ABSTRACT

Extensive diagenetic changes have occurred in deltaic sandstones of the "Gray" interval, Strawn Series (Pennsylvanian) from Taylor County, Texas. The present burial depth of the "Gray" interval is 4500 feet (1370 m), and maximum burial was about 7500 feet (2300 m). Sandstone beds were deposited in delta slope, bar crest, and distributary channel environments in a high-constructive elongate delta. Petrographically, the rocks are fine to very fine, submature to mature, shale-bearing sublitharenite. Except for clay clasts, the framework grains, more than 95% quartz, are very mature mineralogically.

Good primary porosity occurred in sediments deposited in high energy environments such as bar crest and channel. These sediments had large mean grain size (x = 2.8 or .15 mm), contained an average of only 1.49% detrital clay matrix, and were located farther away vertically from shale beds. It is estimated that depositional porosity of about 35% was reduced to 22% by compaction.

Cementation began with authigenic chlorite rims, followed by quartz overgrowths. Calcite cement filled the remaining pore space and replaced unstable grains such as feldspar. Later, dissolution of calcite produced secondary porosity. Further compaction was prevented by quartz cement, so the secondary porosity was preserved. Kaolinite, barite, and ferroan dolomite precipitated as late cements that reduced secondary porosity.

Samples from high energy environments contain the most cement, but they also retain the highest porosity. Original textures and depositional environments in a formation may be used to predict where porosity will survive diagenesis.