ABSTRACT

Many Gulf Coast fields consist of multiple vertically stacked sandstones in which oil and gas are seemingly randomly distributed stratigraphically. Hydrocarbon entrapment is strongly affected by seal competency and possibly by formation pressure, and these factors are in turn controlled by the characteristics of the interbedded shales. In West Fulton Beach field, Aransas County, Texas, the Oligocene, Frio Formation shales were deposited as shelf mudstones and represent flooding events in a barrier bar/strandplain and inner shelf setting. Reservoirs were placed into a high-frequency genetic stratigraphic framework to test the theory that hydrocarbon entrapment is controlled by a hierarchy of maximum flooding surfaces. Cumulative oil and gas production and gas-to-oil ratios were tabulated for individual reservoirs, as well as for all reservoirs in each 5th-, 4th-, and 3rd-order genetic unit, as measures of entrapment.

Total producible hydrocarbons (oil plus gas, in barrels of oil equivalent) increase in volume in successively shallower 3rd-order Frio units (each 800 to 1,000 ft thick), capped by the 600-ft-thick Miocene Anahuac Shale. Likewise, successively shallower 4th-order units (120 to 200 ft thick) within each 3rd-order unit contain greater volumes of total hydrocarbons, as well as greater percentages of gas. This pattern exists independent of shale thickness or reservoir porosity, and it is repeated at the 5th- and 6th-order levels, when viewed on a per-gross-ft-of-sandstone basis. Thus, although appearing random, when evaluated carefully within a stratigraphic framework, total hydrocarbon volumes and oil versus gas distributions follow a systematic pattern tied to their position within a stratigraphic hierarchy.

This finding can be used to more accurately constrain seal risk in exploration or deeper pool drilling and to evaluate hydrocarbon type ahead of the drill bit. Further study is needed to model reservoir filling, incorporating reservoir pressures, to better understand the petrophysical controls on hydrocarbon entrapment. Additionally, observations of lateral changes in the observed cyclic patterns may produce a greater understanding of the effects that facies changes and genetic unit stacking patterns have on seal competency.

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