Subsurface gas- and oil-shale samples of Texas shales from the Permian, Fort Worth, and Maverick basins and San Marcos Arch: core sampling for measured vitrintite-reflectance (Ro) determination: final technical summary report (FY 2010–2014)

Shale samples analyzed for measured vitrinite reflectance during FY 2010–2014 were collected from varying depositional basins in Texas and strata of different ages. They include the Upper Devonian and Lower Mississippian Woodford Shale (Permian Basin), the Lower Pennsylvanian Smithwick Shale (Fort Worth Basin), the Lower Permian shales and Spraberry Formation (Midland Basin), the Lower Cretaceous Pearsall Formation, and the Upper Cretaceous Eagle Ford Shale (Maverick Basin and adjacent area).

Although an approximate trend of increasing vitrinite-reflectance values with depth (i.e., increasing thermal maturity, or rank) occurs in the Eagle Ford Shale of the San Marcos Arch, this pattern is not exhibited with the other units sampled. Moreover, when measured vitrinite-reflectance values are compared to calculated-Ro values of Lower Permian shales and the Spraberry Formation, consistently lower values occur with the measured-Ro data set. Sample values from the remaining three successions studied (with the possible exception of the Smithwick Shale) are characterized by similarly lower-than-expected vitrinite-reflectance values. These low values are probably a result of markedly lean successions and not of the presence of low-rank strata. (Lean = either no vitrinite was present in a sample or it was too small to be measured.) Oil- and gas-shale core samples do not appear to be ideal for measuring vitrinite reflectance primarily because of the fine-grained character of the rock, as opposed to coal, in which vitrinite is sufficiently coarse, visible, and abundant to consistently derive Ro values.