Page 16 - The Petroleum System From Source to Trap

8 Magoon and Dow

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Perrodon's Petroleum System Demais n 's Generative Basin
Perrodon (1980, 1 9 83a,b) and Perrodon and Masse In Demaison (1 984, p. 1), the term generative basin is
(1984) first used the term petroleum system. Since defined as follows:
Perrodon (1 980, 1 9 83b) are in French, and the same
material is covered in a revised and updated version Areas underlain by mature source rocks are called "petroleum
(Perrodon, 1983a) and in Perrodon and Masse (1 984), generative depressions" or "hydrocarbon kitchens." A "genera­
which are in English, we quote these latter publications. tive basin" is defined as a sedimentary basin that contains one
or more petroleum generative depressions. Mapping generative
1
Perrodon (1983a, p. 8 7) states that depressions is achieved by integrating geochemical data
relevant to maturation and organic facies with structural and
The geologic criteria governing the distribution of pools, and in
particular, the combined presence of source rocks, reservoirs stratigraphic information derived from seismic and deep wells.
and seals, generally exhibit a certain geographic extension
which is reflected by the formation of a family of pools, or even Demaison (1984, p. 1 ) describes the success ratios in
better, a petroleum system, a structured set of natural elements exploration of petroleum provinces:
of the same species or having the same function. From the
geographic standpoint, and according to their dimensions and Locales of high success ratios in finding petroleum are called
complexity, these sets are reflected by the existence of a "areas of high potential," "plays," or "petroleum zones." A
petroleum zone or province. rapid worldwide review of 12 sedimentary basins, described in
order of geotectonic style, reveals the following regularities:
In Perrodon and Masse (1984), petroleum system is used 1. The zones of concentrated petroleum occurrence ("areas
in the title and they define it (p. 5) as follows: of high potential") and high success ratios are genetically related
to oil generative depressions or basins. These depressions are
Therefore, a petroleum province can be considered as the final mappable by integrated methods (geology, geophysics, and
result of an organized set of geologic events (in space and in geochemistry).
time) that can be called a petroleum system. In such a system, 2. The largest petroleum accumulations tend to be located
the sequence of subsidence movements and associated flows is close to the center of the generative basins or on structurally
just as decisive as lithologic and geometric factors in the high trends neighboring deep generative depressions.
formation of a group of pools. This concept of the succession of 3. Migration distances commonly range in tens rather than
geodynamics and sedimentary processes which affect hundreds of miles and are limited by the drainage areas of indi­
petroleum potential is developed, and specific examples of vidual structures. Thus the outlines of generative depressions
petroleum systems from the North Sea, the Arabian Platform commonly include most of the producible hydrocarbon accu­
and the Congo Basin are presented. mulations and the largest fields. Unusual cases of long distance
migration are documented on certain foreland basin plates
Concerning basin geodynamics, Perrodon and Masse where stratigraphy and structure permitted uninterrupted
(p. 5) go on to say that updip movement of oil.
These three regularities provide powerful analogs for fore­
In a sedimentary basin it is not only the source rocks, reservoirs
and seals, but the whole sedimentary column which plays an casting areas of high petroleum potential in undrilled or
active and decisive role in the genesis, entrapment, and conser­ sparsely drilled basins.
vation of hydrocarbons. The formation of a petroleum system is Demaison (1984, p. 3) continues his discussion of the
the result of a succession of physical and chemical transforma­
tions (diagenesis, tectonic deformations, compaction, etc.) which generative basin concept by stating that
affect these sediments and closely control the genesis, concentra­
tion and dispersion of hydrocarbons. Important factors which Recognition of generative depressions is achieved by overlaying
control these transformations and even initiate them are the organic facies maps and maturation maps of each key
movements of uplift and subsidence. We will stress the condi­ petroleum source horizon. Maturation maps are compiled from
tions which affect the genesis and growth of these movements, seismic depth maps, near the potential source horizons, and
and note that together they conform to a small number of basic from maturation gradients derived from well data and cali­
.
mechanisms: tectonics, heat flow and gravity. brated time-temperature models . . . Organic facies maps reflect
the stratigraphic distribution of organic matter types within a
Their paper then discusses thermotectonic areas, given source rock unit. They are compiled by integrating
subsidence, sedimentation rates, and sedimentary and kerogen type data in the known paleographic and paleooceano­
climatic factors. With regards to petroleum systems and graphic context.
The geochemical approach, in prospect appraisal, begins by
provinces, Perrodon and Masse (1984, p. 1 8 ) state the investigating whether mature source beds are present in the
following:
drainage area of a trap. A further step consists of mapping areas
of mature source beds and calculating both mature source rock
The petroleum potential of a basin (the formation and preserva­
tion of hydrocarbon pools) is the result of the organization of the volumes and petroleum yield. Lastly, migration pathways can
sedimentary volume and of its evolution in time. Furthermore, be modeled between the mature source-rocks and the trap. This
special attention must be paid to the characteristics and relation­ type of geologic exercise permits a ranking of prospects by the
ships of the flows passing through the sedimentary space: geo­ criterion of degree-of-access to mature source rocks.
thermal flows rising from the mantle and the crust, and flows of The geochemical approach to basin evaluation consists of
the different fluids circulating due to differences in pressure and mapping oil generative depressions or basins and erecting a
available pathways. In the final analysis, all these transfers of matrix of drilling success ratios, volumes of discovered hydro­
energy and fluids themselves appear to be controlled by the carbons and "kitchen" potential. When these correlations have
geodynamics of the basin, i.e., by the characteristics of subsi­ been established they may be used for comparative purposes
dence, whose mechanisms are reflected by different types of and for future evaluation of geologic risk. Application of the
petroleum systems. Some examples follow. "generative basin concept," leading to recognition and predic­
tion of areas of high potential, is the object of this contribution.
These examples include rift basins, platform basins,
passive margin basins, and pull-apart basins. Demaison (1984) makes the following points. First, the