point Seeps” in the area of a, b, and c in Figure 2. Labels a, b, c, d and e in Figure 2 

represent the sites of measurements by Allen et al. [1970]. The study of Mikolaj and Ampaya 

[1973] was confined to the area of d and e, termed the “Isla Vista Seeps” by Wilkinson [1972]. 

Other names include “La Goleta Seeps” [Wilkinson, 1972] or “West Rincon Seeps” [Fischer, 

1977] for the large seep to the southeast of the map, “Goleta” and “West Goleta” [Fischer, 1977] 
A Methodology for Investigation of Natural Hydrocarbon Gas Seepage

 23

for the irregular seep in the middle of the map, and “South Ellwood Seeps” [Fischer, 1977] in the 

area around Platform Holly and the Seep Tents. Holly is an offshore oil platform producing from 

the Monterey reservoir through Sisquoc cap rock. The Seep Tents are a pair of large steel 

pyramids approximately 100 m2 at the base installed on the seafloor by ARCO in 1982 to cap a 

particularly active area of seepage [Rintoul, 1982; Guthrie and Rowley, 1983]. 

Figure 3 shows the structural features which correlate with the seep trends presented in Figure 2. 

Immediately offshore the Coal Oil Point Anticline defines the seepage trend studied by Allen et 

al. [1970] and Mikolaj and Ampaya [1973], who observed oil globules exuding from discrete 

openings through the unconsolidated sediment overlain on a bottom topography of fractured 

shale. The Monterey formation outcrops at the seafloor here in the axis of the Coal Oil Point 

Anticline [Fischer, 1977]. These nearshore seeps are predominantly oil and tar seeps similar to 

those in the Point Conception area [Vernon and Slater, 1963], whereas the outer seep trends 

further offshore tend to be dominated by roughly equal parts of oil and gas [Fischer, 1977]. This 

increase in the ratio of gas to oil moving offshore may reflect stages of seep evolution [Fischer, 

1977], with gas venting off from the relatively younger seeps which become increasingly 

dominated by oil as they mature. Alternatively, this may reflect that while inshore oil seepage 

occurs directly from the Monterey Formation reservoir, further offshore gas buoyancy is required 

to provide a driving mechanism for transport through the Sisquoc cap rock. 

Outside of the inner Coal Oil Point Anticline a second anticline unknown to Fischer [1977] 

coincides with the central trend of seepage in Figure 2. These paired anticlines interpreted as the 

Coal Oil Point fold complex [Bartsch et al., 1996] culminate just south of the point, where 

persistent seepage has occurred in the past [Fischer, 1977]. The central seepage trend 

corresponds to where the second anticline axis and synclinal limbs intersect the seafloor, and 

may be most intense where axial surfaces are cut by cross faults [Bartsch et al, 1996], as 

suggested by Fischer [1977]. The linear feature c in Figure 2 described by Allen et al. [1970] lies 

along the projection of an onshore-offshore fault striking N15°W and dipping 45° east [Fischer 

and Stevenson, 1973; Fischer, 1977]. 

The outermost seep trends align with the axis of the South Ellwood Anticline (Figure 3). This is 

a multiple plunging anticline with a series of structural culminations which control the 

distribution of seepage. Bore hole data suggest that maximum seep intensity following the trend 

of the axial surface correlates to locations where the Sisquoc/Monterey formation contact is more 

shallow [Bartsch et al., 1996]. One culmination lies beneath Platform Holly, a second beneath 

the Seep Tents, and a third in the center of the La Goleta seep field, each of which was a location 

of intense historical seep activity [Fischer, 1977]. Due to this structural relief, Sisquoc outcrops 

on the seafloor around Platform Holly and the Seep Tents along the South Ellwood Anticline 

axis. The structural saddle along this axis may also be responsible for the separation between the 

two outermost seepage trends in Figure 2 [Fischer, 1977]. 
Final Study Report – Luyendyk et al. 

24

5.2 PREVIOUS ESTIMATES OF SEEPAGE FLOW RATES AT COAL OIL 

POINT 

Previous quantitative estimates of seepage in the Coal Oil Point Area have been made [Allen et 

al., 1970; Mikolaj and Ampaya, 1973; Fischer, 1977], but the inherent inaccuracies in these 

estimates are acknowledge by the authors given the difficulties involved in such measurements 

and the possibility that seep rates could fluctuate. Allen et al. [1970] estimated oil seepage in the 

nearshore area (a, b, c, d and e in Figure 2) leaked 50-70 barrels/day (8000-11,000 liters/day) 

with a possible range from 10-100 barrels/day (1600-16,000 liters/day). Mikolaj and Ampaya 

[1973] estimated oil flow of 12.5 barrels/day (2000 liters/day) confined in the area of d and e. 

These studies were concerned only with rates of oil leakage and not gas. Fischer [1977] 

determined flow rates of liquid oil for the entire offshore area of seepage would be 55 to 800 

barrels/day (8800-12,800 liters/day) based on the local rates of Allen et al. [1970], but allowing 

for the higher gas fraction in the outer seep trends would revise this estimate to 25-400 

barrels/day (4000-64,000 liters/day) [Fischer, 1977]. Estimates for the emissions of gaseous 

hydrocarbons from seepage are also available. Hovland et al. [1993] estimated a methane flux of 

400 g m-2 

yr

-1 from the Coal Oil Point area based on the volume of gas collection at the Seep 

Tents (a point source) distributed over the 18 km2 area surveyed by Fischer [1977], and Saxena 

and Oliver [1984] estimated 18 to 152 tons of ROG’s per day are produced by all the Santa 

Barbara Channel seeps. 

6.1 OCEANOGRAPHIC SURVEYS 

To map the distribution of gas seeps offshore of Coal Oil Point for this study, a series of marine 

surveys were undertaken in 1994, 1995, and 1996. Locations of gas seeps associated with the 

offshore seepage trends were pinpointed with acoustic reflection systems which sense the 

presence of gas bubbles in the water from the density contrast between seawater and these 

bubbles [Sweet, 1973; Tinkle, 1973]. Accurate navigation was insured with differential GPS. 

The inshore oil seeps were not mapped for several reasons: the shallow inshore water and kelp 

presented logistical problems for the survey boats, oil seeps lack the distinctive acoustic 

signature of gas seeps since density contrast between oil and water is low, and the inshore oil 

seeps have already been extensively studied in the past. 

Two different frequency acoustic systems, one 50 kHz and the other 3.5 kHz, were utilized for 

the surveys. Since frequency response of the sonar is dependent on bubble size distribution, data 

for the different surveys is not completely comparable. The 3.5 kHz sonar would have a stronger 

return but be more non-linear based on the location of the resonance peak with respect to the 

observed distribution of bubble sizes (see Section 8.1).

The 50 kHz acoustic source was a hull-mounted Raytheon wide-beam transducer (model MCPT 

25-05) operated from the Genoa out of Santa Barbara Harbor. The analog signal was recorded 

with a Raytheon JRC JFF-770 paper chart recorder. The 50 kHz surveys were conducted 

November 20, 1994; December 27, 1994; August 15, 1995; and November 15, 1995. Cruising 

speed for surveys aboard the Genoa varied between 5.6 and 8.4 knots, with an average cruising 

speed of 7.0 knots. The resulting vertical exaggeration of the paper records was approximately 

40:1 given the chart recorder speed. 
A Methodology for Investigation of Natural Hydrocarbon Gas Seepage

 25

The 3.5 kHz acoustic source was a towfish-mounted transducer deployed from the Seawatch, a 

vessel in the fleet of the Southern California Marine Institute operating out of San Pedro, 

California. The transducer was towed at a depth of 10 m at a cruising speed of approximately 5

knots. Analog records were recorded simultaneously using both a Gifft-type wet paper recorder 

and an EPC dry (thermal) paper recorder. In addition, digital data was collected using a SUN IPX 

workstation equipped with an A/D Board and digitizing software developed at the University of 

Hawaii. The A/D was triggered by the same recorder that triggered the transducer. The 

digitization rate was 10 kHz (0.1 msec sample period). The transceivers were operated without 

time varied gain (TVG), and a Krone-Hite filter bandpassed the signal from 3 to 4.8 kHz to 

eliminate excess noise. The 3.5 kHz surveys were conducted July 26-27, 1995; September 28-30, 

1995; and August 15-17, 1996. In addition to this newly acquired data, historical 3.5 kHz analog 

paper records from surveys made by Peter Fischer in 1972 and 1973 were obtained for the 

purpose of comparing the change in seep distribution over a 20 year time period.

6.2 METHOD OF MAPPING SEEP DISTRIBUTION 

The acoustic data acquired during the marine surveys offshore Coal Oil Point provide a cross 

section of the water column and sub-bottom along the navigational track of each survey. The 

delay time of each successive acoustic pulse is related to the depth within the cross section by the 

sound speed, which is approximately 1500 m/s for the seawater and water saturated seafloor 

sediments. Figure 4b illustrates a sample cross section of the water column and sub-bottom for a 

line running roughly west-east from the location of the Seep Tents to the La Goleta Seep field 

along the axis of the South Ellwood Anticline (location of the cross section line in map view is 

shown in Figure 5). This particular cross section is digitally recorded 3.5 kHz data from the 

August 1996 cruise. 
Final Study Report – Luyendyk et al. 

26
A Methodology for Investigation of Natural Hydrocarbon Gas Seepage

 27

The dark columnar bands within the water column are representative of gas seeps. In the case of 

analog paper records, the paper scrolls were scanned on an HP Deskjet scanner at 200 dpi 

resolution to acquire 8 bit digital images which could be processed in a similar manner to the 

image displayed in Figure 4b. The resulting scanned images have a grayscale with 256 levels of 

pixel brightness. Since seeps are represented by dark areas in the images, the grayscale was 

inverted to darkness by subtracting the brightness value from 255. The scanned analog records 

have less dynamic range and saturate at a lower level than the digitally recorded data, but offer 

greater coverage and allow comparison with historical datasets. 

