Book excerpt: The Travis Peak (Hosston) formation constitutes a 1,000- to 5,000-ft-thick clastic wedge that formed two major depocenters along the north flank of the Gulf Coast Basin. The depocenters were dominated by fluvial-deltaic facies. A delta-fringe facies, including tidal flat and nearshore shallow-margin shelf facies, formed around the margins of the clastic wedge. These marginal-marine deposits within the upper Travis Peak are the most important productive facies of the formation within a nine-county area in East Texas. Sandstones in the Travis Peak are mineralogically mature. Low permeability and occlusion of porosity is primarily due to quartz overgrowths, authigenic clay, ankerite, and reservoir bitumen, a high-molecular-weight hydrocarbon residue. Within six Travis Peak gas fields in East Texas, porosity ranges from 8 to 11 percent and water saturation ranges from 28 to 44 percent within intervals of net pay. The permeability-thickness product is low in the southern part of the study area and increases toward the north. Upper limits of permeability range from 0.074 md (median value) to 0.084 md (thickness-weighted average), based on well tests that postdate fracture treatment. Within Chapel Hill field, three reservoir sandstone types were defined; sandstones with greatest lateral continuity were deposited as sandy tidal flats, including associated channel sandstones that trend northwest. Lower energy tidal-flat deposition is characterized by increased mud content of sandstones, and low marine transgression resulted in deposition of mudstone and muddy limestone.