GEOLOGICAL SURVEY CIRCULAR 530

Gravimetric Effects of 

Petroleum Accumulations 

A Preliminary Summary
Gravimetric Effects of 

Petroleum Accumulations 

A Preliminary Summary

By Thane H. McCulloh

GEOLOGICAL SURVEY CIRCULAR 530

Washington 1966
United States Department of the Interior

STEWART L. UDALL, Secretary

Geological Survey

William T. Pecora, Director

Free on application to the U.S. Geological Survey, Washington, D.C. 20242
CONTENTS

Page

Abstract ----------------------------------------- 1

Introduction --------------------------------------- 1

Acknowledgments ------------------------------------ 2

Previous work -------------------------------------- 2

Principal observations and conclusions- ------------------------ 2

The borehole gravimeter -------------------------------- 3

Recommendations ------------------------------------ 3

References- --------------------------------------- 4

III
Gravimetric Effects of Petroleum Accumulations 

A Preliminary Summary

By Thane H. McCulloh

Abstract

Negative gravity anomalies of very local extent and 

with amplitudes of 1.2 milligals or less have been 

observed over some known petroleum and natural gas 

fields in southern California and South Dagestan, 

U.S.S.R. Field evidence, laboratory measurements, 

and theory indicate that these anomalies are mainly 

the result of hydrocarbon pore fluids of densities 

significantly lower than that of water. Gravity meters 

already available have the precision necessary to 

detect some of these anomalies from surface measurements. In addition, a high-precision borehole gravity 

meter has been developed, by the industrial firm of 

LaCoste and Romberg, Inc., that can be used in wells 

with a casing 7 inches or more in diameter and at 

temperatures below 100°C. Field tests indicate that 

the prototype attains a precision in wells of 4^0.015 

milligal for a single measurement. These observations and the new gravimeter should aid in the search 

for new petroleum fields and for new reservoirs in 

known fields that are incompletely explored.

INTRODUCTION

The thinking and interpretation of most users of 

gravimetric methods of prospecting for petroleum 

have been dominated by an assumption that positive 

geologic structures in general produce positive gravity anomalies except where a mineralogically controlled rock-density deficiency (such as arises from 

mineralogically unusual rocks like salt, gypsum, 

coal, or diatomite) reverses the general and normal 

tendency for sedimentary rocks of all types to decrease 

in porosity and increase in density with greater depth. 

This assumption is correct for some structures and 

wholly incorrect for others. Most structural highs 

that contain volumetrically important reservoirs of 

petroleum fluids produce gravity anomalies of smaller 

positive amplitudes than analogous barren structures. 

This difference is due in part or in whole to the relatively low densities of most petroleum and naturalgas reservoirs. It is found even where no mineralogically controlled rock-density deficiency is present.

Gravity measurements by the writer in 1955 (McCulloh, 1957) in the area of the Salt Lake field, California, and in 1958 in the Santa Fe Springs oil field, 

California, showed the presence of negative anomalies 

in those fields, and subsequent investigations indicate 

that small but recognizable gravity minima that seem 

to be attributable to the density deficiency resulting 

from petroleum- or natural gas-saturation are relatively common but have not been widely publicized, 

and many other minima are probably unrecognized 

because they are superimposed on larger gravity 

maxima attributable to pronounced positive structures. 

Such minima are most noticeable in multiple-zone 

oil fields of the type prevalent in the deep Tertiary 

basins of California, characterized by large thicknesses of highly porous petroleum-saturated sandstone. In some extreme cases where total reservoir 

thickness numbers thousands of feet, where the sandstones are young and highly porous, where petroleum 

gravity and gas-oil ratios are high, or where reservoir depth is only hundreds or a few thousand feet, 

conspicuous gravity minima completely unrelated to 

positive gravity anomalies are produced over pronounced anticlinal culminations. A few examples of 

such anomalies in California are those of the Santa 

Fe Springs oil field, the Buena Vista Hills field, and 

the main producing anticlines of the Midway-Sunset 

field.

