Page 85 - The Petroleum System From Source to Trap

4. Genetic Classification of Petroleum Systems 77

The genetic potential, which is a semiquantitative facies, a general knowledge of paleogeography, source
measure of source rock richness, is defined as the S1 + Sz rock depositional models (Demaison et al., 1984; Jones,
yield from Rock-Eva! pyrolysis (Espitalie et al., 1 9 77). 1987), and possible thickness changes (e.g., derived from
The 51 represents the kilograms of hydrocarbons that are seismic data) allow an interpreter to determine whether
thermally distilled from 1 t of rock, and the Sz represents the source rock in the undrilled area is likely to have
the kilograms of hydrocarbons that are generated by shown a higher or lower SPI than those obtained from
pyrolytic degradation of the kerogen in 1 t of rock nearby wells or measured outcrop sections. Therefore,
(Espitalie et al., 1977; Peters, 1986). Using Rock-Eva! SPI calculations often serve to constrain the possible
pyrolysis data of closely spaced whole-rock samples lateral changes in quantitative source rock potential.
(approximately every 10 m) in a given well or measured The concept and calculations necessary to formulate
outcrop section, the average genetic potential for a source cumulative hydrocarbon potential, which we have
rock is determined in the following manner. Using drill renamed SPI for practical reasons, were first applied by
depth, the top and bottom of the gross source rock Tissot et al. (1980) to show the variability in hydrocarbon
interval are determined using a geochemical log (see source potential of Cretaceous black shales in Atlantic
Figures 5.4-5.11 , Chapter 5, this volume). Within this basins. Identical calculations were later expanded to a
gross interval, we determined the individual source rock global geochemical data base by Demaison (1988) and
interval(s) on which the average genetic potential is Demaison and Huizinga (1989). The original purpose for
calculated. Within each source rock interval, the 51 + Sz developing an SPI data base was to create the operational
value for a depth (core) or depth interval (cuttings) is recognition that source rock volume, which is in part a
multiplied by the depth interval to the next deeper function of thickness, is as important as source rock
sample or the sample depth interval, whichever situation richness.
applies, to get the genetic potential rectangle. We Our concept of SPI is different from the "source
calculate as many genetic potential rectangles as potential rating index" proposed by Dembicki and Pirkle
necessary to get through the net source rock interval. (1985). Their parameter is calculated by multiplying the
Then, the genetic potential rectangles are summed and average total organic carbon (TOC) content of the source
divided by the thickness of the net source rock interval to rock, the thickness of mature source rock, and certain
get the average genetic potential. maturity scaling factors. Although the method of
For source rocks that are immature across an entire Dembicki and Pirkle (1985) can be useful for mapping
area, the acquisition of SPis fulfills no useful purpose regional trends from a given source rock in a well-known
because hydrocarbon generation has never taken place in basin, it is not suited for making valid comparisons of
the subsurface. SPI calculations for a given source rock different source rock sections from basins around the
have relevance only where a thermally mature source world because it does not take the variability of kerogen
rock has been identified. The well penetrations or types into account. In contrast, our SPI ranking scale has
measured outcrop sections selected for SPI determina­ been developed for the primary purpose of making these
tion should be located within or immediately adjacent to global comparisons. We have made the ranking of very
the pod of mature source rock. Since SPI is a measure of different source rocks possible by using petroleum
petroleum potential, the magnitude of SPI at a given potential, rather than relative units of effectively realized
location is optimal when the source rock is immature to generation, and by expressing source rock richness in
early mature. A source rock that has reached a "middle terms of the genetic potential (i.e., Rock-Eval 51 + 5 2
oil window rank" or higher maturity shows a signifi­ yield) rather than TOC. Our use of the genetic potential
cantly reduced SPI because the average genetic potential instead of TOC allows for a fundamentally viable
is depleted due to oil expulsion. In certain cases, theoret­ ranking of the petroleum potential of diverse source
ical estimates of the initial SPI of a mature source rock rocks containing dissimilar kerogen types. Thus, a lean
prior to maturation can be calculated using the average gas-prone source rock containing type III kerogen but
genetic potential from an equivalent organic facies having substantial thicknesses (e.g., Tertiary sequence of
located in an area where it is less mature. At a given the Niger delta or Jurassic source rock of the
location, the difference between the initial SPI (when the Barrow-Dampier basin) can be compared on the same
source rock was immature) and present-day residual SPI SPI ranking scale to oil-prone source rocks containing
(source rock is mature) can provide a rough estimate of type I or II kerogen and having lower stratigraphic thick­
the amount of oil expelled from the column of source nesses (e.g., Upper Jurassic-Lower Cretaceous source
rock under 1 m2 (in metric tons HC per square meter). rocks of the North Sea). SPI can also be used for mapping
Each SPI is usually obtained from an individual explo­ regional variations in a given source rock, as discussed
ration well or measured outcrop section; thus, the elsewhere.
resulting value is representative of the source unit at a
specific location in the area of interest. Some source rocks Applications of SPI to Exploration
show nearly uniform richness and thickness over long
distances. Therefore, an SPI determined for an immature A relative source rock ranking system has been
source rock in one area is representative of the original developed by compiling the average SPis of individual
SPI of a thermally mature, lateral equivalent of the same source rocks from various basins (Figure 4.3; Table 4.1).
source rock. Although many other source rocks show For each source rock, the result listed in Table 4.1 has
significant lateral variations in thickness and organic been determined by calculating SPI at as many locations