Organic Metamorphism in the California Petroleum Basins

Chapter A—Rock-Eval and Vitrinite Reflectance

By Leigh C. Price, Mark Pawlewicz, and Ted Daws

U.S. Geological Survey Bulletin 2174–A

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

Bruce Babbitt, Secretary

U.S. Geological Survey

Charles G. Groat, Director

Any use of trade, product, or firm names in this publication

is for descriptive purposes only and

does not imply endorsement by the U.S. Government

Published in the Central Region, Denver, Colorado

Manuscript approved for publication December 15, 1999

Photocomposition by Norma J. Maes
III

Contents

Abstract ................................................................................................................................................ 1

Introduction........................................................................................................................................... 1

Why this study? .......................................................................................................................... 1

Samples and Methods......................................................................................................................... 2

Results.................................................................................................................................................... 5

Los Angeles Basin Central Syncline ........................................................................................ 5

Ventura Basin Central Syncline................................................................................................ 11

Hydrogen-Poor OM Los Angeles Basin................................................................................... 11

Hydrogen-Poor OM Ventura Basin .......................................................................................... 13

Southern San Joaquin Valley.................................................................................................... 16

Hydrogen-Rich OM Los Angeles-Ventura Basins ................................................................. 19

Northern San Joaquin-Southern Sacramento Valley Basins.............................................. 21

Discussion ............................................................................................................................................. 24

HC Generation Potential from the ROCK-EVAL Hydrogen Index......................................... 24

Type III/IV OM Kinetic Differences between Different Basins ........................................... 25

Kinetic Differences between Hydrogen-Rich and Hydrogen-Poor OM............................. 29

Conclusions........................................................................................................................................... 30

Acknowledgments ............................................................................................................................... 31

References ............................................................................................................................................ 31

Figures

1. Plot of total organic carbon content versus burial temperature and depth for

 the Apex-1 wellbore, Los Angeles Basin ...................................................................... 7

2–5. Plots of various petroleum geochemical data from the Apex-1 wellbore:

2. Total organic carbon, and the ROCK-EVAL S1 and S2 peaks versus burial

temperture ................................................................................................................. 8

3. ROCK-EVAL oxygen index, Tmax, production index; and vitrinite 

reflectance versus burial temperaure .................................................................. 9

4. ROCK-EVAL S1 peak versus S2 peak for rocks buried between 80°to 150°C. . 9

5. Total organic carbon and the ROCK-EVAL S1 and S2 peaks versus burial 

temperature for sandstones. .................................................................................. 10

 6. Plots of porosity and percent residual-oil saturation versus depth for

Apex-1 sandstone core............................................................................................ 11

7–11. Plots of various petroleum geochemical data versus burial temperature for 

samples with hydrogen-poor organic matter:

7. Total organic carbon, the ROCK-EVAL: S1, S2, S3 pyrolysis peaks, 

production index, and, Tmax; and vitrinite reflectance, for two 

Ventura Basin wells.................................................................................................. 14

8. Total organic carbon, and the ROCK-EVAL S1 and S2 peaks, Los Angeles 

Basin. .......................................................................................................................... 15

9. Vitrinite reflectance, and the ROCK-EVAL Tmax, production index, and 

S3 peak, Los Angeles Basin..................................................................................... 15

10. Total organic carbon, and the ROCK-EVAL S1 and S2 peaks, Los Angeles 

Basin. .......................................................................................................................... 16

11. Vitrinite reflectance, the ROCK-EVAL Tmax, S3 peak, and production 

index, Ventura Basin. ............................................................................................... 17

12-13. Plots of various petroleum geochemical data versus depth for samples from the 

Southern San Joaquin Valley Basin:

 12. Total organic carbon, and the ROCK-EVAL S1 and S2 pyrolysis peaks............. 18

13. Vitrinite reflectance and the ROCK-EVAL Tmax, production index, and 

 S3 peak ....................................................................................................................... 19
IV

14–15. Plots of various petroleum-geochemical parameters versus burial temperature 

 for samples with hydrogen-rich organic matter, Los Angeles and Ventura Basins:

 14. Total organic carbon, and the ROCK-EVAL S1, S2, and S3 pyrolysis peaks...... 20

15. Vitrinite reflectance, and the ROCK-EVAL Tmax and production index............. 21

16. Plots of various petroleum-geochemical data for shales versus depth, 

Wilmington Field, Los Angeles Basin............................................................................... 22

17. Plots of various petroleum-geochemical data for shales versus depth for a

Central San Joaquin Valley Basin well. ........................................................................... 23

18. Plots of various petroleum-geochemical data for shales versus depth 

for a Sacramento Valley Basin well ................................................................................. 23

 19. Plots of ROCK-EVAL hydrogen index versus kerogen elemental hydrogen to 

 carbon for Monterey shales .............................................................................................. 24

 20. Plot of ROCK-EVAL hydrogen index versus kerogen elemental hydrogen to

 carbon for Los Angeles and Ventura Basin shales ....................................................... 25

21.–22. Plot of vitrinite reflectance, and the ROCK-EVAL S1 peak and production index all 

 versus the ROCK-EVAL S2 peak for shales from the California Petroleum Basins 

 for equilibrium-burial temperatures of:

21 140°- 159.9°C.............................................................................................................. 28

22. 180°- 199.9°C.............................................................................................................. 29

23. Plot of vitrinite reflectance (observed and calculated) versus burial temperature, 

Los Angeles Basin............................................................................................................... 30

Tables

1. Wells and well locations for this study’s sample base ................................................. 3

2. Geothermal gradients for all areas examined in this study ......................................... 6

3. Core analyses (porosity, permeability, and residual-oil ) for deep Apex-1 

sandstones........................................................................................................................... 12

4. ROCK-EVAL and kerogen elemental analyses for various Los Angeles-Ventura 

Basin shales......................................................................................................................... 26
1

Abstract

The results of ROCK-EVAL and vitrinite reflectance analyses of a large sample base from more than 70 wells located in 

three oil-rich California petroleum basins are reported. The 

cores from these wells have a wide range of present-day burial 

temperatures (40° to 220°C). The rocks in these basins were 

deposited under highly variable conditions, sometimes resulting 

in substantially different organic matter (OM) types in rocks tens 

of meters vertically apart from each other in one well. The 

kinetic response of these different OM types to equivalent wellknown burial histories is a pivotal point of this study. 

In the Los Angeles and Ventura Basins, rock organic-richness significantly increased with depth, as did kerogen hydrogen 

content, and the percentage of fine-grained versus coarsegrained rocks. The shales in these basins are perceived as containing primarily hydrogen-rich amorphous OM. In actuality, 

the shallowest 2,000 to 3,000 m of rocks in the basins, and at 

least the upper 6,000 m of rocks in parts of the Los Angeles 

Basin central syncline, are dominated by type III/IV OM. In the 

Los Angeles Basin, mainstage hydrocarbon (HC) generation 

commences in the type III/IV OM at present-day burial temperatures of 85° to 110°C, most likely around 100°C, and is largely 

complete by 220°C. In the Southern San Joaquin Valley Basin, 

mainstage HC generation commences in type III/IV OM at 

150°C and is also largely complete by 220°C. In the Ventura 

Basin, mainstage HC generation commences above 140°C in 

type III/IV OM. The apparent lower temperatures for commencement of HC generation in the Los Angeles Basin are 

attributed to the fact that parts of the basin were cooled from 

maximal burial temperatures by increased meteoric water flows 

during the last glaciations.

All aspects of organic metamorphism, including mainstage 

HC generation, are strongly suppressed in rocks with hydrogenrich OM in these basins. For example, ROCK-EVAL data suggest that mainstage HC generation has not commenced in rocks 

with hydrogen-rich OM at present-day temperatures of 198°C. 

This observation is attributed to much stronger bonds in hydrogen-rich OM compared to types III and IV OM and, therefore, 

significantly higher burial temperatures are required to break 

these bonds. This difference in OM kinetics has profound ramifications for petroleum-geochemical exploration models.

Organic-matter characteristics inherited from original depositional conditions were overlaid on, and at times confused 

interpretation of, characteristics from organic metamorphism in 

all study areas. In all the basins examined in this study, immature fine-grained rocks occasionally had high to very high 

carbon-normalized concentrations of pre-generation indigenous 

bitumen. This unusual characteristic may be due to unique depositional conditions in these basins.

Introduction

Why this Study?

The Los Angeles Basin is the richest petroleum basin by 

sediment volume or area worldwide (Barbat, 1958; Yerkes and 

others, 1965; Wright, 1987; Price, 1994; and many other investigators). Moreover, the Ventura and Southern San Joaquin Valley 

Basins are among the richest basins worldwide. However, a paucity of published petroleum-geochemical data exists for these 

three basins. For example, Keller (1987, p. 30) in her resource 

assessment of the Ventura Basin noted, “For a mature province 

with a long and successful period of hydrocarbon exploration 

and development, very few data are available on geochemistry 

and thermal maturity of possible source rocks in the Ventura 

Basin assessment province.” This conclusion also applies to the 

Los Angeles and Southern San Joaquin Valley Basins. For 

example, Phillipi’s (1965) classic study of the Los Angeles 

Basin includes only 12 samples from two areas (the Dominguez 

and Seal Beach oil-fields). Moreover, vitrinite reflectance (Ro), 

ROCK-EVAL, and gas chromatographic analyses were not 

available at the time of the Phillipi (1965) study. The general 

lack of published petroleum-geochemical data contrasts with the 

fact that geologically, these are the, or among the, most thoroughly studied basins worldwide (Beyer and Bartow, 1987; 

Keller, 1987, and Beyer, 1988a, also see references below). 

Many geologic studies of these basins have been published 

including Jennings and Troxel (1954), Nagle and Parker (1971), 

and Keller (1987) for the Ventura Basin. Studies of the San 

Joaquin Valley Basin include Hoots and others (1954), Hackel 

(1966), Foss and Blaisdell (1968), Bandy and Arnal (1969), Callaway (1971), Foss (1972), Harding (1976), MacPherson (1978), 

Zieglar and Spotts (1978), Ingersoll (1979), Dickinson and Seely 

(1979), Webb (1981), Graham and others (1982), and Williams 

and Graham (1982), Blueford (1984), Graham (1985, 1987), 

Graham and Williams (1985), Bartow (1987), and especially 

Beyer and Bartow (1987) for a complete review of previous 

work. Geologic studies of the Los Angeles Basin include: 

Driver (1948), Edwards (1951), Woodford and others (1954), 

Barbat (1958), Yerkes and others (1965), Brown (1968), Gardett 

(1971), Hill (1971), Harding (1973), Yeats (1973), Crowell 

(1974), Campbell and Yerkes (1976), Schwartz and Colburn 

Organic Metamorphism in the California Petroleum Basins

Chapter A—Rock-Eval and Vitrinite Reflectance

By Leigh C. Price, Mark Pawlewicz, and Ted Daws
2 Organic Metamorphism in the California Petroleum Basis

(1987), Wright (1987), Mayer (1987; 1991), Beyer (1988a), 

Blake (1991), Biddle (1991), Redin (1991), and Yeats and Beall 

(1991). 

The southern California basins provide some of the most 

accurate equilibrium-burial temperatures worldwide, largely 

because of early exploration activities. For example, the Brea 

Olinda field, Los Angeles Basin, was discovered in 1880 (Biddle, 1991). From 1920 to 1950, numerous investigators aggressively pursued accurate determination of present-day 

equilibrium-burial temperatures in different areas, including the 

southern California basins, by closely monitoring production 

temperatures and determining long-term (as much as 3 years) 

shut-in temperatures. This resulted in numerous publications of 

equilibrium-burial temperatures in the southern California 

basins, including Carlson (1930a, b), Van Orstrand (1934), 

French (1940), Benfield (1947), and Moses (1961, 1962). Accurate equilibrium-burial temperatures also were later measured by 

John Castaño and coworkers (Castaño and Sparks, 1974; and 

Hood and Castaño; 1974) and by Wang and Munroe (1982). 

Consequently, we have knowledge of present-day equilibriumburial temperatures, available in few other studies. 

The current study involves a very large sample base composed of core samples from more than 70 wells in the Los Angeles, Ventura, and Southern San Joaquin Valley Basins (table 1), 

as well as cuttings-chips samples from the Apex-1 well, Los 

Angeles Basin, which was drilled with a water-base mud. Several wells from the northern San Joaquin Valley and Sacramento 

Valley Basins were also studied. This study’s sample base 

spanned a wide temperature range, with maximum burial temperatures of 223°C. (Geothermal gradients for the different 

study areas are given in table 2.) Areas of uplift and erosion 

exist in all the basins studied; however, only samples from the 

basin synclinal areas at present-day maximal burial are reported 

here.

Early publications on the Los Angeles-Ventura (California), 

Douala (Cameroons, Africa), Paris (France), and Williston 

(North Dakota) Basins form the foundation for present-day 

petroleum geochemistry. However, 1974 to 1978 U.S. Geological Survey petroleum-geochemical research in the California 

basins resulted in data which fell outside of existing paradigm. 

Some of these data were presented in Walker (1982), Walker 

and others (1983), Price (1983, figs. 3,4) and Price and Barker 

(1985, fig. 4 and table 1), wherein different organic-matter (OM) 

types responded to the same burial conditions in dramatically 

different fashions. The California basins perhaps offer our best 

possibility to examine the thermal response of different OM 

types to the same burial conditions, because of the large burialtemperature range of the rocks there, the large sample base of 

cores, the wide range of OM types in contiguous samples, the 

well-known geologic history, and the accurately determined 

equilibrium-burial temperatures. 

The Los Angeles-Ventura Basins are not the only examples 

of basins having data sets that strongly contradict established 

paradigm, constituted on the basis of original studies in the Los 

Angeles-Ventura, Douala, Williston, and Paris Basins. Investigations in the Williston Basin (Price and Le Fever, 1994) also 

demonstrate that the currently accepted model of oil expulsion 

and accumulation, a model formed on the basis of early studies 

in the Williston Basin, is dysfunctional in the very basin in 

which the model was derived.

In this study, the petroleum-geochemical analyses on the 

rocks of the California basins yielded complex results, for which 

interpretations were difficult. Much greater complexity would 

have been added to this study with any attempted oil to sourcerock correlations. Moreover, sufficient rock-sample bases, 

regarding both maturity and basinal location, are not available 

for these basins to perform such a study.

This is a three part study. This first paper involves ROCKEVAL and vitrinite reflectance (Ro) analyses. The second part 

(Price, 1999) concerns quantitative and qualitative results of the 

extracted bitumen and saturated hydrocarbons (HCS). The third 

part will involve biomarkers, aromatic HCS, aqueous-pyrolysis 

experiments, modeling studies, and whole-rock extraction as 

related to possible low-rank formation of commercial-oil deposits in these basins.

Stratigraphy, stratigraphic relationships, and depositional 

histories are not discussed in detail in these three studies. 

Bostick and others (1978) present detailed stratigraphy and 

burial histories for many of the areas of this study. Stratigraphy 

for all areas of this study is available in California Oil and Gas 

Fields (1973, 1974). Thick Pliocene to Pleistocene sections are 

present in the synclines of the Los Angeles, Ventura, and Southern San Joaquin Valley Basins. The oldest rocks we examine are 

upper mid Miocene in the Los Angeles-Ventura Basins and 

lower Miocene in the Southern San Joaquin Valley Basin. Thus, 

the burial histories of the rocks we studied from these three 

basins are very similar.

Samples and Methods

With the exception of the cuttings-chips samples from the 

Apex-1 wellbore, all samples were core (table 1). All wells were 

drilled with water-based muds. All core samples were cleaned 

by scrubbing them with a brush in warm water until all drilling 

mud was removed. The Apex-1 cuttings-chips samples (originally in packets of 30 or 60 ft (9.1 to 18.2 m) intervals) were 

cleaned in hot water, by an ultrasonic cleaner, to remove all drilling mud. The chips were then lightly rinsed with methanol to 

remove the water and dried in an oven at 50°C for five minutes. 

Iron fragments from cuttings-chips samples were removed 

with a magnet, the chips were then sieved and microscopically 

picked (to 20 mesh) to dominant lithologies (black shales, brown 

shales, and dark or light silty shales or shaley silts, sandstones). 

Samples were then ground and sieved to 100 mesh for ROCKEVAL analysis carried out at the U.S. Geological Survey from 

1980 to 1994.

Many ROCK-EVAL analyses of sandstones and silty sandstones indicated oil staining which was also present in some siltstones. Four to five hundred milligrams of 100 mesh powder 

from many stained samples were solvent extracted for 3 to 7 

days simultaneously together in small packets. These packets 

were made by folding over and stapling shut 5.5 mm diameter 

filter paper and were labeled with a soft lead pencil. The 

extracted samples were then reanalyzed by ROCK-EVAL. 

Reanalyzed oil-stained samples exhibited large decreases (125 
Wilmington Field 3

Table 1. Wells, and well locations (including basins, fields, and areas) from which samples were obtained for 

this study. Latitude and longitude values for some wells are from Bostick and others (1978). Wilimington field 

wells are directionally-drilled wells from cluster sites of piers or offshore man-made islands.

