Diagenesis, Fluid, and Thermal History of Carbonate Mudrocks: An Example from the Lower Lodgepole Formation, Williston Basin

Goldstein, Robert H.; Mitchell, Ray W.

Petrographic and geochemical techniques, typically used to study conventional reservoir rocks, can also be used on carbonate mudrocks in resource plays. Results can elucidate porosity evolution, diagenesis, fluid and thermal history. The fine grain size of the lower Lodgepole lime mudstone precludes the use of many petrographic techniques in the matrix, but optically resolvable diagenetic features can be studied in intraparticle pores, secondary pores (molds and fractures), interparticle pores in grainy facies, and in stratigraphically equivalent mound facies. A complex history of at least 30 diagenetic events is documented for the Scallion and Carrington facies using CL and UV petrography, fluid inclusions, and stable C/O isotopes.

The paragenesis is grouped into 4 major categories - submarine, early burial, late burial, and Laramide/post-Laramide late burial. During early burial, compaction and dewatering are dominant, resulting in fracturing and partial dolomitization of the matrix. In the Scallion, discontinuous spar-filled fractures are common in nodules. Based on the shapes, fractures result from shrinkage by dewatering and compaction. Yellow-orange fluorescent rhombic dolomite is common in clay-rich seams around nodules, in burrows and lining some early fractures. Stable isotope data and position in the paragenesis suggest the dolomite may result from fluid migrating from the underlying Bakken Shale.

The post-Mississippian, pre-Laramide system included pressure solution, moldic pores, and calcite cementation. Fluid inclusion data indicate that some late burial calcite cements precipitated from concentrated brines at temperatures between 84-110 degrees C.

Laramide and later tectonism caused deformation and fracturing. Fractures and pores were enlarged by corrosive fluids and then reduced by calcite and baroque dolomite. The calcite cements formed up to about 130 degrees C. Fluid inclusion data indicate the dolomite precipitated from brines with pulsing temperatures up to 150 degrees C, indicating hydrothermal fluid flow. There is a close association between fracturing, multiple oil migration events, and dolomite precipitation. The geothermometry results can be integrated with organic analyses to improve constraints on basin models.

The observations point to a more dynamic fluid and thermal history than previously hypothesized. Similar studies could be broadly applied to improve understanding of carbonate-rich resource plays elsewhere.

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