These and related observations led the writer to 

formulate the hypothesis that such gravity minima 

are, in important part, the result of hydrocarbon pore 

fluids of densities significantly lower than that of water, 

and in 1963 work was begun to test this hypothesis by 

a critical analysis of the basic factors that control 

the volumetric and mass properties of sedimentary 

rocks in situ, and to evaluate the surface and subsurface gravimetric effects expectable from low-density 

porous reservoir rocks saturated with petroleum 

fluids. A high-precision borehole gravimeter was 

recognized as an important tool in testing this hypothesis and in extending its application to the search for 

deeper and smaller reservoirs than could be detected 

from surface surveys. Accordingly, the industrial 

firm of LaCoste and Romberg, Inc. was asked in 

January 1964 to attempt to modify one of its highprecision sensors to fit a borehole system, the specifications for which were outlined by the writer 

(McCulloh, 1965a).

Such a gravimeter has now been developed as a 

modification of the well-known and patented LaCoste 

and Romberg geodetic gravity meter. A description 

of the instrument and the results of preliminary field 

tests are being prepared for publication; one 

report on mass properties of sedimentary rocks as 

related to petroleum exploration is being published 

(McCulloh, 1966) and another is in preparation. Some 

of the principal conclusions and recommendations are 

summarized here, however, along with a brief descrip-
tion of the previously published investigations that 

relate to gravity anomalies associated with petroleum 

accumulations.

ACKNOWLEDGMENTS

The author is indebted to an unusually large number 

of persons and organizations for assistance, advice, 

and data. Specific identification of their contributions 

must be deferred for the longer publications mentioned 

above, but thanks must be expressed here at least to 

those who contributed most heavily in the development 

and testing of the borehole gravimeter, namely Lucien 

LaCoste, H. B. Parks, and Alvin Saunders of LaCoste 

and Romberg, Inc.; and J. E. Schoellhamer and Earl 

Pampeyan of the U.S. Geological Survey. Thanks are 

due also to the Shell Oil Co., Mobil Oil Co., Standard 

Oil Co. of California, and Lane Wells for their 

cooperation in field tests of the gravimeter.

PREVIOUS WORK

Search of the literature shows that a number of 

others have observed negative gravity anomalies 

associated with petroleum accumulations and a few 

have offered the suggestion that they are the result 

of petroleum pore fluids.

Miller (1931) was apparently the first to notice and 

publicly record gravity minima associated with oilproducing anticlines (in California), and he attributed 

the required deficiency of density to "the compaction 

and rarefaction of the beds caused by folding." Somewhat later, Poletaev (1934), (not available to the writer 

but reported by Tsimel'zon, 1959a), independently 

recognized local gravity minima comparable to those 

of southern California associated with gas-producing 

anticlines in South Dagestan, U.S.S.R., and suggested 

an interpretation "that the rocks saturated with gas 

have a low density." Barton (1938, p. 377-78; 1944) 

and Boyd (1946), apparently independently of each 

other's work, interpreted the prominent gravity minimum over the Lost Hills anticline of the Central 

Valley of California as a product of excessively low 

density diatomaceous shales of Miocene age a few 

hundred to several thousands of feet beneath the surface, even though, in Barton's (1944, p. 13) words, 

"Simple anticlinal arching of the beds would not seem 

competent to produce so simple and so sharp an anomaly as the Lost Hills minimum." Tsimel'zon (1956a, 

1956b, and 1959a) reexamined the anticlinal gravity 

minima of South Dagestan first noted by Poletaev and 

concluded that they result from "zones of fracturing 

(high porosity)" of carbonate rocks of Cretaceous 

age in the crests and steep flank parts of the anticlines, a conclusion not greatly unlike that reached 

by Miller in southern California 25 years earlier. 

Most recently, Medovskiy and Komarova (1959) examined several Russian anticlinal minima, including 

those of South Dagestan, in terms of the possible 

local gravitational effects of the low densities of the 

petroleum and natural gas filling the pores of the 

reservoir rocks of these folds. Medovskiy and Komarova (1959, p. 676), concluded: "Thus, these experimental operations allow us to assume that local 

gravity minima above the crests of structures are 

caused by gas-oil deposits," a reiteration of the longburied view attributed to Poletaev (1934). This conclusion, based mainly on a supposed temporal change

in gravity indicated by comparison of measurements 

at a producing pool, using early torsion-balance data 

and recent gravity meter surveys, was not accepted 

by Nemtsov (1962), mainly because of the low accuracy 

of the gravimetric surveys (according to Brod and 

Vasilev, 1958, Russian gravimetric surveys ordinarily 

have a maximum precision of only about 0.3 mgl). 

