Diagenesis of Cotton Valley Sandstone (Upper Jurassic), East Texas: Implications for Tight Gas Formation Pay Recognition

Abstract

The Upper Jurassic Cotton Valley sandstone comprises a thick siliciclastic sequence in the East Texas basin. These sediments were deposited in a dominantly progradational sequence of shallow marine and fluvial-deltaic environments. Along the eastern flank of the basin, natural gas is produced from low porosity and low-permeability reservoirs which have been stimulated by massive hydraulic fracturing. Cotton Valley sandstones generally are very fine-grained, well-sorted quartz arenites and subarkoses. Principal framework constituents are monocrystalline quartz and feldspars. The sandstones have a complex diagenetic history and are cemented by authigenic quartz, calcite, phyllosilicates, and iron oxides. The most common paragenetic sequence of pore-fill minerals is (1) development of clay coats on grains, (2) formation of syntaxial overgrowths, (3) dissolution of unstable grains followed by precipitation of phyllosilicates, (4) precipitation of calcite in relict primary and secondary pores, and (5) replacement of grains by calcite resulting in a poikilotopic texture. Clean coarser grained sandstones may be cemented very early by calcite and progress to stage 5 with only intermediate episodes of grain and cement dissolution. Cotton Valley sandstones are classified by R-mode factor analysis into three groups which can be related to porosity characteristics and, therefore, can be used to predict potential reservoir rock. Type I rocks are tightly cemented by quartz and calcite and make a poor reservoir. Type II rocks have a high phyllosilicate content and abundant microporosity that may produce gas. Type III rocks have a high content of unstable grains and have well developed secondary porosity which can be of reservoir quality. Although there is a great overlap in rock types due to vertical and lateral inhomogeneity of the formation, end-member lithologies can be discriminated on the basis of log calculations of porosity and water saturation. Depositional environments were a major control on the diagenesis of the sands. Clean, well-winnowed sands deposited in high-energy environments became tightly cemented by silica overgrowths and sparry calcite. Sands deposited in lower energy environments contained detrital clays that inhibited nucleation of overgrowths and helped preserve porosity.