Page 121 - The Petroleum System From Source to Trap

5. Applied Source Rock Geochemistry 115

SUMMARY Acknowled gm ents W e thank L. B. Magoon and W. G. Dow
f o r their invitation to prepare this paper and G. J. Demaison f o r
Source rock characterization using geochemical logs his s u ggestion that it be written. The following people
and maps is an exercise in sedimentary basin analysis contributed timely reviews that improved the manuscript: L. B.
with the objective of identifying the pod of active source Magoon, 5. C. Teerman, D. K. Baskin, T. A. Edison, G.J.
rock. The pod of active source rock contributes hydrocar­ Demaison, J.T. Smith, and W.G. Dow. Tables 5.1, 5.2, and 5.3
bons to the petroleum system. were improved by input from D. K. Baskin, L. B. Magoon, and
This chapter emphasizes (1) terms used to describe J. Miles. Concepts for Figures 5.2 and 5.3 were provided by T.
source rocks; (2) sampling, preparation, and analysis A. Edison and 5. C. Teerman, respectively. 5. D. Northam and
criteria; (3) geochemical logs and their use to describe B. R. Barden coordinated the production of figures and text.
source rocks and petroleum shows in one dimension; W e thank E. L. Couch and N. Schneidermann for their support
and (4) geochemical maps and their use for interpolating and the management of Chevron Overseas Petroleum Inc. f o r
between one-dimensional control points for a three­ permission to publish this work.
dimensional understanding of the petroleum system.
Proper use of terms is critical for clearly describing
petroleum systems. Some examples of source rock terms References Cited
include richness, kerogen type, thermal maturity,
product generated, time generated, and provenance or
depositional environment. Source rock organic richness Alpern, B., 1970, Classification petrographique des constitu­
can be poor, fair, good, very good, or excellent (Table ants organiques fossiles des roches sedimentaires: Revue
5.1). Kerogen can be described as type I, II, III, or IV Institut Francais du Petrole et Annual Combustion Liquid,
Paris, v. 25, p. 1233-1266.
based on elemental analysis (Table 5.2). Organic petrog­ Bandurski, E., 1982, Structural similarities between oil-gener­
raphy provides information on organic matter type and ating kerogens and petroleum asphaltenes: Energy
thermal maturity, but is currently too imprecise to Sources, v. 6, p. 47-66.
describe generative potential. Thermal maturity is Barker, C., 1979, Organic geochemistry in petroleum explo­
divided into immature, mature, and postmature based ration: AAPG Continuing Education Course Note Series
and 10, 159 p.
on such parameters as vitrinite reflectance, T max'
thermal alteration index (Table 5.3). A source rock can be Baskin, D. K., 1979, A method of preparing phytoclasts for
described as potential (could generate oil), effective vitrinite reflectance analysis: Journal of Sedimentary
(generated or currently generating oil), or spent Petrology, v. 49, p. 633-635.
(generated oil). A spent source rock can still generate gas. Baskin, D. K., and K. E. Peters, 1992, Early generation charac­
An inactive source rock is not generating oil today, but in teristics of a sulfur-rich Monterey kerogen: AAPG Bulletin,
v. 76, p. 1-13.
the past it was an active source rock. The term "marine Bernard, B. B., 1978, Light hydrocarbons in marine sediments:
source rock" implies marine deposition, while the terms Ph.D. dissertation, Texas A&M University, College Station,
"marine organic matter" and "marine kerogen" could TX, p. 53-62.
imply an origin from marine organisms. A marine source Bostick, N. H., and B. Alpern, 1977, Principles of sampling,
rock might contain dominantly land plant organic preparation, and constituent selection for microphotom­
matter. etry in measurement of maturation of sedimentary organic
Geochemical logs of closely spaced Rock-Eva! matter: Journal of Microscopy, v. 109, p. 41-47.
pyrolysis and TOC, vitrinite reflectance, lithology, mud Cassani, F., and G. Eglinton, 1986, Organic geochemistry of
log gas, and related data are indispensable tools in the Venezuelan extra-heavy oils, 1. Pyrolysis of asphaltenes: a
sedimentary basin evaluation process. Useful geochem­ technique for the correlation and maturity evaluation of
crude oils: Chemical Geology, v. 56, p. 167-183.
ical logs require adherence to proper procedures for Clementz, D. M., G. J. Demaison, and A. R. Daly, 1979, Well
sample selection, preparation, analysis, and interpreta­ site geochemistry by programmed pyrolysis: Proceedings
tion. These logs identify petroleum source rocks (as of the 11th Annual Offshore Technology Conference,
potential, effective, or spent), the thermal maturation Houston, OTC 3410, v. 1, p. 465-470.
gradient (including immature, mature, and postmature Cook, A. C., and N. R. Sherwood, 1991, Classification of oil
zones), and in situ and migrated petroleum shows. shales, coals and other organic-rich rocks: Organic
Because of the rapid and inexpensive screening Geochemistry, v. 17, p. 211-222.
methods used, it is practical to generate libraries of Daly, A. R., and J. D. Edman, 1987, Loss of organic carbon
geochemical logs that progressively reduce the risk asso­ from source rocks during thermal maturation (abs.):
AAPG Bulletin, v. 71, p. 546.
ciated with petroleum exploration as a petroleum Demaison, G. J., 1984, The generative basin concept, in G.J.
province becomes more thoroughly sampled. Logs from Demaison and R. J. Murris, eds., Petroleum Geochemistry
various locations can be used to map the pod of active and Basin Evaluation: AAPG Memoir 35, p. 1-14.
source rock, regional variations in organic facies, and the Demaison, G. J., A. J. J. Holck, R. W. Jones, and G. T. Moore,
volume of generated petroleum. This information can be 1983, Predictive source bed stratigraphy; a guide to
used as input to refine mathematical basin models. regional petroleum occurrence: Proceedings of the 11th
Finally, the two-step procedure consisting of screening World Petroleum Congress, London, v. 2, p. 17-29.
followed by detailed geochemical analyses on selected Demaison, G. J., and B. J. Huizinga, 1991, Genetic classifica­
samples reduces cost and simplifies interpretation. tion of petroleum systems: AAPG Bulletin, v. 75, p.
1626-1643.