Nemtsov's (1962) further conclusion that "Amplitudes 

of model gravitational effects of oil pools in the oil 

field under consideration ... range from 0.005 to 

0.025 mgl, the average being 0.007 to 0.010 mgl" 

discouraged his hopes regarding gravimeteric identification of such reservoirs, except possible natural 

gas reservoirs.

PRINCIPAL OBSERVATIONS AND CONCLUSIONS

The results of some field and laboratory studies of 

mass properties of sedimentary rocks and of the 

gravimetric effects of petroleum and natural-gas 

accumulations (McCulloh, 1966) are briefly summarized as follows:

1. Precise and detailed gravimeter surveys show that 

small local gravity minima are associated with 

some known shallow to intermediate depth natural 

reservoirs of petroleum and natural gas (Miller, 

1931; Tsimel'zon, 1956a, 1956b, and!959a). Such 

gravity minima range in amplitude from more 

than 1 millagal to values that are scarcely detectable.

2. Under reservoir temperature and pressure, the 

densities of pure water, brine, and petroleums 

and natural gases of various compositions vary 

widely but reveal that a sedimentary rock of any 

porosity is notably less dense when saturated 

with petroleum fluid of 30°API gravity, or higher, 

and a gas-oil ratio of 500 cuftper bbl, or higher, 

than when saturated with pure water or brine. 

For example, the density contrast between rock 

of 30-percent porosity saturated with water and 

the same rock saturated with hydrocarbon fluid 

at a temperature and pressure appropriate to a 

depth of 4,000 feet ranges from 0.11 g per cm^ 

(for petroleum of 30° API gravity and 500 cu ft 

per bbl gas-oil ratio) to 0.29 g per cm3 (for pure 

methane).

3. Consideration of the hypo thetic al gravimetric 

effects of well-explored reservoirs indicates that 

all porous rocks containing low-density petroleum 

and natural gas produce relatively negative gravity anomalies, but that many such effects are 

masked or attenuated to the point of concealment. 

Factors that tend to conceal the negative gravimetric effect of a reservoir are (a) small volume, 

(b) great depth, (c) low porosity, (d) low petroleum 

gravity and low gas-oil ratio, (e) high interstitial 

water content (f) large area-volume ratio, (g) 

strong density contrasts between other nearby 

rock masses of large volume, and (h) gravimetric 

effects of pronounced distortion of isopycnic surfaces outside the reservoir because of very recent deformation of young rocks or very complete 

consolidation of lithologically heterogeneous 

rocks of large structural relief, including the 

positive anomalies associated with some structural highs. Factors that tend to make the nega-
tive gravimetric effects of a reservoir conspicuous are also, in the main, factors which tend to 

make a reservoir a commercially attractive 

target. These are (a) large volume, (b) high 

porosity, (c) shallow depth, (d) high API gravity 

petroleum, high gas content, or both, (e) large 

reservoir volume-area ratio, and (f) low interstitial water content. Additional factors that make 

the negative gravitational effects conspicuous are 

(a) geologic simplicity in the rocks surrounding 

the reservoir, (b) negligible to moderate structural relief and distortion of isopycnic surfaces, 

and (c) thorough consolidation of surrounding 

nonpetroliferous rocks.

4. While the low densities, characteristic of the hydrocarbon pore fluids of petroleum and natural-gas 

reservoirs, lower the bulk densities in situ of 

reservoir rocks sufficiently to account for many 

of the relatively negative gravimetric effects observed over known oil fields, several other natural 

factors also tend to produce the same kinds of 

effects. These factors are not examined per se 

in this report nor are the observed negative gravimetric effects of known oil fields compared quantitatively with hypothetical effects.

5. Most reservoir rocks are sufficiently dense in spite 

of the low-density hydrocarbon pore fluids that 

other, more porous, water-saturated rocks may 

be equally or even less dense. However, sandstones of the maximum probable or possible 

porosity at any given depth, those that are in general late Cenozoic in age, possess almost uniquely 

low densities when saturated with petroleum of 30° 

API or higher and gas-oil ratio of 500 cu ft per 

bbl or higher. The nearly unique densities of these 

rocks are probably sufficient reason in themselves 

to label the rocks as petroleum bearing if a satisfactory method is available to measure density in 

boreholes, unless salt, coal, or diatomite are 

known or suspected constituents, or unless abnormally high aqueous pore-fluid pressure has dilated 

the rock.