To convert the cross section data into a map view plot of seep intensity distribution, a line profile 

of relative seep intensity was constructed as shown in Figure 4a. The line profile is obtained by 

scaling the intensity within a 20-30 m depth window of the cross section relative to mean 

background intensity values and normalizing by the average intensity of the outgoing acoustic 

pulse which is the dark band across the top of the record. This can be represented in equation 

form as: 

 

δ = α – β (6.2.1) 

 γ - β

Here δ is the normalized darkness profile as shown in Figure 4a, α is the intensity within the 

water column window, β is the mean intensity within the water column window in the absence of 

seepage, and γ is the mean intensity of the outgoing acoustic pulse. 
Final Study Report – Luyendyk et al. 

28

For the scanned analog images, mean values of darkness were taken as the proxy for intensity. 

For the digital 3.5 kHz data such as that displayed in Figure 4, the standard deviation of the 

signal over a given time window was taken as the proxy for intensity. Standard deviation is the 

root mean square deviation and represents relative energy of the signal since it is equal to the 

square root of the sum of energy over all frequencies from Parseval’s theorem. The range of data 

for calculating intensity of the outgoing pulse was considered to be the first 50 points of each 

shot trace, corresponding to the 5 msec pulsewidth of the sonar based on the 10 kHz digitization 

rate. The range from 125 to 275 points within each shot trace approximately corresponded to the 

depth window of 20-30 meters in the water column based on two way travel time for a seawater 

velocity of 1500 m/s and a towfish depth of 10 m. 

The relative seep intensity δ profile data, as shown in Figure 4a, were subsequently gridded at 

100 by 100 meters. Separate gridded data sets were calculated for the various surveys, both 

analog and digital, in order to present a series of seep distribution maps over time. The gridded 

data was contoured using a tension spline surface algorithm [Smith and Wessel, 1990] and 

displayed as a relative intensity plot of seep distribution. An estimated threshold level for the first 

contour above the level of noise was selected as 0.05 for the digital data and 0.1 for the scanned 

analog data.

7.0 SEEPAGE DISTRIBUTION MAPS

The seep distribution displayed in Figure 5 for the August 1996 3.5 kHz digital data is similar to 

the distribution reported by Fischer [1977] (Figure 2) with respect to the various structural trends 

(Figure 3), but the coverage is not as extensive as his. Note that the Coal Oil Point seep trend 

mapped by Fischer [1977] (Figure 2) represents oil seepage and should not be directly compared 

to Figure 5 which plots the distribution of gaseous seepage. The seepage due south of Coal Oil 

Point in Figure 5 between 34°23’ and 34°24’ North and -ll9°52 to -1l9°54’ West is not as 

extensive as the Goleta seepage trend reported by Fischer [1977] (Figure 2). However, Fischer

[1977] claims seepage within this trend was intermittent on the 10 to 20 year time scale between 

the surveys on which he based his map, acquired from 1946 to 1977, with the most consistent 

seepage occurring in the area we show due south of Coal Oil Point (Figure 5). 

The outer seepage trends are also less extensive and noncontiguous in Figure 5 as compared with 

Figure 2. In particular, part of the South Ellwood seep trend is absent in the area to the west 

around Platform Holly, and the West Rincon or La Goleta seep trend identified in Figure 2 is 

more tightly confined along the anticline axis. Fischer [1977] proposed a progressive shrinking in 

the extent of seepage based on his own comparison of data from 1946-47, 1953-56, and 1972-73 

(1976); the August 1996 3.5 kHz digital data (Figure 5) appears to corroborate his hypothesis. 

However, Fischer [1977] does not rigorously define his method for judging what constitutes 

seepage or describe how he arrived at the seep distribution mapped in Figure 2. In addition, 

navigational techniques available for the historical surveys of the seeps were less accurate and 

precise than modern differential GPS. Therefore, the accuracy of the seep distribution in Figure 2 

is unknown. 

To allow a more one-to-one comparison of seep distribution as mapped by Fischer with the 

distributions determined from our own surveys, the 3.5 kHz analog scrolls acquired by Fischer in 
A Methodology for Investigation of Natural Hydrocarbon Gas Seepage

 29

1972-73 were borrowed and scanned to obtain these analog records in a digital file format. To 

make the comparison more direct, our own Figure 6 shows the comparison of seep distribution 

plotted from analog 3.5 kHz data collected by Fischer in 1973 (Figure 6a) and our survey in July 

1995 (Figure 6b) along the axis of the South Ellwood Anticline. The box on the map outlines the 

13 km2 region where the navigational coverage is similar, and track lines from which data from 

the two surveys are plotted are also shown. Figure 7 shows the same comparison, but within the 

13 km2 boxed region of Figure 6. From Figure 7 it is apparent that the distribution of gaseous 

seepage decreased within a few kilometers of Holly over the 22 year time period spanned from 

1973 to 1995. 
Final Study Report – Luyendyk et al. 

30

Fischer [1977] stated that a progressive decrease in the extent of seepage occurred offshore Coal 

Oil Point due to oil production from Holly. This seems to imply shrinking inward of the seep 

trends toward the platform. However, what is actually observed in Figure 7 is the disappearance 

of seepage outward from the platform while peripheral seepage is left intact. The observed 

decrease in seepage outward away from Holly contradicts slightly Fischer’s [1977] original 

statement. However, this could be explained as the result of oil production from platform Holly 

gradually being pushed further and further afield. On the other hand, if one is to believe the 

accuracy of the seep distribution mapped by Fischer [1977] (Figure 2) going back to the 1950’s, 

then Fischer’s statement is true for other seep trends in the Coal Oil Point area but not for those 

in the immediate vicinity of Holly.

Since the comparison with Fischer’s data was only valid within a limited portion of the survey 

area, we also conducted comparisons of our own data sets for the series of surveys that we had 
A Methodology for Investigation of Natural Hydrocarbon Gas Seepage

 31

acquired. Figure 8 shows the distribution of seepage compiled from 3.5 kHz analog and digital 

data obtained in the summer of 1995 during July and September. The combined coverage of 

these surveys, as shown by the track lines, contains a few gaps compared to the coverage in 

August 1996. This may explain some of the differences between the seep distributions displayed 

in Figure 5 and Figure 8. An example is the gap in the 1995 coverage roughly due south of Coal 

Oil Point (Figure 8). Although GPS navigation insures that seeps are accurately located, gaps and 

holes in the plots of seep distribution should be considered suspect where coverage is absent. 

Furthermore, since our method for locating gas seeps maps bubbles at 20-30 m depth within the 

water column rather than at the seafloor source, the shearing effect of water currents visible in 

cross section is expected to shift the displayed distribution of seepage west-southwest of the seep 

sources given the predominantly westward local current flow which shifts slightly south with ebb 

tide [Fischer, 1977; Kolpack, 1977]. Given bubble rise velocities (see Appendix 1) and 

maximum current velocities [Fischer, 1977] are on a similar order of about 20 cm/sec, seep 

bubbles would be expected to shift a horizontal distance on the order of the water depth by the 

time they reach the sea surface, implying a maximum shift in position of approximately 30 m by 

the time the bubbles reach the 20-30 m depth window from a typical 60 m depth. 

Figure 5 and Figure 8 are similar but with some differences. Both distributions show the 

dominant La Goleta, or West Rincon, seepage trend along the South Ellwood Anticline axis as 

well as an amorphous region of seepage to the south of Coal Oil Point. Meanwhile seepage 
Final Study Report – Luyendyk et al. 

32

around Holly is absent, suggestive of the long term change in seep distribution demonstrated in 

Figure 6 and Figure 7. Neglecting the seeps south of Coal Oil Point where different coverage 

precludes a comparison, the seep distribution along the South Ellwood Anticline Axis appears to 

remain relatively stable over a one year time scale but with some fluctuations. For example, the 

tongue of seepage east of the Seep Tents in Figure 8 is absent in Figure 5, as is the seepage west 

of Holly, although this may have been skirted by the August 1996 navigation. 

Short term changes in seep distribution may occur in addition to long term variation, but an 

appropriate cause for explaining these changes is currently unavailable. Wilkinson [1972] 

proposed seasonal variations might affect seep distribution, but both Figure 5 and Figure 8 

present seepage during the summer months, and although episodic seepage events could also 

provide an explanation there is no evidence of activity, such as an earthquake, which might 

trigger such an event during this time period. 

Another short term process previously proposed to affect seepage is tidal forcing. Mikolaj and 

Ampaya [1973] claimed a correlation between the tidal height, or hydrostatic head, and flow 

rates of oil at the inshore Isla Vista Seeps (d and e in Figure 2). This would tend to cause cyclic 

variation in seep emission rates. To examine if emission from gas seeps varied with tidal height, 

similar profile lines collected at high and low tide were compared. Such a comparison of profiles 

for two 50 kHz survey lines collected through the intense La Goleta seep field is shown in Figure 

9a with the corresponding navigation displayed in Figure 9b. The high tide data was collected at 

approximately 10:00 am local time on November 20, 1994 while the high tide at Santa Barbara 

Harbor on this day peaked at 5.8 feet (1.8 m) at 9:18 am. The low tide data was collected 

approximately at noon on December 27, 1994; low tide on this day at Santa Barbara Harbor 

peaked at 0.6 feet (0.2m) at 12:05 pm. The difference in tidal height for the two records was 

therefore approximately 5 feet (1.5m). 
A Methodology for Investigation of Natural Hydrocarbon Gas Seepage

 33

The two profiles in Figure 9a appear similar in detail in the left half of the plot, albeit with some 

slight shift. The right half of the plot shows some variation with a larger emission at low tide. 

However, this may be due to slight divergence of the two track lines to the west as shown in 

Figure 9b. Also, since the tracks were taken approximately one month apart, variations other than 

those caused by tide may be present. However, other compared profiles, including those 

compared for different tidal heights on the same day, appeared quite similar to the profiles in 

Figure 9a. If the profiles of relative sonar intensity obtained using equation (6.2.1) are an accurate 

representation of relative volumes of seep emission, it would appear that volume emission rates 

of gaseous seeps are relatively insensitive to tidal cycles. In addition, emission remained 

relatively constant over the one month time period between November 20, 1994 and December 

27, 1994. 
Final Study Report – Luyendyk et al. 

