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An Integrated Reservoir Quality Evaluation From the Desmoinesian-Age Colony Granite Wash, Anadarko Basin, Oklahoma

Abstract

Colony Wash is a significant Desmoinesian age “granite wash” field in western Oklahoma, Anadarko Basin. Understanding the impact of diagenesis on the formations reservoir quality is a key component of success in the play. The Colony Wash sandstones and shales display a near-classic stratigraphic submarine fan sequences and related stacked turbidite with distal fan lobe facies. Vertically, the rocks show variable lithofacies and reservoir qualities. Using 5 cored intervals, we evaluated the diagenetic history and reservoir quality. From each of the key wells, samples representing proximal channel/levee deposits and distal cyclic turbidites were studied through XRD, XRF, core plug porosity and permeability measurements, thin-section quantitative point-count-analysis, and FE-SEM. The maximum paleotemperature was determined through clay authigenesis and pyrolysis measurements on associated organic-rich shales supporting our proposed paragenesis schema. Five depositional facies are present: fine sandy debris flow, coarse sandy debris flow, medium graded turbidite bed, channel levee and debris flow slumps. The best reservoirs are associated with sandy debris flows – generally, all are classified as litharenite sandstones. All facies have been altered by early-stage Fe-chlorite and late-stage Fe-dolomite; additional secondary pore-filling minerals include quartz cement, illite, pyrite, and Ti oxides. In addition to minor intergranular porosity, samples display micro-porosity within the chlorite aggregates, rock fragments, and detrital clay. Chlorite grain coating and continuity controls the extent of quartz overgrowths, and thus in those samples where grain-coating chlorite is less abundant, quartz overgrowths are more common. Chlorite formation is linked to the presence of volcanic rock fragments and ferromagnesium minerals. Carbonate cements are influenced by the presence of detrital calcite and dolomite fragments, availability of the Fe in the system, temperature, the concentration of bicarbonate and pH. Because Fe-dolomite is principally responsible for degrading reservoir quality, forward modeling of Fe-dolomite cement is an important variable when evaluating prospects within the Colony Wash.