Page 82 - The Petroleum System From Source to Trap

74 Demaison and Huizinga

improve our hydrocarbon volume forecasting ability." source rock sequence is stratigraphically continuous,
This sobering opinion was echoed by Ulmishek (1986, p. thick, and widespread (Gonzalez and Holguin, 1992).
63): 'Tectonic basin classifications are limited in their This source rock was deposited in a deep water anoxic
effectiveness of rating the richness of basins." Grunau environment during a major marine transgression. In
(1987, p. 254) went even further in stating, "A classifica­ contrast, the South Florida basin contains thin Lower
tion system which is based on measurable relevant Cretaceous source beds that are erratically distributed,
criteria, and which is tuned to the needs of the explo­ both vertically and laterally. They were deposited in
rationist, has not yet been developed." The reason the ponded shallow water, hypersaline, anoxic "basins" that
analog approach does little to improve our hydrocarbon occurred inside the carbonate platform during a period
volume forecasting ability is that source rock volumes of overall low sea level. Furthermore, the geothermal
and richness are unrelated to tectonic style. In other gradients tend to be lower in the South Florida basin.
words, tectonically "look-alike" basins of similar size do
not necessarily contain identical volumes of mature Because source rock volume and richness are
source rock and, thus, do not possess analogous regional unrelated to tectonic style, we felt that a genetic classifi­
charging potential. cation scheme based on the processes of petroleum
Furthermore, Bois et al. (1982) and Ulmishek and formation, migration, and entrapment had to be devised
Klemme (1990) have documented that paleolatitudinal to supplement conventional tectonic classifications. The
and paleoclimatic considerations are more important geologic framework of a basin provides only the natural
than tectonic style in explaining the world's geographic setting for these physicochemical processes to interact in
distribution of prolific source rocks and major petroleum a manner that can lead to either concentration or disper­
reserves. For instance, they show that highly favorable sion of petroleum. The geologic history of a basin is the
paleogeographic factors led to the development of "program" that sequentially activates these physico­
prolific Upper Jurassic and middle-Upper Cretaceous chemical processes during the passage of geologic time
source rocks in the greater Tethyan realm, which is the (Perrodon, 1980).
earth's best endowed region in terms of petroleum Three important geologic factors control the accumu­
reserves (75% of the oil and 61% of the gas). The impor­ lation of petroleum in the subsurface and thus are
tance of source rock distribution, rather than tectonic essential to the existence of viable petroleum systems:
style, is illustrated by the following examples.
1. Volumetrically adequate petroleum generation,
1. In peri-Andean foreland basins, middle-Upper occurring during or after the time of trap
Cretaceous source rocks show an exponential decrease in formation;
petroleum potential from north to south. The tectonic 2. Favorable migration drainage geometry, leading to
style, which cau be classified as a foreland ramp, and the focused movement of hydrocarbons into traps
regional seals of each are similar. However, source rock rather than to dispersion and loss of hydrocarbons
richness and thickness, as well as discovered Cretaceous­ in either subsurface migration "waste zones" or up
sourced petroleum reserves, tend to wane from north to to the surface;
south. 3. The existence of volumetrically adequate traps,
2. Coastal basins along the Brazilian Atlantic margin capable of retaining their petroleum charge from
show unequal petroleum richness, despite extensive the earliest time of trap filling to the present day.
exploration. The Campos basin alone contains 65% of all
Brazilian reserves (Bacoccoli et al., 1989; Mohriak et al., Our plan is to classify genetically the main types of
1990), whereas the neighboring Espirito Santo and petroleum systems and to qualify and quantify, when­
Santos basins hold only minor reserves. The unequal ever possible, the key factors that control the occurrence,
occurrence of petroleum in Brazilian coastal basins is abundance, and habitat of oil and gas.
dependent on variations in the thickness and regional
distribution of Lower Cretaceous lacustrine source rocks
deposited in early rift settings. Otherwise, the tectonic THE PETROLEUM SYSTEM
style, the effectiveness of regional seals, and source rock
maturity levels are similar in all the basins along this A petroleum system is a dynamic petroleum generating
South Atlantic margin. and concentrating physicochemical system, functioning
3. Carbonate platform systems, whether associated in a geologic space and time scale. A petroleum system
with passive margins or foreland basins, also show requires the timely convergence of certain geologic
extreme variations in petroleum richness, ranging from elements and events essential to the formation of
prolific (e.g., Campeche-Reforma basin in Mexico and petroleum deposits (e.g., mature source rock, expulsion,
central Arabian basin) to marginal (e.g., South Florida secondary migration, accumulation, and retention)
basin and Adriatic basin). The differences in petroleum (Nijhuis and Baak, 1990). This definition is similar to
richness between the Campeche-Reforma basin and the earlier concepts of "source-reservoir oil system" (Dow,
South Florida basin are caused not so much by differ­ 1 9 74), "petroleum system" (Perrodon, 1 9 80, 1 9 83;
ences in tectonic styles but more by inequalities in source Perrodon and Masse, 1984; Magoon, 1987, 1988), "hydro­
rock volumes and basin geothermal parameters. In the carbon machine" (Meissner et al., 1984), and "indepen­
Campeche-Reforma basin, the prolific Upper Jurassic dent petroliferous system" (Ulmishek, 1986). A petroleum