Page 108 - The Petroleum System From Source to Trap

102 Peters and Cassa

late petroleum from larger drainage areas compared to Lawrence, 1990; Hester et al., 1990). These methods are
vertically drained systems. For this reason, lower limits most reliable within small areas where wireline response
are used to define SPI categories for laterally drained has been calibrated to geochemical data.
systems (low, SPI < 2; moderate, 2 � SPI < 7; high, SPI ;:: Geochemical logs for eight exploratory wells are
7) than for vertically drained petroleum systems (low, included to show their usefulness for detecting free
SPI < 5; moderate, 5 � SPI < 15; high, SPI ;:: 15) (see figure hydrocarbons and identifying source rocks. The first
4.4 of Lewan, Chapter 4, this volume). three geochemical logs (Figures 5.4-5.6) are from three
SPI is a measure of the petroleum potential of a source wells (1, II, and ill) that are in the same area and demon­
rock and ideally is determined from thermally immature strate the lateral continuity of two different source rocks.
rock. After a source rock shows a favorable SPI rating, The last five geochemical logs (Figures 5.7-5.11) are from
maps of SPI and thermal maturity are used to evaluate wells that are in different areas, but are used as examples
which areas of a basin have the highest petroleum of different ways to identify and evaluate a source rock.
charge. Areas with the highest charge are most likely to
be nearest the source rock where it is the most thermally Wells I through III
mature, or nearest the pod of active source rock. The high-quality geochemical log for well I is based
Conversely, areas most likely to have the lowest charge on closely spaced Rock-Eva! pyrolysis and TOC data
are farthest from the mature source rock, or farthest from supplemented by vitrinite reflectance (Figure 5.4).
the pod of active source rock. Whether this charge is Closely spaced samples allow a critical evaluation of
mostly gas or mostly oil is determined from the kerogen source and reservoir rock intervals (note the wider
type and maturity. Demaison and Huizinga (1991; sample spacing in the C formation, a Lower Cretaceous
Chapter 4, this volume) provide a complete discussion of reservoir rock). The penetrated section contains two
migration drainage and entrapment styles for different
petroleum systems and show how to estimate the SPI for source rocks. The Upper Cretaceous B formation source
source rocks, even when they have undergone thermal rock interval at 780-1540 m is a potential source rock that
has the capacity to generate significant quantities of oil
maturation beyond the immature stage.
(SPI = 42 t HC/m2). The T max versus depth trend is
slightly depressed through this interval, probably
Mass Balance Calculations because this sulfur-rich kerogen undergoes thermal
degradation at lower temperatures than many type II
Mass balance calculations, either by accumulation (or
prospect) or petroleum system, can be used to provide kerogens. Because the Lower Cretaceous is at maximum
another comparison of the amount of petroleum burial depth, the F formation source rock at 3120-3620 m
generated with the amount that has accumulated. The is an active source rock that is presently generating oil
geochemical data for screening can also be used for SPI (SPI ;:: 8 t HC/m2). The production or productivity index
calculations (Demaison and Huizinga, Chapter 4, this (PI) gradually increases below about 3200 m, reflecting
volume) and for mass balance calculations as suggested the onset of generation, which is also indicated by T max
by Schmoker (Chapter 19, this volume), whose technique and Ra data. Vitrinite is generally absent in the carbonate
is used in many of the case studies in this volume. section and in the strata containing particularly
hydrogen-rich kerogen. PI anomalies (e.g., at 100--600 m
and 1600-3050 m) are "mathematical artifacts" caused by
EXAMPLES relatively low Sz yields where S1 yields may be slightly
Geochemical Logs elevated by small quantities of organic drilling additives
or minor shows. The F formation penetrated in well I is
Geochemical logs are among the most valuable tools presently an active source rock.
for basin analysis, yet few examples are given in the liter­ The geochemical log for well II, which is located in the
ature (e.g., Clementz et al., 1979; Espitalie et al., 1977, same basin about 120 km southeast of well I (Figure 5.5),
1984, 1 9 87; Peters, 1986; Magoon et al., 1 9 87, 1 9 88). shows that the Upper Cretaceous potential source rock is
Proper use of geochemical logs allows identification of thicker than in well I. This potential source rock is still
the following features in penetrated intervals: immature and shows a similar source potential index
(SPI = 40 t HC/m2) to that in well I. The Lower Creta­
• Occurrence of potential, effective, and spent as well ceous source rock in well II is thicker and shows more
as active and inactive source rocks discrete zones of higher and lower source potential than
• Main stages of thermal evolution: diagenesis in well I. The total thickness of the Lower Cretaceous
(immature), catagenesis (mature), and metagenesis interval in well II is 700 m, but the net source rock
(postmature) zones thickness is only about 550 m and shows an SPI of 25 t
• Occurrence of varying amounts of in situ and HC/m2. Only the deeper portions of the Lower Creta­
migrated petroleum ceous source rock are actively generating petroleum
(because the onset of petroleum generation for this
When geochemical logs are unavailable, geophysical source rock occurs at 0.6% Ra). Stratigraphically equiva­
wireline logs and interpretive techniques can be used as lent Lower Cretaceous source rocks buried more deeply
qualitative indicators of organic content (e.g., Passey et adjacent to this trap are the probable source for hydro­
al., 1 9 90; Schmoker and Hester, 1 9 83; Stocks and carbon shows in the Lower Cretaceous sandstone in well