ABSTRACT

The Lower Cretaceous Travis Peak (Hosston) Formation contains oil and gas reservoirs distributed from Texas to Mississippi. Much of the formation from 6,800 to 10,100 ft (2,100 to 3,100 m) in East Texas has low permeability (less than 0.1 millidarcy), but a few thin zones near the top of the formation have as much as 100 millidarcy permeability. Low permeability is caused by compaction, extensive precipitation of authigenic cements during burial, and minor pressure solution.

Travis Peak sandstones in East Texas are fine- to very fine-grained quartzarenites and subarkoses 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 cements were clays, primarily illite, that coated detrital grains with tangentially oriented crystals. Next, extensive quartz cement (averaging 15.5 percent of the rock volume) occluded much of the primary porosity. Relatively late-stage cements include ankerite, which commonly has a dolomitic nucleus, a second generation illite, and chlorite. Ankerite and dolomite are most abundant in marginal-marine deposits near the top of the Travis Peak and less abundant in the deeper, fluvial part of the formation. Feldspar dissolution and albitization of plagioclase were also relatively late diagenetic reactions. Plagioclase composition determined by microprobe ranges from An₂₄ to An_0.1, but 83 percent of the analyses indicated a composition of less than An₅.

Reservoir bitumen, a solid, immobile hydrocarbon accumulation, fills pores in sandstones in the upper 300 ft (90 m) of the Travis Peak. The volume of reservoir bitumen is as much as 19 percent. The reservoir bitumen has a H/C ratio of 0.79 to 0.90, suggesting that the bitumen formed by deasphalting of oil, which caused heavy molecules to precipitate.

Permeability decreases with depth below the top of the Travis Peak. Porosimeter-measured porosity is the best predictor of permeability, and there is a low but statistically significant inverse correlation between total volume of cement and permeability.