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

Hugh J. Mitchell-Tapping

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 Schneider and Corey 1 E. M. Strode well. This well initially tested 491 BOPD from a 34-ft interval at 9,519 ft. The field has an oil-water contact at subsea 9,090 ft and a gas-oil contact at subsea 8,980 ft. Ten wells were originally drilled in the 800-ac field, but only six oil and one gas wells were put on production. The estimated in-place reserves in 1969 were 6.4 million bbl oil, 1.3 million bbl condensate, and 14.97 bcf gas, with an estimated primary recovery of 22%. The total cumulative oil production to 1986 was 3.19 million bbl oil. The grainstones of the producing int rval are composed of ooids, pisoids, pelecypod fragments, grapestones, and fecal pellets. This shallow marine bar deposit has been extensively leached and cemented. Fracturing and brecciation have 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 have 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 gr ater than had been initially calculated.

AAPG Search and Discovery Article #91029©1989 AAPG GCAGS and GC Section of SEPM Meeting, October 25-27, 1989, Corpus Christi, Texas.