Abstract

Fine-grained quartzarenites (and subar-koses) in the Lower Cretaceous Travis Peak Formation were extensively modified during burial diagenesis. Timing of diagenetic events can be constrained by combining petrographic and geochemical data with data on subsidence and thermal history. The geothermal gradient in the East Texas study area is 38 °C/km now, but it may have been as high as 44 °C/km when the Travis Peak was deposited because of elevated heat flow caused by crustal stretching associated with rifting of the Gulf of Mexico.

Illite rims and dolomite were the first authigenic minerals to precipitate in Travis Peak sandstones. Dolomite probably formed soon after deposition at about 25 °C from water with a δ18O composition near SMOW. Next, extensive quartz cement, averaging 17% of the rock volume in well-sorted sandstones, occluded much of the primary porosity. The average δ18O composition of the quartz overgrowths indicates that they precipitated from meteoric fluids at temperatures of between 55 and 75 °C, at depths of 1 to 1.5 km. Dissolution of orthoclase and albitization of plagioclase followed quartz cementation and occurred prior to mid-Cretaceous movement of the Sabine Uplift. Illite, chlorite, and ankerite precipitated after feldspar diagenesis. The ankerite may have precipitated over a range of temperatures from about 80 to 125 °C, from fluids with δ18O composition of about +2‰ (SMOW); +2‰ is the average present composition of Travis Peak water.

Most diagenesis ended when oil migrated into the Travis Peak. Later de-asphalting of the oil by solution of gas filled much of the remaining porosity with reservoir bitumen in some zones near the top of the formation.

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