Diagenesis and burial history of the lower cretaceous Travis Peak Formation, East Texas
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Sandstone in the Travis Peak (Hosston) Formation has been extensively modified by burial diagenesis. Permeability in much of the formation has been reduced to less than 0.1 md as a result of compaction extensive precipitation of authigenic minerals, and minor pressure solution. Thin zones of higher porosity and permeability occur mainly near the top of the formation: porosity and permeability decrease with depth below the top. The Travis Peak Formation in East Texas is approximately 2,000 ft (600 m) thick; depth to the top of the formation ranges from 5,800 ft (1,770 m) to 9,400 ft (2,870 m). Travis Peak sandstone is fine- to very fine-grained quartzarenite and subarkose having an average composition of Q₉₅F₄R₁. Plagioclase feldspar is more abundant than orthoclase, and chert and low-rank metamorphic rock fragments are the most common lithic components. The first authigenic cement to precipitate was illite, which coated detrital grains with tangentially oriented crystals. Next, extensive quartz cement, averaging 17% of the rock volume in well-sorted sandstone, occluded much of the primary porosity. Quartz cement is most abundant in the lower Travis Peak, in well-connected sandstone beds that were deposited in braided streams. Oxygen-isotopic composition of quartz overgrowths indicates that they precipitated from meteoric fluids at temperatures of 130° to 165°F (55° to 75°C). These temperatures equate to depths of 3,000 to 5,000 ft (900 to 1,500 m). Dissolution of orthoclase and albitization of plagioclase followed quartz cementation and occurred prior to mid-Cretaceous movement of the Sabine Uplift. An abrupt loss of orthoclase occurs at 1,200 ft (365 m) below the top of the Travis Peak, and albitization is more extensive deeper in the formation. Illite (a second generation), chlorite, and ankerite precipitated after feldspar diagenesis; these late authigenic phases incorporate ferrous iron released by thermal reduction of iron compounds. Ankerite was derived primarily from early dolomite cement, but it incorporated some light carbon from maturation of organic matter and radiogenic strontium from feldspar dissolution. The oxygen-isotopic composition of pore fluids evolved during ankerite precipitation from -4 o/oo to +3 o/oo (SMOW); +3 o/oo is the present composition of Travis Peak water. Oil migrated into Travis Peak reservoirs about 65 mya from shale in the Bossier Formation. Later deasphalting of the oil filled much of the remaining porosity in some zones near the top of the formation with reservoir bitumen.