Controls on Permeability Distribution in Lower Cretaceous Travis Peak Formation, East Texas

Shirley P. Dutton, Timothy N. Diggs

Porosity and permeability decrease significantly with depth in low-permeability, gas-bearing Travis Peak sandstones on the western flank of the Sabine uplift. The Travis Peak Formation is about 2,000 ft thick and is buried to depths of 6,000-10,000 ft. Thin-section and core-analysis data from 3,550 ft of core from 25 wells indicate that both depositional environment and diagenetic history influenced the present permeability distribution. Reservoir sandstones are divided into two main types: fluvial sandstones deposited in a sand-rich environment and paralic sandstones deposited in mud-rich environments. Paralic sandstones are finer grained than fluvial sandstones (average of 0.12 mm vs. 0.17 mm, matrix-free sandstones only) and thus probably had lower original permeabilit .

Porosity and permeability were reduced initially by compaction and later by extensive cementation. Porosity decreases at a rate of 2.3% per 1,000 ft, starting from an average of 13.4% at 6,000 ft, and permeability is reduced from an average of 10 to 0.001 md between 6,000 and 10,000 ft. Correlation between porosimeter porosity and permeability is good (R = 0.89). Precement porosity does not vary significantly with depth, indicating that decreasing permeability is not a function of greater compaction with depth. The main cause of the permeability trend is increased quartz cement with depth in all facies, from an average of 14.5% at 6,000 ft to 19.4% at 10,000 ft. Paralic and fluvial sandstones contain similar volumes of quartz cement; but because paralic sandstones have more authigenic dolomite, ankerite, illite, and chlorite, they average 3% more total cement. As a result, a comparison of clean sandstones at any given depth indicates that permeability is about one order of magnitude higher in fluvial than in paralic sandstones. Predicted values of permeability (measured under in-situ pressure conditions) in fluvial sandstones vary from 24 md at 6,000 ft to 0.002 md at 10,000 ft.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.