LOS ANGELES BASIN

 Santa Fe Springs Field

Union Oil "Bell" No. 62, lat 33 ̊56'34" N., long 118 ̊03'55" W.

Mobil (General Petroleum) "Santa Fe" No. 46A, lat 33 ̊56'31" N., long 118 ̊03'25" W.

Shell "GH & N" No. 15, lat 33 ̊56'45" N., long 118 ̊04'22" W.

ARCO (Richfield Oil Corporation), "Elliott" No. 2, lat 33 ̊56'44" N., long 118 ̊04'53" W.

Union Oil "Bell" No. 38, lat 33 ̊56'39" N., long 118 ̊04'00" W.

Mobil (General Petroleum) "Santa Fe" No. 243, lat 33 ̊56'21" N., long 118 ̊03'33" W.

Union Oil "Bell" No. 100, lat 33 ̊56'36.8" N., long 118 ̊03'54" W.

Union Oil "Bell" No. 107, lat 33 ̊56'43.5" N., long 118 ̊04'01.5" W.

Texaco Foix No.6, sec. 6, T. 3 S., R. 11 W.

Jordon No. 7, NW1/4 NW1/4 NW1/4 sec. 6, T. 3 S., R. 11 W.

Jordon No. 8, NW1/4 NW1/4 NW1/4 sec. 6, T. 3 S., R. 11 W.

Northwest Plunge Santa Fe Spring Field

Chevron (Standard Oil of California), "Houghton Comm. One" No. 1, lat 33 ̊57'18" N., long 118 ̊5'10.5" W., sec. 36, T. 2 S., R. 12 W.

 Long Beach Field

Shell Stakemiller Rose No. 1, sec. 29, T. 4 S., R. 12 W.

Richfield Malcom Davis #8, SE sec. 19, T. 4 S., R. 12 W.

Shell Alamitos 48A, sec. 29, T. 4 S., R. 12 W.

Texaco Long Beach Airport-1, sec. 20, T. 4 S., R. 12 W.

 Central Syncline

Chevron (Standard Oil Company of California), "Carlin Comm." No. 1, lat 33 ̊54'19" N. long 118 ̊13'07" W., NW1/4 NW1/4

 sec. 14 T. 3 S., R. 13 W.

Western Gulf "Pacific Gas and Electric" No. 1, lat 30 ̊57'11.9" N., long 118 ̊14'34.4" W., NE1/4 SE1/4 sec. 33, T. 2 S., R. 13 W.

American Petrofina Central Core Hole-2, SW1/4 NE1/4 sec. 4, T. 3 S., R. 13 W.

 Whittier Field

Standard Oil of California MW-101, sec. 26, T. 2 S., R. 11 W.

Standard Oil of California MW-62, NE1/4 NW1/4 sec. 26, T. 2 S., R. 11W.

Standard Oil of California MW-100, NE1/4 NE1/4 sec. 26, T. 2 S., R. 11W.

 Huntington Beach Field

Standard Oil Calif. Lomita Land and Water Co., sec. 19, T. 5 S., R. 11 W.

 Dominguez Field

Standard Oil Calif. Del Amo-1, sec. 8, T. 4 S., R. 13 W.

Shell (Dominguez-Oil), Reyes-1, sec. 33, T. 3 S., R. 13 W.

Shell (Dominguez-Oil), Reyes-27A, sec. 34, T. 3 S., R. 13 W.

Samples and Methods
4 Organic Metamorphism in the California Petroleum Basis

 Dominguez Field—continued

Shell (Dominguez-Oil), Reyes-60, sec. 34, T. 3 S., R. 13 W.

Shell (Dominguez-Oil), Reyes-97, sec. 33, T. 3 S., R. 13 W.

Shell (Dominguez-Oil), Reyes-100, sec. 34, T. 3 S., R. 13 W.

Shell (Dominguez-Oil), Reyes-109, sec. 34, T. 3 S., R. 13 W.

Shell (Dominguez-Oil), Reyes-141, sec. 34, T. 3 S., R. 13 W.

Shell (Dominguez-Oil), Reyes-147, sec. 33, T. 3 S., R. 13 W.

 Wilmington Field

 WJ141 WD107 D308

 WD401 WD114 D102

 WC533 WD204 D605

 WD401 WC603 C403

 Seal Beach Field and Evirons

Texaco Bixby Ranch-1, sec. 36, T. 4 S., R. 12 W.

Texaco Bixby Ranch-2 (NCT-1), sec. 36, T. 4 S., R. 12 W.

Texaco Bryant Estate-1, SW Corner sec. 1, T. 5 S., R. 12 W.

Continental Oil McGrath-18, sec. 10, T. 5 S., R. 12 W.

 Baldwin Hills Area

Shell Baldwin Hills Community-1, sec. 9, T. 2 S., R. 14 W.

 Northeast Flank

C.C.M.O. "Bandini" No. 1, lat 33 ̊59'16.5" N., long 118 ̊08'23.6" W., NW1/4 NE1/4 sec. 21, T. 2 S., R. 12 W.

ARCO (Richfield Oil Corporation) "Vail" No. 1, lat 34 ̊00'16.5" N., long 118 ̊08'17" W., NE1/4 NE1/4 sec. 16, T. 2 S., R. 12 W.

ARCO (Richfield Oil Corporation) "UP Unit" No. 1., lat 34 ̊00'09" N., long 118 ̊08'45.5" W., NE1/4 NW1/4 sec. 16, T. 2 S., R. 12 W.

 Anaheim Nose

Mobil (General Petroleum Corporation), "Librown" No. 1, lat 33 ̊53'26" N., long 118 ̊01'44.5" W., sec. 21, T. 3 S., R. 11 W.

General Petroleum La Mirada 46-1, sec. 16, T. 3 S., R. 11W.

General Petroleum McNally-1, sec. 22, T. 3 S., R. 11 W.

Standard Oil Calif. (Chevron), Pacific Community-1, sec. 26, T. 3 S., R. 11 W.

VENTURA BASIN

Ventura Avenue Field

Getty Lloyd-183, Ventura Avenue Field, Ventura Basin, NW1/4 NW1/4 sec. 26, T. 3 N., R. 23 W.

Shell Taylor 316, NE1/4 NE1/4 sec. 29, T. 3 N., R. 23 W.

Shell Taylor 315, NW1/4 NW1/4 NW1/4 sec. 28, T. 3 N., R. 23 W.

Shell Taylor 395.

 Central Syncline

Standard Oil Calif. (Chevron) Limoneira-1, sec. 30, T. 3 N., R. 22 W.

Chevron (Standard Oil Company of California) "Maxwell" No. 1, lat 34 ̊14'35"N., long 119 ̊15'06" W., sec. 23, T. 2 N., R. 23 W.

Superior Oil Company "Limoneira" No. 1, lat 34 ̊16'07" N., long 119 ̊11'48" W., sec. 9, T. 2 N., R. 22 W.

Table 1—continued
Results 5

SOUTHERN SAN JOAQUIN VALLEY BASIN

 Paloma Field

Ohio (Marathon) Oil KCLA 72-4, sec. 4, T. 32 S.,R. 26 E. (MDB)

Ohio Oil A-1, sec. 31, T. 31 S., R. 26 E. (MDB)

Ohio Oil KCLA 8, sec. 32, T. 31 S., R. 26 E. (MDB)

Ohio Oil KCLA 61-31, sec. 31, T. 31 S., R. 26 E. (MDB) 

Ohio Oil KCL-F-71-10, sec. 10, T. 31 S., R. 26 E. (MDB)

Superior Anderson 56, sec. 35 T. 31 S., R. 26 E. (MDB)

KCL-74-3, sec. 35 T. 31S., R. 1, sec. 3, T. 32 S., R. 26 E. (MDB)

Coles Levee/Canal Fields

Shell Canal 21-14, sec. 14, T. 30 S., R. 25 E. (MDB)

Arco CLA 67-29, Coles Levee A, sec. 29, T. 30 S., R. 25 E. (MDB)

Southern Rim

Arco RSS 84-26, sec. 26, T. 11 N., R. 20 W.

Ohio Oil Mitchell Ranch-1, sec. 3, T. 32 S., R. 29 E. (MDB)

Tenneco-Superior Sand Hills 64X, sec. 34, T. 32 S., R. 28 E. (MDB)

MIDDLE SAN JOAQUIN VALLEY BASIN (JACALITOS FIELD)

Standard Oil of California 67-17E, Jacalitos Oil Field, sec. 17, T. 21 S., R. 15 E. (Fresno County), sec. 17, T. 21 S., R. 15 E. (MDB) 

SACRAMENTO VALLEY BASIN (TRACY GAS FIELD)

Amerada Farmers Development Lands (FDL)-1, sec. 15 T. 2 S., R. 5 E. (MDB)

Table 1—continued

to 500 percent or more) in total organic carbon (TOC) and in the 

S1 and S2 pyrolysis peaks, either as normalized to TOC or in 

absolute concentrations (ppm by rock weight). However, these 

data are not presented here. Rocks with TOC values less than 0.5 

percent are also largely excluded. Although some immature finegrained rocks were oil-stained (discussed in Price, 1999), others 

had abnormally high indigenous-HC concentrations. Analyses of 

all such fine-grained rocks are included here. 

Vitrinite reflectance (Ro) analyses were carried out by kerogen isolation by HCl-HF rock maceration, floating the kerogen 

in hydrogen bromide during centrifuging, water-rinsing, and 

freeze-drying the float kerogen. Microscopic analyses were 

largely performed from 1975 to 1984. All samples had at least 

30 readings; almost all had 50–70 readings; some selected samples had up to 120 readings. Representative examples of the statistical distribution of the Ro histograms, and the number of 

readings per sample, are in Bostick and others (1978). Many Ro

analyses of these same rocks published in Bostick and others, 

(1978) followed expected behavior regarding Ro increase with 

increasing burial temperature. However, other samples yielded 

abnormally low Ro values for their burial temperatures (discussed in Walker, 1982; Walker and others, 1983; and Price and 

Barker, 1985). Thus, these samples were sent to commercial 

(service) geochemical laboratories and to other investigators to 

verify the in-house measurements. Some samples had as many 

as 20 such separate measurements. Unlike previously reported 

“round-robin” petroleum-geochemical studies (Claypool and 

Magoon, 1985; Dembicki, 1984; and Isaacs and Rullkötter, 

1993), in this case excellent interlaboratory agreement was consistently reported: anomalously low Ro values. 

Results

Los Angeles Basin Central Syncline

The Apex-1, a 6,332.6 m wellbore, drilled in the central 

syncline of the Los Angeles Basin, is the deepest well in the 

basin. Rock lithologic characteristics of the Apex-1 wellbore 

bear on its petroleum geochemistry. In this well, from 1,067 m 

(the shallowest sample) to 3,350 m, gravel, sandstone, and silty 

sandstone constitute 70 to 95 percent of the material in the sample packets. From 1,067 m to 3,012 m, the only fine-grained 

rocks were silt; shale or shaley silt were absent. Shale was first 

encountered at 3,073 m. Conversely, deeper than 3,350 m, shale 

and shaley-silt constitute 50 to 80 percent of the sample packets. 

Similar lithologic changes versus depth were present in other 

areas of the Los Angeles (and Ventura) Basins and are due to the 

basins “sanding up” late in their history (upper-Miocene to 
6 Organic Metamorphism in the California Petroleum Basis

Table 2. Equations for geothermal gradients in the areas examined in this study. Temperatures are in °C and depths are in 

feet. Depths are given in feet and not meters, because all the original sample depths were in feet, thus allowing more direct 

comparison of other sample bases with that of this study and Price (1999). Log-header (nonequilibrium) temperatures were 

corrected by the formula in Price and others (1981).

Basin

 Field/Area

 Equation for

 Geothermal Gradient

 Reference Source

Los Angeles

 Santa Fe Springs

 ̊C = 23.3 ̊ + 0.0107 x depth Bostick and others (1978) Shallow production temperatures and

deep and shallow well shut-in 

temperatures.

Los Angeles

 Long Beach

 ̊C = 23.0 ̊ + 0.01066 x depth French (1940) Production emperatures. Surface temperature of 23.0 ̊C assumed.

Los Angeles

 Whittier

 ̊C = 21.7 ̊ + 0.01052 x depth Van Orstrand (1934)

and Carlson (1930a, b)

Shallow production 

temperatures.

Los Angeles

 Central Syncline

 ̊C = 23.3 ̊ + 0.008797 x depth 

(for 0-12,300 ft)

 ̊C = 131.5 + 0.0110 (depth-12,310 ft)

(for >12,300 ft)

This study Corrected nonequilibrium log-header 

temperatures.

Los Angeles

 Wilmington

 ̊C = 21.1 + .017002 x depth Mayuga (1970) Deep production temperatures.

Los Angeles,

 Anaheim Nose

 ̊C = 23.3 + 0.00086 x depth 

(for 0-6,000 ft) 

 ̊C =74.9 + 0.0104 x (depth - 6,000 ft)

(for >6,000')

Bostick and others (1978) Deep well shut-in temperatures.

Los Angeles,

 Santa Fe Springs

 ̊C = 137 + 0.01222 x (depth -10,000 ft)

(for 10,000 ft -15,000 ft)

Bostick and others (1978) Corrected nonequilibrium temperatures.

Los Angeles,

 Los Angeles 

 ̊C = 23.3 + 0.0102 x depth Bostick and others (1978) Deep well shut-in temperatures.

Los Angeles,

 Huntington Beach

 ̊C = 23.0 + 0.013479 x depth Van Orstrand (1934) Production temperatures. Surface temperature of 23.0 ̊C assumed. 

Los Angeles,

 Dominguez

 ̊C = 23.0 + 0.010620 x depth French (1940) Production temperatures. Surface temperature of 23.0 ̊C assumed.

Los Angeles,

 Seal Beach

 ̊C = 23.0 + 0.011188 x depth French (1940) Production temperatures. Surface temperature of 23.0 ̊C assumed.

Los Angeles,

 Inglewood

 ̊C = 23.0 + 0.010318 x depth Walker (1982) Corrected nonequilibrium temperatures for Standard Oil Calif.-Packard 

55 well.

Surface temperature of 23.0 ̊C 

assumed.

Ventura,

 Central Syncline

 ̊C = 21.0 + 0.0071 x depth Bostick and others (1978) Deep well shut-in temperatures.

Ventura,

 Ventura Avenue

 ̊C = 21.1 + 0.008335 x depth John Castaño, Shell Oil,

oral commun., 1985

Six month shut-in temperatures from six separate deep wells.

Southern 

Sacramento Valley,

 Tracy

 ̊C = 22.2 + 0.00851 x depth This study Corrected nonequilibrium log-header 

temperatures

Northern

San Joaquin Valley,

 Jacalitos

 ̊C = 22.4 + 0.01009 x depth Van Orstrand (1934) Shallow production temperatures

Southern

San Joaquin Valley,

 Paloma

 ̊C = 22.0 + 0.009186 x depth Hood and Castaño (1974) Production and shut-in temperatures

Southern

San Joaquin Valley,

 Coles Levee

 ̊C = 23.0 + 0.09658 x depth Moses (1961)

Southern

San Joaquin Valley,

 Tejon Area

 ̊C = 20.1 + 0.007686 x depth Castaño and Sparks (1974)
Results 7

TEMPERATURE, IN °C

DEPTH, IN KILOMETERS

50

0 0.2 0.4 0.6 0.8

70

90

110

130

150

170

1.0

2.0

3.0

4.0

5.0

6.0

190

210

230

Pliocene). The shallow coarse-grained rocks resulted from WHOLE ROCK TOC

heavy erosion of crystalline rock from adjacent highland areas 

concurrently being uplifted. Upper Miocene rocks were still 

present at the bottom of the Apex-1 wellbore at 6,325 m.

One consequence of this “sanding up” in these basins is a 

strong decrease in rock organic richness with decreasing depth, 

from the increase in gravels and sands. This is apparent in figure 

1, where TOC of the cleaned unpicked cuttings chips is plotted 

versus depth for random samples. Average TOC contents for 

rocks deeper than 3,350 m largely varies between 0.4 and 0.8 

percent (fig. 1). The lower TOC values within this burial interval 

are from samples with higher sand contents, and higher TOC 

values are from samples with higher silty-shale and shale contents. Shallower than 3,350 m, average TOC contents dramatically decrease and remain low due to the high sand and gravel 

content of these rocks. Essentially the upper 3,350 m of the Los 

Angeles Basin central syncline is a sand and gravel pit. This has 

strong implications for meteoric water flow in the recent geologic past.

A second important lithologic characteristic of the Apex-1 

wellbore is that rock samples, especially samples from deeper 

than about 3,500 m, often exhibited evidence of turbidite deposition. For example, sandstone, silt, or shale were swirled with 

each other in the same cutting chip (nonlinear depositional 

boundaries between different rock types in the same chip). In 

addition, grain size was poorly sorted in many chips. Poorly 

sorted silts were the most common fine-grained rocks in the 

wellbore, and shales contained occasional sand-size grains. 

