Big Hill field is in the Frio sand trend on the western flank of the Big Hill salt dome. Multiple reservoirs in Miocene and Oligocene sandstones are on the downthrown side of a regional, up-to-the-coast growth fault across which early Miocene and older sediments increase in thickness by 57%. One reservoir in the Oligocene Hackberry is bounded by two growth faults and an unconformity (Hackberry unconformity). The hydrocarbons trapped in this reservoir evidently were generated within the surrounding rocks. An isopachous map of the interval between the top of the Frio and the Hackberry unconformity indicates that growth of the Big Hill salt dome occurred prior to the close of Frio time, and that the crest of the dome was north of the present-day salt spine. This map also sug ests the presence of a buried, down-to-the-coast growth fault which traverses the western flank of Big Hill field but which does not intersect any wells.

Pressure-performance histories of two reservoirs and of two wells producing from other reservoirs are shown graphically in order to illustrate the problems involved in explaining wells that are in pressure communication. Four gas-fluid contacts in a continuous Marginulina sandstone reservoir differ in elevation by 600± ft. These original gas-fluid contacts were established by the migration of hydrocarbons into a complexly faulted area. Accumulation of oil downdip from these gas-fluid contacts can be explained reasonably in terms of gravity-segregation effects.

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