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The Upper Jurassic Cotton Valley Sandstone is a thick siliciclastic unit in the East Texas basin. Sandstones and shales of this unit were deposited in shallow marine and fluvial-deltaic environments and exhibit progradational successions of facies. Along the eastern flank of the basin, natural gas is produced from Cotton Valley reservoirs characterized by low porosity and permeability, which have been stimulated by massive hydraulic fracturing.
Cotton Valley sandstones are generally very fine-grained, well-sorted quartz arenites and subarkoses. Principal framework constituents are monocrystalline quartz and feldspars. The sandstones have had a complex diagenetic history and are cemented by authigenic quartz, calcite, phyllosilicates, and iron oxides. The most common paragenetic sequence of pore-fill minerals was (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 framework grains by calcite resulting in a poikilotopic texture. Clean coarser grained sandstones may have been cemented very early by calcite and progressed directly o stage 5 with only intermediate episodes of grain and cement dissolution.
Cotton Valley sandstones are classified by R-mode factor analysis into three groups that can be related to porosity characteristics. Therefore, the groups can be used to predict potential reservoir rock. Type I rocks are tightly cemented by quartz and calcite and make poor reservoirs. Type II rocks have a high phyllosilicate content and abundant microporosity, and 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 characteristics of the three rock types owing to vertical and lateral inhomogeneity of the sandstones, end-member lithologies can be discriminated using log calculations of porosity and water saturation.
Depositional facies controlled the diagenesis of the Cotton Valley Sandstone. Clean, well-winnowed sands deposited in high-energy environments became tightly cemented by silica overgrowth and sparry calcite. Sands deposited in lower energy environments contained detrital clays that inhibited nucleation of overgrowths and helped preserve porosity.
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