However, well-sorted shales or sandstones also were present in 

the wellbore. Beyer (1988a), Redin (1991), and many other 

investigators have noted that turbidity currents played major 

roles in Los Angeles Basin sedimentation. Debris flows, fluidized-sediment flows, and grain flows also were significant 

(Redin, 1991). This infilling of the Los Angeles Basin central 

syncline by mass-transport sedimentation strongly affected rock 

OM characteristics (fig. 2). 

The TOC content and the carbon-normalized (mg/g) 

ROCK-EVAL S1 and S2 (hydrogen index) peaks are plotted versus burial temperature and depth for rocks from the Apex-1 in 

figure 2. Only four samples have hydrogen index (HI) values 

above 225 (the maximum value being 294). Deeper than 5,650 

m (200°C), a strong decrease of hydrogen index (HI) values 

occurs from intense HC generation. However, many of the figure 2 samples shallower than 5,250 m (and thus not affected by 

intense HC generation) also have low HI values (80 to 150), 

values characteristic of type IV OM. Many samples also exhibit 

HI values between 150 and 200 (type IV /III OM) and only a 

few samples have HI values over 200 (type III OM). The dominance of types IV and IV/III OM in these rocks precludes the 

possibility that they be considered as oil-source rocks. Additional ROCK-EVAL data (not shown here) collected from two 

other wellbores from 612 to 4,182.9 m in the Los Angeles Basin 

central syncline (Carlin Community-1, and Pacific Gas and 

Electric-1, table 1), also exhibit low TOC and low HI values, 

also indicating a lack of source rocks there.

Most past investigators have concluded that one of the most 

important reasons the Los Angeles Basin is so oil-rich, is that the 

basin has abnormally rich source rocks. However, there is 

almost no published data supporting this widespread assumption, an assumption which the HI data of figure 2 suggest is not 

completely accurate. That at least 6,332.6 m of rocks in the Los 

Angeles Basin central syncline contain no hydrogen-rich OM is, 

to say the least, unexpected.

Our large ROCK-EVAL data base demonstrated that at different sites in the Los Angeles and Ventura Basins, the organic 

richness of the rocks there (as measured by TOC and HI values) 

increased dramatically with depth. In figure 2, TOC and HI values appear to be largely invariant with depth, contradicting this 

generally observed trend. However, the figure 2 data are from 

fine-grained rocks picked from the total sample. If TOC values 

of the total (unpicked) sample are considered (fig. 1), then rock 

organic richness at the Apex-1 site clearly does increase with 

depth, especially beyond 3,350 m. This general increase in rock 

organic richness with depth in these basins is caused by: (1) an 

increase in fine-grained rocks with depth; (2) possibly a general 

shift of depositional conditions for shallower rocks (resulting in 

lower HI values) compared to the depositional conditions for 

deeper rocks which resulted in higher HI values (350-800); (3) 

increasing hydrogenation of kerogen with increasing depth; and 

(4) lower sedimentation rates for deeper rocks compared to shallower rocks, resulting in less dilution of the OM deposited with 

the sediment. This last feature is thought to be an important 

overlooked depositional control of sediment organic richness 

(Caroline Isaacs, then U.S. Geological Survey, written commun., 

1997). 

As stated, one may argue that the HI data of figure 2 are 

largely depth-invariant to about 200°C, where HI values 

strongly decrease with depth from mainstage HC generation. 

However, as portrayed in figure 2, we interpret the data otherwise. From 1,000 m to 2,440 m, maximal HI values increase 

from 105 to 220. We attribute this increase both to kerogen 

hydrogenation and to changes in depositional conditions with 

Figure 1. Plot of TOC (total organic carbon) versus burial temperature 

in °C and burial depth in kilometers for unpicked cleaned samples from 

random cuttings chips from the Apex-1 wellbore. Depth axis changes 

scale because of a dog-leg in the geothermal gradient.
8 Organic Metamorphism in the California Petroleum Basis 6080

100

120

140

0

40

80

120

160

200

240

280

1.0

2.0

294

4.0

5.0

6.0

S1mg/g OC HYDROGEN INDEX

90

110

130

150

170

50

70

190

210

230

3.0

DEPTH, IN KILOMETERS

TEMPERATURE, IN °C

CIHG

0.5

1.0

1.5

2.0

2.53.0

0

20

40

TOC

HI values over this burial range (fig. 2). Moreover, production 

indices (fig. 3, PI) are abnormally high for these burial temperatures, ranging between 0.20 and 0.45. These production index 

(PI) values are characteristic of those of much deeper samples in 

figure 3. If such intense HC generation were occurring from 85° 

to 140°C, HI values should be concurrently decreasing. Instead, 

like the S1 values, HI values strongly increase over this interval 

(fig. 2), exactly the opposite of expected behavior. This dual 

increase of S1 and HI values for rocks between 80° and 140° C is 

more apparent in figure 4. The relationship between S1 and HI is 

interpreted as evidence of varying depositional conditions controlling the organic richness of rocks, and thus partly controlling 

the increase in S1 values, over this burial interval. In other 

words, the rocks are becoming more organic-rich with depth. 

This increase in S1 values from 80° to 140°C is also partly 

attributed to a characteristic of fine-grained rocks from the California oil basins discussed both below and in Price (1999). 

Abnormally high carbon-normalized contents of immature 

indigenous HCS and bitumen are present in some rocks of these 

basins at ranks below mainstage HC generation. The parameter(s) controlling this feature may have been operative when the 

rocks of the 80° to 150°C interval were deposited, apart from a 

general increase in organic richness with depth. The high HC 

concentrations in rocks over 85° to 140°C are neither from oil 

staining nor contamination during drilling. As noted above, the 

Apex-1 was drilled with a water-based mud and sandstones 

between 85° and 140°C have normal S1 and S2 values (fig. 5), 

precluding the possibility of contamination from staining or 

drilling.

Returning to figure 2, S1 values strongly decrease between 

140°and 150°C, correlating with the decreasing HI values over 

this interval. The correlation of decreasing S1 values with 

Figure 2. Plot of TOC (total organic carbon), and the TOC-normalized (milligrams per gram, mg/g OC) S1 and S2 (HYDROGEN INDEX) ROCK-EVAL 

peaks versus burial temperature in °C and depth in kilometers for siltstones and shales from the Apex-1 wellbore, central syncline, Los Angeles Basin. 

Depth axis changes scale because of a dog-leg in the geothermal gradient (table 2). Trends defined by solid lines are discussed in text. The number 

294 in the hydrogen index plot is an offscale high value. "CIHG" in the carbon-normalized S1 plot is the commencement of intense HC generation by 

that measurement.

increasing depth. At 2,440 m, HI values begin decreasing to 

minimum values of about 70 (type IV OM) around 2,500 to 

2,700 m. Hydrogen indices then increase with depth to maximal 

values of 230 at 4,054 m, with two outliers at 276 and 294. 

Hydrogen indices of 200–230 are still hydrogen-poor, type III 

OM; however; over a three-fold increase has occurred in maximal HI values from 2,700 m to 4,054 m. This significant 

increase is again attributed both to changes in depositional conditions and to kerogen hydrogenation with increasing depth . A 

significant decrease of maximum and minimum HI values from 

142° to 165°C are not due only to HC generation, because maximal HI values increase from 140 at 165°C to 190 by 175°C. 

Thus, these changes in HI values over the temperature interval 

of 142° to 165°C are attributed partly to changing depositional 

conditions. Beyond 5,578 m (196°C), HI values strongly 

decrease to minimal values of 40 to 60 by 223°C (6,320.9 m), 

and we attribute the decrease to intense HC generation. 

The ROCK-EVAL S1 peak normalized to organic carbon 

(S1 mg/g OC, fig. 2) is an indirect measure of the HC coefficient 

as would be determined from solvent extraction. The S1 peak 

demonstrates significant increases from 1,070 m to 3,934.8 m, 

attributed here to both HC generation and to varying organicgeochemical characteristics from changing depositional conditions. Commencement of intense HC generation (CIHG, fig. 2) 

occurs at 85°C by the S1 data of figure 2, in contrast to Phillipi’s 

(1965) value of 120°C. Nevertheless, other data from this paper 

and from Price (1999) support the lower temperature value. 

However, the S1 increase from 85° to 140°C is also related to 

two factors other than HC generation.

For example, the S1 values of figure 2 maximize at 100 to 

140 mg/g OC at 140°C. These are extreme values for OM with 

so little capacity for HC generation as demonstrated by the low 
Results 9

0

20

40

60

80

100

120

140

410

420

430

440

450

460

470

0

0.1

0.2

0.3

0.4

0.5

0.4

0.6

1.0

1.5

OXYGEN INDEX Tmax Pl Ro

TEMPERATURE, IN °C

70

50

90

110

130

150

170

190

210

230

3.0

4.0

5.0

6.0

1.0

2.0

DEPTH, IN KILOMETERS

168

CIHG

140 80-140°C

140°C-150°C

120

100

80

0

4080

120160200240280

320

HYDROGEN INDEX

60

40

20

0

S1 mg/g OC

decreasing HI values is also demonstrated in figure 4. This 

decrease is attributed to varying depositional conditions affecting OM characteristics, rather than to intense HC generation, 

which should result in increasing S1 values versus decreasing HI 

values, exactly opposite the observed trends in figures 2 and 4. 

From 150° to 195°C, S1 values increase with depth (fig. 2), 

which we attribute to intense HC generation over this interval. 

ROCK-EVAL S1 values decrease from 195° to 223°C at well 

bottom. Hydrogen index values also strongly decrease over this 

interval from continuing intense HC generation. The S1

decrease is due to loss of generated HCS from the rocks and is 

not due to HC thermal destruction. This HC loss is from both 

HC expulsion and HC loss to the drilling mud in drilling operations (a topic discussed in Price, 1999). The loss of generated 

HCS may first commence at about 140°C in the Apex-1 wellbore, but intensifies with depth from increasing amounts of generated HC gases, especially above 195°C.

Katz (1983) demonstrated that the ROCK-EVAL oxygen 

index (mg S3 peak/g OC) is not a valid measurement in low 

TOC rocks with elevated CaCO3 concentrations, because of 

CaCO3 thermal decrepitation. Katz (1983) also noted that the 

oxygen index is a valid parameter in rocks with moderate to high 

TOC values and low CaCO3 concentrations. Both of Katz’s 

(1983) observations are consistent with results of our studies. 

The fine-grained rocks of the California basins largely have 

moderate TOC contents (1 to 5 percent) and low CaCO3 contents. Both the oxygen index and Tmax data from the Apex-1 

rocks broadly indicate that organic metamorphism proceeded in 

these rocks over all burial temperatures: In figure 3, oxygen 

indices consistently decrease over all burial temperatures to low 

values (7 to 30) at well bottom. The Tmax values of most samples 

are rather tightly distributed in the swath defined in figure 3, 

increasing with depth to values (450°C and above) at well bottom to be expected from the high burial temperatures (>220°C) 

of these samples. 

In the Tmax plot of figure 3, at depths beyond about 3,700 

m, light-colored silts (the crosses in the fig. 3 Tmax plot) invariably had low Tmax values, some abnormally so. These samples 

Figure 3. Plot of ROCK-EVAL oxygen index (S3 mg/g OC),Tmax, production index (PI) values; and Ro versus burial temperature in °C and burial depth 

in kilometers for siltstones and shales from the Apex-1 wellbore. Depth axis changes scale because of a dog-leg in the geothermal gradient (table 

2). Trends defined by solid lines discussed in text. Crosses in the Tmax plot are low-TOC siltstones. "CIHG" in the PI plot is the commencement of 

intense HC generation as indicated by that measurement. The number 168 in the oxygen-index plot refers to a sample point with an offscale value 

of 168.

Figure 4. Cross plot of TOC-normalized ROCK-EVAL S1 peak values (S1

mg/g OC) versus hydrogen index for Apex-1 shales and silts with burial 

temperatures of 80° to 150°C. Samples from 80° to 139.9°C are shown by 

dots and are largely enclosed by the solid lines. Samples from 140.0° to 

150.0°C are shown by crosses and are largely enclosed by the dashed 

lines and stippling.
10 Organic Metamorphism in the California Petroleum Basis

50

70

0

0.02

0.040.060.08

0.100.12

0.14

0

40

80

120

160

200240

280

90

1.0

2.0

3.0

4.0

5.0

6.0

110

130

150

170

190

210

230

TEMPERATURE, IN °C

DEPTH, IN KILOMETERS

TOC S1 & S2 ppm

S1

S2

had the lowest TOC values of the sample base (0.73 to 1.20 

percent), but made up only a small percentage of the total sample base. The cause of the abnormally low Tmax values in these 

silts is unknown, as is the cause of the abnormally high Tmax

values (464° to 475°C) of the six core samples at depth (silts 

and shales).

ROCK-EVAL oxygen indices and Tmax values, both in this 

well and in general, were the best maturity indices of the study, 

usually indicating progressive organic metamorphism with 

increasing temperature, when other maturity indices failed to do 

so. For example, vitrinite reflectance (Ro) values for the Apex-1 

(fig. 3) are nearly invariant from 110° to 188°C and provide no 

indication of the changes taking place in the OM of this well over 

this interval. From 188° to 223°C, Ro values increase tightly with 

depth. Ro suppression has been documented in hydrogen-rich 

OM in general (Price and Barker, 1985) and especially in the 

hydrogen-rich OM of the California basins (Walker, 1982; 

Walker and others, 1983; Price and Barker, 1985). However, the 

Ro plot of figure 3, and other Ro data of this study, demonstrate 

that Ro in the hydrogen-poor OM of the California basins, can 

also be significantly suppressed compared to expected values.

Production indices (fig. 3, PI) for Apex-1 rocks increase 

moderately from 50° to 81°C from the slightly increasing S1

peak values over this interval (fig. 2). The large increase in 

production indices above 85°C, as discussed above, is attributed to both changing depositional conditions and to HC generation. The decrease in production indices over 140° to 150°C, 

as also discussed above, is attributed principally to variable 

depositional conditions. The significant increase in production 

indices from 150° to 223°C is attributed solely to mainstage 

HC generation.

Occasional Apex-1 cuttings chips below 4,000 m had discrete pieces of vitreous tar interbedded with the sediments, suggesting that oil macro seepage was active at the depositional site 

of such rocks. Oil shows were encountered in the Apex-1 from 

shallow depths to well bottom (Arco personnel, Bakersfield 

Exploration Office, oral commun., 1985). This fact was 

reflected by occasional pieces of tar on the surface of some cuttings chips or by discrete pieces of tar in varying amounts in the 

cuttings chips packets. All tar-bearing chips (external or internal) were excluded from ROCK-EVAL analysis. These oil 

shows would seem to contradict the low S1 and TOC values of 

the sandstones in figure 5. However, when the samples shown in 

figure 5 were picked, all brown-stained (oil-bearing) sandstones, which were a small percentage of the total grains, were 

excluded from analysis.

Figure 6 shows porosity and percent residual-oil-saturation 

analyses from Core Laboratories for conventional core of a thick 

sandstone from 5,701.6 to 5,728.2 m in the Apex-1 and is a 

graphic example of the oil shows encountered at depth in the 

Apex-1. Porosities range from low (2.6 percent) to moderate 

(12.8 percent) values. Most sands contained no oil. However, 

the measurable to moderate residual-oil concentrations in some 

sands, at burial temperatures of 203°C, and higher, are unexpected by accepted paradigm concerning HC thermal stability. 

Table 3 presents porosity, permeability, and percent residual-oil 

saturation analyses from Core Laboratories for sandstones from 

sidewall core from between directionally drilled uncorrected 

vertical depths of 18,410 to 19,826 ft (corrected depths in meters 

of 5,483.4 m, 194.1°C, to 5,912.9 m, (209.7°C). These analyses 

augment the data shown in figure 6. The 5,890.0 m sample 

(208.9°C) with a porosity of 31.3 percent, a permeability of 

Figure 5. Plot of TOC and the ROCK-EVAL S1 (dots) and S2 (crosses) pyrolysis peaks (in ppm by rock weight) for sandstones from 

the Apex-1 wellbore versus burial temperature in °C and depth in kilometers. Depth axis changes scale because of a dog-leg in 

the geothermal gradient (table 2). Although there is an increase in TOC, S1, and S2 values deeper than about 2,000 m, the low values 

in these parameters at depth preclude the possibility of a general organic contamination of Apex-1 rocks.
Results 11

4

8

12

0

0

10

20

30

5710

5716

5722

5728

5704

DEPTH, IN METERS

POROSITY,

IN PERCENT

PERCENT RESIDUAL

OIL SATURATION

studied. ROCK-EVAL and Ro analyses are presented in figure 7 

for two deep wells in the Ventura Basin Central Syncline (“Santa 

Clara Trough” of Nagle and Parker, 1971), the Chevron Maxwell-1 (5,394 m) and the Superior Limoneira-1 (5,710 m), and 

for one shallow well, the Chevron Limoneira-1. The deepest 

data point from the Chevron Limoneira-1 is at 2,730 m. Both 

maximal TOC and HI values increase significantly beyond 4,270 

m, demonstrating that the rocks become more organic-rich with 

depth. As stated, this trend was observed at other sites in the Los 

Angeles and Ventura Basins, including the Ventura Avenue Field 

(Ventura Basin) and along the entire southwestern shelf of the 

Los Angeles Basin from the Dominguez to the Huntington 

Beach Fields. Increases in organic richness with depth at these 

sites are assumed to be largely due to decreasing sedimentation 

rates with depth, an increase in the input of marine OM, continuously increasing kerogen hydrogenation, and subordinately to 

increasing percentages of fine-grained rocks (versus sandstones), 

all versus increasing depth.

