Resource Optimization...Characterization of Downdip Frio Shoreface/Shelf Sandstone Reservoirs: Red Fish Bay Field, South Texas

Located in Nueces County, 5 mi southeast of Aransas Pass in Corpus Christi Bay, Red Fish Bay field has yielded more than 19 MMbbl of liquid hydrocarbons and 103 Bcf of gas from more than 50 stacked Frio Sandstone reservoirs since field discovery in 1950. This study forcused on delineating bypasssed hydrocarbons in 19 upper Frio reservoirs. Red Fish Bay field exemplifies the tremendous bypassed hydrocarbon potential in supposedly depleted sandstone reservoirs of the Downdip Frio BarrierIStrandplain Sandstone on the San Marcos Arch play in SouthTexas. The field is located in Nueces County, 5 mi southeast of Aransas Pass in Corpus Christi Bay. Since field discovery in 1950, Red Fish Bay field has yielded more than 19 million barrels (MMbbl) of liquid hydrocarbons (oil and gas condensate) and 103 billion cubic feet (Bcf) of gas from more than 50 stacked Frio sandstone reservoirs. The trapping mechanism is an anticlinal closure on the footwall of a regional growth fault, in addition to a facies change to more shaly sediments to the east and southeast. An additional 640 Bcf of gas and 395 thousand barrels (Mbbl) of gas condensate have been produced from the more laterally extensive Common 10 reservoir, which extends to Mustang Island field, 3 mi southeast of Red Fish Bay field. This study focused on delineating bypassed hydrocarbons in upper Frio reservoirs in Red Fish Bay field. The 19 reservoirs targeted by this study account for 80 percent of total field production, although these reservoirs were recently abandoned in favor of deeper Frio targets. The upper Frio sandstone reservoirs are 7,300 to 8,500 ft deep and consist of stacked and amalgamated shoreface to inner-shelf sandstones interbeddedwithshelfsiltstones andmudstones. Stratigraphic analysis, geological mapping, and evaluation of well-log and production data document underexploited regions within most of the oil and gas reservoirs. Although most of the 182 exploration and development wells have only old electric logs available, 8 wells of post- 1970's vintage have complete logging suites. These eight key wells were subjected to detailed petrophysical analysis. Analytical results were compared with available coredata to corroborate the log interpretations, and a log response data base was established to allow extrapolation of the petrophysical analysis to the remaining older wells throughout the field. Mapping of log-derived porosity and hydrocarbon saturation values confirms the bypassedhydrocarbon opportunities delineated by geologic mapping. Moreover, the eight key wells targeted deeper Frio pay and postdate abandonment of the upper Frio reservoirs. Analysis of these logs confirms that bypassed hydrocarbons remain within the upper Frio reservoirs, and 21 upper Frio recompletion opportunities were identified in currently active wells. During late 1997, the operator acted on one of our recommended recompletion opportunities. This well tested at 200 barrels per day (bbl/d) and was completed to produce 80 bbl/d. The additional production from this well also represents an increase of more than 200 percent from the 60-bbl/d total field production at the time of this study. Average recovery efficiencies are 12 percent for oil reservoirs under primary depletion, 25 percent for waterflooded oil reservoirs, and nearly 50 percent for underexploited gas reservoirs. Conservative estimates of the remaining hydrocarbon potential indicate that 37.5 million stock-tank barrels (MMSTB) of oil remains in the 17 nonwaterflooded oil reservoirs exclusive of zones 5-A and 5-B and 14 Bcf of gas remains in the underexploited (<85 percent recovery) gas reservoirs. The better developed oil and gas reservoirs have attained primary recovery efficiencies of approximately 25 and 90 percent, respectively. Using these primary recovery factors, recommended recompletion and infill drilling strategies are expected to add an estimated 7 MMSTB of oil and 11 Bcf of gas to the ultimate recovery from Red Fish Bay field. An estimated 52 MMSTB of mobile oil will remain in all of the studied oil reservoirs after attainment of these recovery factors. Implementation of secondary and tertiary recovery programs should further increase ultimate hydrocarbon recovery.