Sequence-Stratigraphic Controls on Subsurface Fluid Movement: An Example of Selective Charging of Stacked Hydrocarbon Reservoirs, West Fulton Beach Field, Texas

Paul R. Knox

A study of 41 stacked Frio Formation barrier/strandplain oil and gas reservoirs within West Fulton Beach field, Aransas County, documents the effect of 3rd-, 4th-, and 5th-order flooding surfaces on the vertical migration of hydrocarbons and the selective charging of stacked reservoir sandstones. Flooding surfaces were identified from regional, high-resolution stratigraphic analyses constrained by existing barrier/strandplain depositional-facies and facies-tract models. Recognition of flooding surfaces within a field or prospect area aids in predicting the likelihood of a reservoir's having been charged with hydrocarbons and may identify the dominant hydrocarbon phase in a prospective reservoir.

Vertical trends of total hydrocarbon production, gas production, and gas/oil ratios in West Fulton Beach field define cyclic patterns related to the stratigraphic framework. The dominant pattern is an upward increase in both total producible hydrocarbons (oil plus gas, in BOE) and total gas production in 4th-order sequences below 3rd-order flooding surfaces. A secondary pattern is an upward increase in the gas-oil ratios of individual reservoir sandstones (5th-order sequences) below 4th-order flooding surfaces. These patterns suggest that (1) 3rd-order flooding surfaces seal vertical migration of both oil and gas along a bounding fault, presumably because of increased shale content in fault gouge, (2) 4th-order flooding surfaces are gas-selective baffles within a 3rd-order sequence an distribute oil and gas under the main seal, and (3) 5th-order flooding surfaces act as baffles to selectively fill stratigraphically higher reservoirs in a 4th-order sequence with more gas than oil. Potential oil and gas exploration and deeper-pool targets should be evaluated within a sequence stratigraphic framework to more accurately predict the potential for hydrocarbon charging and the gas-oil ratio within prospective reservoirs.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995