Minimum and maximum values of the ROCK-EVAL S1

peak normalized to TOC in figure 7 are invariant versus depth 

from 1,325 m to 4,225 m (131°C) where they begin to increase 

slightly. However, this deep increase is not from HC generation 

but is due instead to a shift to hydrogen-rich OM at depth. Note 

that the production index, especially maximal values, largely 

decrease slightly with depth, instead of increasing, as would be 

expected from HC generation. Moreover, neither the low production indices (0.05 to 0.07) nor the low carbon-normalized S1

values of 18 to 37 for the three deepest samples at about 5,000 

m are indicative of HC generation proceeding. However, Tmax

and Ro do increase with depth and the oxygen index decreases 

with depth (fig. 7). These three trends demonstrate that organic 

metamorphism is proceeding in these rocks, even though mainstage-HC generation has not commenced in figure 7 by 154°C. 

However, the Ro data do exhibit scatter, with a poor correlation 

coefficient (r2 = 0.474). Moreover, Ro values of 0.40 to 0.43 percent at 150°C are significantly suppressed, and as with the Ro

data from the Apex-1 rocks, Ro data in Santa Clara Trough rocks 

are not a valid measure of the actual organic-metamorphic ranks 

of the rocks.

The lack of HC generation in the deep, high-temperature 

rocks of the Santa Clara Trough is because all maturity indices, 

including mainstage-HC generation, are suppressed in rocks 

with hydrogen-rich OM, compared to rocks with hydrogen-poor 

(types IV and III) OM throughout the Los Angeles and Ventura 

Basins. For this reason, ROCK-EVAL analyses from rocks of 

these two basins were split into two categories and plotted separately versus burial temperature: (1) rocks with HI values <300, 

and (2) rocks with HI values >300. 

Hydrogen-Poor OM Los Angeles Basin

The TOC values and the ROCK-EVAL S1 and S2 (HI) 

pyrolysis peak values normalized to TOC are plotted in figure 8, 

versus burial temperature in °C, for rocks with hydrogen-poor 

OM (HI values <300), for all wells analyzed in the Los Angeles 

Basin. These values are not plotted versus burial depth because 

of highly variable geothermal gradients throughout the Los 

Angeles Basin. The dots are samples with HI values <200 and 

Figure 6. Plot of porosity in percent and percent residual-oil saturation 

versus depth in meters for a 26.6 m section of conventional core from a 

thick sandstone in the Apex-1 wellbore. Analyses by Core Laboratories.

1,375 millidarcies, and a residual-oil saturation of 41.6 percent 

is unexpected. The table 3 data also clearly demonstrate a 

greater thermal stability of oil than portrayed by current paradigm. The data of figure 6 and table 3 also dictate that the 

decrease of S1 values from 195° to 223°C in figure 1 cannot possibly be from C15+ thermal destruction.

Lastly, as discussed above, the Apex-1 data demonstrate 

that variable depositional conditions, even for this one (hydrogen-poor) OM type, can cause wide variance, versus depth, in 

different petroleum-geochemical parameters, including TOC 

(figs. 1, 2), the ROCK-EVAL S1 peak (fig. 2), the production 

index (fig. 3), and the hydrogen-index (fig. 2). Indeed, the substantial variations of these latter three parameters versus depth, 

variations at times due solely to variable-depositional conditions, 

greatly obfuscate the degree of control that organic metamorphism actually has had on these parameters. This strong overprint by original depositional conditions on organic-geochemical 

characteristics, even within one OM type, was observed in other 

instances in this study. 

Ventura Basin Central Syncline

ROCK-EVAL analyses of the Apex-1 fine-grained rocks 

provided a defined HC generation profile, albeit overlaid by OM 

characteristics inherited from depositional conditions. However, 

such defined generation profiles were not the case at all the sites 
12 Organic Metamorphism in the California Petroleum Basis

Table 3. Core analyses. Permeability (Perm.) in millidarcies (md); porosity (Por.) in percent; residual oil and water (in percent of porosity); and residual oil/water ratios, all for sidewall core of the Apex-1 wellbore. Uncorrected and corrected depth in feet are given, 

as well as corrected depth in meters, for the directionally drilled well.

Corrected

Depth,

Meters

Corrected

Depth,

Feet

Uncorrected

Depth,

Feet

Perm.,

md

Por.

%

__Residual Fluid__

Oil Water

Oil/

Water

5,483.4 17,990.1 18,410 486 19.0 8.9 54.7 0.16

5,492.5 18,020.1 18,440 74 24.5 5.7 66.1 0.09

5,493.7 18,024.0 18,444 103 16.9 0.0 52.7 ---

5,500.1 18,045.1 18,465 392 23.4 5.1 78.6 0.06

5,501.6 18,050.0 18,470 42 24.6 5.3 82.1 0.06

5,522.4 18,118.0 18,538 69 22.9 7.0 55.5 0.13

5,527.2 18,134.0 18,554 19 12.9 26.4 54.3 0.49

5,527.8 18,136.0 18,556 24 15.4 21.4 48.7 0.44

5,530.9 18,146.0 18,566 316 25.0 0.0 83.2 ---

5,535.2 18,160.0 18,580 183 20.6 0.0 54.4 ---

5,536.4 18,163.9 18,584 73 19.7 3.6 66.0 0.05

5,554.0 18,221.8 18,642 936 32.6 40.2 56.3 0.71

5,568.6 18,269.7 18,690 374 21.6 10.2 44.9 0.23

5,588.0 18,333.5 18,754 1,184 15.2 7.9 59.9 0.13

5,588.6 18,335.5 18,756 457 26.7 5.2 67.8 0.07

5,591.4 18,344.5 18,765 241 26.4 17.8 58.7 0.30

5,592.3 18,347.4 18,768 758 32.4 6.8 54.3 0.13

5,606.3 18,393.4 18,814 529 33.9 19.8 69.3 0.29

the squares are samples with HI values of 200 to 300. Apex-1 

data between 80° and 140°C were excluded from figure 8 to 

gauge commencement of intense HC generation versus burial 

temperature for hydrogen-poor OM at sites other than the Apex1. Most of the data above 175°C in figure 8 is from the Apex-1.

The TOC values are invariant versus depth and range from 

0.25 to 4.0 percent (fig. 8). Rocks with TOC values of 0.25 to 

0.50 percent were largely light colored silts. Most rocks with 

TOC values of 0.5 percent or less have HI values of 200 or less. 

Maximum S1 values increase noticeably at 105°C, which is 

taken as commencement of mainstage HC generation (CIHG in 

the S1 plot of fig. 8). Minimum S1 values increase slightly from 

40° to 132°C and increase sharply thereafter. Maximum HI values increase acutely from 39° to about 90°C. Minimum HI values also increase acutely, but from 39° to about 142°C. Both 

these increases in HI values are attributed both to the increasing 

kerogen hydrogenation and to rocks with hydrogen-poor OM in 

the Los Angeles Basin generally becoming more organic-rich, 

both versus increasing depth. The sharp decrease in minimum 

and maximum HI values above 140°C is from intense HC generation at these elevated temperatures.

Oxygen indices decrease versus burial temperature for 

rocks with hydrogen-poor OM in the Los Angeles Basin (fig. 9). 

However, the range of values at low burial temperatures is large 

(about 90). At 125°C, the variation in oxygen indices begins 

decreasing to much more limited ranges above 200°C. ROCKEVAL Tmax increases with burial temperature (fig. 9). However, 

again scatter in the data is significant. Although the range of 

variation in Tmax values decreases with increasing burial temperature, the range remains large at even high burial temperatures 

(about 12°C at 200°C burial temperature). Nonetheless, the 

oxygen index and Tmax trends do demonstrate that organic metamorphism is proceeding in the rocks even at temperatures as 

low as 39° to 90°C. Maximum and minimum values of the production index are invariant from 40°C to 125° and 150°C 

respectively, where sharp increases take place. Thus, by production indices, mainstage HC generation commences in these 

rocks by 125° to 150°C. Vitrinite reflectance increases, with 

significant scatter, from minimum values of 0.3 percent at 40°C 

to 0.62 percent at 190°C, a suppressed Ro value at that temperature. The Ro data beyond 190°C are from the Apex-1.

Regarding the large variance in ROCK-EVAL Tmax values 

discussed directly above, Caroline Isaacs reports (U.S. Geological Survey, written commun., 12/97) that in the Cooperative 

Monterey Organic Geochemistry Study (CMOGS, Isaacs and 

Rullkötter, 1993), a variance in Tmax values of 5°–8°C was 
Results 13

present for the same rock measured by the same laboratory, but 

at different time periods. For all laboratories measuring a given 

rock, the 95 percent confidence limit of Tmax values was about 

20°C (roughly +/-10°C). We have seen the same kind of poor 

ROCK-EVAL reproducibility in our laboratory, regardless of the 

steps taken in calibration, in the samples reported on here, as 

well as in other sample suites, for example Price and others 

(1984). These observations carry two implications we wish to 

comment on: (1) Within this study’s rocks, Tmax distributions 

versus rank may actually be quite tight, if Tmax could be precisely measured. (2) Although ROCK-EVAL is a wonderful 

screening tool, it is a most imprecise instrument, an observation 

which calls into question the precision of ROCK-EVAL-based, 

computer-produced HC generation studies.

By figures 8 and 9, commencement of intense HC generation is vague, occurring at 105°C by the S1 data (fig. 8) and 

somewhere between 125° and 150°C by the production index 

data (fig. 9). However, we believe that the S1 data reflect reality 

regarding commencement of intense HC generation, and that the 

trend in the production index data from 40° to 120°C is an artifact from changing depositional conditions. From 40° to 105°C, 

S1 values are essentially invariant and largely range from 15 to 

30 mg/g, normal to slightly elevated values for immature rocks. 

However, over much of this same burial interval, HI values are 

low, largely 50 to 105 from 40° to 90°C. As a result, production 

indices for most samples between 40° and 105°C are unusually 

high, ranging from 0.025 to 0.150 (dashed lines, production 

index (PI) plot, fig. 9). By contrast, production indices for 

immature but more organic-rich rocks are commonly between 

0.025 and 0.060. Thus, detection of commencement of intense 

HC generation by the production index in figure 9 is masked by 

two OM characteristics derived from deposition: (1) moderately 

high amounts of carbon-normalized HCS deposited in some of 

these rocks, and (2) low HI values in shallow rocks, HI values 

which increase with increase in depth. Thus, the significant 

increase in S1 values at 105°C in figure 8 is taken as commencement of intense HC generation in Los Angeles Basin finegrained rocks.

Hydrogen-Poor OM Ventura Basin

TOC and the ROCK-EVAL S1 and S2 (hydrogen index) 

pyrolysis peaks normalized to TOC are shown in figure 10 for 

rocks with hydrogen-poor OM (HI values <300) from the Ventura Basin, all versus burial temperature in °C. These values, 

Corrected

Depth,

Meters

Corrected

Depth,

Feet

Uncorrected

Depth,

Feet

Perm.,

md

Por.

%

__Residual Fluid__

 Oil Water

Oil/

Water

Table 3.—continued 

5,625.4 18,456.0 18,877 650 31.9 15.1 64.6 0.23

5,643.9 18,516.8 18,938 1,526 22.2 0.0 80.2 ---

5,651.8 18,592.8 18,964 110 16.4 0.0 58.7 ---

5,667.0 18,592.7 19,014 1,432 18.0 25.0 50.0 0.50

5,667.0 18,592.7 19,014 283 27.6 9.4 73.2 0.13

5,758.3 18,892.0 19,315 186 33.1 5.1 65.9 0.08

5,829.4 19,125.4 19,550 218 16.2 0.0 55.6 ---

5,835.5 19,145.2 19,570 151 19.3 7.8 52.9 0.15

5,871.2 19,262.5 19,688 144 24.1 0.0 74.3 ---

5,876.0 19,278.4 19,704 56 23.6 10.6 46.6 0.23

5,877.2 19,282.3 19,708 32 13.5 0.0 61.5 ---

5,880.9 19,294.4 19,720 49 22.8 0.0 50.0 ---

5,890.0 19,324.2 19,750 73 17.6 0.0 52.8 ---

5,890.0 19,324.2 19,750 1,375 31.3 41.6 51.2 0.81

5,891.2 19,328.1 19,754 477 37.5 14.4 48.3 0.30

5,897.3 19,348.0 19,774 46 20.5 2.9 63.4 0.05

5,899.1 19,353.9 19,780 103 23.2 10.8 40.5 0.27

5,901.5 19,362.0 19,788 27 24.7 6.5 66.8 0.10

5,909.0

5,912.9

19,386.4

19,399.4

19,813

19,826

 35 

 212

22.2

22.1

 0.0 51.8

 4.1 22.2 

 ---

0.18
14 Organic Metamorphism in the California Petroleum Basis

again, are not plotted versus depth because of variable geothermal gradients in the Ventura Basin. The dots are samples with 

HI values <200, and the squares are samples with HI values of 

200 to 300. Both minimum and maximum TOC values increase 

gradually with depth. However, maximum TOC values increase 

more strongly over 95° to 105°C. Minimum HI values increase 

slightly from 50° to 130°C and then increase more strongly in 

the deepest samples. Maximum HI values increase dramatically 

from 50° to 105°C. As discussed above, these TOC and HI 

increases versus depth are attributed to organic richness increasing with depth, a characteristic feature throughout the Los Angeles and Ventura Basins. 

Minimum values of the S1 pyrolysis peak decrease slightly 

from 50° to 90°C and then increase slightly, but continuously, 

from 90° to 140°C (fig. 10). Maximum S1 values increase 

strongly from 50° to 120°C, with three outliers. Beyond 120°C, 

S1 values strongly decrease to low values (7–20) at the highest 

burial temperatures. The low S1 values between 130° and 140°C 

preclude the commencement of mainstage-HC generation in 

these rocks. The increase in S1 values between 50° and 120°C 

coincides with the strong increase in maximal HI values over 

much of this burial interval. Both increases are attributed to 

increasing organic richness versus depth.

The low S1 values between 130° and 140°C strongly contrast with much higher S1 values in both the Apex-1 wellbore 

(fig. 2) and in the Los Angeles Basin as a whole (fig. 8) at equivalent burial temperatures. Thus, HC generation appears to be 

suppressed in the Ventura Basin compared to the Los Angeles 

Basin. Phillipi (1965) also concluded that HC generation was 

suppressed in the Ventura Basin compared to Los Angeles Basin 

(see his figures 8a and 8d), due to the lower average geothermal 

gradient in the Ventura Basin (26.6°C/km) compared to Los 

Angeles (39.1°C/km). However, this does not explain why 

rocks at equivalent burial temperatures in the two basins have 

such disparate characteristics regarding commencement of HC 

generation, as demonstrated by Phillipi’s (1965) figures 8a and 

8d and by the data of this study (figs. 2, 8, and 10). Reasons for 

these differences are discussed below. For now, we conclude 

that mainstage HC generation has not begun in the Ventura 

Basin by 140°C (fig. 10). 

With the exception of eight elevated values, oxygen indices 

in Ventura Basin rocks with hydrogen-poor OM tightly decrease 

versus increasing temperature (two solid lines, fig. 11). Even if 

the elevated oxygen indices are incorporated (dashed line, oxygen index (OI) plot, fig. 11), oxygen indices still uniformly 

decrease with temperature. Moreover, seven of the eight samples 

with elevated oxygen indices falling outside of the two solid lines 

have among the lowest TOC values of the figure 10 sample base 

(0.75 to 1.18 percent). These elevated oxygen indices may be 

due to a contribution of CO2 from CaCO3, a contribution which 

becomes evident in lower TOC rocks (Katz, 1983). ROCKEVAL Tmax, and Ro, regularly increase with burial (fig. 11). 

However, as in previous cases, Tmax has an unacceptably large 

range of values (12°C). Although increasing Ro correlates moderately well with increasing burial temperature (r2 = 0.72), Ro

values of 0.42 to 0.44 percent at 130° to 144°C are significantly 

suppressed from values expected for hydrogen-poor OM at these 

burial temperatures. Thus, Ro does not yield a valid measurement of the true rank of these rocks. The oxygen-index, Tmax, and 

Ro data, however, do demonstrate that organic metamorphism is 

ongoing over the entire temperature range of figure 11.

