ABSTRACT

Umbrella Point field exemplifies bypassed hydrocarbon potential in a mature Middle Frio Sandstone play in the nearshore Texas Gulf Coast. Located in Chambers County near the northern shore of Trinity Bay, the field has yielded 17 million barrels of oil and 100 billion cubic feet of gas from 18 stacked Frio sandstones since discovery in 1957. Most of the 36 wells were abandoned in the 1970's, and little has been done to optimize drainage patterns, to control the encroachment of water, or to prevent gas-cap expansion. Stratigraphic analysis and geological mapping, combined with analysis of individual well-production histories, were used to locate underexploited areas within these reservoirs. Two modern log suites and limited core data were used to define criteria for estimating original hydrocarbons in place and drainage areas.

The Middle Frio in the area consists of stacked distributary-channel, delta-front, shoreface, and shore-parallel bar sandstones interbedded with shales. Hydrocarbons are trapped in a rollover anticline in the footwall of a growth fault underlain by diapiric shale. Original hydrocarbons in place at Umbrella Point were calculated volumetrically at 60 million barrels of oil and 300 billion cubic feet of gas. Remaining recoverable hydrocarbon potential is estimated at 32 million barrels of oil and 173 billion cubic feet of gas. Average recovery efficiency is 28.2 percent for oil and 33.5 percent for gas. The best recoveries have been from the zones characterized by uniform sandstone deposits that were redistributed during small-scale transgressions between depositional cycles.