ABSTRACT

Five major lithofacies were identified in the Hosston Formation of Trawick Field, Nacogdoches County, Texas. These were identified based upon vertical changes in sedimentary structures, textures, and lithologic composition. Facies identified in the Hosston Formation included: (1) a clean, crossbedded, tidal-channel facies, (2) a sandy, bioturbated, tidal-flat facies, (3) thin, rapidly deposited, crevasse splay deposits, (4) interdistributary bay sands, silts, and muds and (5) partially reworked distal deltaic sandstones. Distinct differences are observed between the upper and lower Hosston facies due to the transition from a lower, more continental fluvio-deltaic system to an upper, marginal marine delta-fringe system. In the lower Hosston, only the channel, crevasse splay, and interdistributary bay facies are significant. The regressive nature of the formation is documented through changes in color and detrital constituents as well as in the presence of prodelta muds and shallow carbonates found exclusively in the upper Hosston.

The Hosston Formation has characteristically low porosity and permeability values. Variations in the reservoir quality of Hosston sands is controlled both by environment of deposition and diagenesis. Two diagenetic pathways are postulated. In the first, extensive silica and phyllosilicate cementation dramatically reduce reservoir potential and porosity values rarely exceed 2 percent. In the second, early hydrocarbon migration inhibits additional cementation and therefore suppresses further porosity reduction. Samples following this pathway have porosities of up to 30 percent. Late stage diagenetic events include cementation by calcite and anhydrite as well as isolated occurrences of intergranular albite cementation. Channel sands of the lower Hosston maintain the highest consistent reservoir qualities.