The production index (fig. 11) supports the pervious conclusion that mainstage-HC generation has not begun by 140°C in 

Ventura Basin rocks with hydrogen-poor OM. Excluding four 

points with values above 0.150, production indices broadly 

decrease with temperature, from 40° to 140°C. Three of the 

0

240

20

40

0

200400

600

40

80

1.0

2.0

3.0

DEPTH, IN KILOMETERS

TEMPERATURE, IN °C

4.0

5.0

120

0

0.05

0.1

0.15

410

420

430440

0.3

0.4

0.5

50

60

70

80

90

100

110

120

130

140

150

r2=.474

TOC

S1

mg/g OC HI OI PI Tmax Ro

Figure 7. Plot of TOC (total organic carbon), TOC-normalized values (mg/g OC) of the ROCK-EVAL S1, S2 (HI, hydrogen index) and S3 (OI, oxygen index) 

peaks; the ROCK-EVAL production index (PI) and Tmax; and Ro, all versus burial temperature in °C, and depth in kilometers, for siltstones and shales 

from two deep wellbores from the Ventura Basin central syncline ("Santa Clara Trough"). Trends defined by solid lines are all explained in the text.
Results 15

400

410

420

430

Tmax

40

60

80

TEMPERATURE, IN °C

100

220

200

180

160

140

120

440

450460470

0

0.1

0.2

0.30.40.5

0.2

0.5

1.0

2.0

50

100

150

200

Pl Ro OXYGEN INDEX

CIHG

CIHG 100

120

140

40

60

80

TEMPERATURE, IN °C

0

1.0

2.0

3.0

4.0

0

10

20

30

405060

70

0

50

100

150

200250300

TOC S1 mg/g OC

160

180

200

220

HYDROGEN INDEX

Figure 8. Plot of TOC (total organic carbon), and the TOC-normalized (milligrams per gram, mg/g OC) S1 and S2 (HYDROGEN INDEX), ROCK-EVAL 

peaks, all versus burial temperature in °C for siltstones and shales with HI values <300 from the Los Angeles Basin. Trends defined by solid lines are 

discussed in text. Circles represent samples with HI values below 200, squares are samples with HI values of 200 to 300. "CIHG" in the carbon-normalized S1 plot is commencement of intense HC generation by that measurement.

Figure 9. Plot of ROCK-EVAL Tmax and production indices (PI), Ro, and the TOC-normalized value of the ROCK-EVAL S3 peak (OXYGEN INDEX), all 

versus burial temperature in °C for siltstones and shales with HI values <300 from the Los Angeles Basin. Trends defined by solid lines are discussed 

in text as are dashed lines in the PI (production index) plot. Dots are samples with HI values below 200, squares are samples with HI values of 200 to 

300. The vertical arrow and "CIHG" in the PI plot is the range of the commencement of intense HC generation by that measurement.
16 Organic Metamorphism in the California Petroleum Basis TEMPERATURE, IN °C 90

0

1.0

2.0

3.0

0

10

203050

150

250

TOC

50

60

70

80

100

110

120

130

140

150

S1mg/g OC HYDROGEN INDEX 

four points with elevated production indices had very low HI 

values (75–125), and the fourth point at 120°C was a 175 HI 

rock with the S1 maximum in figure 10. Thus the four high production indices of figure 11 are attributed as analytical artifacts, 

with no relation to HC generation. Moreover, production indices 

of 0.05 to 0.08 between 130° and 140°C are not indicative of, 

and argue against, the commencement or occurrence of mainstage-HC generation.

Southern San Joaquin Valley

TOC contents and TOC-normalized ROCK-EVAL S1 and 

S2 (hydrogen index) peaks are shown in figure 12 for shale cores 

from six wells in the Paloma field, Southern San Joaquin Valley 

Basin (Maricopa-Tejon subbasin of Beyer and Bartow, 1987). 

The data from 1,232.6 m (59°C) to 3,666.0 m (132°C) are from 

three wells laterally within 1,980 m (6,500 ft) of one another. 

The data from 3,666.0 m (132°C) to 4,347.8 m (153°C) are 

from three more widely separated wells and the data from 

4,347.8 m (153°C) to 6,498.8 m (218°C) are from the 6,547.4 m 

(21,482 ft) deep Ohio Oil KCL “A” 72–4 wellbore. Total 

organic carbon values fluctuate widely with depth (fig. 12). 

From 1,232.6 to 1,398.3 m, TOC increases dramatically (0.36 to 

2.55 percent) followed by a decrease to low values (0.06 to 0.36 

percent) from 1,511.7 to 2,389.2 m. From 2,759.7 to 4,062.5 m, 

TOC again dramatically increases, to a maximal value of 5.01 

percent. Both minimal and maximal TOC values follow these 

trends, trends we attribute to variable depositional conditions at 

this site. From 4,062.4 to 6.493.8 m (all samples from the 72–4 

wellbore), maximal TOC values decrease as portrayed by the 

solid curve; however, the seven samples defining this curve are 

only a small part of the sample population. Most samples from 

4,062.4 to 6.493.8 m, actually range from 0.6 to 2.10 percent 

TOC and are largely invariant versus depth.

Minimal and maximal values of the TOC-normalized 

ROCK-EVAL S1 peak increase gradually (with several outliers) 

to 170° and 140°C respectively (fig. 12). The gradual increase 

in maximal S1 values from 60° to 140°C is ascribed principally 

to low-level HC generation. The significant increase in maximal 

S1 values above 140°C is due to mainstage-HC generation. 

Between 180° and 217°C, S1 values range from 15 to 214 mg/g. 

This very large range of S1 values is attributed to: (1) more than 

a five-fold difference in original HI values (45 to 240 between 

155° to 182°C, HI plot, fig. 12) with the originally higher HI 

rocks yielding significantly higher S1 values; and (2) variable 

losses of generated HCS to expulsion and to the drilling mud. 

These last two parameters also account for the significant 

decline in maximal and minimal S1 values from 203° to 218°C. 

The four elevated S1 values between 128° and 141°C are attributed to higher than normal amounts of bitumen incorporated into 

the rocks at deposition, and not to HC generation.

The HI fluctuates widely from 1,232.6 m to 4,413.6 m (fig. 

12), which is attributed to highly variable depositional conditions. From 1,232.6 to 2,130.8 m, with one exception (caused by 

that sample’s low TOC value, 0.09 percent), HI values strongly 

decrease from 341 to 20, and then increase from 2,130.8 to 

3.973.5 m, with reversals, to a maximal value of 590. From 

Figure 10. Plot of TOC (total organic carbon), and the TOC-normalized (milligrams per gram, mg/g OC) S1 and S2 (HYDROGEN INDEX), ROCK-EVAL peaks versus burial temperature in °C for siltstones and shales with HI values <300 

from the Ventura Basin. Trends defined by solid lines are discussed in the text. Dots are samples with HI values 

below 200, squares are samples with HI values of 200 to 300.
Results 17

TEMPERATURE, IN °C

80

50

60

70

415

425435

50

150

250

0.05

Tmax Ol 

0.100.15

0.3

0.4

0.5

90

100

110

120

130

140

150

r2=

0.72

Pl Ro 

3,973.5 to 4,376.7 m, HI values again decrease to a minimum of 

82. From 4,376.7 m to 5,181.8 m, HI values are invariant with 

depth and range widely from 43 to 211 (fig. 12, dashed lines). 

Beyond 5,181.8 m, both maximal and minimal HI values 

decrease with depth, as portrayed by the solid lines. Moreover, 

the range of HI values significantly decreases over this depth. 

Both these features are due to mainstage-HC generation taking 

place, documented by the increase in both S1 values (fig. 12) and 

production indices (fig. 13) over this interval.

As is evident from the HI values of figure 12, the rocks at 

Paloma deeper than 4,376.7 m have a mixture of types III and 

IV OM. Considering their respective HI values, rocks shallower 

than 4,376.7 m were deposited under variable conditions. LowTOC, low-HI rocks were deposited from 1,511.7 to 2,389.2 m. 

In contrast to the rocks between 1,511.7 to 2,389.2 m with S2

values of 50–520 ppm (by rock weight, not shown), Paloma 

rocks between 3,190.5 to 4,182.0 m are a more impressive 

source section, with elevated TOC and HI values and S2 values 

of 4,380 to 27,380 ppm (not shown). The top of the upper to 

middle Miocene Monterey Formation in the KCL “A” 72–4 

wellbore was at 2,654.4 m, where both TOC and HI values demonstrate dramatic increases (fig. 12). The TOC and HI values 

maximize in figure 12 over 3,804.6 to 4,182.0 m, which is the 

lower-middle portion of the upper Miocene Antelope member of 

the Monterey Formation. However, from 4,238.0 to 5,596.8 m 

(the base of the Monterey Formation) HI values are modest (primarily 45 to 165, with some values as high as 240, fig. 12). 

Thus, a large part of the Monterey Formation at Paloma appears 

to be not especially organic rich.

One might attribute the strong decrease in HI values from 

3,973.5 to 4,376.7 m to intense HC generation in the type II OM, 

rather than to a shift in depositional facies. However, this is not 

the case. Over this depth interval, neither S1 values (fig. 12) nor 

production indices (fig. 13) proportionately increase, given that 

this proposed decrease in the HI values (values of 300 to 600 

decreasing to 50 to 150) is from HC generation. Moreover, optical analyses of isolated kerogens between 4,376.7 to 6,493.8 m 

showed them to be composed of the same kerogen type, with 

either no, or only small, amounts of algal kerogen.

ROCK-EVAL oxygen indices at Paloma strongly decreased 

from 1,232.6 to about 3,900 m (fig. 13). Elevated oxygen indices (174 to 633) of three shallow samples are attributed to low 

(0.06 to 0.23) TOC values and thermal decrepitation of calcite in 

the samples (Katz, 1983). Samples from 1,852.8 to 2,389.2 m in 

figure 13 also have low TOC values (0.19 to 0.36 percent). 

However, these low TOC values do not appear to significantly 

affect the elevated (87 to 136) oxygen indices of these samples. 

From 3,916.5 to 5,263.8 m, maximum and minimum oxygen 

indices are invariant. The Paloma field was the only area of this 

study where, unaccountably, oxygen indices did not uniformly 

decrease with depth over the entire sample interval. From 

5,263.8 m to well bottom, oxygen indices generally decrease 

Figure 11. Plot of ROCK-EVAL Tmax, the TOC normalized (milligrams per gram) S3 peak (OI-oxygen index), and production indices (PI); and Ro, versus 

burial temperature in °C for siltstones and shales from the Ventura Basin with HI values <300. Trends defined by solid lines, and the dashed line in 

the OI plot, are discussed in text. Dots are samples with HI values below 200, squares are samples with HI values of 200 to 300.
18 Organic Metamorphism in the California Petroleum Basis

0

0.5

1.0

1.5

2.02.5

3.0

3.5

4.0

0

20

TOC

TEMPERATURE, IN °C

DEPTH, IN KILOMETERS

50

70

90

110

130

150

170

406080

100

120

140

0

60

120

180

240

300

360

420

480

1.0

2.0

3.0

4.0

590

214

4.37

4.64

5.01

179

5.0

6.0

CIHG

190

210

S1 mg/g OC HYDROGEN INDEX

with depth, with two exceptions at about 5,300 m and less so at 

6,070 m, where maximum oxygen indices unaccountably 

increase. 

Maximum and minimum production indices (fig. 13) 

increase slightly from 1,232.6 m to about 3,750 m, where they 

dramatically increase from the commencement of intense HC

generation. After maximizing at 205°C, production indices 

decrease from 205° to 217°C due to significant losses of generated HCS from either expulsion or loss to the drilling mud, corresponding to similar decreases in the S1 peak (fig. 12). Samples 

with high production indices outside the defined trend from 85°

to 90°C are due to abnormally low HI values over this interval 

(fig. 12). Samples between 127° to 133°C with high production 

indices outside the defined trend (fig. 13) are from high values of 

the S1 peak, in turn attributed to high amounts of HCS incorporated into the rocks at deposition. 

ROCK-EVAL Tmax values are moderate (427° to 435°C) to 

highly elevated (455° to 563°C) from 1,232.6 to 2,265.5 m (fig. 

13), not reflecting the true immature rank of these samples. Of 

the seven shallow samples with highly elevated Tmax values, six 

are due to the low TOC and/or low HI values of the samples. 

From 2,760 to 3,700 m, Tmax values are both reasonable for the 

burial temperatures and regularly increase with burial temperature. From 3,704.4 m, maximal and minimal Tmax values are 

invariant to 4,740.3 m and to 5,078.9 m respectively, mirroring 

similar behavior of the oxygen indices over this interval (fig. 

13). With further increase in depth, Tmax values uniformly 

increase (fig. 13). However, beyond 5,078.9 m, the range of Tmax

is unacceptably large. For example, from 5,181.8 to 5,437.3 m 

(stippled band), Tmax varies from 428° to 455°C, or from pre-HC 

generation to post mature. Even discounting extreme low and 

high values, the range of Tmax in these deeper samples is 18°C 

for the same OM type throughout this section, certainly an unacceptable level of precision for any analytical tool.

The Paloma Ro data from this study (crosses, solid line, 

fig. 13) agree moderately well with previously published data 

(dots, dashed line, Hood and Castaño, 1974). In this study, five 

samples shallower than 2,760 m macerated for kerogen did not 

yield sufficient vitrinite for analysis. Moreover, the recovered 

vitrinite from two samples shallower than 3,100 m, had unrealistically high Ro values for their rank. Thus only samples 

deeper than 3,230 m yielded realistic Ro values. Both data sets 

have good correlation coefficients (r2 = 0.930 this study; r2 = 

0.957, Hood and Castaño, 1974). However, both data sets yield 

unrealistically low Ro values at surface intercept (22°C): 0.135 

percent for this study, 0.078 percent for Hood and Castaño 

(1974), versus expected values of 0.25–0.29 percent. Assuming intense HC generation to commence at Paloma at 150°C 

from the production index plot (fig. 13), yields the expected Ro

value of 0.60 percent.

The Paloma samples provide a well-defined HC-generation profile for hydrogen-poor OM, commencing at 150°C and 

largely complete by 220°C. However, like the Apex-1 wellbore 

where a thick sample sequence was also available, organic 

metamorphic trends at Paloma are strongly overlaid by OM 

characteristics inherited from deposition. In this case, a stratigraphic section with hydrogen-rich OM lies between 3,508.7 m 

(127.7°C) and 4,238.0 m (148.0°C), the burial temperature 

range where we might expect commencement of mainstage generation in the type III/IV OM. Thus it could be suggested that 

mainstage generation in hydrogen-poor OM would be observed 

below 150°C at Paloma, if this OM type were present in the section. Two points argue against this hypothesis. First, in this section of rocks with hydrogen-rich OM, a sample is present at 

4,075.6 m (144.8°C) with type III OM (HI = 238, fig. 12). This 

sample has a TOC-normalized S1 value of 21.7 and a production index of 0.084, values not characteristic of intense HC generation. Thus, even the type III OM at these burial temperatures 

has not commenced intense HC generation. Second, beyond 

150°C, S1 values (fig. 12) and production indices (fig. 13) dramatically increase. However, the values of these two indices 

just beyond 150°C are not that much greater than the background values of type III/IV OM at temperatures as low as 

80°C. Both considerations dictate that HC generation could not 

Figure 12. Plot of TOC (total organic carbon), and the TOC-normalized (milligrams per gram, mg/g OC) S1 and S2 (HYDROGEN INDEX), ROCK-EVAL 

peaks versus burial temperature in °C and depth in kilometers for siltstones and shales from six wellbores in the Paloma field, Southern San Joaquin 

Valley Basin. Trends defined by solid and dashed lines are discussed in text. Numbers to the right of each plot are values for the respective samples 

which fell off the scale. "CIHG" in the carbon-normalized S1 plot is the commencement of intense HC generation by that measurement.
Results 19

TEMPERATURE, IN °C

90

110

70

174

0

20

40

6080

100120140

OXYGEN INDEX

160

410

420

430

440450

460

0

0.10.2

0.30.40.5

0.60.70.80.30.50.7

1.0

1.5

1.0

2.0

3.0

4.0

5.0

6.0

130

150

170

190

210

633

500

563

510

506

467

207

DEPTH, IN KILOMETERS

Tmax PRODUCTION INDEX Ro

r2=0.930

r2=0.957

CASTANO

USGS

CIHG

50

have started significantly before 150°C for the type III/IV (or II) 

OM at Paloma.

