Subsurface stratigraphy and depositional systems, Lower Cretaceous Travis Peak Formation, Shelby and Nacogdoches counties, east Texas

Gas prices have prompted academic interest into the area to gain a full understanding of depositional processes to further comprehension of depositional systems and to achieve maximum production from tight gas sands of the Travis Peak Formation in East Texas.; Three cores were described to provide a datum with which to compare shared characteristics. Cores from the #2 Alton Lilly and the #1 J.O. Pate from western Shelby County and the #1 Manuel Herrera from eastern Nacogdoches Comity yielded strikingly similar sedimentary structures, cementation, and texture to enable the assignment of groups of related lithologies to facies. The presence of cross-bedding, clay drapes, the amount of detrittal clay matrix and authigenic clay, sorting, and grain size were the determining factors in identifying facies. These parameters distinguished facies changes from lithologic changes that allowed for the development of a depositional model that would complement work done in previous studies. The presence of bidirectional herringbone crossbedding, little bioturbation and light cementation was the basis for identification of the Crossbedded, Laminated Sandstone Facies. Pervasive dolomite cement, detrital clay, bioturbation, and an overall lack of porosity defined the Dolomitic Sandstone Facies. Sections of finer grained siltstone with interbedded claystones, heavy bioturbation, and authigenic clay came to define the Dark, Bioturbated Siltstone Facies. These facies were common throughout the core study, an anomalous facies was identified by dolomitic mottling, heavy detrital clay, and heavy bioturbation and root traces. This facies was of interest as it was the only non-marine fades identified in the study. Dutton recognized many variations of fluvial fades that were not recognized here. The Mottled, Bioturbated Sandstone Facies was only identified in the #1 J.O. Pate core, and was minor in comparison to tidal channel facies recognized in the Crossbedded, Laminated Sandstone Facies in the remainder of that core. Interfingering of non-marine fluvial and fine-grained shelf facies was expected in the interval, but this was the only evidence of either, as there were no shales encountered in any of the cores.; Identifications of sedimentary fabrics and their assignment to fades necessitated the addition of some processes to the working model for the Travis Peak. These additions are: (1) presence of a strong paralic and diminished fluvial/deltaic influence within upper Travis Peak sequences, (2) recognition of a tidal dominance within channeloid sandstones on the south and southeastern margins of Travis Peak depositional areas, (3) more distal sand bodies are density flows that occur during higher order (4th or 5 th) tidal (neap?) cycles that transport sand into deeper basinal areas.; Facies identified within this study closely parallel those identified and described in button's (1991) Paralic Depositional System. Whereas the names were different, their lithologies were quite similar. Each of the fades identified were compared to button's and direct corollaries were soon realized. Sedimentary structures, cementation, mineralogy, lithology thickness, and petrography provide a depositional history characterized by supratidal coastal plain root mottled sandstones, tidal channel sandstones with associated clay drapes, subaerial tidal deltas, and siltstones deposited as distal marine deposits. The presence of cementing minerals became an important factor in the diagenetic history of the various fades. Cement mineralogy became an important issue in differentiating the histories of the fades, as it was realized that some of the sand bodies experienced very different diagenetic histories that either permitted or impeded pervasive cementation, based upon their fades relationships.; Identification of the Crossbedded, Laminated Sandstone Facies is the key as far as optimal porosity and permeability for three reasons. These are: (1) early dolomitization prevents...