Dolomitization and its Impact on Porosity Evolution in the Middle Bakken, Elm Coulee Field, Williston Basin

Abstract

The shallow marine, mixed-siliciclastic-carbonate deposits of the middle Bakken (MB) Formation in Elm Coulee field are dolomitized to varying degrees, and the dolomite content ranges from 5 to 70%. This study addresses the dolomitization process in Elm Coulee field and its impact on porosity evolution of the MB dolomites. Petrography, XRD, and stable isotope (C and O) analyses from 8 wells across the field were used to constrain the dolomitization process. Routine core analysis, mercury injection porosimetry, N2 adsorption analysis, and FE-SEM studies were performed to determine total porosity, pore-size distribution, and pore types. Dolomite crystals with planar-s to planar-e fabrics, stable isotope results, presence of late anhydrite cement, and overdolomitization indicate that dolomites in Elm Coulee field are the product of seepage-reflux dolomitization in mesohaline conditions. Relationships between dolomite content, total porosity, and pore size suggest that the porosity evolution in the MB is closely linked with the dolomitization process and is mainly dependent on the degree of dolomitization. Progressive increase in the dolomite content (up to 60%) results in an overall increase in the development of intercrystalline pores, total porosity, pore size, and dolomite crystal size. However, with further increase in dolomite content beyond 60% (overdolomitization), total porosity and pore size decrease as the size of dolomite crystals increases. The dolomitization process involves open-system replacement of precursor calcite, and is dependent on the external supply of Mg2+ and (CO3)2- ions, and the rate of precursor calcite dissolution and dolomite precipitation. When the rate of calcite dissolution equals the rate of dolomite precipitation, porosity of the resulting dolostone mimics that of the precursor limestone. When calcite dissolution exceeds the rate of dolomite precipitation, it result in an increase in porosity. This explains the increase in porosity in the MB with the increase in dolomite content, inasmuch as dolomite precipitation is the rate-limiting step. Pore size is generally controlled by crystal size, shape and sorting. Therefore, the degree of dolomitization, which affects the dolomite crystal size, controls the average pore size in the MB dolomites. However, in the overdolomitization stage, dolomite crystals form interlocking fabrics and porosity is lost due to formation of dolomite cement overgrowths.

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