Page 105 - The Petroleum System From Source to Trap

5. Applied Source Rock Geochemistry 99

this chapter, although the reader should be aware that asphaltenes, followed by biomarker analysis of the
these studies are likely to impact our understanding of generated bitumen (e.g., Cassani and Eglinton, 1986).
kerogen. Biomarker and other correlation technologies, such as
Asphaltenes in bitumen are lower molecular weight stable carbon isotope analysis and pyrolysis-gas chro­
fragments of kerogen and may be intermediates between matography, are among the most powerful tools for
kerogen and bitumen. For example, although mapping petroleum systems to reduce exploration risk,
asphaltenes are soluble in polar solvents, they show particularly when oils migrate large distances from their
elemental compositions similar to associated kerogens pod of active source rock or when more than one source
(Orr, 1986) and similar distributions of hydrocarbons rock pod exists in the basin fill. Based on these finger­
(Bandurski, 1982; Pelet et al., 1985), including steranes printing techniques, the level of certainty for a petroleum
and triterpanes (Cassani and Eglinton, 1986). system is determined. This level of certainty indicates the
Lipids can be incorporated into kerogen during diage­ confidence that the petroleum from a particular accumu­
nesis, but many survive as free constituents in the lation came from a specific pod of active source rock.
bitumen and are known as molecular fossils, biological
markers, or biomarkers. Biological markers are complex
organic compounds composed of carbon, hydrogen, and SCREENING METHODS
other elements which show little or no change in
structure from their parent organic molecules in living Sedimentary basin analysis (Magoon and Dow,
organisms (Peters and Moldowan, 1993). Chapter 1, this volume) of frontier areas begins with
geologic and geophysical reconnaissance. Early evalua­
tions focus on sample and data collection to assess the
Expelled Products presence of thick sedimentary sequences, regional hydro­
carbon seals, and appropriate reservoir lithologies. Maps
Petroleum expelled from an active source rock,
(primary migration) (Lewan, Chapter 11, this volume) can using well control, outcrop, and geophysical data must
migrate along a fault plane or permeable carrier bed be prepared or revised.
(secondary migration) (England, Chapter 12, this volume) Geochemical screening analyses are practical explo­
to a porous reservoir rock (Morse, Chapter 6; Jordan and ration tools for rapid and inexpensive evaluation of large
Wilson, Chapter 7, this volume) capped or surrounded numbers of rock samples from outcrops and wells.
by a comparatively impermeable seal (Downey, Chapter Outcrop samples from measured stratigraphic sections
8, this volume) that together form a trap (Biddle and are better than random outcrop samples because they
Wielchowsky, Chapter 13, this volume). Examples of can easily be made into a geochemical log that can be
how this happens are described in the case studies in this compared to nearby geochemical logs of wells. Rock
volume. Factors controlling the quantities of petroleum samples from wells include drill cuttings, sidewall cores,
needed to saturate the pore space in a source rock prior and conventional cores, in order of decreasing
to expulsion and the efficiency of expulsion are poorly abundance. Large numbers of analyses of these rock
understood and represent active research topics (e.g., samples are used to make geochemical logs to evaluate
Wilhelms et al., 1990; Mackenzie and Quigley, 1 9 88). the thickness, distribution, richness, type, and thermal
Accurate estimates of these quantities will improve mass maturity of source rocks in the basin fill. Evaluating the
balance calculations. source rock in the basin fill is an important part of sedi­
Shows of petroleum are proof of a petroleum system mentary basin analysis. The next step is to identify the
and when encountered during drilling are useful explo­ pod of active source rock, which is the first step in evalu­
ration clues, particularly when they can be quantified ating a petroleum system.
and regionally mapped. Cuttings or cores that bubble or The most effective screening method for large
bleed oil and gas during removal from the well are called numbers of rock samples from wells and outcrops
"live" shows, in contrast to the asphaltic staining of combines Rock-Eva! pyrolysis and TOC measurements.
"dead" shows. The quality of shows can be evaluated by These data are usually supplemented by vitrinite
their fluorescence under ultraviolet light, by the color of reflectance and spore coloration results to construct
organic solvent extracts, or by the geochemical screening detailed geochemical logs (see Figures 5.4-5.11).
methods described later. Quantitative bitumen or hydro­ Chapter Appendix B describes key criteria for useful
carbon yields from reservoir rocks assist in distin­ geochemical logs. These include proper ( 1 ) sample
guishing between commercial and noncommercial spacing, (2) sample quality and storage, and (3) sample
subsurface petroleum occurrences (Swanson, 1981). preparation.
Oils inherit biomarker distributions similar to those in
the bitumen from the source rock, thus allowing oil-oil Rock-Eval Pyrolysis and T o tal Organic
and oil-source correlation or "fingerprinting" and paleo­ Carbon
reconstruction of source rock depositional conditions
(Peters and Moldowan, 1 9 93). An advantage of Total organic carbon (TOC, wt. %) describes the
biomarkers is their resistance to biodegradation by quantity of organic carbon in a rock sample and includes
aerobic bacteria in the reservoir. For heavily biodegraded both kerogen and bitumen. roc can be determined in
oils where biomarkers have been partially altered, corre­ several ways, and geologists should be familiar with the
lation sometimes requires sealed tube pyrolysis of advantages and disadvantages of each (Chapter