6. Hypothetical subsurface gravimetric effects of 

well-explored petroleum reservoirs of moderate 

and small size suggest that most promising extensions of the relatively negative surface gravimetric effects of reservoir rocks can be expected 

from the application of a borehole gravimeter (or 

gravity gradiometer) having a sensitivity equal 

to that of modern surface gravimeters.

7. All the characteristics that tend to cause petroleum 

and natural-gas reservoirs to possess low densities exist in some degree irrespective of the 

reservoir's relationship to structure. Reservoirs 

not associated with prominently positive geologic 

structures (for example, stratigraphic traps, 

fracture-porosity traps, reservoirs formed by 

lateral permeability barriers, fault traps of negligible structural relief, and anticlinal traps of 

small closure) produce some negative gravity 

anomalies that maybe large enough to be conspicuously identifiable. .Reservoirs associated with 

salt domes or with other structures in which 

density-deficient rocks are present also produce 

negative gravimetric effects, but these may be

indiscernible or discernible only with difficulty 

against the background of effects of the other 

strong density contrasts.

THE BOREHOLE GRAVIMETER

As previously mentioned, the new borehole gravimeter is a modification of the well-known and patented 

LaCoste and Romberg geodetic gravity meter. It is 

designed for use in wells with a casing of 7 inches or 

more in diameter and at temperatures below 100°C. 

It has been tested to depths of 4150 feet in a Shell Oil 

Co. well at Santa Fe Springs oil field, California. A 

single measurement requires only 3 to 5 minutes, and 

test results indicate that its precision is +0.015 milligal. With some instrumental refinement and further 

operating experience, it appears possible to approach 

closely an ultimate precision in wells of +0.005 milligal. ~

Ideally, the precision of a borehole gravimeter 

should be +0.001 milligal to permit effective use in 

the full variety of applications outlined in petroleum 

exploitation and exploration (McCulloh, 1965a, 1966). 

The precision of the prototype instrument, however, 

is sufficient to detect the anomalies expectable from 

many petroleum accumulations.

Borehole measurement of gravity should prove exceptionally helpful in the evaluation and exploitation 

of new reservoirs and in the elucidation of many problems of importance to petroleum reservoir engineering (see, for example, McCulloh, 1965b regarding in 

situ measurement of density and porosity). Moreover, 

subsurface gravimetric measurements are certain to 

be of great and novel value in exploring for deeper 

pools and lateral extensions of known pools in partly 

explored oil fields, particularly those of the stratigraphic entrapment, fracture-porosity, or unpredictable residual- and solution-porosity types. Lastly, 

systematic gathering and analysis of subsurface gravimetric data could prove highly beneficial, in conjunction with surface gravity data, in support of any wildcat exploration campaign, but particularly in exploration in basins or regions where much is already known, 

from previous drilling, about subsurface structural 

and stratigraphic variations and isopycnic configurations.

Although the focus in the development of the borehole gravimenter has been on petroleum exploration, 

the instrument should also prove applicable to the 

search for many other minerals.

RECOMMENDATIONS

The data, observations, and co nc lus io n s of the 

studies referred to in this report lead to several specific recommendations concerning studies applicable to 

petroleum exploration:

1. Precise gravity maps of many petroliferous regions 

should be examined, preferably first by suitable 

trend-surface analyses or graphical or numerical 

derivative procedures, for evidence of local gravity minima of small amplitude associated with 

known and explored oil fields in various regions. 

By this means, an empirical basis could be established for seeking previously unrecognized reservoirs by direct gravimetric detection.
2. Further efforts should be made to calculate from 

density models of well-explored oil fields their 

gravimetric effects for comparison with gravity 

variations actually observed over these fields. 

By this means, an improved understanding could 

be reached of factors that control the sizes and 

shapes of both the small-amplitude and the larger 

amplitude subsurface gravity anomalies produced 

by reservoirs of petroleum and natural gas and 

their surrounding rock masses, and a basis would 

be established for judging the reliability of such 

models.

3. Efforts should be continued to improve the sensitivity and accuracy of the borehole gravimeter, 

reduce its diameter, and extend its temperature 

(and therefore depth) tolerance.

