Rock Types, Pore Types, and Hydrocarbon Exploration: ABSTRACT

Abstract

A proposed exploration-oriented method of classifying porosity in sedimentary rocks is based on microscopic examination of cores or cuttings. Factors include geometry, size, abundance, and connectivity of the pores. The porosity classification is predictive of key petrophysical characteristics: porosity-permeability relationships, capillary pressures, and (less certainly) relative permeabilities. For instance, intercrystalline macroporosity typically is associated with high permeability for a given porosity, low capillarity, and favorable relative permeabilities. This is found to be true whether this porosity type occurs in a sucrosic dolomite or in a sandstone with pervasive quartz overgrowths. This predictive method was applied in three Rocky Mountain oil plays. Subtle pore throat traps could be recognized in the J sandstone (Cretaceous) in the Denver basin of Colorado by means of porosity-permeability plotting. Variations in hydrocarbon productivity from a Teapot Formation (Cretaceous) field in the Powder River basin of Wyoming were related to porosity types and microfacies; the relationships were applied to exploration. Rock and porosity typing in the Red River Formation (Ordovician) reconciled apparent inconsistencies between drill-stem test, log, and mud-log data from a Williston basin wildcat. The well was reevaluated and completed successfully, resulting in a new field discovery. In each of these three examples, petrophysics was fundamental for proper evaluation o wildcat wells and exploration plays. End_of_Article - Last_Page 845------------