"Porosity pod"; concepts for successful stratigraphic exploration of fine-grained sandstones

Sandstones in the Schuler Formation of the Cotton Valley Group (Upper Jurassic) and the Hosston Formation (Coahuilan Cretaceous) have yielded commercial production of gas and condensate in north Louisiana, East Texas, and southwestern Arkansas since the middle 1940s. Exploration for these reservoirs was confined to anticlines, fault closures, and up-dip reentrants of blanket strandline sandstones. Geologic work resulted in the "porosity-pod" hypothesis, which was the basis for drilling where conventional structures were not present. Commercial production from wells drilled on this basis substantiates this hypothesis. The widespread distribution of test wells makes possible the application of this approach wherever porosity pods are present in favorable sediments. Porosity-pod reservoirs exist in areas of monoclinal dip because of local depositional variations within lenticular sandstones. Gas and condensate are present either throughout the pod or above a water contact. In areas examined, porosity pods are present at random throughout the approximately 3,500 ft of the combined Schuler and Hosston Formations. Optimum trends for porosity-pod development are mapped from results of computer analysis of digitized sonic logs. The total thickness of sandstones and the number of discrete porous pods meeting criteria for porosity and minimum thickness are used as trend indicators. Zones are selected for testing on the basis of cross plots of effective porosity versus water saturation determined by computer analysis of digitized BHC sonic-gamma-ray and dual laterologs. Commercial production from Hosston and Cotton Valley sandstones requires a synergistic drilling, logging, evaluation, and completion program. Commingled multiple-zone completions are usually necessary to obtain adequate deliverability rates.