Hydrogen-Rich OM Los Angeles-Ventura Basins

The TOC contents and S1, S2 (HI, hydrogen index), and S3

(OI, oxygen index) TOC-normalized ROCK-EVAL peaks for all 

samples with hydrogen-rich OM (HI>300) for the Los Angeles 

and Ventura Basins are shown in figure 14. Again, the data are 

plotted versus burial temperature, and not burial depth, because 

of variable geothermal gradients in these basins. The data are 

subdivided into two populations in figure 14 (and 15): samples 

with HI values of 300 to 400 (dots) and samples with HI>400 

(squares). Total organic carbon values are invariant with depth 

and range from 1 to 7 percent. Minimum carbon-normalized S1

values increase slightly from 60° to 200°C (4 mg/g to 8 mg/g), 

whereas maximal S1 values increase significantly from 60° to a 

maximum of 60 to 70 at about 115°C. However, beyond 115°C, 

maximal S1 peak values strongly decrease to values of 20 to 30 

mg/g around 200°C. This deeper decrease is unexpected. 

These, and other data we discuss, suggest that mainstage-HC 

generation is not occurring in these rocks with hydrogen-rich 

OM at temperatures of 150° to 200°C. Thus, the general 

increase in S1 values from 60° to 115°C cannot be from intense 

mainstage HC generation. Instead we attribute this shallow 

increase in S1 values either to low-level HC generation occurring 

over 60° to 115°C and/or possibly to changing depositional conditions (increasing organic richness) in the rocks with depth.

As discussed in Price (1999), strong qualitative changes 

occur in the bitumen and saturated HCS from rocks with hydrogen-rich OM over burial temperatures of 50° to 120° C, demonstrating that a low-level HC generation is occurring in these 

rocks. However, this low-level HC generation does not usually 

result in significant quantitative changes, such as the increase in 

S1 peak values from 60° to 115°C in figure 14. Note that maximal HI values, like the S1 peak, strongly increase from 60° to 

115°C (fig. 14). This is another example of the general increase 

in organic richness of rocks with depth in the Los Angeles and 

Ventura Basins and would suggest that the increase in S1 values 

(fig. 14) is due more to increasing organic richness with depth, 

than to low-level HC generation.

Minimum oxygen indices moderately decrease from 60°C 

to about 140°C (fig. 14), whereas maximal oxygen indices are 

invariant over this interval. From 140° to 200°C, maximal oxygen indices significantly decrease. This general decrease suggests that organic metamorphism is occurring in these rocks over 

these burial temperatures of 50° to 200°C, even though intense 

HC generation has not yet commenced. This conclusion is supported by the fact that Tmax increases with depth over the entire 

sample set (fig. 15). However, as in previous sample sets, the 

range of Tmax values (about 18°C at any given burial temperature) is unacceptably large. Minimum production indices (fig. 

15) are invariant over the entire sample set. Production indices, 

in general, are invariant to 150°C; however, maximum production indices increase slightly with depth to about 160°C and then 

strongly decrease with depth to 200°C. The low production 

indices (0.01 to 0.07) from 160° to 200°C are further evidence, 

along with the low S1 values and high HI values in this interval, 

that significant HC generation is not occurring in these deeper 

rocks.

Vitrinite reflectance values of this sample set are invariant 

with depth, ranging from 0.20 to 0.48 percent and are highly 

suppressed (Price and Barker, 1985) at high burial temperatures. 

Moreover, at any given burial temperature in figure 15, samples 

with higher HI values (the squares) usually have lower Ro values 

than do the lower HI samples (the dots).

Figure 13. Plot of Ro and ROCK-EVAL oxygen-indices, Tmax, and production indices, versus burial temperature in °C and depth in kilometers for siltstones and shales from six wellbores in the Paloma field, Southern San Joaquin Valley Basin. Trends defined by solid lines are discussed in the text, 

as is the stippled band in the Tmax plot. Numbers at the top-right of the Tmax and oxygen indices plots are values for the respective samples which are 

off the scale. Crosses and solid line in the Ro plot are U.S. Geological Survey data, dots and dashed line are Ro data from Hood and Castaño (1974). 

Correlation coefficients versus depth (r2

) are shown for both data sets. "CIHG" in the production index plot is the commencement of intense HC generation by that plot.
20 Organic Metamorphism in the California Petroleum Basis TEMPERATURE, IN °C

90

110

130 834

150

170

190

50

70

0

1

23

4

TOC

5

6

7

0

10

20

30

405060

300

400500

600

700

0

25

50

75

100

125

S1 mg/g OC HI OI

ROCK-EVAL and Ro data for rocks from the Wilmington 

field, Los Angeles Basin are given in figure 16. Shales with high 

S1 values (70 to 184 mg/g OC) over 620 to 1,250 m are delineated by crosses. Solvent extraction demonstrates that at least 

some of these samples are stained by migrated oil from reservoirs interbedded with the shales, probably via cap-rock leakage. 

Although this topic is discussed in Price (1999), we note that the 

intense faulting and fracturing in the Wilmington anticline 

(Mayuga, 1970) would greatly facilitate such cap-rock leakage. 

Thus, vertical migration of reservoired oil is probably the principal cause of the shallow elevated S1 values and production indices shown by crosses in figure 16. However, elevated amounts 

of HCS in these immature shales, a moderately common occurrence in the shales of California basins, could also be contributing to these values. In any case, analyses in Price (1999) 

demonstrate that the high concentrations of these shallow HCS 

are not from HC generation.

The TOC values in figure 16 are depth invariant. The S1

values for unstained samples (the dots) increase slightly from 

600 to 1,800 m, an increase is attributed to an increase in organic 

richness over this depth interval, from original depositional conditions. This increase in organic richness is also reflected by an 

increase in HI values over part of this interval. Low-level HC 

generation over this interval may also be partially contributing to 

these increasing S1 values. A persistent decrease in S1 values 

from 60 to 25 over 1,800 to 3,150 m is unexpected and demonstrates that intense HC generation is not responsible for the shallower increase in S1 values. This conclusion is supported by the 

slight, but persistent, decrease in production indices in the 

unstained samples over the entire burial interval. Moreover, 

from 150° to 198°C, production indices are low (0.013 to 0.081) 

as are S1 values (7–33). The HI values are high (400–700) and 

are depth invariant. These data, from a single depositional site 

in the Los Angeles Basin, demonstrate that HC generation has 

not commenced in these rocks with hydrogen-rich OM at temperatures as high as 198°C.

ROCK-EVAL Tmax increases, with scatter, from 400° to 

410°C at 620 m to 430° to 440°C at depth, demonstrating that 

some form of organic metamorphism is occurring in these samples with increasing burial temperature. This metamorphism is 

also reflected by the significant decrease in oxygen indices, 

from 80 to 100 at 620 m to about 30 by 1,680 m. Vitrinite 

reflectance data at Wilmington exhibit scatter and are highly 

suppressed at moderate to elevated burial temperatures. In fact, 

Ro values of 0.21 to 0.29 percent from 181° to 198°C demonstrate that Ro suppression in some hydrogen-rich OM can be 

quite severe. Moreover, as discussed above, the Ro data of figure 16 result from readings by numerous service companies, 

and investigators both within and outside of the U.S. Geological Survey. 

The data of figures 14 to 16, and other analyses in Price 

(1999), demonstrate that not only Ro, but almost all aspects of 

organic metamorphism, including mainstage HC generation, are 

suppressed in some hydrogen-rich OM, even at highly elevated 

burial temperatures. We believe that such wholesale suppression of intense HC generation is not uncommon, which has significant ramifications for petroleum-geochemical exploration 

models.

Figure 14. Plot of TOC (total organic carbon), and the TOC-normalized (milligrams per gram, mg/g OC) S1, S2 (HI-hydrogen index), and S3 (OI-oxygen 

index) ROCK-EVAL peaks versus burial temperature in °C for siltstones and shales with hydrogen indices of 300 and greater from the Los Angeles 

and Ventura Basins. Trends defined by solid lines are discussed in text. Dots are samples with HI values of 300 to 400, squares are samples with HI 

values of 400 or above.
Results 21

400410

430

Tmax

TEMPERATURE, IN °C 110

130

420

440

0

0.05

0.10

0.150.200.30

0.40

0.50

Pl Ro

50

70

90

150

170

190

Northern San Joaquin-Southern Sacramento Valley

Basins

Figure 17 presents Ro and ROCK-EVAL data for shale core 

from the Standard Oil of California 67–17E wellbore, one of the 

deeper wells in the Jacalitos oil field, Northern San Joaquin Valley Basin. These data help illustrate that as one proceeds north 

from the Southern San Joaquin Valley to the Sacramento Valley 

Basin, organic richness in the fine-grained rocks continuously 

decreases in any given stratigraphic interval. Rocks between 

914 m to 1,200 m in figure 17 belong to the middle and lower 

Miocene Temblor Formation and have low to moderate TOC values (0.33 to 1.00 percent) and, excepting one sample, low to 

moderate HI values (78 to 230). The one exception (HI = 433) 

also has a high TOC-normalized S1 peak (65 mg/g) and a low 

TOC value (0.40 percent). The high HI and S1 peak values are 

most likely analytical artifacts from the low TOC value. The 

equivalent to the Temblor Formation in the Paloma 72–4 well 

(fig. 12) was the middle and lower Miocene Carneros Formation 

(5,621.5 m to 5,835.1 m). In the Paloma well, TOC values 

ranged from 0.9 to 2.7 percent, and probably were higher before 

mainstage HC generation, contrasting with the lower TOC values (0.33–1.00 percent) of the Temblor Formation in the Jacalitos well. Three samples between 1,200 to 1,400 m in figure 17 

have elevated TOC values (2.35 to 5.27 percent) and moderate 

HI values (260 to 304). These are reasonable organic richnesses. 

However, these three rocks are from the Oligocene to upper 

Eocene Kreyenhagen Formation which is much more organicrich further south in the Great Valley (Milam, 1985). 

Beyer and Bartow (1987) noted that the thick section of 

marine Neogene rocks in the Southern San Joaquin Valley contrasts with a thin entirely nonmarine section of equivalent age 

rocks in the north of the basin. This difference in depositional 

conditions explains the decrease in organic richness in the Neogene rocks northward. Eocene and Upper Cretaceous rocks are 

deeper than about 1,400 m in figure 17, and TOC and HI valFigure 15. Plot of Ro and ROCK-EVAL Tmax and production index (PI) values, versus burial temperature in °C for siltstones and shales with hydrogenindices of 300 and greater from the Los Angeles and Ventura Basins. Trends defined by solid lines are discussed in text. Dots are samples with HI 

values of 300 to 400, squares are samples with HI values of 400 or above.
22 Organic Metamorphism in the California Petroleum Basis

0480

50

100

0

400

800

TOC S1 mg/g OC

DEPTH, IN KILOMETERS

1.0

60

75

184

1.5

2.0

2.5

3.0

0

0.10.20.3

400

420

440

0

20

40

60

80

100

0.150.200.300.40

Hl Pl Tmax Ol Ro

100

125

150

175

200

TEMPERATURE, IN °C

ues both decrease (compared to values from the shallower 

Kreyenhagen Formation rocks) ranging from 0.56 to 2.00 percent and 72.2 to 118, respectively. 

Production index (PI), Tmax, oxygen indices (OI), and Ro

values are scattered and (or) exhibit unexpected trends in figure 

17. Oxygen indices are probably heavily influenced by changes 

in organic facies with depth. Also, the reduced TOC values of 

some samples, may allow a significant contribution from 

decomposition of CaCO3 to the S3 peak (Katz, 1983). Trends in 

the oxygen indices are not likely due to organic metamorphism. 

The significant scatter in production indices largely occurs in the 

low to moderate TOC and low HI rocks which can have elevated 

production indices for immature ranks because of their low HI 

values. The shallow trend of Tmax values decreasing with depth 

correlates with, and is due to, HI values increasing with depth 

over the same interval. We have found that often, immature 

rocks with low HI values have higher Tmax values than rocks with 

higher HI values at equivalent ranks. The Ro data are scattered, 

and the solid line is our interpretation of the most probable correct trend. The Tmax and Ro data suggest that maturities are too 

low in the deepest rocks of figure 17 for significant levels of HC 

generation to have occurred.

ROCK-EVAL and Ro data are presented in figure 18 for 

shale core from the Amerada Hess FDL-1 well, Tracy Gas Field, 

southernmost Sacramento Valley Basin. This well, drilled in 

1935, has historical significance, for it is the first commercial gas 

well of the Sacramento Valley Basin and opened that gas province. Beyer (1988b) provides a through overview of the Sacramento Valley Basin as well as references on previous work in the 

basin. Samples from 350 to 1,000 m (fig. 18) are from postEocene undifferentiated nonmarine strata and have very low 

TOC values (0.08 to 0.20 percent). The moderate S1 peak and 

high HI values of these rocks are analytical artifacts from the 

low TOC values. Upper Cretaceous rocks deeper than 1,000 m 

have much higher TOC values compared to the shallower rocks. 

The S1 values of Upper Cretaceous rocks are very low (around 6 

mg/g) at 1,000 m and decrease slightly with depth to even lower 

values of 2 to 3 mg/g at 3,000 m. This mirrors similar decreases 

in both TOC (values of 2 to 3 percent at 1,100 m, to 1.2 to 1.5 

percent by 3,000 m) and HI values (about 100 at 1,000 m, to 40 

to 50 at depth). Although TOC values of these Upper Cretaceous 

rocks are moderate to elevated, HI values are very low (41 to 

111, type IV OM), except for one sample at 217. Moreover, S1

peak values are low (1.6 to 7.8 mg/g, except for one sample at 

13.1 mg/g). These are very organic-poor rocks with essentially 

“dead” carbon. 

In the shallower rocks from the FDL-1 well, oxygen indices 

are very high (316 to 700, except for one sample at 160, fig. 18) 

and are due to the very low TOC values of these rocks. Oxygen 

indices regularly decrease in the Upper Cretaceous rocks from 

values around 100 to 125 at 1,000 m to around 60 at well bottom. Vitrinite reflectance increases regularly with, and correlates moderately well (r2 = 0.80) to, depth. These decreasing 

oxygen indices and increasing Ro values both demonstrate that 

organic metamorphism is proceeding with burial. Production 

indices are low (primarily between 0.025 to 0.050) and are 

largely invariant versus depth, as are Tmax values. The FDL-1 

Tmax and Ro data, like the Jacalitos Tmax and Ro data, demonstrate 

inadequate maturities for significant levels of HC generation in 

the deepest samples.

Figure 16. Plot of TOC (total organic carbon); the TOC-normalized (milligrams per gram, mg/g OC) S1, S2 (HI-hydrogen index), S3 (OI-oxygen index) 

ROCK-EVAL peaks; the ROCK-EVAL production index (PI) and Tmax; and Ro , all versus burial temperature in °C and depth in kilometers for shales from 

various wells in the Wilmington field, Los Angeles Basin. Trends defined by solid lines are discussed in text. Crosses are immature (pre-HC generation) samples with large amounts of indigenous or stained HCS. The sample with 184 to the far right of the S1 plot is the S1 value of that sample, 

which fell off scale of the S1 plot.
Results 23

DEPTH, IN KILOMETERS

0.5

270 513

611

30

40

1.0

1.5

2.0

2.5

3.0

0

2

4

0

20

40

TOC S1 mg/g OC Hl Ol Pl Tmax Ro

0

100

200

50

100150

.025.075.125.175

410420

430

440

0.30

0.40

0.50

TEMPERATURE, IN °C

50

60

70

80

90

100

680

565

300

542

530

350

575

371

700

330

316

363

398

366

r2=.80

DEPTH, IN KILOMETERS

0.9

1.0

1.1

433

1.2

1.3 5.27

1.4

1.5

1.6

1.7

0

10

2030

4050

6050

100

150

200

250300

50

100150

200

.025.050.075.100

.125

.150

400

410420

430440

TOC S1 mg/g OC

0

1 

2 3 

0.30.4

0.50.6

55

60

65

70

75

80

Hl Ol Pl Tmax Ro

TEMPERATURE, IN °C

Figure 17. Plot of TOC (total organic carbon); the TOC-normalized (milligrams per gram, mg/g OC) S1, S2 (HI, hydrogen index), and S3 (OI-oxygen index), 

ROCK-EVAL peaks; the ROCK-EVAL production index (PI) and Tmax; and Ro versus burial temperature in °C and depth in kilometers for shales from the 

Standard of California 67-17E wellbore, Jacalitos field, Central San Joaquin Valley Basin. Solid lines defining trends are explained in text. Numbers 

in the TOC (5.27), and HI (433) plots are values for samples which fell offscale. Stratigraphy, from California Oil and Gas Fields (1973), is as follows: 

bottom of the lower and middle Miocene Temblor Formation and top of the middle and upper Eocene and lower Oligiocene Kreyenhagen Formation 

about 1,204 m; top of the lower to middle Eocene Avenal Formation, about 1,432 m; top of the Upper Cretaceous Moreno Formation, about 1,600 m.

