ABSTRACT

Reservoir geometry and heterogeneity are fundamentally controlled by channel style. Two channel styles existed during Travis Peak deposition: (1) meandering (high sinuosity) channels, and (2) straight (low sinuosity) channels. Each is restricted to a specific geographic area, a result of the long term stability of the loci of sediment input into the ancestral Gulf of Mexico. Reservoir models for meandering and straight fluvial systems have been developed from cores and logs using knowledge of depositional environment and channel style. Predictions of reservoir dimensions in the Travis Peak Formation are based on geologic modeling. The geologic models are supported by reservoir simulation studies performed as part of several staged field experiments sponsored by the Gas Research Institute.

Reservoirs that are developed in high sinuosity systems originate as point bars and as crevasse splay deposits. They are isolated within flood basin mudrocks. Point bar reservoir sandstones average 12 ft in thickness and cover approximately 300 acres. Point bar sandstones are usually compartmentalized by mud drapes, particularly in their upper portions. The drapes are sand-rich, and do not appear to act as barriers to fluid flow when wells are fracture stimulated. Crevasse splay sandstones are laterally extensive (>640 acres) but have lower reservoir quality and are thinner than associated point bar sandstones.

Reservoirs developed in the low sinuosity system originate as in-channel braid bars and as crevasse splays. Channel sandstones are lithologically homogeneous. The average thickness of a single, low sinuosity channel sandstone is 8 ft. Such deposits are rare because successive channels stack vertically. As a result of channel avulsion and vertical stacking, reservoir sand bodies are very large (at least 5000 acres) and thick (12 to 45 ft). Pressure relationships between successive channels and associated crevasse splay deposits are complex.

Wells drilled in Harrison and northeast Panola counties penetrate high sinuosity channel sands while wells drilled in the rest of the study area penetrate low sinuosity channel sands. Within each area, sand bodies originating in channel and crevasse splay environments can be differentiated using a variety of wireline log responses. The volume of sand (channel sand, and channel plus overbank sand) is significantly greater in the low sinuosity system than in the high sinuosity system throughout the whole Travis Peak interval. However, the percentage of sand in the Travis Peak section increases with increasing depth in both areas. In terms of resource potential, the lower portions of the Travis Peak are extremely important because these sands are gas-saturated.