Page 99 - The Petroleum System From Source to Trap

Ma g cxm, L. B, and W. G. Dow, eds., 1994, The petroleum
system-from source to trap: AAPG Memoir 60.

Chapter 5

Applied Source Rock G e ochemistry

Kenneth E. Peters* Mary Rose Cassa
Chevron Overseas Petroleum Inc. California Enuironmcntal Protection Agcncv
San Ra111011, Ca/ifomia, U.S.A. Ocpart111cnt of Toxic SllhstaiiCcs Co11trol
Berkeley, Cali(tmiia, U.S.A.

Abstract

Applied organic geochemistry provides the information needed to make maps of the richness,
type, and thermal maturity of a source rock. These maps are a necessary step toward determig
the stratigraphic and geogrphic extent of a pod of active source rock in a petroleum system, and
they are base on geochemical analyses of rock samples from outcrops and wells that are displayed
on logs. These geochemical well logs are based on Rock-Eva! pyrolysis, total organic carbon,
vitrinite reflectance, and other rapid, inexpensive "screening" methods. The logs define (1)
potential, effective, and spent petroleum source rock; (2) the thermal maturation gradient, including
immature, mature, and postmature zones, and (3) in situ and migrated petroleum shows. Useful
geochemical logs require proper sample selection, preparation, analysis, and interpretation.
Detaed studies, including oil-source rock correlations by biomarker and supporting technques,
are undertak on selected samples only after the screening methods are completed.

INTRODUCTION well site sampling, type of samples (core, sidewall,
cuttings), sample spacing, sample preparation proce­
The goal of this chapter is to show how geochemical dures, and methods of analysis and interpretation.
"screening" technology is applied to petroleum explo­
ration. This chapter provides a conceptual framework for
later discussions in this book by defining key terms used SOURCE ROCK PROPERTIES AND
to describe source rock characteristics and reviewing TERMS
principles and recent developments in source rock Sedimentary rocks commonly contain minerals and
geochemistry. Major emphasis is placed on (1) criteria for organic matter with the pore space occupied by water,
sampling, preparation, and analysis of rocks and oils; (2) bitumen, oil, and / or gas. Kerogen is the particulate
geochemical logs; and (3) geochemical maps. fraction of organic matter remaining after extraction of
The main contribution of organic geochemistry to pulverized rock with organic solvents. Kerogen can be
sedimentary basin analysis is to provide analytical data isolated from carbonate- and silicate-bearing rocks by
to identify and map source rocks. These maps include treatment with inorganic acids, such as HCI and HF (e.g.,
the richness, type, and thermal maturity of a source rock Durand, 1980). This is only an operational definition
and are a necessary step toward determining the strati­ because the amount and composition of insoluble
graphic and geographic extent of a pod of active source organic matter or kerogen remaining after extraction
rock in a petroleum system. The volume, richness, and depends on the types and polarities of the organic
thermal maturity of this pod of active source rock deter­ solvents. Kerogen is a mixture of macerals and reconsti­
mines the amount of oil and gas available for traps. tuted degradation products of organic matter. Macerals
Because of this, maps that show the pod of active source are the remains of various types of plant and animal
rock reduce exploration risk (e.g., Demaison, 1984). matter that can be distinguished by their chemistry and
Geochemical well logs are essential for mapping active by their morphology and reflectance using a petro­
source rocks. These logs plot various geochemical para­ graphic microscope (Stach et a!., 1982). This term was
meters versus depth and can be made from surface originally applied to components in coal but has been
sections and during or after drilling. Certain criteria that extended to sedimentary rocks. Palynomorphs are
are largely omitted from the literature must be met to resistant, organic-walled microfossils such as spores,
ensure useful geochemical logs. These criteria include pollen, dinoflagellate cysts, and chitinozoa.

•Present address: Mobil Exploration and Producing Technical
Center, Dallas, Texas, U.S.A 93