Page 107 - The Petroleum System From Source to Trap

5. Applied Source Rock Geochemistry 101

Several factors based on the experience of the analyst Reflectograms
are weighed in the process of selecting vitrinite particles. Reflectograms (Figure 5.3) are frequency plots of the
Ro trends established above and below the sample can be reflectance of all macerals measured in the polished
used to eliminate certain populations of macerals from kerogen slide. Unlike Ra histograms, reflectograms may
consideration. Because TAl and Ro are related (Jones and provide an idea of the difficulty in selecting vitrinite
Edison, 1978), a measured TAl can be used to estimate particles for measurement. Selection of the correct
the R0 of the vitrinite population. This process is not particles for measurement may be difficult when there is
always reliable, however, because TAl is commonly no clearly predominant population of telocollinite
measured on less than a dozen palynomorphs and these (Figure 5.3). R0 must be determined using vitrinite
might represent recycled organic matter or contamina­ because other macerals mature at different rates (e.g.,
tion from drilling mud. Dow, 1 9 77b). However, R0 can be extrapolated from
Reliability of Ro measurements from single samples reflectance measurements of some macerals other than
increases when supported by independent maturity telocollinite, such as exinite (Alpern, 1970).
parameters (e.g., T AI and T m a x) and R0 versus depth
trends established by multiple samples in a well. For
can be used to support Ro, particularly in
example, T m ax
the thermally mature stage. In situ vitrinite in some INTERPRETIVE TECHNIQUES
samples can be overwhelmed by recycled (high Source Potential Index
maturity) or caved (low maturity) particles. Selection of
these particles as the "true" vitrinite might result in During the source rock assessment phase of sedimen­
anomalous values compared to the Ro trend established tary basin evaluation, geologists commonly rely on the
by samples from other depths. As an extreme example, quantity, type, and thermal maturity of organic matter as
some Alaskan wells show little change or even a criteria to indicate favorable risk for significant
decrease in Ro with increasing depth at shallow well petroleum charge. However, source rock volumetrics
depth due to shedding of recycled (high Ro) organic (thickness and lateral extent) must not be ignored. An
matter from Mesozoic highlands into thermally "cool" oil-prone source rock dominated by type I or II kerogen
Tertiary basins. and showing excellent genetic potential (e.g., S1 + Sz > 10
Ro cannot be measured in rocks that lack vitrinite. mg HC/ g rock) may be too thin to charge economically
Vitrinite is derived from land plants and is not common significant oil accumulations. Source potential index (SPI)
in rocks older than Devonian because abundant land (Demaison and Huizinga, 1 9 9 1 ) is defined as the
plants had not yet evolved. Reflectance can be measured quantity of hydrocarbons (metric tons) that can be
on graptolites in lower Paleozoic rocks (Link et al., 1990). generated in a column of source rock under one square
Some oil-prone source rocks that formed on broad meter of surface area (Demaison and Huizinga, Chapter
marine carbonate shelves (e.g., Jurassic of Saudi Arabia) 4, this volume). SPI (or "cumulative hydrocarbon
or in large lakes (e.g., Lower Cretaceous of West Africa) potential," according to Tissot et al., 1980) is a simple
contain only small amounts of vitrinite due to limited method for ranking source rock productivity because it
tenigenous organic matter input. The reflectance of solid integrates both source rock richness and thickness. A
bitumen has been calibrated to R0 and is particularly relative source rock ranking system has been developed
useful in vitrinite-poor carbonate rocks (Jacob, 1989). by compiling the average SPI values for source rock units
Evidence suggests that large amounts of bitumen from around the world.
(Hutton et al., 1980) and oil-prone macerals (Price and The SPI is relevant only where a pod of active source
Barker, 1985) retard the normal increase of vitrinite rock has been established. The entire source rock interval
reflectance with maturity. Low Ra values can result from must be sampled and systematically logged at closely
poor polishing, whereas high values are typical of spaced intervals using Rock-Eval pyrolysis. Samples that
oxidized vitrinite. were positively picked (Chapter Appendix B) or affected
by substantial caving should be avoided. Gross thickness
Ro Histograms of the source rock must be corrected for well deviation,
structural complexities, and nonsource units lacking
R0 histograms show the frequency distribution of significant hydrocarbon generative potential (S1 + Sz < 2
reflectance measurements determined on about 50-100 mg HC/g rock) to get net source rock thickness. The
vitrinite particles in each polished kerogen preparation. samples should be representative of the organic facies in
The random mean R0 is determined from these the area of interest. Samples should show no evidence of
histograms. Ra versus depth plots use the random mean contamination or migrated oil (Chapter Appendix B) and
value and refer only to the population of organic particles should be from thermally immature or early mature
identified by the analyst as vitrinite. Likewise, the portions of the source rock. SPI values determined from
standard deviation for these R0 values represents the thermally mature or postmature sections can be low
repeatability by which the analyst can select these because of petroleum expulsion. Although specific
particles. Because R0 values based on fewer than 50 source rock densities should be used, a density of 2.5
particles can be unreliable, we recommend that R0 t/ m3 is used for most SPI calculations.
histograms be examined for all samples. Laterally drained petroleum systems tend to accumu-