Bimodal Porosity in Oolitic Reservoir--Effect on Productivity and Log Response, Rodessa Limestone (Lower Cretaceous), East Texas Basin

Abstract

Running Duke field, Houston County, Texas, is on a small domal structure in the East Texas basin. The field currently consists of five gas wells producing from the Lower Cretaceous Rodessa limestone. Seven additional gas completions have been plugged. The Rodessa yields water-free gas production from zones with water saturations in excess of 60%. Porosity ranges from 7 to 14%. The depositional model depicted for the Rodessa in this field starts with the development of an active organic buildup. This was successively overlain by ooid-skeletal-peloid lime sands, lagoonal lime muds, and subsequently by anhydrite and shale. Six lithofacies, based on core study, were recognized for the Rodessa. Each facies has a uniform diagenetic history in the field, making porosity development predictable for each facies. The one grainstone facies forms the reservoir in the field and has two distinct diagenetic sequences; one consists of a marine isopachous cement followed by a later partial infilling of pore space by sparry calcite. The resulting intergranular porosity has a unimodal pore-size distribution. In the other sequence, a discontinuous meteoric phreatic cement is followed by sporadic patches of sparry cement. The resulting pore-size distribution is bimodal with intergranular macropores and intercrystalline micropores within peloids and micritized ooids. Water-free gas production is limited to the rock with the bimodal pore system: macropores hold movable gas and micropores hold bound water within grains. Where core data are not available, cross-plotting techniques using geophysical logs are suggested as methods for separating the productive from the nonproductive grainstone.