4. Precise ocean-bottom gravimeter surveys or traverses over seismically or acoustically determined structures should be investigated as a 

means of predicting, before drilling, the relative 

productive potential of such structures from their 

local gravimetric effects. Determination of generalized sub-ocean-bottom structure in water of 

moderate (and even great) depth has now become 

comparatively simple and reliable. Analysis of 

precise gravity measurements made on the ocean 

bottom over potential trapping structures offers 

the prospect of greatly reducing exploration risks 

or greatly enhancing ability to bid wisely in competition over an offshore parcel, the structure 

of which is moderately well understood.

5. Where known stratigraphic traps and fracture 

porosity reservoirs are suspected of having unexploited extensions, precise gravity surveys 

should be conducted to determine the feasibility 

of locating such suspected extensions by their relatively negative gravimetric effects.

6. Because of their probable future critical importance in investigations of the variations of the 

underground vertical gradient of gravity, conventional cores from wells already drilled should be 

carefully conserved.

REFERENCES

Barton, D. C., 1938, Gravitational methods of prospecting, _in_ Dunstan, A. E., ed., Science of Petroleum, v. 1, sec. 8: London, Oxford Univ. Press, 

p. 364-381.

____1944, Lost Hills, California an anticlinal minimum, in Barton, D. C., Case histories and quantitative calculations in gravimetric prospecting: 

Am. Inst. Mining Metall. Engineers Tech. Pub. 

1760, Petroleum Technology, v. 7, no. 6, p. 9-15.

Boyd, L. H., 1946, Gravity-meter survey of the Kettleman Hills-Lost Hills trend, California: Geophysics, 

v. 11, no. 2, p. 121-127.

Brod, I. O., and Vasilev, V. G., 1958, Fundamental 

problems in the field of the method of prospecting 

and exploration of oil and gas: Geologiya Nefti i 

Gaza, no. 2, p. 1-6; translation in Petroleum Geology, v. 2, no. 2-A, p. 103-111.

McCulloh, T. H., 1957, Simple Bouguer gravity and 

generalized geologic map of the northwestern part 

of the Los Angeles Basin, California: U.S. Geol. 

Survey Geophys. Inv. Map GP-149.

____1965a, The promise of precise borehole gravimetry in petroleum exploration and exploitation: 

ECAFE, 3d symposium on the development of petroleum resources of Asia and the Far East, Tokyo, 

Japan, Nov. 1965. Also in press (1966) as a U.S. 

Geological Survey circular.

____1965b, A confirmation by gravity measurements 

of an underground density profile based on core 

densities: Geophysics, v. 30, no. 6, p. 1108-1132.

____1966, Mass properties of sedimentary rocks and 

gravimetric effects of petroleum and natural-gas 

reservoirs: U.S. Geological Survey Prof. Paper 

528-A. (In press)

Medovskiy, I. G., and Komarova, G. M., 1959, Possible nature of local gravity minima over oil and 

gas pools: Geologiya Nefti i Gaza, no. 11, p. 50- 

52; translation in Petroleum Geology, v. 3, 

no. 11-B, p. 674-676.

Miller, R. H., 1931, Analysis of some torsion-balance results in California: Am. Assoc. Petroleum 

Geologists Bull., v. 15, no. 12, p. 1419-1429.

Nemtsov, L. P., 1962, The prospects of high-precision 

gravity prospecting in the search for oil: Priklad. 

Geofizika, no. 35, p. 142-156; English translation 

by Associated Technical Services.

Poletaev, S. P., 1934, Review of gravity work by NGRI 

in south Dagestan from 1927 to 1931 by the North 

Caucasus Conference of Oil Geologists Trust. Oil 

fields of Dagestan [In Russian].

Tsimel'zon, I. O., 1956a, Nature of local gravity anomalies of the Apsheron Peninsula: Priklad. Geofizika, no. 14, p. 14-22 [in Russian].

____1956b, Geologic interpretation of gravity anomalies of Kobystan: Priklad. Geofizika, no. 15, 

p. 109-126 [in Russian].

____1959a, Use of gravimetry for direct exploration 

for oil and gas: Geologiya Nefti i Gaza, no. 12, 

p. 41-46; translation in Petroleum Geology, v. 3, 

no. 12-B, p. 728-733.

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