34

It may be that short term variations in seep distribution are a second order effect in comparison to 

the type of long term variation illustrated in Figure 6 and Figure 7. This assumption was made 

when compiling July and September 1995 3.5 kHz data into a single plot of seep distribution 

(Figure 8). Similarly, all the 50 kHz data collected over a one year period during surveys in 

November 1994, December 1994, August 1995, and November 1995 were incorporated into a 

single dataset to represent seep distribution averaged over this one year period (Figure 10). In 

part, combining the different surveys into common datasets was motivated by lack of 

navigational coverage. Combining different surveys provides more complete coverage to allow 

better spatial comparisons of changes in seep distribution for the whole Coal Oil Point offshore 

area. However, at the same time combining the data tends to obscure time variations in seep 

distribution which may vary for different surveys. 

The distributions of seepage based on the August 1996 digital 3.5 kHz data, July and September 

1995 analog and digital 3.5 kHz data, and November 1994 through 1995 50 kHz data are plotted 

together in Figure 11. It is questionable whether comparing data at different frequencies is 

entirely appropriate since frequency response varies for different bubble distributions as 

discussed later in Section 8. However, the three surveys do present similar distributions of 

seepage. The La Goleta or West Rincon seep trend running roughly east from the Seep Tents 

along the axis of the South Ellwood Anticline is remarkably consistent. The Goleta Seep trend 

south of Coal Oil Point is more variable, in part due to difference in coverage, but is nevertheless 

a persistent site of seep activity. Variable seep activity is evident west and north of Holly in the 

areas previously designated as South Ellwood and West Goleta Seeps [Fischer, 1977] (see Figure 

2). The immediate vicinity of Platform Holly is consistently barren of seepage in contrast to 1973 

(Figure 6a and Figure 7a). 
A Methodology for Investigation of Natural Hydrocarbon Gas Seepage

 35

The map in Figure 11 seems to suggest that although seepage may be somewhat variable, 

dominant trends of seepage are remarkably persistent and fairly stable over the short time scale 

between these surveys. This is in contrast to the long term change demonstrated in Figure 6 and 

Figure 7. These dominant seepage trends correlate with the told axes shown in Figure 2 and 

discussed previously in Section 5.1. Previous seepage in the proximity of Platform Holly has 

apparently disappeared since 1973 which may provide evidence of oil production reducing 

natural seepage [Fischer and Stevenson, 1973; Fischer, 1977]. 

8.1 QUANTIFYING SEEP EMISSION RATES - THEORETICAL 

CONSIDERATIONS 

For our study, submarine gas seeps were located by acoustic sensing of the prolific gaseous 

component of seepage which is so prevalent in the outer seep trends. We also desired to obtain 

quantified estimates of emission volumes of this gaseous seepage based on our data. Previous 

estimates of liquid petroleum seepage in the Coal Oil Point area were determined based on the 

sizes of surface slicks of an assumed thickness and direct collection of oil extruded at the 

seafloor [Allen et al., 1970; Mikolaj and Ampaya, 1973]. Remote sensing techniques have also 

been employed to detect surface slicks [Kraus et al., 1977; Estes et al., 1985] and estimate the 

corresponding oil emission [Clester et al., 1996]. 

Quantifying the gaseous portion of seepage from our backscattered acoustic data is more 

problematic and requires calibrating relative sonar return in terms of the rate of gas emission. 
Final Study Report – Luyendyk et al. 

36

Acoustic backscatter from bubble clouds is dependent on both the number and sizes of entrained 

bubbles, and acoustic frequency [Medwin, 1977a, b; Clay and Medwin, 1977; Urick, 1983]. The 

frequency dependence of acoustic parameters on bubble radii allows bubble size distributions to 

be inferred from acoustic backscatter at different frequencies [Vagle and Farmer, 1992]. 

Backscatter data have advantages over in situ measurements because they can be collected 

efficiently over relatively large water volumes [Akulichev et al., 1986; Vagle and Farmer, 1992]. 

However, estimating bubble distributions from backscatter data requires a complicated data 

inversion and involves numerous experimental uncertainties. 

For simplicity we consider the active sonar equation in the form used by Vagle and Farmer 

[1992], equation (2): 

This equation states that for active sonar, the received level (RL) relative to the source level (SL) 

is dependent on the target strength (TS) of the reflector and the two way transmission loss (2TL). 

Transmission loss arising from the presence of bubbles is neglected in equation (8.1.1). Target 

strength is expressed [Vagle and Farmer, 1992]: 

where Mυ is the scattering cross section per unit volume and U is the insonified volume, which 

depends on characteristics of the sonar such as pulse length, beam pattern (solid angle), and 

range to the target. If sonar parameters are constant, target strength depends only on scattering 

cross section per unit volume Mυ, which is defined [Vagle and Farmer, 1992]: 

σs is the acoustic scattering cross section of an individual bubble and n(a).da is the number of 

bubbles per unit volume with radii between a and a+da. 

The bubble scattering cross section σs has a pronounced peak which is much higher than the 

geometrical cross section (πa

2 for spherical bubbles) at the radius which corresponds to the 

resonance frequency with respect to the insonifying acoustic source. Scattering cross section σs is 

given by [Medwin, 1977a, b]: 

where ƒR and ƒ are resonant and source frequency respectively, and δ is the damping constant. 

Figure 12a shows theoretically calculated values of scattering cross section for 3.5 kHz and 50 
A Methodology for Investigation of Natural Hydrocarbon Gas Seepage

 37

kHz sources. If bubble clouds are tightly packed, bubble-bubble interactions and multiple 

scattering will broaden the resonant peak [Vagle and Farmer, 1992]. 

The expression for scattering cross section in equation (8.1.4) is valid for bubble radii which are 

small with respect to the acoustic wavelength in the medium [Akulichev et al., 1986]. For our 3.5 

kHz and 50 kHz acoustic sources, the acoustic wavelengths in seawater are approximately 40 cm 

and 3 cm respectively. The distribution of bubble sizes for the La Goleta Seep field was 

estimated by analyzing a video of gas bubbles rising through a grid of known dimension placed 

at the sea surface. This distribution of bubble sizes is shown in Figure 12b. However, distribution 

of seep gas bubble sizes is not necessarily homogenous throughout the offshore Coal Oil Point 

area. 
Final Study Report – Luyendyk et al. 

38

Mean bubble radii for the observed distribution of 108 bubbles was 0.6 cm. This is sufficiently 

small in comparison to the acoustic wavelengths used but large compared to typical bubble sizes 

in near surface ocean waters [Wu, 1981] arising from natural processes other than submarine gas 

seepage. Bubble radii decrease with depth as does the radii of the resonant peak, both due to 

increasing ambient pressure. Measured bubble radii presented in Figure 12b were taken at the 

ocean surface and the calculated theoretical curves in Figure 12a are likewise based on surface 

values. The theoretical calculation was also performed assuming air was the bubble gas 

composition, whereas the true composition of the seep gas is given in Table 1. The parameters in 

equation (8.1.4) are dependent on properties of the gas composition such as density and specific 

heat [Medwin, 1977a], but these are second order effects. 

The peak of the observed bubble size distribution (Figure 12b) is well away from the 

theoretically calculated 50 kHz resonant peak but overlaps the 3.5 kHz peak (Figure 12a). Since 

our interest is in estimating volumes of gas entrained as bubbles, we are most interested in the 

larger radius bubbles as they contain the most volume (increasing as a

3 for spherical bubbles). 

However, overlap of the 3.5 kHz resonant peak shown in Figure 12a with the observed 

distribution of bubble sizes in Figure 12b suggests contribution by smaller radii bubbles may 

represent a significant portion of the sonar return for this frequency. 

The integral in equation (8.1.3) implies that the solution for volume scattering cross section Mυ is 

non-unique and solving for the bubble size distribution based on sonar return is therefore a 

complicated inverse problem. The approach of Vagle and Farmer [1992] was to use multiple 

frequencies to derive the bubble size distribution. However, their technique was designed for 

measuring microbubbles (a ≤ 150 μm) much smaller than the observed distribution of seep gas 

bubbles in Figure 12b. In addition, they used six frequencies as opposed to our two, and of those 

two the resolution and dynamic range of the (analog) 50 kHz data is limited compared to the 

digital 3.5 kHz data. 

8.2 EMPIRICAL CALIBRATION OF SEEPAGE RATES 

Faced with these difficulties and the complicated data inversion of Vagle and Farmer [1992], it 

was decided to attempt a simple empirical calibration of the 3.5 and 50 kHz data. To this end, 

sonar data was collected during a series of calibration experiments over artificial sources 

releasing gas at known rates. For the 3.5 kHz system an experiment was conducted off San 

Pedro, California in November, 1996 during which a canister of compressed nitrogen gas was 

connected via lengths of hose to diffusing nozzles deployed on the ocean floor in approximately 

40 m water depth. Flow rates were varied for a series of sonar passes and directly measured using 

two flow meters, then averaged and plotted as the abscissa in Figure 13a. Although both digital 

and analog data were acquired during this experiment, a problem with the digitization equipment 

resulted in only one good digital data point being obtained. 

The 50 kHz calibration was conducted separately in concert with the 50 kHz data acquisition 

cruises in November 1994, December 1994, August 1995, and November 1995. Each separate 

data point in Figure 13b was obtained during one of these cruises. The artificial source for these 

experiments were tanks of compressed air with a capacity of 71.4 ft3 (2021 liters) of gas at 3300 
A Methodology for Investigation of Natural Hydrocarbon Gas Seepage

 39

psi pressure attached to leaking SCUBA regulators. The tanks were deployed in approximately 

60 m water depth, and flow rate was inferred by measuring the initial and final tank pressure and 

the intervening time. 

By plotting the averaged emission rates E against the mean values of normalized intensity profile 

δ obtained over some area A, calibration coefficients were obtained based on empirical fits to the 

data. Average values δ • A verses emission rates E are presented in Figure 13a and b for the 3.5 

kHz and 50 kHz systems respectively. The ordinate axis in each plot is the mean value of relative 

intensity profile δ multiplied by the survey area A over which the δ profile value is averaged. 
Final Study Report – Luyendyk et al. 

40

Area A is calculated as the product of sonar swath width with the length of the analysis window 

along the survey track. 

