Development of organic porosity in the Woodford Shale with increasing thermal maturity

Abstract

Abstract Using a combination of focused ion beam milling and scanning electron microscopy we describe the evolution of secondary organic porosity in eight Woodford Shale (Late Devonian–Early Mississippian) samples with mean random vitrinite reflectance values ranging from 0.51% Ro to 6.36% Ro. Organic porosity was observed to be absent in samples with vitrinite reflectance values of up to 0.90% Ro with the first appearance of secondary pores starting with the 1.23% Ro sample. Porosity in the organic matter was unexpectedly absent in a sample with a vitrinite reflectance of 2.00% Ro; however, organic pores were again found in samples with higher thermal maturities. Porosity, when present, did not appear to be uniformly distributed among the organic matter that was within less than a micron of each other suggesting important differences in composition of the organic matter. Thin regions of organic matter were observed between grains raising the possibility that small amounts of the deposited organic matter were compacted between grains to form thin layers and/or the structures are part of the secondary organic matter (interpreted to be post-oil bitumen) which was left behind as a residue during oil migration through the shale. Some regions of porous organic matter appeared to be grain protected whereas others did not which indicates that these non-protected porous organic regions may be stress supporting with porosity intact under in situ reservoir conditions. These observations suggest that thermal maturity alone is insufficient to predict porosity development in organic shales, and other factors, such as organic matter composition, complicate porosity development.