Page 124 - The Petroleum System From Source to Trap

118 Peters and Cassa

CHAPTER APPENDIX A:
Kerogen T y pes

There are four principal types of kerogens found in coals lower thermal maturity than other type II kerogens (Orr, 1986;
and sedimentary rocks which are defined using atomic H/C Baskin and Peters, 1992). Type II kerogens are also dominated
versus 0/C or Rock-Eva! HI versus OI diagrams (see Figures by liptinite macerals.
5.1 and 5.2.)
Type III
Type I Immature type III kerogens show low atomic H/C (<1.0)
and high 0/C ($0.3). Type III kerogen is called gas-prone
Immature type I kerogens are oil prone, show high atomic
H/C (�1.5), low 0/C (<0.1) (Figures 5.1 and 5.2), and generally because it yields some hydrocarbon gas but little oil during
have low sulfur content. These kerogens are dominated by maturation. This term is misleading because type III kerogens
liptinite macerals, but vitrinites and inertinites can be present in actually yield less gas than types I and II. Some thick deltaic
lesser amounts. Type I kerogens appear to be derived from deposits dominated by type III kerogen have generated
extensive bacterial reworking of lipid-rich algal organic matter, substantial oil (e.g., Mahakam Delta in Indonesia, U.S. Gulf
commonly, but not always, in lacustrine settings (e.g., Eocene Coast, and offshore West Africa), primarily from liptinite
Green River Formation). Botryococcus and similar lacustrine macerals that may represent only a small portion of the
algae and their marine equivalents, such as Tasmanites, can be kerogen.
major contributors to type I kerogens.
Type IV
Type II Type IV kerogen is "dead carbon" showing very low atomic
H/C (about 0.5-0.6) and low to high 0/C ($0.3). These
Immature type II kerogens are oil-prone (e.g., Jurassic of
Saudi Arabia) and show high atomic H/C (1.2-1.5) and low kerogens are dominated by inertinite macerals that generate
0 /C compared to types III and IV. Sulfur is generally higher in little or no hydrocarbons during maturation. Type IV kerogens
type II compared to other kerogens. Type II S kerogens (e.g., can be derived from other kerogen types that have been
Miocene Monterey Formation) show high sulfur (e.g., 8-14 wt. reworked or oxidized.
%; atomic S/C � 0.04) and appear to generate petroleum at

CHAPTER APPENDIX B:
Key Factors Affecting Accuracy of Geochemical Logs

We recommend Rock-Eva! pyrolysis and TOC analyses the residue lacks useful volatile compounds and only the
every 10-20 m and vitrinite reflectance data every 100-200 m kerogen can be analyzed reliably. Samples stored for long
throughout each well. Closer sample spacing results in better periods are generally reliable, provided they are clean and were
geochemical logs. The strength of the pyrolysis and TOC stored under conditions restricting the growth of fungus. Core,
screening approach lies in sheer numbers of analyses. Trends sidewall, and outcrop samples can be brushed or scraped to
are established by statistically significant amounts of data, and remove mudcake, residues from marking pens, or weathered
occasional anomalies become obvious (e.g., Figure 5.6). Incom­ surfaces.
plete geochemical logs based on isolated measurements are of Outcrops are commonly weathered, resulting in altered
little exploration value. organic matter. Outcrops should be systematically collected
Because screening analyses are inexpensive, it is practical to (e.g., every 2 m vertically) from fresh cuts, such as cliffs,
generate libraries of detailed geochemical logs. As provinces roadcuts, or river banks. In general, outcropping rocks with
become better explored, libraries of logs progressively reduce high dips are more deeply weathered than those with low dips.
exploration risk by clarifying the three-dimensional distribu­ Fist-sized samples (5 x 5 x 8 em) are sufficient for most source
tions of organic facies, thermal maturity, and prospective reser­ rock analyses. Accurate positioning (within about 10 m) can be
voirs. Consistent scales for geochemical logs simplify compar­ obtained using a commercially available hand-held satellite
isons of source rock intervals between wells. positioning system. Sample locations should be recorded on a
measured section, outcrop sketch, or photograph.
Rock Sample Preparation Samples can be screened in the field by sedimentologic
Rock sample quality generally decreases in the following features or using a commercially available portable pyrolysis
order: conventional whole core, sidewall core, drill cuttings, system. Shales or dense, fine-grained micritic carbonates are the
and outcrops. Cuttings can be contaminated by particulate or best candidates for source rocks. Good source rocks containing
fluid (e.g., oil-based mud) drilling additives or can contain rock type I or II kerogen are commonly thinly laminated and range
chips "caved" from higher in the section during drilling. from yellowish brown to grayish brown or brownish black
Cuttings polluted with diesel can be cleaned with a solvent, but (GSA Rock Color Chart, 1979). However, good source rocks can