Reservoir Quality of the Three Forks Formation, Williston Basin — An Integration of Geologic and Engineering Data

Abstract

Wireline log and routine core analysis does not resolve the differential reservoir quality in the upper Three Forks reservoirs. This paper demonstrates the value of examining the qualitative geologic data in light of quantitative engineering data to delineate controls on reservoir quality. Five pairs of samples representing differential fluid saturations of several possible reservoir facies were described in core and petrographic thin section. These observations were integrated with routine core analysis, mercury intrusion porosimetry and nitrogen gas adsorption-desorption. Intervals characterized by massive to diffuse lamination fabrics, abundant cements, and relatively large detrital grain sizes correspond to water saturated intervals of less favorable reservoir quality. These features indicate a high primary porosity that early diagenetic fluids favored, resulting in preferential cementation and porosity occlusion. Oil saturated samples with better reservoir quality are associated with heterogeneous, clay mottled to rippled dolomudstones. Flow baffles inherent to these fabrics inhibited early diagenesis resulting in relative porosity preservation compared to more homogeneous intervals. Subtle differences in pore size, shape and interconnectivity result from these geological differences. Permeability is consistently an order of magnitude better in oil saturated samples. Samples with less than 4.5% porosity had the highest tortuosities and were water saturated. Subtle differences such as micro- to nanopore proportions, average pore diameters, and interconnectivity in samples with porosity over 4.5% determined fluid saturations. Samples with very high proportions of nanoporosity had the least tortuous, equidimensional pore systems but remained water saturated due to the very small average pore diameters. Localized conditions including solid reservoir bitumen precipitation and over pressuring due to proximity to source rock may inhibit recovery.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015