An example is shown in Figure 14 for the case of a 3.5 kHz analog record. The scanned image of 

a plume is presented in Figure 14b for an artificial source releasing 2.45 m3/day of gas. Each 

column of the 20-30 m depth analysis window is averaged to produce the corresponding δ-profile 

of the plume shown in Figure 14a. The mean δ value δ = 0.28 is then multiplied by the length of 

the analysis window (30 m) and swath width of the sonar to obtain the value of δ • A averaged for 

the plot in Figure 13a. Swath width of the sonar is determined at the midpoint of the 20-30 m 

depth window based on the beam pattern (solid angle) of the sonar and was 15 m for both the 3.5 

kHz and 50 kHz systems. Length of the analysis window varied with the width of the artificial 

source plume, but was selected to be wide enough to encompass the whole plume profile. 
A Methodology for Investigation of Natural Hydrocarbon Gas Seepage

 41

The error bars in Figure 13a and b represent the standard deviations of δ • A and emission rate E

for separate sets of sonar passes. Linear least squares fits through the origin were matched to the 

set averages. For the 50 kHz data, the fit was only applied to the two lowest emission rates 

because the data appeared to saturate with increasing values of δ • A. The calibration coefficients 

for converting relative sonar return to seep gas flux were taken as the slopes of the linear fits in 

Figure 13a and b for the 3.5 kHz and 50 kHz data respectively. 

For the 3.5 kHz sonar, the calibration coefficient was c3.5 = 0.2 ± 0.06 m3

/m2.day, and for the 50 

kHz system c50 = 0.12 ± 0.02 m3

/m2.day. Error of these coefficients is calculated (Appendix 2) 

from the error in the slope of the fit curve, but the actual error in the estimates may be larger due 

to additional sources of error unaccounted for in the calculation (see Appendix 2). 

The lack of digital 3.5 kHz data from the November 1996 calibration experiment prevented the 

calculation of a separate digital coefficient, but the single good digital data point obtained falls 

close to lying on the calibration curve for the analog data in Figure 13a. Comparison of similar 

analog and digital profiles from July 1995 suggests the digital S profile data should by scaled 

upward by about 30% to correspond to the analog data. This is probably because the lack of 

dynamic range of the analog data causes premature saturation of the primary signal used to 

normalize the darkness profiles. 

Although the approach for arriving at the above calibration coefficients is wholly empirical and 

admittedly crude, it was hoped it could provide an order of magnitude estimate of gas seep 

emission rates. Since the seep distributions mapped in Figure 5, Figure 8, Figure 10 and Figure 

11 plot the intensity distributions greater than 0.1 δ for analog data (both 3.5 and 50 kHz) and 

0.05 δ for the digital 3.5 kHz data, these levels correspond to 20 ± 6 liters/m2

•day, 12 ± 2 

liters/m2• day, and 13 ± 4 liters/m2•day respectively. We can also add additional contours and replot these distributions in terms of the calibrated seep flux levels. This is done for the August 

1996 3.5 kHz digital data (Figure 15), July and September 1995 3.5 kHz analog and digital data 

(Figure 16), and 1994-1995 50 kHz analog data (Figure 17) respectively.
Final Study Report – Luyendyk et al. 

42
A Methodology for Investigation of Natural Hydrocarbon Gas Seepage

 43

8.3 EMISSION ESTIMATES 

The total emission of seep gas from offshore Coal Oil Point for each survey can be estimated by 

summing the values over the gridded data set and multiplying by the appropriate calibration 

coefficient: 

The area of each grid node was 100 by 100 m, or 104 m

2

, and only δ values greater than 0.1 or 

0.05 for analog and digital data respectively were included for the estimate. The total emission 

estimates (Table 2) are 10.7 ± 3 x 104

 m3/day of gas for August 1996, 20.3 ± 6 x 104 m3/day for 

July and September 1995, and 9.9 ± 2 x 104 m3/day for the 50 kHz data spanning 1994 and 1995. 
Final Study Report – Luyendyk et al. 

44

The distributions of emissions plotted in Figures 15, 16, and 17 demonstrate that volumetric flow 

from seepage in the offshore Coal Oil Point area is greatest above the culminations along the 

South Ellwood Anticline axis (Figure 3) and also due south of Coal Oil Point along what may be 

the trace of the fault described in Section 5.1 noted by Fischer [1977]. Comparison of flow rates 

for the different surveys seem to indicate flow rates are higher by a factor of two in July and 

September, 1995 compared with August 1996 and the 50 kHz data from 1994 and 1995. 

However, it is possible that the difference between these estimates arises from errors. Although 

the estimated error in seep flux calculated in Appendix 2 is only 20-30%, perhaps it should be 

revised upwards to 50% in order to encompass the observed range of estimates and account for 

additional errors not accounted for in the calculation (see Appendix 2). 

It is certainly likely that there is some fluctuation in seepage rates, but our ability to accurately 

observe such fluctuations probably depends on their scale in both time and in volume. For 

example, the decrease in areal cover of seepage in the 13 km2

 area around Holly over a 20 year 

time period presented in Figure 7 would be accompanied by a concomitant decrease in seepage 

calculated in Figure 18. Using the arbitrary 50% error suggested above, the estimated change in 

gaseous emission (Table 2) for this area declined from 5.1 ± 2.5 x 104 

m3/day in 1973 to 2.3 ± 1.2 

x104 m3

/day in 1995. 

If the individual estimates calculated from Figures 15, 16, and 17 are sufficiently accurate and 

precise, this represents a significant variation in seep flux on a short term (1 year) time scale as 

well. Neglecting the 50 kHz data which may differ due to its different frequency response, the 

apparent reduction of seepage between July - September 1995 and August 1996 could represent 
A Methodology for Investigation of Natural Hydrocarbon Gas Seepage

 45

part of a longer term trend of seep decline suggested by Figure 18. On the other hand, it might 

result from natural cyclic variations. The total seep emission for August 1996 (Figure 15) is 

actually larger than the 50 kHz estimate for 1994 and 1995 (Figure 17) which contradicts a 

hypothesis of decreasing rates of seepage over time. However, if Platform Holly is used to 

reference a demarcation line separating the further offshore seeps of the South Ellwood Seepage 

Trend to the south from the more inshore Goleta Seepage Trend to the north, spatial variations in 

seep emission are observed as well rather than a coherent variation for the entire offshore area. 

South of Holly navigational coverage extending to the southeast along the South Ellwood 

Anticline axis is sufficiently robust to allow a good comparison of seep distributions from the 

different surveys. The seep distribution mapped south of Holly from the 50 kHz data (Figure 17) 

presents a larger areal extent of seepage and higher rate of gas emission than the August 1996 

data (Figure 15). However, north of Holly the seepage covers less area and has a lower emission 

volume. It should be noted again, however, that differences in the frequency response between 

the 50 kHz and 3.5 kHz data may prevent a direct comparison. 

Comparison between emission volume estimates from 3.5 kHz data from August 1996 (Figure 

15) and July and September 1995 (Figure 16) are consistently different by a factor of two both 

north and south of Platform Holly. South of Holly the decrease in seep area along the South 

Ellwood Anticline may partly explain the lower emission in August 1996, but flow rates are also 

lower. In fact, north of Holly seepage covers a larger area in August 1996 but emission is still 

lower, although the dissimilarity in navigational cover makes this interpretation more 

problematic. However, it appears seepage varies independently both spatially and volumetrically. 

Causes other than error which could adequately explain the observed short term variations in 

seep distributions and fluctuations in emission rates remain elusive. Whether the apparent 

variability in seep distribution and emission rate fluctuate randomly or cyclically over time, or 

rather are part of a longer term monotonically decreasing trend is unclear. A long term trend 

producing the alteration of seepage around Holly displayed in Figure 18 suggests a monotonic 

decrease since this area is consistently barren of seepage in all our surveys. 
Final Study Report – Luyendyk et al. 

46

Time series of average monthly emission volumes collected at the Seep Tents presented in Figure 

19a also show a long term decline in emission rate. However, there are again shorter term 

fluctuations within this trend which defy an easy interpretation. The initial rate of gas collection 

at the Seep Tents following their installation in 1982 was 1,050,000 ft3 (30,000 m3

) of gas per 

day [Guthrie and Rowley, 1983]. Subsequent production increased due to the addition of flaps to 

the Seep Tents structure, causing the increase observed in 1987 (Figure 19a) [J. S. Hornafius, 

pers. comm., 1996]. Average production from 1983 to 1990 was 1,270,000 ft3/day (36,000 

m3

/day) [Hoviand et al., 1993]. 
A Methodology for Investigation of Natural Hydrocarbon Gas Seepage

 47

Around 1989-1990 a gradual decrease in seep emissions began (Figure 19a) which by 1994 had 

declined to about half of the peak production during 1987-1989. The sharp roll-off in Figure 19a 

after 1994 was caused by a mechanical failure of the line from one of the Seep Tents which was 

subsequently repaired but again failed, causing the spike in the time series in 1995 [J. S. 

Hornafius, pers. comm., 1996]. The mechanical problems affecting the Seep Tents prevent 

comparisons at the times of our surveys in 1995 and 1996. The second order fluctuations during 

the overall decline between 1989 and 1994, however, are not explained by mechanical factors. 

Oil production is a reasonable mechanism for producing the progressive long term declines in 

seepage illustrated by Figure 18 and Figure 19a. From 1983 to 1995, annual oil production from 

Holly gradually decreased from 500 x106 liters/year to about 200 x106 liters/year (Figure 19b) 

with corresponding gas production of 300 to 500 ft3/barrel (50 to 90 liters of gas/liter of oil) [J. S. 

Hornafius, pers. comm., 1996]. Figure 19b is a smooth curve, lacking sharp spikes in production 
Final Study Report – Luyendyk et al. 

48

that might account for the dips in seepage in 1990 and 1992 in Figure 19a. Furthermore, gas reinjection, which might also effect seepage rates, was not practiced at this time. 

It is interesting to speculate on other possible causes which could account for the second order 

variations which accompany the overall trend of declining seep emission demonstrated by Figure 

18 and Figure 19a. Short period tidal fluctuations might by partially responsible. Long period 

cyclic variations could include seasonal affects associated with gravity drive from groundwater 

reservoir pressure or water temperature altering oil viscosity and methane gas solubility. Such 

mechanisms might also account for the observed short term variations between Figures 15, 16, 

and 17. The longer term trend of a 50% reduction in seepage within 13 km2 of Holly over a 20 

year period (Figure 18) and the 50% decline at the Seep Tents over a 5 year period (Figure 19a) 

probably arises due to oil production, and suggests a total decline of gaseous emission on the 

order of 50,000±20,000 m3

/day since 1973! 