Figure 18. Plot of TOC (total organic carbon); the TOC-normalized (milligrams per gram, mg/g OC) S1, S2 (HI, hydrogen index), and S3 (OI, oxygen 

index) ROCK-EVAL peaks; the ROCK-EVAL production index (PI) and Tmax; and Ro versus burial temperature in °C and depth in kilometers for shales 

from the Amerada Hess FDL-1 wellbore, Tracy gas field, Sacramento Valley Basin. Solid lines defining trends are discussed in text. Any numbers 

on the right-hand side of the above plots are values which fell off the scale for the respective sample. Stratigraphy, from California Division of Oil 

and Gas Fields (1973), is as follows: bottom of the post-Eocene undifferentiated nonmarine strata and top of the Upper Cretaceous Panoche Formation, about 1,036 m.
24 Organic Metamorphism in the California Petroleum Basis

Beyer (1988b, p. 30) had noted that the Mesozoic marine 

rocks of the Sacramento Valley Basin ".....generally contain 

type III kerogen and less than 1 percent total organic carbon...". 

However, previously published petroleum-geochemical analyses of Sacramento Basin rocks are limited to Jenden and Kaplan 

(1989), who provided TOC analyses from rocks of five wells 

along a north-south strike of the length of the Sacramento Valley Basin (as deep as about 3,350 m (11,000 ft)). Their Mesozoic rock samples had TOC values primarily around 1 percent 

with some values as high as 2 percent. Tertiary rocks generally 

had values substantially below 1 percent, although, several had 

values between 1 and 2 percent. Jenden and Kaplan (1989) presented microscopic kerogen analyses which demonstrated a 

dominance of higher plant (woody) OM and intertinite, and an 

absence of algal OM in these rocks. Their kerogen analyses 

thus agree with the low HI values of these age rocks in figure 

18. The significant decreases in rock organic richness, at least 

of rocks penetrated by the drill, northward in the Great Valley of 

California, is reflected by the transition from a rich oil province 

in the southern San Joaquin Valley Basin to a mediocre (regarding discovered reserves) dry-gas province in the Sacramento 

Valley Basin.

Discussion

HC Generation Potential from the ROCK-EVAL Hydrogen 

Index

Baskin (1993, 1997, in press) noted that previous studies 

(Katz and Elrod, 1983; Grabowski, 1984; Crossey and others, 

1986; Katz and Pheifer, 1986; Burwood and others, 1988; Scott 

and Hussain, 1988) have demonstrated that ROCK-EVAL HI 

values often do not correlate well with kerogen elemental hydrogen to carbon (H/C) ratios. On the other hand, ROCK-EVAL HI 

values have been classically portrayed as correlating very well 

with kerogen H/C ratios (Tissot and Welte, 1984, their figure 

V.1.11; p. 511). The reasons for this lack of correlation between 

H/C and HI have been discussed by various investigators 

(including Horsfield and Douglas, 1980; Peters, 1986; and 

Larter, 1988), and largely have been attributed to erroneous 

ROCK-EVAL analyses. However, the details of these discussions will not be reviewed here. 

This lack of correlation appears to be pronounced in suites 

of extremely immature source rocks from both the Santa Maria 

and Ventura Basins (Baskin 1993; in press) and also from the 

offshore California Borderland basins (Katz and Elrod, 1983). 

Figure 19 from Katz and Royle (1993) demonstrates this point. 

Kerogen elemental H/C ratios are invariant versus a large range 

of ROCK-EVAL HI values (100 to 750). Moreover, Baskin (in 

press) presents similar plots (figs. 3, 7, 8, and 9; also see fig. 3 in 

Baskin, 1997). It could thus be argued that the data and conclusions of this paper are invalid, because, on the basis of figure 19, 

the ROCK-EVAL HI would appear to be an inappropriate measurement of HC generation potential, at least for California 

Miocene source rocks.

In this light, a random suite of samples from this study was 

assembled and subjected to elemental kerogen and ROCKEVAL analyses. Three criteria had to be met in this sample 

suite. First, the samples had to be immature, not having entered 

the HC-generation stage. The lack of HC generation was principally determined by either low production indices (generally 

0.07 or less) or by low values of the carbon-normalized S1 peak 

(20–50). However, some samples either had very low starting 

HI values (and hence higher production indices) or had abnormally high concentrations of indigenous (pre HC-generation) 

bitumen. In fact, some samples were purposefully chosen for 

this latter characteristic. In such cases, the rank of these samples 

was set by numerous other samples, at nearly the same depth as 

the sample in question, from the same wellbore. The second criterion was that as wide a range of HI values as possible be chosen for this sample suite (based on previously existing ROCKEVAL analyses). The third criterion was that the isolated kerogen had to be available from kerogen macerations performed 

from 1974 to 1977 for the Bostick and others (1978) study. Thus 

the samples chosen for ROCK-EVAL versus H/C analysis in this 

study were truly random, determined principally by kerogen 

macerations carried out more than 20 years ago. Note that some 

of the samples with high HI values (≥400) could be deeply buried and at burial temperatures substantially above 100°C (table 

4).

Once the sample suite was selected, newer, replicate, 

ROCK-EVAL analyses were performed on the rocks to optimize 

analytical reproducibility. This was because the older ROCKEVAL analyses had been performed over a 16 year time period. 

The elemental kerogen analyses were carried out concurrently 

with the newer ROCK-EVAL analyses. ROCK-EVAL HYDROGEN INDEX

600

500

400

800

0.6

ELEMENTAL H/C

700

300

200

100

0

0.8

1.0

1.2

1.4

1.61.8

B. KATZ SUITE

KG SUITE

Figure 19. ROCK-EVAL hydrogen index plotted versus kerogen elemental hydrogen to carbon (H/C) ratios for the "KG" suite of the Cooperative 

Monterey Organic Geochemistry Study (CMOGS, Isaacs and Rullkötter, 

1993) and for a supplemental sample suite collected by Barry Katz. All 

data collected by Barry Katz.
Discussion 25

ROCK-EAL HYDROGEN INDEX 400

600

500

300

200

100

0 0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

ELEMENTAL H/C

170

112

The results of these analyses are plotted in figure 20 where 

there is a reasonable agreement between increasing ROCKEVAL HI values and kerogen elemental H/C ratios, as delineated 

by the two solid lines. Thus for the samples of this study, 

ROCK-EVAL HI is taken as a representative measurement of 

hydrogen richness, and thus the HC generation potential. This 

agreement between the ROCK-EVAL HI and kerogen elemental 

H/C ratios was also supported by "spot check" visual-kerogen 

analyses, which demonstrated all suspected hydrogen-poor OM 

(types III/IV OM by ROCK-EVAL HI to be structured (“terrestrial”) kerogen and all hydrogen-rich OM to be amorphous kerogen. 

The triangles in figure 20 represent samples with large 

amounts of extractable bitumen (samples with asterisks, table 4). 

As delineated by the dashed lines and stippled pattern, these 

samples plot in a trend almost perpendicular to the trend of the 

main body of samples (the dots), and despite a wide range of H/

C ratios (0.83 to 1.26), are invariant versus HI. However, this 

stippled trend is opposite to the invariant trend in figure 19, and 

as reported by Baskin (1993; in press), where elemental H/C is 

invariant versus large ranges of HI. Thus, based on the limited 

sample base of figure 20 (6 samples), large concentrations of 

indigenous bitumen appear to obfuscate expected HI versus H/C 

trends. However, high indigenous bitumen concentrations do 

not appear to explain the spurious trend of figure 19. 

Although we have no explanation for the trend of figure 19, 

this trend may be linked to the fact that the rank of the figure 19 

samples ranges from extremely low to low, whereas the rank of 

the figure 20 samples is substantially higher. Elemental kerogen 

H/C ratios demonstrate excellent correlation with ROCK-EVAL 

hydrogen indices for Lower Mississippian-Upper Devonian 

Bakken shale sample suites, where the Bakken shales are immature and have wide ranges in hydrogen-richness (L. C. Price, 

Figure 20. Plot of kerogen elemental H/C (hydrogen/carbon) ratios versus the ROCK-EVAL hydrogen index for the samples of table 4. Triangles 

are samples with high contents of pre-HC-generation indigenous bitumen. Two of these samples ("170", "112") had much lower HI values (170 

and 112) after solvent extraction. Stippled pattern and solid lines are explained in the text.

unpub. data). For example, HI values can range from 75 to 850 

in 12.2 m (40 ft) of section. However, these suites of Bakken 

shales, although decidedly immature, have been exposed to significantly higher burial temperatures than the immature Miocene 

shales of the California petroleum basins, which yield such poor 

kerogen H/C to ROCK-EVAL HI correlations (fig. 19). Thus, it 

would appear that at least some source rocks must be exposed to 

minimal burial temperatures of perhaps 70° to 80°C before the 

ROCK-EVAL HI becomes a valid measure of kerogen hydrogen 

richness.

As stated, the kerogens of figure 20 were macerated from 

1974 to 1977. A split of each core sample was taken years later 

and powdered for ROCK-EVAL analysis. This explains some of 

the variance in figure 20, because some degree of facies variance 

exists even within one core sample. A split, for ROCK-EVAL, 

of the powdered rock used for kerogen maceration would have 

decreased the scatter in figure 20. However, such splits were not 

available. Lastly, we agree with Baskin’s (1993, 1997, in press) 

principal conclusions: ROCK-EVAL is a fast, inexpensive 

screening tool which allows large numbers of samples to be analyzed. However, kerogen elemental H/C ratios provide the most 

accurate estimate of kerogen (and thus source-rock) richness, 

although this analysis is much more expensive and time consuming. When possible, ROCK-EVAL HI values should be calibrated against kerogen H/C ratios, such as in figure 20.

Two of the figure 20 triangles have numbers (112 and 170) 

beside them. These numbers are HI values for solvent-extracted 

splits of the two samples (all HI values in figure 20 are from 

unextracted rocks). Thus, most of the HI value from the two 

unextracted samples under discussion are clearly from the 

higher-molecular-weight portion of the high bitumen concentrations (the effect of Clementz, 1979). Solvent extraction of the 

two samples resulted in reduced HI values. However, such solvent extraction removes the asphaltenes, which are small pieces 

of kerogen. Moreover, many unpublished analyses by L. Price, 

and related to the effect described by Price and Clayton (1992), 

demonstrate that the asphaltenes in source rocks are intimately 

associated with kerogen and not distributed throughout the mineral matrix of source rocks. Some investigators recommend running ROCK-EVAL and elemental H/C analyses only on rocks 

which have been solvent-extracted to determine generation 

potential. However, such extractions remove the asphaltenes 

from the kerogen, which in many California shales constitute a 

significant part of the shale’s HC-generation potential. We may 

conclude from this discussion that unresolved problems clearly 

remain concerning the proper analytical steps for characterization of source-rock HC-generation potential.

Type III/IV OM Kinetic Differences between Different 

Basins

The apparent differences in reaction kinetics between the 

type III/IV OM of the Los Angeles, Ventura, and Southern San 

Joaquin Valley Basins warrant discussion. By the S1 data of the 

Los Angeles Basin Apex-1 well (fig. 2), mainstage HC generation commences by 85° to 90°C (CIHG, Commencement of 

Intense Hydrocarbon Generation), which is supported by the 

production indices for the Apex-1 (fig. 3, CIHG). Solvent-
26 Organic Metamorphism in the California Petroleum Basis

Table 4. ROCK-EVAL and kerogen elemental analyses for randomly selected samples from the Los Angeles and 

Ventura Basins, California. Wells and field or basin areas, and basin, are shown for each sample, as are depth in 

feet and burial temperature in °C. Depth is given in feet and not meters because the original sample depths were in 

feet. HI is the ROCK-EVAL hydrogen index; H/C is the kerogen atomic elemental hydrogen to carbon ratio; S1 mg/g 

OC is the ROCK-EVAL S1 pyrolysis peak normalized to total organic carbon (TOC); PI is the ROCK-EVAL production 

index (S1/S1+S2); and %S is weight percent sulfur in the kerogen. Kerogen carbon and hydrogen contents were 

determined at the USGS laboratories. Kerogen sulfur and iron contents were determined by Huffman Laboratories, 

Golden, Colorado. Iron was assumed to be pyrite and pyritic sulfur was subtracted from total sulfur. Samples with 

asterisks over the depth values had high concentration of indigenous pre-HC-generation bitumen.

Well Depth

in feet

Temp.

 °C

HI H/C S1

mg/g OC

PI %S

Wilmington Field, Los Angeles Basin

WD401 2411 62.1 350 1.18 20.2 0.054

WJ141 2569* 64.8 456 1.31 121 0.210

WD401 2764 68.1 372 1.24 4.47 0.012

WD107 3017 72.4 104 0.78 4.62 0.042

WC533 3083 73.5 228 1.21 10.2 0.043

WD107 3414* 79.1 460 1.23 100 0.179

WD114 3828 86.2 312 1.12 4.11 0.013

WD204 4116* 91.1 410 1.22 79.9 0.163

WC603 4294 94.1 230 1.07 18.4 0.074

WC603 4531 98.1 195 1.11 10.7 0.052

WC603 5019 106.4 298 1.16 4.31 0.014

WC603 5261 110.5 433 1.24 92.3 0.176

D308 5455 113.8 433 1.27 36.7 0.078 4.90

D102 6071 124.3 476 1.29 46.9 0.090 4.66

D605 8607 167.4 484 1.24 26.0 0.051 13.77

C403 9412 181.1 492 1.18 5.41 0.011

C403 10,387 197.7 390 1.18 12.1 0.030

Long Beach Field, Los Angeles Basin

Shell Alamitos-48A 8061-8071 109.0 80 0.86 2.59 0.031

Richfield Malcom Davis 9985 129.4 345 1.15 24.9 0.067

Richfield Malcom Davis 10,065 130.3 389 1.20 40.0 0.093

Richfield Malcom Davis 10,210 131.8 526 1.25 14.6 0.027 4.63

Shell S. Rose-1 10,583 135.8 524 1.24 41.2 0.073 3.58

Shell S. Rose-1 10,931-10,941 139.6 503 1.20 33.4 0.062 2.33

extraction analyses of Los Angeles Basin rocks with type 

III/IV OM (Price, 1999) also suggest that mainstage HC 

generation begins at 100°C there. By the maximum carbonnormalized S1 values for all Los Angeles Basin rocks with 

type III/IV OM (fig. 8), mainstage HC generation begins by 

105°C, although maximum carbon-normalized values of 

production indices (fig. 9) suggest 125° to 150°C for this 

event. Thus, these different data sets suggest that mainstage HC 

generation commences somewhere from 85° to 130°C, most 

likely around 100° C, for type III/IV OM in the Los Angeles 

Basin.

In contrast, commencement of intense HC generation in 

type III/IV OM is not observed in the Ventura Basin by 140°C 

(figs. 10, 11), and only by 145°C at Paloma (Southern San 
Discussion 27

Joaquin Valley Basin; figs. 12, 13). Differences of 45°C for the 

onset of intense HC generation are significant. We envision only 

four possible explanations why HC generation begins at such 

lower temperatures in the Los Angeles Basin. First, the analyses 

performed are not reflecting reality. This is unlikely because 

these are standard widely used petroleum-geochemical analyses 

of a very large sample base. Second, kinetic differences may 

exist between the type III/IV OM in the three basins. This also 

appears unlikely because qualitative characteristics of the 

extracted bitumen (Price, 1999) suggest the same starting OM in 

the three basins. Third, rocks now at 85° to 100°C (and deeper) 

in the Los Angeles Basin incorporated unusually high concentrations of HCS at deposition compared to shallower rocks. Such a 

HC concentration increase with depth would give the appearance of commencement of intense HC generation. This normally would be an unlikely explanation. However, significant 

Table 4.—continued

Well Depth

in feet

Temp.

 ̊

C

HI H/C S1

mg/g OC

PI %S

Anaheim Nose, Los Angeles Basin

Mobil Librown-1 8826-8834 104.3 148 0.97 13.5 0.084

Mobil Librown-1 9018-9030 106.3 180 0.97 8.6 0.046

Santa Fe Springs Field, Los Angeles Basin

Mobil Santa Fe-243 9494-9502 124.9 320 1.02 9.68 0.029

Union Bell-107 12,570 157.8 422 1.16 13.9 0.032 1.65

Bandini Field, Los Angeles Basin

C.C.M.O. Bandini-1 10,070-10,090 126.1 193 1.05 28.6 0.129

Ventura Avenue Field, Ventura Basin

Lloyd-183 12,705-12,720 127.0 436 1.23 18.7 0.041 10.28

Lloyd-183 13,067-13,097* 130.1 433 1.16 82.1 0.160 5.58

Lloyd-183 13,843-13,867 136.6 421 1.18 23.9 0.054

Lloyd-183 14,546-14,564 144.2 393 1.20 25.4 0.061 3.84

Lloyd-183 15,064-15,074 146.7 291 1.13 24.9 0.079

Ventura Basin Central Syncline

Chevron Maxwell-1 4343-4352* 51.9 454 1.07 77.4 0.146

Chevron Maxwell-1 6058-6068* 64.0 495 0.96 54.5 0.099

Chevron Maxwell-1 8568-8580 81.9 147 0.92 4.19 0.028

Chevron Maxwell-1 9997-10,014 92.0 170 0.81 9.09 0.051

Chevron Maxwell-1 10,394-10,402 94.8 263 1.15 10.1 0.037

Chevron Maxwell-1 10,763-10,774 97.5 163 0.94 7.35 0.043

Superior Limoneira-1 13,080-13,098 113.9 232 1.17 14.2 0.058

Superior Limoneira-1 13,988 120.3 288 1.26 7.95 0.027

Chevron Maxwell-1 15,100-15,112 128.3 247 1.12 12.6 0.049 <0.50

Chevron Maxwell-1 16,493-16,503 138.1 315 1.11 12.1 0.037 1.02

Chevron Maxwell-1 16,694-16,697 141.4 405 1.17 23.8 0.056 <0.50

Superior Limoneira-1 18,428-18,438 151.9 351 1.10 22.8 0.061

Superior Limoneira-1 18,711-18,721 153.9 383 1.11 26.8 0.066 2.14
28 Organic Metamorphism in the California Petroleum Basis

S1 mg/g OC PRODUCTION INDEX

80

120

0

100

HYDROGEN INDEX

40

0

0.5

0.4

0.3

0.2

0.1

0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

200

300

400500

600

700

BURIAL

TEMPERATURE

140–159.0° C

r2=0.744

r

Ro

2=0.805

increases in organic richness occur in the Los Angeles (and Ventura) Basins with depth. Moreover, as discussed in Price (1999), 

many examples exist throughout the California oil basins of finegrained immature rocks with abnormally high concentrations of 

indigenous immature HCS. A fourth explanation, and one that 

we prefer, is that some parts of the Los Angeles Basin have been 

hotter and have cooled significantly in the recent geologic past. 

