Page 89 - The Petroleum System From Source to Trap

80 Demaison and Huizinga

exists between the magnitudes of the SPis of source rocks
and the associated petroleum reserves of that petroleum
system. Source rocks with high SPis, as defined in Figure
4.4, are statistically associated with large petroleum
reserves and production from giant oil and gas fields
(Figure 4.3; Table 4.1). In contrast, areas containing a
source rocks with low SPis, as defined in Figure 4.4, are
N'
associated with either small petroleum reserves and
production from subgiant oil and gas fields or are
nonproductive (Figure 4.3; Table 4.1). Although the use
Ill
! of SPI categories that shift with respect to the size of
� s-llfffit::'::::::::::·.:::_::::·: drainage areas (Figure 4.4) shows great promise as an
-....;.;�::::::::: : :::::::: operationally viable shortcut for estimating regional
charging capacity, we will continue to evaluate and
refine this sliding SPI scale as more data become
available.
In the genetic classification of a petroleum system, the
DRAINAGE AREA ability of a source rock to expel hydrocarbons is
Sman �--------------� Large
(common In vertically (common In laterally described by the charge factor (Figure 4.1), which semi­
drained systems) drained systems) quantitatively characterizes the total amount of hydro­
carbons available for entrapment (i.e., regional charge).
The three terms used to describe the charge factor (super­
Figure 4.4. Preliminary source potential index (SPI) charged, normally charged, or undercharged) are provi­
classification. sionally assigned according to the magnitude of SPI
(high, moderate, or low SPI, respectively, as defined in
as possible, within or adjacent to the pod of mature Figure 4.4).
source rock, and averaging these values. Although these After a source rock showing a favorable SPI rating is
averaged results are tabulated in numerical order from identified, it is economically important to delineate the
highest to lowest, only substantial relative differences are specific areas that have the highest charging capacity.
considered important in the SPI comparison of two or This is primarily achieved by the integration of SPI and
more source rocks. Some provinces contain more than thermal maturity maps. The evaluation of regional
one source rock, which are listed separately in Table 4.1 trends in hydrocarbon charging capacity is also
(e.g., the Miocene and Eocene-Oligocene source rocks of enhanced with the aid of paleogeographic maps, a
the San Joaquin basin), or some may not all appear due knowledge of source rock depositional models
to lack of data (see Talukdar and Marcano, Chapter 29, (Demaison et al., 1984), and a recognition of migration
this volume). styles. This information will help in interpolating the
In the appraisal of a petroleum system, the source charging capacity for areas occurring between locations
rock must be evaluated in the context of the overall struc­ of known SPI and possibly assist in extrapolating trends
tural and stratigraphic framework, which determines the away from measured SPI data points.
migration style. We have developed a preliminary SPI
classification (Figure 4.4), which has been empirically
derived from worldwide measurements of SPI and a QUALIFICATION OF
general knowledge of the discovered hydrocarbon MIGRATION-ENTRAPMENT
reserves that are likely to be derived from these specic
source rocks. In vertically drained petroleum systems, General Considerations
SPis are classified as low (SPI < 5), moderate (5 � SPI <
15), and high (15 � SPI). In contrast, laterally drained Because expulsion and migration mechanisms are
petroleum systems are significantly less demanding on relatively inefficient, the existence of petroleum accumu­
the magnitude of SPI. Traps in laterally drained systems lations requires that migrating hydrocarbons be focused
tend to focus migrating petroleum from larger drainage from a much larger volume of mature source rock into a
areas relative to those in vertically drained systems. much smaller trap. Lateral oil movement along the
Because areal extent is also critical in determining source carrier bed is an important migration mechanism.
rock volume, we tentatively propose lower limits to However, reservoir systems cannot function as efficient
define the SPI categories in laterally drained systems: lateral drains unless they are continuously overlain by
low (SPI < 2), moderate (2 � SPI < 7), and high (7 � SPI). unbreached sealing lithologies. These beneficial
So far, our database includes SPis from only a limited reservoir-seal associations are often regionally extensive
number of laterally drained petroleum systems (e.g., wherever marine transgressive sequences rest on marine
Central Arabia, Maturin, West Siberia, Illinois, Oriente, regressive or continental clastic sedimentary rocks.
and Williston). Thus, the SPI categories for laterally Widespread evaporitic sequences overlying reservoir
drained systems are subject to future refinement. units in carbonate platform systems can also function in
In extensively explored areas, a positive correlation the same favorable manner.