Pitfalls in Geological Mapping Within Unconventional Plays

Abstract

Historically geologists have identified oil and gas prospects by mapping a conventional reservoir's water saturation (Sw), porosity (Phi) and thickness (H). These three measurements proved to be reliable predictors of reservoir productivity and could be integrated into a single calculation, the SoPhiH map. In conventional plays a SoPhiH map can be used to quickly identify sweetspots. In tight oil plays, however, SoPHiH maps can mislead operators as to where the highest yielding reservoirs are due to the complex nature of unconventional reservoirs and the necessity to hydraulically stimulate the rock. One example is the first bench of the upper Three Forks in the Williston Basin, where core helium porosities and water saturations, when averaged over the entire reservoir interval, are relatively consistent over large areas. Since averaged Sw and Phi do not change significantly, areas with the greatest reservoir thickness calculate the highest SoPhiH. Operators have targeted areas with the thickest first bench but unfortunately the zones with the highest calculated SoPhiH ultimately proved to have some of the poorest production. This is due in part to the variability of different lithologies within the Three Forks and associated effective porosity and permeability. Careful identification and separation of lithologies and facies within the first bench and measurements of reservoir properties such as capillary pressure and brittleness within each facies are critical in identifying potential sweetspots. Unconventional production is impacted by many factors that are not included in SoPhiH maps and these maps should be used with care.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016