9.1 DISCUSSION

The data presented here demonstrate both spatial and temporal variation in the distribution and 

emissions of natural gaseous marine hydrocarbon seepage offshore of Coal Oil Point. The 

dominant spatial constraint on seep distribution is structural control as addressed in Section 5.1. 

The distribution of offshore seepage mapped from the August 1996 3.5 kHz data with respect to 

the various structural trends is presented in Figure 20. A north-south cross section of the 

subsurface geology offshore Coal Oil Point interpreted from well data after Bartsch et al. [1996] 

is presented in Figure 21a. The elongated linear seep trend furthest offshore is tightly confined to 

the South Ellwood Anticline Axis with the points of most intense seepage at structural 

culminations where relief of the Sisquoc-Monterey contact is more shallow. This is illustrated in 

the interpreted geologic cross section along the axial trend of the South Ellwood Anticline 

(Figure 21b, after [Bartsch et al., 1996]). 
A Methodology for Investigation of Natural Hydrocarbon Gas Seepage

 49

The further inshore seepage area due south of Coal Oil Point also coincides with the structural 

culmination along an anticline axis but is more disperse, possibly due to fault splays at the 

intersection of cross faults with the anticline axis and synclinal limbs [Bartsch et al., 1996]. 

Reverse faulting such as that observed for the South Ellwood Offshore oil field [Division of Oil 

and Gas, 1991] is perceived to be associated with the tight compressive folding offshore (Figure 

21a). Such faults could provide migration conduits for seepage in addition to the dense network 

of crestal fractures in the anticline axes inferred by Fischer and Stevenson [1973] from the 

absence of seismic reflections. 
Final Study Report – Luyendyk et al. 

50

Thickness of the overburden of unconsolidated sediment has also been proposed to control 

seepage distribution [Fischer and Stevenson, 1973; Fischer, 1977]. To examine the correlation 

between seep locations and sediment overburden, an isopach of the Quaternary sediment 

thickness produced from 3.5 kHz data is also presented in Figure 20. Cover by the Quaternary 

sediment becomes thin or absent towards the southeast portion of the map, but this thin sediment 

veneer is fairly uniform throughout a region of variable seep distribution. This suggests structural 

control is more likely the dominant constraint on seep configuration, although thinning of the 

Quaternary cover may reflect where sediment overburden is truncated over structural highs which 

correspond to seepage. 

Comparison of the seep distributions mapped from different surveys reveals variations in seepage 

patterns. Some minor variations in position on the order of 30 m (see Section 7) may be due to 

changes in the direction and velocity of ocean currents which could shift the position of seep gas 

bubbles in the water column relative to their seafloor source. In addition, errors in the mapped 

seep distribution may result as an artifact of different navigational coverage. However, some 

changes in seep distribution patterns are likely to be real. A case in point is the disappearance of 

seepage around Platform Holly between 1973 and 1995 illustrated by Figure 7. Seepage at Holly 

was prolific in 1973, but while navigational cover of this area is robust for all surveys from 1994 

through 1996 (Figures 5, 8, and 10), seepage is consistently absent (Figure 11), suggesting a long 

term disappearance of the seeps at this site. 

Changes in seep distribution could result from drying up of seep source hydrocarbons or changes 

in subsurface migration pathways acting as seepage conduits. Creation of fault-induced fracture 

pathways associated with seismic activity, exposure of new seep vents through erosion, or 

sealing of active vents by viscous residual tar are all possible mechanisms which could affect 

seep distribution. However, the spatial coincidence between the location of Platform Holly and 

the disappearance of seepage between 1973 and 1994 suggests that the anthropogenic effect of 

oil production is probably largely responsible for this disappearance. 

Oil production has been proposed as a mechanism for reducing seepage as the reservoir of source 

hydrocarbons is drawn down by producing wells [Landes, 1973; Kvenholden and Harbaugh, 

1983]. Draw down of reservoir hydrocarbons could lead to reduction of reservoir pressure which 

could cause declines in seepage rates since velocity of fluid flow in a porous medium is 

proportional to the pressure gradient and inversely proportional to fluid viscosity from Darcy’s 

Law [Craft and Hawkins, 1959]. Reduction of reservoir pressure is proposed as a factor in 

reducing seepage in the vicinity of Coal Oil Point [Fischer and Stevenson, 1973; Fischer 1977], 

but production could also reduce seep emissions by diminishing the quantity of hydrocarbons 

available for seepage. Either of these mechanisms could account for decrease in areal coverage of 

seepage and also decline in emission volumes. 

Figure 22a presents reservoir pressure measurements corrected to a common depth of 3881 ft 

(1180 m) subsea for various sites around Platform Holly shown in map view in Figure 22b. 

Figure 22a shows a decline in reservoir pressure over time beginning approximately in 1981. The 

initial pressure is only slightly above hydrostatic, and pressure within the reservoir drops below 

hydrostatic fairly early in the production history. If the pressure gradient in the reservoir is not 
A Methodology for Investigation of Natural Hydrocarbon Gas Seepage

 51

super-hydrostatic, pressure should not be sufficient to drive the vertical migration of seepage. 

Therefore, initial declines in reservoir pressure below hydrostatic may have reduced pressure 

driven seepage in the spatially restricted area around Holly. 

 

Buoyancy of gaseous phase hydrocarbons dominate fluid potential gradients in the shallow 

subsurface [Clarke and Cleverly, 1991] and may be another appropriate mechanism for driving 

the seepage offshore Coal Oil Point given the gaseous nature of these seep trends. Buoyancy of 

the gas phase could also transport oil through the Sisquoc cap rock from the underlying Monterey 

Formation reservoir. The velocity of buoyant transport would be essentially constant if not 

coupled with pressure drive, and seep flux would then depend only on the volume of 

hydrocarbons undergoing transport. The diversion of migrating hydrocarbons to production could 

reduce available seep volumes, thus possibly accounting for some of the disappearance of 
Final Study Report – Luyendyk et al. 

52

seepage around Holly and reduction in emissions at the Seep Tents. 

Gas production from the Monterey Formation Reservoir beneath Holly commenced in 1977 prior 

to which gas was re-injected. Subsequently this practice ceased which may partially explain the 

declines in reservoir pressure beginning in 1981 (Figure 22a). Analysis of δ13C of seep gas 

collected at the Seep Tents indicates it is compositionally consistent with the produced gas from 

the Monterey Formation reservoir [J. S. Hornafius, pers. comm., 1996]. The gas to oil production 

ratio from the reservoir gradually increased over time from 300 to 500 ft3/barrel (50 to 90 liters 

of gas/liter of oil) due to the exsolution of gas with declining reservoir pressure [J. S. Hornafius, 

pers. comm., 1996]. Therefore a significant quantity of gas was removed from the reservoir in 

conjunction with oil production (Figure 19b). If the seep gas originated in the produced Monterey 

reservoirs, this represents a significant removal of gaseous hydrocarbons available for seepage. 

The decline in seepage volumes at the Seep Tents beginning in 1989 (Figure 19a) also 

demonstrates the trend of decreasing seepage which may be the result of ongoing production 

drawing down the reservoir hydrocarbons and reducing reservoir pressure. The period of steady 

seep emissions at the Seep Tents preceding the gradual decline which began in 1989 might be 

explained as a lag time. Seepage could be decreasing progressively outward away from Platform 

Holly as production is pursued further and further afield from Holly’s central location. The 

disappearance of seepage immediately adjacent to Holly (Figure 7) supports this hypothesis. 

Declines in reservoir pressure also appear to exhibit a spatial correlation with the distance to 

Holly based on the data in Figure 22. This reduction in the pressure drive for seepage, as 

proposed by Fischer [1977], would support a progressive decrease in seepage outward away from 

Holly. The changes in seep distribution are accompanied by simultaneous changes in gas 

emission. The disappearance of seepage from around Platform Holly caused an estimated 

reduction in seepage of 2.8 ± 1.4 x 104

 m3/day between 1973 and 1995 (Table 2, Figure 18), 

while emission similarly declined at the Seep Tents by 2.5 x 104 m3/day between 1989 and 1995 

(Figure 19a). 

In addition to the anthropogenic effects of oil production, various other mechanisms could 

account for fluctuations in seep emission. Seismic activity is no doubt important, and could cause 

episodic seepage events such as that observed by Field and Jennings [1987]. However, there is 

only anecdotal evidence (e.g.- [Fischer, 1977]) of seepage events associated with seismic activity 

in the Coal Oil Point Area. Storm surges and mud diapirism could trigger another type of 

episodic seepage event associated with degassing from sediments [Hovland et al., 1993], but this 

is unlikely for Coal Oil Point where the seep gas migrates from a deep reservoir rather than 

occurring in shallow sediments. 

Tidal fluctuations might cause periodic oscillations in seepage rates, as observed by Mikolaj and 

Ampaya [1973] for the nearshore Isla Vista oil seeps, but although gas seep emission might have 

been higher at low tide in Figure 9, this was not clear-cut. Gas collection at the Seep Tents 

(Figure 19a) does not appear to exhibit periodic tidal cycles although spectral analysis of gas 

production records from the Seep Tents could not be used to determine a dependence on tides 

since the daily averages of gas volume would alias the shorter tidal periods and tidal pumping of 
A Methodology for Investigation of Natural Hydrocarbon Gas Seepage

 53

gas already trapped by the Seep Tents structure might be incorrectly interpreted as variation in 

emissions from the seafloor seep source. In addition, interpretation of short term changes in 

Figure 19a is complicated by mechanical factors (see Section 8.3). 

If cyclic seasonal variations or spiky episodic events are visible within the Seep Tents production 

record (Figure 19a) they are a second order effect in comparison to the dominant trend of 

declining seep emission. Seasonal changes in groundwater reservoir pressure associated with 

gravity-driven pumping by meteoric waters which oscillate with drought cycles could 

conceivably affect seepage rates. However, Monterey formation waters have sea water salinities 

[J. R. Boles, pers. comm., 1997], which suggests a lack of mixing with meteoric recharge water, 

thus reducing the likelihood that gravity flow affects seepage significantly. Seasonal changes in 

water temperature changing oil viscosity or methane solubility could influence oil and gas 

seepage rates respectively, but evidence is lacking for seasonal variations in seepage, although it 

is a possible mechanism for varying seepage rates. 

