Effect of Fractures on Reserve Calculations as Determined by Petrology: Birthright Field, Hopkins County, Texas: ABSTRACT

Abstract

ABSTRACT Birthright field in Hopkins County, Texas produces oil and gas from Jurassic carbonate grainstone deposits of the upper Smackover Formation. The field was discovered in 1965 by the drilling of the well Schneider and Corey #1 E. M. Strode. This well initially tested 491 BOPD from a 34 ft interval at 9519 ft. The field has an oil-water contact at subsea 9090 ft and a gas-oil contact at subsea 8980 ft. Ten wells were originally drilled in the 800 acre field but only 6 oil and 1 gas wells were put on production. The estimated inplace reserves in 1969 were 6.4 MMBO, 1.3 MMBC, and 14.97 BCF, with an estimated primary recovery of 22%. The total cumulative oil production to 1986 was 3.19 MMBO. The grainstones of the producing interval are composed of ooids, pisoids, pelecypod fragments, grapestones, and fecal pellets. This shallow marine bar deposit has been extensively leached and cemented. Fracturing and minor brecciation has taken place causing many grains to be broken and sheared. Many grains have been coated by early diagenetic fibrous rim-cement indicative of leaching in vadose and phreatic conditions. These conditions have caused significant variations in the structural conditions and sizes of the coated grains. Anhydrite and sparry calcite has subsequently filled many of the pores, especially below the oil-water contact. It is thought that these factors may have affected actual recoverable reserves, and that petrology can be an effective tool in reservoir evaluation especially where recovered reserves are greater than had been initially calculated.