Basin Description and Classifications of Petroleum Reservoir

Because of the burial and temperature requirements needed for the maturation of organic matter, most petroleum will be found in sedimentary basins. Sedimentary basins are depressions on the earth's surface, caused by subsidence, that receive greater-than-average sediment thicknesses.

Most basins have sediment fills in excess of 2 kilometers, and some may contain 10 or more kilometers of sedimentary rock. This is usually sufficient for at least part of their contained organic matter to mature to petroleum.

However, being within the "oil window" is not enough. The petroleum richness of sedimentary basins, or even the presence of petroleum at all, is also highly dependent on source rock and reservoir development, migration pathways, geothermal regime, style and timing of trap development, and the presence of good sealing lithologies. The age of the sedimentary rocks within a basin is also of some importance.

Even though petroleum reserves can be found in rocks of all ages, most giant fields and most of the world's reserves occur in sequences, of Late Mesozoic and Cenozoic age (Figure 1). Paleozoic rocks probably had potential to generate hydrocarbons equal to that of these younger rocks, but there has been more time in which to destroy all or part of the petroleum through uplift and erosion (Halbouty et al, 1970).

Figure 1

Petroleum enrichment, the incidence of giant fields, and the habitat of petroleum within sedimentary basins can be related to structural, sedimentological, and

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geothermal settings, which can be used to describe a number of petroleum basin types.

There are several general ways in which sedimentary basins can be grouped (Figure 2).

Figure 2

They can be divided on the basis of their underlying material or crust:

continental crust, which is relatively light, granitic and underlies most continental areas; or,

intermediate crust, compositionally between granite and basalt and occurring along continent-ocean margins.

They may also be grouped according to the stability and movement of this underlying crust, as either;

cratonic basins, developed on the stable parts of continents away from continental margins;

divergent-margin basins, formed along continental margins where the sea floor is spreading and rift-drift (extensional) movements occur; or,

convergent-margin basins, formed along continental margins where continents and/or oceans are in collision and some ocean crust may be consumed.

For the purpose of petroleum exploration, however, we need a finer-tuned classification scheme such as the ten-part basin classification scheme based on the work of Huff (1980) and Klemme (1980), which is summarized in Figure 3 .

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Figure 3

Worldwide reserves can be related to their location within a petroleum basin, regardless of its basin type (Figure 4).

Figure 4

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Most petroleum is found along a basin's flanks, either along hinges that mark the break between the basin and normal sediment thicknesses of the shelf, or along mobile rims. A sizeable amount of petroleum, about 18%, also occurs in extrabasinal settings. For example, in the central United States, a regional stratigraphic high that received thinner-than-average sediment persisted for most of the last 600 million years. Yet this region, the Cincinnati arch, is a major petroleum province and has some giant field production.

Oil usually becomes lighter and gas more abundant with depth in most sedimentary basins. Oil also becomes lighter and gas more prevalent laterally toward a basin's center. The heaviest crude is typically found along basin margins. This lateral and vertical distribution of oil and gas is of considerable importance to exploration. Part of this pattern may be attributed to increased thermal maturation with depth. However, another explanation is that the lighter gas displaces earlier formed oil that had already accumulated in the trap (Gussow, 1954). When the trap becomes full to its spill point, the oil is displaced and moves upward toward the basin's flanks.

Exploration of a Petroleum Basin

Petroleum exploration can be divided into a series of critical information phases. With each step, there is a progressively increasing data base, from which to evaluate the petroleum prospects of a region.

Phase I is the stage of early surface mapping and reconnaissance geophysics (Figure 1).

Figure 1

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It begins with the unexplored basin. To varying degrees, there may be some previous knowledge of surface geology and structures. There may also have been reports of surface indications (e.g., surface seeps, springs, asphaltic vein-fillings, gas detected in water wells, etc.) to encourage the exploration. Surface evidence of petroleum has been important in the discovery of nearly every major onshore petroleum province in the world (Levorsen, 1979), although there are also major areas with abundant surface evidence that have proven to be subcommercial (e.g., Cuba and Morocco).

At this stage, the geologist's role is to obtain a more detailed knowledge of surface structures (i.e., potential traps) and evaluate other aspects critical to the exploration task, such as sedimentary facies, continentality, and possible metamorphism. The exploration geologist must work closely with the geophysicist to relate the surface stratigraphic and structures to the subsurface. At this stage, a geologic analog is often used to compare the unexplored basin to other producing "look-alike" basins which appear to have common geologic characteristics.

Phase II is the stage of seismic survey (Figure   2 ).

Figure 2

(This is the initial step, in offshore exploration.) During this stage, more data is obtained on the depth configuration of potential traps and hopefully some knowledge of the character and volume of the sedimentary fill is gained. It has generally been observed that the chances of finding commercial oil is roughly in proportion to the total sediment volume (Levorsen, 1979), particularly if most of it lies within the depth range of the oil and gas window (Klemme, 1980). The volume of subsurface shale (source potential) is also evaluated.

Phase III is the stage of exploratory or "wildcat" drilling, which establishes for the first time a detailed sampling of the sediment character (reservoir, source and caprock potential), maturation, and the geothermal regime (Figure   3 ).

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Figure 3

The potential for a discovery exists at this stage, since the most promising prospects, usually surface or seismically detected subsurface structures are drilled first. However, even a dry hole is not necessarily a total failure. It can supply a large amount of data (e.g., subcommercial shows; water-filled reservoir downdip from a possible pinchout, etc.) that, if intelligently studied, may lead to the placement of new wildcat wells.

Phase IV, the discovery phase, follows the successful completion of some wildcat wells (Figure   4 ).

Figure 4

At this stage, reservoirs are established and hydrocarbon types may be linked to certain stratigraphic units and/or trap types. Further wildcat drilling in less developed parts of the basin may be guided in part by the play and petroleum zone concepts. A play is defined as a group of geologically similar, "look-alike" prospects, usually at fixed horizons sharing common stratigraphic features (lithology, unconformity). A

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basin may also be divisible into discrete petroleum zones. These are sediment volumes whose contained pools show several characteristics in common. Application of the play and petroleum zone concepts usually causes the success ratio of drilling (discovered fields/tested prospects; or bbls. found/thickness drilled) to improve during the discovery stage. Many of the basin's largest fields will have been discovered, and exploration for more subtle traps may commence.

Phase V, the production phase, begins to provide exploration geologists with reserve estimates and a history of the hydrocarbon potential of the basin ( Figure   5 ). There is enough information to work out field-size distribution patterns, which may help guide further exploration as the area matures. Both the field size of new discoveries and the success rate of drilling typically tapers off during this stage.

Figure 5

Commonly, not all of a sedimentary basin is at the same stage of drilling and development at the same time. Part of the basin may be maturely drilled, while other areas that may have appeared initially less geologically favored, or were less accessible, may still be only semi-mature or untested. Also, shallower depths may have been thoroughly tested and have established production, while at the same time deeper stratigraphic horizons may be only at the seismic survey or wildcat stages of development. It is significant that new discoveries are still being made in sedimentary basins where drilling and development have proceeded for 50 or more years.

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