The anthropogenic effect of oil production drawing down hydrocarbons within the reservoir 

seems to be the most likely candidate for producing the observed long term declines in seep 

emissions demonstrated by Figure 18 and Figure 19a. Ostensibly this long term trend should 

continue into the future if production continues. The data presented here offers a baseline for 

future comparison. Current estimates of the gaseous seep emissions (Table 2) for the Coal Oil 

Point area range from 9.9 ± 5.0 x 104

 m3 of gas per day for the 50 kHz data (Figure 17) to 20.3 ± 

10.0 x 104 m3 of gas per day for 3.5 kHz data from July-September 1995 (Figure 15) based on an 

empirical calibration of relative sonar return with an arbitrary 50% error (see Section 8.3). 

Some variation in these estimates may be due to error (see Appendix 2). However, it is likely that 

additional variations in both spatial distribution of seepage and volume emission occur at time 

scales other than that of the observed long term decline. Unfortunately, causes for these shorter 

term variations are not as obvious. If the data in Figures 15, 16, and 17 are accurate we are seeing 

changes in emission by a factor of two over a one year period. This represents a significant 

fluctuation in seepage. 

The better constrained emission data from the Seep Tents (Figure 19a) demonstrates a 50% 

reduction in seepage but on a more reasonable five year time span. However, this is emission 

from a point source, and the decrease in area (about 20% along the South Ellwood Anticline Axis 

South of Holly) between July-September 1995 (Figure 16) and August 1996 (Figure 15) could 

partially account for a similar reduction in flux on a shorter one year time scale for the entire 

offshore area. 

9.2 SEEP METHANE FLUX

Annual flux of methane based on the above estimates of seep gas flux would range from about 

2.4 ± 1.2 x 1010 to 5.0 ± 2.5 x 1010 g/year where we have incorporated the arbitrary 50% error 

assigned in Section 8.3. This assumes a seep gas composition consistent with Table 1 (90% 

methane by volume). No effort is made to account for dissolution of methane within the water 

column which could represent a significant sink for the methane gas before it reaches the 
Final Study Report – Luyendyk et al. 

54

atmosphere [Cynar and Yayanos, 1992]. It is assumed that emission of liquid oil which 

accompanies the gaseous seepage forms rims around the gas bubbles which inhibit dissolution by 

creating a double boundary layer. Methane solubility in water is also temperature dependent, but 

this is a fairly insignificant effect, decreasing about 20% from 10° C to 20° C [Lange et al., 

1941]. 

The above estimate of methane flux from the Coal Oil Point Seeps is insignificant in comparison 

to an estimated total annual methane flux of 540x1012 g/year [Cicerone and Oremland, 1988]. 

Therefore, the significance of natural seepage as a source of methane in the global budget is 

dependent on how widespread seeps are. Evidence of gas in marine sediments [Judd and 

Hovland, 1992] suggests submarine seepage of methane may be an ubiquitous occurrence on the 

worlds continental shelves [Hovland and Judd, 1988]. However, seepage rates as high as those 

observed for the Coal Oil Point area are likely to be uncommon [Hovland et al., 1993]. 

Hovland et al. [1993] estimated worldwide seepage contributed from 8 xl012 to 65 x 1012 g of 

methane annually to the global budget, which represents a significant input. To arrive at this 

estimate, Hovland et al. [1993] assumed natural gas seeps are log-normally distributed. Wilson et 

al. [1974] previously made this assumption for liquid petroleum seeps. The assumption is based 

on the fact that quantities such as oil field sizes, basin hydrocarbon reserves, and well-production 

rates are log-normally distributed, and seepage rates should be functions of many of the same 

parameters. However, Kvenholden and Harbaugh [1983] questioned this conclusion on the 

grounds that hydrocarbon accumulations smaller than producing fields could still account for 

seepage. 

To examine the size distribution of gas seeps in the Coal Oil Point area, histograms of gas flux 

for each grid node (l00 x 100 m) were obtained for the 50 kHz data, August 1996 3.5 kHz data, 

and July and September 1995 3.5 kHz data. These histograms are presented in Figure 22a. The 

bins on the histograms were only calculated for values of gas flux above the background level 

(approximately 10 liter/m2

 day). In Figure 21b, a log-log plot of gas flux values verses 

cumulative percent of the gas flux distribution is presented. Such a plot should yield a straight 

line for log-normally distributed data [Hovland et al., 1993]. Each dataset fits a straight line 

reasonably well, but exhibits some curvature that may suggest the data is not lognormal, or may 

arise due to errors in the measurement technique. The fact that the distribution of emission for 

the July and September 1995 3.5 kHz data levels off at the higher flow rates (Figure 23) may 

suggest saturation of the analog portion of the data, and therefore imply errors in the emission 

estimate from this data set. 
A Methodology for Investigation of Natural Hydrocarbon Gas Seepage

 55

9.3 SEEPAGE OF LIQUID PETROLEUM

Seepage of liquid petroleum also accompanies the flux of gas from the Coal Oil Point Seeps. The 

rims of oil film which surround the venting seep gas bubbles at the seafloor are buoyed upward 

through the water column to the surface resulting in slicks. Oil seepage accompanying gas flow 

from the outer seep trends was estimated to be about 80 barrels/day (12,800 liters/day) [Clester et 

al., 1996] based on an oil gas ratio of 90 cm3 of oil per m3 of gas [Clester, pers. comm., 1996]. 

Given the estimates for gas emission above, corresponding seepage of oil (Table 2) would range 

from about 9000 to 18,000 liters (55 to 110 barrels) per day with an arbitrary 50% error (Section 

8.3). This is in addition to the 50 to 70 barrels per day (8000 to 11,000 liter/day) estimated by 
Final Study Report – Luyendyk et al. 

56

Allen et al. [1970] for the nearshore oil seeps. Total seepage of liquid petroleum for the entire 

offshore area could thus range from about 105 to 180 barrels (17,000 to 29,000 liters) per day. 

This range encompasses the estimate of 150 barrels per day (24,000 liters/day) of Clester et al. 

[1996] and is within the span of the estimate by Fischer [1977] of 25 to 400 barrels (4000 to 

64,000 liters) per day. 

The magnitudes of the above estimates demonstrate the prolific nature of natural seepage 

offshore Coal Oil Point and illustrate the potential of significant contributions from seepage to 

the marine environment. Previous work has already shown this is true on a global scale. For 

example, previous estimates of the total mass flux of liquid petroleum into the marine 

environment range from 0.25 million metric tons per year [Kvenholden and Harbaugh, 1983] to 

0.6 million metric tons per year [Wilson et al., 1974]. This represents up to 10% of the input 

from both natural and anthropogenic sources of petroleum, whose total input to the marine 

environment is roughly 6 million metric tons per year [Kvenholden and Harbaugh, 1983]. 

9.4 SEEP FLUX OF ROG’S 

Reactive organic gases (ROG’s) are also emitted by the Coal Oil Point seeps. These include all 

chain hydrocarbons higher than methane and ethane. ROG’s are of concern for issues of air 

quality because they are precursors to smog forming ozone. Approximately 150 g of nonmethane hydrocarbons are emitted for each m3 of seep gas based on the gas composition in Table 

1. Using the above estimates of seep gas emission (Table 2), flux of non-methane hydrocarbons 

from the gaseous component of seepage offshore of Coal Oil Point contributes 15 to 30 metric 

tons per day to the local atmosphere, plus or minus 50%. 

In addition, evaporation of volatiles from the surface slicks of liquid petroleum introduces 

additional ROG’s to the atmosphere. The seep oil which forms the surface slicks has about 10 

API Gravity and is composed (by weight) of 30% aromatic compounds, 27% saturates, 26% 

NSO compounds, and 20% asphaltic compounds [J. S. Hornafius, pers. comm., 1996]. The 

saturated and aromatic compounds (about 50% of the seep oil) evaporate whereas the nonvolatile 

NSO and asphaltic compounds are left as residual tar. The emission of ROG’s due to evaporation 

from the surface slicks is approximately 444 g of ROG’s per liter of oil. This represents a flux 

from 4.0 to 8.0 (± 50%) metric tons per day. 

The sum of ROG emissions estimated from both gaseous and liquid components of seepage is 20 

to 40 (± 50%) metric tons per day. This is within the range of the estimate of Saxena and Oliver 

[1984] (16.2 to 137 metric tons per day) which included all Santa Barbara Channel seeps (not 

just those at Coal Oil Point). This estimate represents a significant contribution and has important 

repercussions for air quality in Santa Barbara County. 
A Methodology for Investigation of Natural Hydrocarbon Gas Seepage

 57

10. CONCLUSIONS 

In the Santa Barbara Channel, natural marine hydrocarbon seeps offshore Coal Oil Point are 

principally responsible for beach tar along the coastline [Welday, 1977; Hartman and Hammond, 

1981] and a significant contributor of pollution to the atmosphere [Saxena and Oliver, 1984; 

Killus and Moore, 1991]. This thesis examines spatial and temporal changes observed in the 

distribution and emission of these seeps, presents estimates of their prolific volumetric discharge, 

and discusses possible mechanisms which may be responsible for these observations. 

In addition to the issue of local pollution, seepage may also be an important process on a global 

scale, introducing significant quantities of liquid petroleum [Wilson et al., 1974; Kvenholden and 

Harbaugh, 1983] and methane [Hovland et al., 1993] to the marine environment. Estimates of 

quantities associated with seepage are also important in assessing volumes of fluid flow of 

gaseous or liquid hydrocarbons within the earth’s crust which is significant since fluids can act as 

markers and tracers of diagenetic processes. Fluid movement can also accompany earthquake 

swarms and may therefore be an important factor influencing seismic activity and structural 

evolution. These factors make this subject an important topic of scientific inquiry. 

The results of this study show that the spatial distribution and discharge rates of the natural 

hydrocarbon seeps offshore Coal Oil Point are primarily governed by the offshore geologic 

structure. In addition, distribution and rates of seepage vary over time. An inferred long term 

trend of declining areal extent of seepage and reduction of seepage volumes spatially coincident 

with offshore oil platform Holly support the notion that drawing down of reservoir hydrocarbons 

through production has led to a decline in natural seepage. This effect is estimated to have 

reduced natural seep emissions within 13 km2 of Holly approximately 50,000 ± 20,000 m3

/day 

between 1973 and 1995 with a corresponding decrease in areal cover from 0.9 to 0.4 km2. 

