Evolution of Porosity and Permeability in the Lower Cretaceous Travis Peak Formation, East Texas (1)

Abstract

Porosity and permeability decrease significantly with depth in low-permeability, gas-bearing Travis Peak sandstones on the western flank of the Sabine arch, east Texas. The Travis Peak Formation is approximately 2000 ft (610 m) thick and is buried to depths of 6000-10,000 ft (1830-3050 m). Reservoirs consist chiefly of sandstones of fluvial and paralic origin. Thin-section and core-analysis data from a total of 3680 ft (1122 m) of core from 26 wells indicate that both depositional environment and diagenetic history influenced the present distribution of porosity and permeability. Porosity in clean sandstones decreases from an average (geometric mean) of 16.6% at 6000 ft (1830 m) to 5.0% at 10,000 ft (3050 m). Permeability is reduced four orders of magnitude over the same depth range, from an average (geometric mean) of 10 md at 6000 ft (1830 m) to 0.001 md at 10,000 ft (3050 m). Loss of porosity by mechanical compaction took place mainly within the first 3000 ft (900 m) of burial, prior to significant cementation. The observed decline in porosity and permeability with depth between 6000 and 10,000 ft (1830 and 3050 m) results from (1) increasing quartz cement, (2) decreasing secondary porosity, and (3) increasing overburden pressure that closes narrow pore throats. At any given depth, average permeability is 10 times higher in clean fluvial than in clean paral c sandstones. Clean fluvial and paralic sandstones contain similar volumes of quartz cement, but paralic sandstones are finer grained and contain an average of 7% more total cement.