Other considerations support this hypothesis.

Numerous investigators have maintained that the Los 

Angeles Basin is currently at maximal burial temperatures, 

including McCulloh and others (1978), Bostick and others 

(1978), Price and Barker (1985), Naeser and others (1987), and 

Beyer (1988a). However, this is not a known fact, because one 

of the weakest areas in petroleum geochemistry is still accurate 

determination of paleoburial temperatures. In our opinion, no 

consistently dependable tool exists to achieve this end. The Ro

data of this study certainly illustrate this point. In fact, correct 

determination of even present-day burial temperatures has been 

a serious problem in many past petroleum-geochemical studies, 

because log-header drilling temperatures, rather than correctedequilibrium temperatures, were often used in these studies 

(Price, 1983). Only in the last five to seven years has this latter 

problem become more widely recognized and steps taken to correct it. In reality, we have no valid way to determine if presentday Los Angeles Basin burial temperatures are maximal. Geologic inferences simply do not answer the question.

If such a cooling occurred, it had to occur in the very recent 

geologic past. This is because the most intense structuring in the 

basin occurred from the early Pliocene to Recent (Biddle, 1991), 

the probable time of emplacement of most of the oil fields. 

Moreover, high-ground water flow is the only possible cause of 

such a recent cooling. The present arid climate of the Los Angeles Basin and environs cannot support significant ground-water 

flow. However, during the last glaciations, 10,000 to 70,000 

years ago, much larger ground-water flow occurred. These 

ground-water flows were aided by (1) the ring of mountains 

(which serve as a watershed) surrounding the Los Angeles 

Basin, some peaks of which are over 3,000 m, and (2) by the 

very high sand and gravel content of the shallower Los Angeles 

Basin rocks (fig. 1, and accompanying discussion), rocks which 

are highly transmissible fluid conduits for high ground-water 

flow.

Evidence of past elevated meteoric water flow is rampant in 

the reservoired oils of the Los Angeles Basin, many of which are 

highly degraded (Bruce Bromley, Unocal, oral commun., 6/94); 

and Jeffrey and others, 1991). An example of oil biodegradation 

is also given in Price (1999). Such extensive biodegradation 

could not occur under present climatic conditions, especially 

given the short emplacement times of the oils. These degraded 

oils could only have resulted from much greater ground water 

flows in the recent geologic past. One other convincing line of 

evidence supports such ground water flow. The Los Angeles 

Basin is the smallest basin in the world with major reserves 

(3,367 km2, 1,300 mi2; Price, 1994). Yet, as demonstrated by 

table 2, and as previously noted by Bostick and others (1978), 

much different geothermal gradients exist in different basin 

areas. Given the basin’s very small area, it is very difficult to 

explain how such different temperature gradients exist there. 

However, enhanced meteoric water flows would have been 

channeled into basinal areas which could best transmit fluids and 

thus would preferentially cool relative to other areas. Therefore, 

Figure 21. Plot of Ro, and the ROCK-EVAL production index and S1 peak 

normalized to organic carbon (mg/g OC), all versus the ROCK-EVAL hydrogen index for rocks from the California petroleum basins at equilibrium-burial temperatures of 140° to 159.9°C. Samples are from the 

Ventura central syncline and the Ventura Avenue field of the Ventura 

Basin; from the Whittier, Long Beach, Wilmington, Santa Fe Springs, and 

Seal Beach fields of the Los Angeles Basin; from the Baldwin Hills area; 

the Apex-1 and the Long Beach Airport-1 wellbores; from various wells 

in the Anaheim nose and northeast flank areas of the Los Angeles Basin; 

and from wells in the Paloma field, Southern San Joaquin Valley Basin. 

All samples are core, except for the Apex-1 samples (cuttings chips). 

The curved line in the Ro plot results from logarithmic regression analysis of the data and has a correlation coefficient of r2

= 0.805 to the data. 

The curved line in the production index plot results from logarithmic regression analysis of the data and has a correlation coefficient of r2

 = 

0.744 to the data. The lines in the S1 pyrolysis-peak plot define the principal sample population.
Discussion 29

Ro PRODUCTION INDEX

100

200

0

HYDROGEN INDEX

80

120

40

0

0.8

0.6

0.4

0.2

0

1.2

1.0

0.8

0.6

0.4

0.2

300

400

500

600

700

800

S1 mg/g OC

BURIAL

TEMPERATURE, 

180–199.9 °C

r2=0.867

the variable gradients provide strong evidence to us of large 

ground-water flow in the basin in the recent geologic past. 

Kinetic Differences between Hydrogen-Rich and Hydrogen-poor OM

Data from this study, (Price, 1999), and other investigators, 

demonstrate the strong suppression of organic metamorphism in 

hydrogen-rich OM compared to type III/IV OM for the same 

burial histories and conditions in these basins. For example, Ro, 

ROCK-EVAL S1 pyrolysis peak values (normalized to TOC, mg/

g OC), and production indices are all plotted versus ROCKEVAL HI in figure 21 for all samples of this study buried 

between 140° to 159.9°C. This temperature interval normally 

would be high enough to initiate mainstage HC generation, 

which would be reflected by significant increases in production 

indices and S1 peak values. In figure 21, as HI increases, the 

three indices of organic metamorphism progressively decrease. 

Samples with HI values of less than 200 are demonstrably in 

mainstage HC generation; samples with HI values greater than 

250 clearly are not. Equivalent plots were made for samples 

between 160° to 179.9°C (not shown) and 180° to 199.9°C (fig. 

22), with the same results, as evident in figure 22. Moreover, 

Tmax also decreased with increase in HI values for these same 

samples, when plotted over these three burial temperature intervals.

Other investigators have also noted a strong suppression of 

different aspects of organic metamorphism in the hydrogen-rich 

OM of these basins. For example, Walker (1982) and Walker 

and others (1983) found very strong Ro suppression in the “nodular shale” of the Los Angeles Basin, a phosphatic organic-rich 

(high TOC and HI values) source rock at the base of the upper 

Miocene Modelo Formation occurring on the northwest shelf of 

the Los Angeles Basin. Ro data, versus burial temperatures, 

from Walker (1982) are given in figure 23. The observed Ro values are highly suppressed for their burial temperatures, compared to the values expected for the burial conditions of these 

rocks.

One may argue that the cooling proposed to have occurred 

in the Los Angeles Basin (discussed above) renders meaningless 

any conclusions regarding differential reaction kinetics between 

hydrogen-rich and types III/IV OM. However, this is not the 

case, because commencement of intense HC generation has not 

yet occurred in rocks with hydrogen-rich OM at 198°C at Wilmington. Even if significant cooling has occurred at Wilmington, 

198°C is still far too high for commencement of intense generation by accepted models. Moreover, no evidence of significant 

cooling exists in the Ventura or Southern San Joaquin Valley 

Basin central synclines. However, intense HC generation commences in the Paloma field rocks with type III/IV OM by 145°C, 

yet this event has not occurred by 150°C in the Ventura Avenue 

field rocks with hydrogen-rich OM. Many of the present-day 

Los Angeles Basin equilibrium-burial temperatures are most 

probably not maximal, and thus this temperature data base is 

somewhat flawed. However, even within the temperature limitations of this data base, one may only conclude that large differences exist in the reaction kinetics between the hydrogen-rich 

and type III/IV OM of this data base.

Jarvie and Lundell (in press), based on kinetic considerations from open-system pyrolysis of two suites of immature 

Figure 22. Plot of Ro, and the ROCK-EVAL production index, and S1 peak 

normalized to organic carbon (mg/g OC), all versus the ROCK-EVAL hydrogen index for rocks from the California petroleum basins at equilibrium-burial temperatures of 180° to 199.9°C. Samples are from the 

Wilmington field, the northwest plunge of the Santa Fe Springs field 

(Houghton Community-1), and the Apex-1 in the Los Angeles Basin; and 

the KCLA 72-4 wellbore, Paloma Field, Southern San Joaquin Valley Basin. The curved line in the Ro plot results from logarithmic regression 

analysis of the data and has a correlation coefficient of r2

 = 0.867 to the 

data. The lines in the S1 pyrolysis peak and production index plots define 

the principal trends of the data.
30 Organic Metamorphism in the California Petroleum Basis TEMPERATURE, IN °C

120 0.1

0.15

Ro

100

110

130

140

150

0.2

0.30.40.5

0.70.9

PREDICTED

OBSERVED

Monterey Formation source rocks, concluded that kerogen 

atomic oxygen to carbon (O/C) ratios were a dominant control of 

the timing (burial temperatures) for commencement of intense 

HC generation from these kerogens. Kerogens having higher O/

C ratios and lower H/C ratios generated earlier than kerogens 

having lower O/C ratios and higher H/C ratios. Thus, open-system laboratory experiments seem to confirm the data of this 

study from the natural system: type III/IV OM (high-oxygencontent OM) enters intense HC generation before more hydrogen-rich oxygen-poor OM.

Differences between the different OM types (hydrogen-rich 

versus hydrogen-poor) have been discussed with examples provided, by Price (1991, p. 532–534). Price (1991) attributed these 

differences in OM reactivity to different bond strengths in the 

different OM types. Tissot and Welte (1984, p. 151–154) discuss the chemical bonds in the different OM types of sediments. 

As one passes from hydrogen-rich OM to type III (or IV) OM, 

the oxygen in the OM goes from ester bonds to exclusively carboxylic acid bonds (in type III OM). Esters have much greater 

bond strengths than carboxylic acids. Thus, more hydrogen-rich 

(ester-rich) OM requires higher ranks to react than hydrogenpoor OM.

Jarvie and Lundell (in press) also noted that a modest variation in kerogen sulfur to carbon (S/C) ratios among these highsulfur kerogens had no effect on the timing of HC generation. 

This observation is contrary to the widely held opinion that highsulfur kerogens enter mainstage HC generation at lower burial 

Figure 23. Plot of observed Ro data versus burial temperature in °C from 

the nodular shale (northwest shelf of the Los Angeles basin) versus expected Ro values given the burial histories of the rocks. Data from Walker 

(1982).

temperatures (“earlier”) than low-sulfur kerogens (Orr, 1986; 

Baskin and Peters, 1992). Jarvie and Lundell (in press) recommended reexamination of the widely held belief concerning 

early HC generation from sulfur-rich kerogens. Data from our 

study supports Jarvie and Lundell’s (in press) position on this 

matter. Fourteen of the hydrogen-rich kerogens of table 4 were 

randomly selected for sulfur analysis with the intent of demonstrating that the hydrogen-rich OM of the Los Angeles and Ventura Basins was largely sulfur-poor OM. Kerogens from twelve 

of the samples had low to moderate sulfur concentrations, from 

0.50 to 5.86 weight percent sulfur (table 4). Two samples, D605 

(8,607 ft) and Lloyd-183 (12,705–12,720 ft), had high sulfur 

concentrations (13.77 and 10.28 weight percent of the kerogen, 

respectively).

The ROCK-EVAL S1 carbon-normalized peak and production indices for these two samples (26.0 mg/g OC and 0.051 

(Tmax = 426°C) for D605, and 18.7 mg/g OC and 0.041 (Tmax = 

425°C) for Lloyd-183) demonstrate that neither rock has begun 

HC generation. This in spite of the high burial temperatures of 

these two samples (Lloyd-183: 127°C, D-605: 167°C). Clearly, 

some sulfur-rich kerogens from early to early mid Miocene 

shales in the California basins do not appear to commence premature HC generation, contrary to accepted paradigm. Also, 

sulfur-rich kerogens clearly exist in some parts of the Los Angeles and Ventura Basins. However, a far greater sample base than 

two samples is needed for definitive insight on the matter.

Conclusions

1. In the Los Angeles and Ventura Basins, rock organic richness 

significantly increases with depth from increases in : TOC, 

kerogen hydrogen content, the percentage of fine-grained 

versus coarse-grained rocks, and most probably from other 

unrecognized controls.

2. Los Angeles Basin shales are perceived to be dominated by 

hydrogen-rich amorphous OM. In reality, large rock volumes contain only types III/IV OM, including the upper 

6,000 m of parts of the Los Angeles Basin central syncline, 

the shallowest 3,000 m of which is essentially a sand and 

gravel pit.

3. ROCK-EVAL HI values are a representative measurement 

of the HC generation potential for the rocks of this study, as 

demonstrated by a cross plot of ROCK-EVAL HI and kerogen elemental hydrogen to carbon ratios. Visual kerogen 

analyses corroborate this finding.

4. In type III/IV OM, by ROCK-EVAL data, mainstage HC 

generation begins at present-day (cooled) burial temperatures of about 100°C in the Los Angeles Basin, at 145°C in 

the Southern San Joaquin Valley Basin, and somewhere 

above 140°C in the Ventura Basin. In Los Angeles and the 

Southern San Joaquin Valley, HC generation is largely complete in type III/IV OM by 220°C.

5. The lower temperatures for commencement of mainstage 

HC generation in the Los Angeles Basin are attributed principally to cooling of parts of that basin from previous maximal burial temperatures due to elevated meteoric water 

flows during the last glaciations. These water flows would 

have also resulted in degradation of much of the reservoired 
Acknowledgments 31

oil in that basin and created the variable geothermal gradients observed throughout the basin.

6. All aspects of organic metamorphism, including mainstage 

HC generation, are strongly suppressed in the hydrogen-rich 

OM of these basins compared to the type III/IV OM there. 

For example, HC generation has not commenced in the 

hydrogen-rich OM of these basins by 200°C. Moreover, 

decreasing values of maturity indices, such as the carbonnormalized S1 ROCK-EVAL peak, production indices, Tmax, 

and Ro, all correlate well with increasing HI values for given 

burial temperature ranges. This suppression is attributed to 

stronger bonds in hydrogen-rich OM compared to type III/

IV OM, and has pronounced relevance to petroleumgeochemical exploration models. 

7. Increasing Tmax and decreasing oxygen indices both reflect 

ongoing organic metamorphism in the hydrogen-rich OM of 

the basins at all burial temperatures, a metamorphism not 

revealed by some other maturity indices of this study. However, for all OM types, the ranges of Tmax and oxygen indices at any given burial temperature are unacceptably large, 

due to the lack of precision of the ROCK-EVAL instrument.

8. Pronounced OM characteristics inherited from deposition, 

overprint, and confuse, the interpretation of OM characteristics from HC generation in the rocks of these basins.

9. Immature shales with all OM types in the California basins 

can have high values of carbon-normalized immature indigenous HCS and bitumen, including rocks with type IV OM. 

10. Coarse-grained rocks in and adjacent to oil fields in the California basins almost invariably have high concentrations of 

HCS and bitumen, apparently from oil staining.

11. Organic richness in fine-grained rocks significantly 

decreases northwards from the Southern San Joaquin Valley 

Basin towards the Northern San Joaquin and Sacramento 

Valley Basins.

12. Wide ranges observed in ROCK-EVAL S1 peak values at 

any burial temperature after commencement of mainstage 

generation in type III/IV OM are hypothesized as due to significantly different original HI values (for example, 50 versus 250) and to loss of generated HCS to expulsion, or to the 

drilling mud during drilling operations.

13. We have no widely applicable tool for dependable and accurate estimation of paleoburial temperatures, which remains 

the greatest weakness in petroleum geochemistry.

14. The hypothesis that sulfur-rich OM generates HCS at substantially lower burial temperatures than sulfur-poor OM is 

not supported by two samples in this study. However, more 

analyses are needed to support this finding.

Acknowledgments

 We are indebted to Caroline Isaacs, Dan Jarvie, and Barry 

Katz for reviews of the manuscript. Access to their experience 

in the California petroleum basins is greatly appreciated.

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