Presumably this trend should continue into the future with ongoing production. 

An estimate of current seepage for the entire Coal Oil Point offshore area (summarized in Table 

2) suggests oil seepage is presently on the order of 20,000 - 30,000 liters/day and gas emission on 

the order of 1-2 x 105

 m3/day. This includes emission of methane, a greenhouse gas (2.4-5 x 1010

g/year), and ROG’s which are precursors to smog forming ozone (20-40 metric tons/day). This 

confirms the notion that seepage is a significant natural source of hydrocarbons in the local 

environment and strengthens the case that such processes may also be important at the global 

scale. 
Final Study Report – Luyendyk et al. 

58

APPENDIX 1: CALCULATION OF BUBBLE RISE VELOCITIES

Bubble rise velocities are calculated from [Wallis, 1974] equation (26): 

 

where v∞ is the terminal bubble rise velocity, g is acceleration due to gravity, a is bubble radius, 

∆ρ is the density difference between the bubbles and the surrounding (continuous) medium, and 

ρc is the density of the continuous medium. Equation (A1.1) is valid for relatively large bubbles 

which assume a “spherical cap” and flat base during ascent. Viscous and surface tension forces 

are neglected, and terminal velocity is due to the balance between form drag and buoyancy. Good 

experimental agreement has been observed for equation (A 1.1) for air bubbles in water. For 

methane bubbles in seawater, ρCH4 << ρH20, so ∆ρ ≈ ρc and thus v∞ ≈ √ga. For the mean radius a 

= 0.6 cm of the observed seep gas bubble distribution (Figure 12b), v∞= 24 cm/sec. 
A Methodology for Investigation of Natural Hydrocarbon Gas Seepage

 59

APPENDIX 2: ERROR CALCULATION OF SLOPE

Error in the calibration coefficient c used in equation (8.3.1) is taken from the standard deviation 

in slope for a linear least squares fit given by Taylor [1982]: 

 

where 

and 

Additional possible sources of error not accounted for by this calculation include the following: 

(1) Systematic errors in equipment including errors due to poor resolution and low dynamic 

range of the analog data. 

(2) Errors in interpretation, such as mistaking seeps for schools of fish or kelp, which can 

have similar acoustic signatures. 

(3) Errors arising from differences in navigational coverage. 

(4) Errors due to drift of seep bubbles in the water column. 

(5) Errors from unaccounted for dissolution of seep gases within the water column. 

(6) Errors due to variations in gas composition from that presented in Table 1. 

(7) Errors in processing such as rounding errors, filtering errors, and errors resulting from 

interpolation, averaging, and contouring operations. 

(8) Errors related to acoustic response including: 

(a) Errors resulting from variations in bubble distribution. 

(b) Affect of different acoustic frequencies. 

(c) Variation in gas composition. 

(d) Changes in ambient pressure. 

(e) Multi-bubble reverberations. 

(f) Transmission loss of acoustic energy due to the presence of bubbles. 

(g) Effects of resonance such as failure to excite bubbles to resonance due to 

insufficient pulse width. 
Final Study Report – Luyendyk et al. 

60

REFERENCES 

Akulichev, V. A., Bulanov, V. A., and Kienin, S. A., 1986. “Acoustic sensing of gas bubbles in 

the ocean medium”, Soviet Phys. Acoustics, v. 32, p. 177-180. 

Allen, A. A., Schlueter, R. S., Mikolaj, P. G., 1970. “Natural Oil Seepage at Coal Oil Point, 

Santa Barbara, California”, Science, v. 170, p. 974-977. 

Bartsch, E. C., Gurrola, L. D., Francis, R. D., Quigley, D. C., Hornafius, J. S., and 

Luyendyk, B. P., 1996. “Structural Control of the Spatial Distribution of Hydrocarbon 

Seeps in the Northern Santa Barbara Channel, California”, in EOS Trans. Amer. 

Geophys. Union, v. 77(46), p. F419. 

California Division of Oil and Gas, 1991. California Oil and Gas Fields, Volume II, Southern, 

Central, and Offshore California, 3rd edition, Publication No. TR12, Sacramento. 

Cicerone, R. J., and Oremland, R. S., 1988. “Biogeochemical Aspects of Atmospheric Methane”, 

Global Biogeochemical Cycles, v.2(4), p. 299-327. 

Clarke, R. H., and Cleverly, R. W., 1991. “Petroleum seepage and post accumulation migration”, 

in Petroleum Migration, Geol. Soc. Sp. Publ. No. 59, W. A. England and A. J. Fleet, 

Editors, Geological Society of London, p. 265-271. 

Clay, C. S., and Medwin, H., 1977. Acoustical Oceanography, John Wiley and Sons. 

Clester, S. M., Hornafius, J. S., Scepan, J., and Estes, J. E., 1996. “Quantification of the 

Relationship Between Natural Gas Seepage Rates and Surface Oil Volume in the Santa 

Barbara Channel” in EOS Trans. Amer. Geophys. Union, v. 77(46), p. F420. 

Cordell, R. J., 1972. “Depths of Oil Origin and Primary Migration: A review and critique”, 

AAPG Bull., v. 56(10), p. 2029-2067. 

Craft, B. C. and Hawkins, M. F., 1959. Applied Petroleum Reservoir Engineering, Prentice-Hall, 

Englewood Cliffs, New Jersey, 437 p. 

Cynar, F. J., and Yayanos, A., 1992. “The distribution of methane in the upper waters of the 

Southern California Bight”, J. Geophys. Res., v. 97(C7), p. 11269-11285. 

Dando, P. R., and Hovland, M., 1992. “Environmental effects of submarine seeping natural gas”, 

Continental Shelf Research, v. 12(10), p. 1197-1207. 

Doligez, B. (ed.), 1987. Migration of Hydrocarbons in Sedimentary Basins, IFP Exploration 

Research Conference. 
A Methodology for Investigation of Natural Hydrocarbon Gas Seepage

 61

Du Rouchet, J., 1981. “Stress Fields, a key to oil migration”, AAPG Bull., v. 65, p. 74-85. 

England, W. A., and Fleet, A. J., 1991. Petroleum Migration, Geological Society Special 

Publication No. 59, Geological Society of London. 

Eremenko, N. A., and Michailov, I. M., 1974. “Hydrodynamic Pools at Faults”, Bulletin of 

Canadian Petroleum Geology, v. 22(2), p. 106-118. 

Estes, J. E., Crippen, R. E., and Star, J. L., 1985. “Natural oil seep detection in the 

Santa Barbara Channel, California, with Shuttle Imaging Radar”, Geology, v. 13, p. 

282-284. 

Field, M. E., and Jennings, A. E., 1987. “Seafloor gas seeps triggered by a northern California 

earthquake”, Marine Geology, v. 77, p. 39-51. 

Fischer, P. J., 1977. “Oil and tar seeps, Santa Barbara basin, California”, in California Offshore 

Gas, Oil, and Tar Seeps, California State Lands Commission, Sacramento, CA, p. 1-62. 

Fischer, P. J, and Stevenson, A. J., 1973. “Natural Hydrocarbon Seeps along the Northern Shelf 

of the Santa Barbara Basin”, in Santa Barbara Channel Revisited, AAPG Trip 3, Annual 

Meeting AAPG, SEPM, SEG. 

Gibbs, J. F., Healy, J. H., Raleigh, C. B., and Coakley, J., 1973. “Seismicity in the Rangely, 

Colorado, area: 1962-1970”, Bull. Seis. Soc. Amer., v. 63(5), p. 1557-1570. 

Guthrie, L. D., and Rowley, P. R., 1983. “Containment of Naturally Occurring 

Subsea Hydrocarbon Emissions - A Project Review”, 15th Offshore Technology 

Conference, Paper 4446, p. 33-38. 

Hartman, B., and Hammond, D., 1981. “The use of carbon and sulfur isotopes as correlation 

parameters for the source identification of beach tar in the southern California 

borderland”, Geochemica et Cosmochemica Acta, v. 45, p. 309-319. 

Healy, J. H., Rubey W. W., Griggs, D. T., and Raleigh, C. B., 1968. “The Denver Earthquakes”, 

Science, v. 161, p. 1301-1310. 

Hill, D. P., 1977. “A Model for Earthquake Swarms”, Journal of Geophysical Research, v. 82(8), 

p. 1347-1352. 

Hovland, M., and Judd, A. G., 1988. Seabed Pockmarks and Seepages, Impact on Geology, 

Biology, and the Marine Environment, Graham and Trotman. 

Hovland, M., Judd, A. G., and Burke, R. A., 1993. “The Global Flux of Methane from Shallow 

Submarine Sediments”, Chemosphere, v. 26, p. 559-578. 
Final Study Report – Luyendyk et al. 

62

Hubert, M. K., and Rubey, W. W., 1959. “Role of fluid pressure in mechanics of overthrust 

faulting”, Bull. Geol. Soc. Amer., v. 70, p. 115-166. 

Hunt, J. M., 1979. Petroleum Geochemistry and Geology, W. H., Freeman Co. 

Isaacs, C. M., and Peterson, N. F., 1987. “Petroleum in the Miocene Monterey Formation, 

California” in Siliceous Sedimentary Rock-Hosted Ores and Petroleums, Hem, J. R., 

(ed.), Van Nostrand Reinhold, New York, p. 83-116. 

Judd, A. G., and Hovland, M., 1992. “The evidence of shallow gas in marine sediments”, 

Continental Shelf Research, v. 12, p. 1081-1095. 

Killus, J. P., and Moore, G. E., 1991. “Factor analysis of hydrocarbon species in the south central 

coast air basin”, Journal of Applied Meteorology, v. 30, p. 733-743. 

Kolpack, R. L., 1977. “Relationship of migration of natural seep material to oceanography of 

Santa Barbara Channel”, in California Offshore Gas, Oil, and Tar Seeps, California State 

Lands Commission, p. 226-255. 

Kraus, S. P., Estes, J. E., Atwater, S. G., Jensen, J. R., and Volimers, R. R., 1977. “Radar 

Detection of Surface Oil Slicks”, Photogrammetric Engr. and Remote Sensing, v. 43(12), 

p. 1523-1531. 

