Dolomitization of Madison Limestone in the Green River Basin, Wyoming; Geochemical Evidence for Low Temperature Diagenesis and the Relation to Regional Porosity Trends

McLaughlin, J. F.; Quillinan, Scott A.; Surdam, Ronald; Bentley, Ramsey; Ganshin, Yuri

The Mississippian Madison Limestone is one of Wyoming's principal hydrocarbon and CO2 reservoirs. Previous studies have identified optimal reservoir zones of permeability and porosity within dolostones of the Madison Limestone, deemed the dolomitized grainstone fairway. The fairway is characterized by porous dolomitic packstone and grainstone, byproducts of early diagenesis and depositional location within a regional carbonate ramp. This study - funded under U.S. Department of Energy National Energy Technology Laboratory award DE-FE0002142 - uses petrophysical, isotopic, geochemical, and petrographic data of core samples and reservoir fluids from a new stratigraphic test well in the central Green River Basin to describe the geologic and reservoir characteristics of the Madison Limestone. These data show characteristic dolostone reservoirs in the Madison Limestone at the test well, and extend the dolomitized grainstone fairway across portions of the Rock Springs Uplift.

In the study area, the Madison Limestone is grossly composed of two sections; the top is non-porous limestone, and the base is a porous dolostone.δ18O (-2.3 to 2.4‰) of core samples indicate dolomite crystals formed in a low temperature, evaporitic, near-surface environment. This correlates to regional trends, and suggests that optimal reservoir zones were the result of regional, pre-burial diagenesis. Later diagenesis, specifically secondary mineralization of calcite and anhydrite, has decreased porosity in small portions of the dolomitic Madison. Mineralized areas record notable differences in trace element concentrations, decreases in isotopic ratios and log porosity, and are mostly independent of dolostone lithofacies. This suggests that petrophysical distinctions in porosity can be the result of both cyclic eustasy-related lithofacies and late-stage mineralization.

On the Rock Springs Uplift, the dolomitic Madison has a geologic and diagenetic history comparable to optimal, regional reservoirs defined as the dolomitized grainstone fairway. Isotopic and geochemical analyses show dolostones were formed during early, regional diagenesis resulting in optimal reservoir zones. Correlation of the new and regional data extends diagenetic similarities and optimal reservoir zones in the Madison Limestone across portions of the Rock Springs Uplift, and across central Green River Basin.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013