--> --> Diagenesis, Porosity and Petroleum Source Beds of Birdbear Formation, McKenzie County, Williston Basin, North America

AAPG Rocky Mountain Section Meeting

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Diagenesis, Porosity and Petroleum Source Beds of Birdbear Formation, McKenzie County, Williston Basin, North America

Abstract

The Birdbear Formation in the Williston Basin is a carbonate-evaporite sequence that extends across western North Dakota, eastern Montana, southern Manitoba and southwestern Saskatchewan. Approximately 180 vertical and horizontal Birdbear wells have cumulatively produced over 25 million barrels of oil equivalent in North Dakota, and Birdbear production also extends into eastern Montana and southern Saskatchewan. This study evaluates the degree of diagenesis and its effects on porosity and permeability, as compared to the depositional patterns/facies that enhanced the reservoir characteristics of the Birdbear Formation in McKenzie County North Dakota, in relation to its hydrocarbon potential in the Williston Basin of North America. This study examined: 1) core and thin section (Thirteen cores and over 50 thin sections); 2) geochemical analysis of prospective petroleum source beds (TOC Rock- Eval program pyrolysis); 3) wireline log correlations and porosity distribution mapping; 4) compiled standard core-plug porosity-permeability measurements and 5) nuclear magnetic resonance (NMR) spectroscopy measurements. The aim of the study was to determine the role of deposition and diagenesis on porosity types, lateral and vertical distribution of reservoir porosity, lateral and vertical distribution of prospective source beds and anhydrite seals. The Birdbear has two distinct units; 1) a lower carbonate dolomitelimestone (boundstone to wackestone) unit, with discontinuous dolomitized reservoir and abundant organic material that is being evaluated as potential source material for hydrocarbon production using TOC Rock Eval Pyrolysis; and 2) an upper anhydrite-carbonate (packstone) unit that has the ability to entrap migrating fluids within the study area. Rocks in the upper section of the lower carbonate unit were exposed to high diagenetic alteration that enhanced permeability and porosity types that includes, intra-crystalline, inter-granular, with the most dominant being solution enhanced/ moldic vuggy porosities. These were confirmed by thin section analysis that revealed varying degree, types of porosity and NMR spectroscopy analysis from marked producible fluids indications and porosity maps across McKenzie County (center of the Williston Basin). The rocks in the formation showed marked selective or partial dolomitization, micritization and dissolution of calcite cement from inclusion of brines, creating excellent reservoir rock qualities of porosity and permeability (average limestone porosity at 1.74%, from a range of 0.48% to 2.26% and average dolomite porosity of 8.69%, from a range of 0.86% to 29.85%, with permeability range of 20-85 md). Hydrocarbon production in the Birdbear Formation has been documented to be from restricted intervals of varying lithological thicknesses of 1-2 foot thick reservoirs in the upper Birdbear, with 10+ foot thick reservoirs within the lower “B” porous zone, which are more prominent reservoirs for production. TOC Rock-Eval program pyrolysis data indicate fair to good distribution of prospective source beds (TOC range from 1.06% to 11.5%, with a maximum HI value of 182.8 and OI value of 11.9) that are thermally mature and within oil generating window with Tmax value of 452. This indicates the Birdbear Formation may be a selfsourced hydrocarbon system within the Williston Basin.