ORIGIN AND DISTRIBUTION OF POROSITY IN THE BLOYD SANDSTONE, SOUTHERN BOSTON MOUNTAINS, ARKANSAS

Pennsylvanian sandstone units in the northern part of the Arkoma basin are prolific natural gas reservoirs. Variations in porosity and permeability limit the productive capability of these units in some wells. The middle Bloyd sandstone (Morrowan) is one of these units with great potential as a reservoir, but porosity values and distribution within the sand are poorly known.

Recent studies immediately north of the basin have identified the middle Bloyd sandstone in a number of well-exposed sections. Three depositional facies have been identified in these sections: 1) Large-scale, uni-directional, cross stratification, 2) Small-scale, bidirectional, cross stratification, and 3) Ripple-laminated stratification (Antia, 2006). The porosity distribution was evaluated within each of these facies and among the three facies in a set of three outcrops in order to pave the way for prediction of these trends in the subsurface. This goal was accomplished primarily by performing point counts of pore spaces in thin section under the microscope.

The origin of the porosity was determined by studying the composition and texture of the rock petrographically. Petrographic analysis showed the middle Bloyd to be a fine to medium-grained sublitharenite, containing a significant percentage of metamorphic rock fragments (MRFs). These MRFs are metastable in many diagenetic environments and have partially dissolved to increase porosity. Also, authigenic clays have served to preserve porosity by blocking the nucleation sites for quartz cement.

In the Arkoma basin, the middle Bloyd sandstone is located between two sandstone units that are already producing natural gas. Therefore, the middle Bloyd has a very high potential to be a reservoir also. The results of this project will be very helpful to petroleum geologists who must determine optimum well locations. For example, they will be able to exclude facies that are less porous.