Krooss, B. M., Leythaeuser, D., and Schaefer, R. G., 1992. “The Quantification of 

Diffusive Hydrocarbon Losses Through Cap Rocks of Natural Gas Reservoirs - A 

Reevaluation”, AAPG Bull., v. 76(3), p. 403-406. 

Kvenholden, K. A., 1993. “Gas Hydrates - Geological Perspective and Global Change”, Reviews 

of Geophysics, v. 31(2), p.173-187. 

Kvenholden, K. A., and Harbaugh, J. W., 1983. “Reassessment of the rates at which oil from 

natural sources enters the marine environment”, Marine Environmental Research, v. 10, 

p. 223-243. 

Lacroix, A.V., 1993, Unaccounted-for sources of fossil and isotopically-enriched methane and 

their contribution to the emissions inventory: A review and synthesis, Chemosphere, v. 

26, p. 507-557. 

Landes, K. K., 1973. “Mother Nature as an Oil Polluter”, AAPG Bull., v. 57(4), p. 637-641. 

Lange, N. A., Forker, G. M., Burington, R. S., 1941. Handbook of Chemistry, Handbook 

Publishers, Inc., Sandusky, Ohio. 

Leythauser, D., Schaefer, R. G., and Yukler, A., 1982. “Role of diffusion in primary migration of 

hydrocarbons”, AAPG Bull., v. 66(4), p. 408-429. 
A Methodology for Investigation of Natural Hydrocarbon Gas Seepage

 63

Link, W. K., 1952. “Significance of Oil and Gas Seeps in World Oil Exploration”, AAPG Bull., 

6(8), p. 1505-1540. 

MacDonald, G. J., 1990. “Role of Methane Clathrates in Past and Future Climates”, Climate 

Change, v. 16, p. 247-281. 

Machel, H.G., and Burton, E. A., 1991. “Causes and Spatial Distribution of 

Anomalous Magnetization in Hydrocarbon Seepage Environments”, AAPG Bull., v. 

75(12), p. 1864-1876. 

McAuliffe, C. D., 1979. “Oil and Gas Migration - Chemical and Physical Constraints”, AAPG 

Bull., v. 63(5), p.761-781. 

Medwin, H., 1977a. “Counting bubbles acoustically: a review”, Ultrasonics, p. 7-13. 

Medwin, H., 1977b. “Acoustical determinations of bubble-size spectra”, Journal of the 

Acoustical Society of America, v. 62 (4), p. 1041-1044. 

Mikolaj, P. G., and Ampaya, J. P., 1973. “Tidal Effects on the Activity of Natural Submarine Oil 

Seeps”, Marine Technical Society Journal, v. 7, p. 25-28. 

Nisbet, E. G., 1990. “The end of the ice age”, Canadian Journal of Earth Science, v. 27, p. 148-

157. 

Price, L. C., 1976. “Aqueous Solubility of Petroleum as Applied to its Origin and Primary 

Migration”, AAPG Bull., v. 60(2), p. 2 13-244. 

Price, L. C., 1979. “Aqueous Solubility of Methane at Elevated Pressures and Temperatures”, 

AAPG Bull., p. 1527-1533. 

Reed, W. E., and Kaplan, I. R., 1977. “The Chemistry of Marine Petroleum Seeps”, Journal of 

Geochemical Exploration, v. 7, p. 255-293. 

Rintoul, B., 1982. “ARCO Caps Santa Barbara Channel Seep”, Pacific Oil World, 74(11), p. 6-9. 

Roberts, W. H., and Cordell, R. J., 1980. Problems of Petroleum Migration, AAPG Studies in 

Geology No. 10. 

Santa Barbara Air Pollution Control District, 1994. “The 1994 Clean Air Plan”, Appendix D., 

Table 3.2, Santa Barbara, CA. 
Final Study Report – Luyendyk et al. 

64

Saxena, P., and Oliver, W. R., 1984. “Estimation of reactive organic gas emissions from offshore 

oil and gas seeps in the Santa Barbara Channel”, Systems Applications, Inc., report 

SYSAPP-84/197, prepared for the U. S. Environmental Protection Agency, San 

Francisco, CA, 25 pp. 

Shine, K. P., Derwent, R. G., Wuebbles, D. J., and Morcrette, J. J., 1990. “Radiative forcing of 

climate”, in Climate Change, the IPCC Scientific Assessment, edited by J. T. Houghton, 

G. J. Jenkins, and J. J. Ephraums, p. 41-68, Cambridge University Press, New York. 

Sibson, R. H., 1996. “Structural Permeability of fluid-driven fault-fracture meshes”, Journal of 

Structural Geology, v. 18(8), p. 1031-1042. 

Smith, W. and Wessel, P., 1990. “Gridding with continuous curvature splines in tension”, 

Geophysics, v. 55, p. 293-305. 

Straughan, D. and Abbott, B. C., 1972. “The Santa Barbara oil spill: ecological changes and 

natural oil leaks”, In: Water pollution by oil (Hepple, P. (Ed.)), Institute of Petroleum, 

London, p. 257-262. 

Sweet, W. E., 1973. “Marine Acoustical Hydrocarbon Detection”, 5th Offshore Technology 

Conference, Paper 1803. 

Sylvester, A. G., Smith, S. W., and Scholz, C. H., 1970. “Earthquake swarm in the 

Santa Barbara Channel, California, 1968”, Bull. Seismological Society of America, v. 

60(4), p. 1047-1060. 

Taylor, J. R., 1982. An Introduction to Error Analysis, The Study of Uncertainties in Physical 

Measurements, University Science Books, Oxford University Press. 

Tedesco, S. A., 1995. Surface Geochemistry in Petroleum Exploration, Chapman and Hall. 

Tinkle, A. R., Antoine, J. W., and Kuzela, R., 1973. “Detecting natural gas seeps at sea”, Ocean 

Industry, v. 8, p. 139-142. 

Tissot, B., 1987. “Migration of Hydrocarbons in Sedimentary Basins: A Geological, 

Geochemical, and Historical Perspective”, in Migration of Hydrocarbons in Sedimentary 

Basins, edited by B. Doligez, 2nd IFP Exploration Research Conference, p. 1-19. 

Urick, R. J., 1983. Principles of Underwater Sound, Mcgraw-Hill, 3rd Edition. 

Vagle, S., and Farmer, D. M., 1992. “The measurement of bubble-size distributions by acoustical 

backscatter”, Journal of Atmospheric and Oceanic Technology., v. 9, p. 630-644. 

Vernon, J. W., and Slater, R. A., 1963. “Submarine tar mounds, Santa Barbara County, 

California”, AAPG Bull., v. 47, p. 1624-1627. 
A Methodology for Investigation of Natural Hydrocarbon Gas Seepage

 65

Wallis, G. B., 1974. “The Terminal Speed of Single Drops or Bubbles in an Infinite Medium”, 

International Journal of Multiphase Flow, v. 1, p. 491-511. 

Watson, R. T., Rodhe, H., Oeschger, H., and Siegenthaler, U., 1990. “Greenhouse 

gases and aerosols”, in Climate Change, the IPCC Scientific Assessment, edited by J. 

T. Houghton, 0. J. Jenkins, and J. J. Ephraums, p. 4 1-68, Cambridge University 

Press, New York. 

Welday, E. E., 1977. “Oil and Tar on Santa Barbara region beaches”, in California Offshore Gas, 

Oil, and Tar Seeps, California State Lands Commission, Sacramento, CA, p. 347-371. 

Wilkinson, E. R., 1972. “California Offshore Oil and Gas Seeps”, California Division of Oil and 

Gas, Pubi. No. TRO8, 11 p. 

Wilson, R. D., Monaghan P. H., Osanik, A., Price, L. C., and Rogers, M. A., 1974. “Natural 

Marine Oil Seepage”, Science, v. 184, p. 857-864. 

Wu, J., 1981. “Bubble Populations and Spectra in Near Surface Ocean: Summary and Review of 

Field Measurements”, J. of Geophys. Res., v. 86(C1), p. 457-463. 
Final Study Report – Luyendyk et al. 

 66

ACKNOWLEDGEMENTS 

This work was supported by funding from 

the University of California Energy Institute 

Grant #08960643 

and 

the Mineral Management Service 

Grant # 13093 

The author would like to express his appreciation to 

Dr. J. Scott Hornafius, Mobil Oil Corporation, Professor Bruce Luyendyk, Professor 

James Boles, Professor Jordan Clark, Professor Libe Washburn, Professor Robert 

Francis, Professor Rachel Haymon, Erik Bartsch, Fred Sanford, Gordon Cota, 

Gretchen Mullendore, David Salazar, Priscilla D’Aquino, and the Seawatch crew 

for invaluable assistance in the completion of this work. 
The Department of the Interior Mission

As the Nation's principal conservation agency, the Department of the Interior has responsibility for most 

of our nationally owned public lands and natural resources. This includes fostering sound use of our 

land and water resources; protecting our fish, wildlife, and biological diversity; preserving the 

environmental and cultural values of our national parks and historical places; and providing for the 

enjoyment of life through outdoor recreation. The Department assesses our energy and mineral 

resources and works to ensure that their development is in the best interests of all our people by 

encouraging stewardship and citizen participation in their care. The Department also has a major 

responsibility for American Indian reservation communities and for people who live in island territories 

under U.S. administration. 

The Minerals Management Service Mission 

As a bureau of the Department of the Interior, the Minerals Management Service's (MMS) primary 

responsibilities are to manage the mineral resources located on the Nation's Outer Continental Shelf 

(OCS), collect revenue from the Federal OCS and onshore Federal and Indian lands, and distribute 

those revenues. 

Moreover, in working to meet its responsibilities, the Offshore Minerals Management Program

administers the OCS competitive leasing program and oversees the safe and environmentally sound 

exploration and production of our Nation's offshore natural gas, oil and other mineral resources. The 

MMS Royalty Management Program meets its responsibilities by ensuring the efficient, timely and 

accurate collection and disbursement of revenue from mineral leasing and production due to Indian 

tribes and allottees, States and the U.S. Treasury. 

The MMS strives to fulfill its responsibilities through the general guiding principles of: (1) being 

responsive to the public's concerns and interests by maintaining a dialogue with all potentially affected 

parties and (2) carrying out its programs with an emphasis on working to enhance the quality of life for 

all Americans by lending MMS assistance and expertise to economic development and environmental 

protection.