Bend Arch–Fort Worth Basin - General Stratigraphy

General Stratigraphy

Hydrocarbon production from Ordovician, Mississippian, and Early Pennsylvanian rocks is mostly from carbonate rock reservoirs, whereas production in the Mid-Pennsylvanian through Low-Permian is mostly from clastic rock reservoirs. The sedimentary section in the Fort Worth Basin is underlain by Precambrian granite and diorite. Cambrian rocks include granite conglomerate, sandstones, and shale that are overlain by marine carbonate rocks and shale. No production has been reported from Cambrian rocks. The Silurian, Devonian, Jurassic, and Triassic are absent in the Fort Worth Basin.

From Cambrian to Mississippian time, the Fort Worth Basin area was part of a stable cratonic shelf with deposition dominated by carbonates. Ellenburger Group carbonate rocks represent a broad epeiric carbonate platform covering most of Texas during the Early Ordovician. A pronounced drop in sea level sometime between Late Ordovician and Mississippian time resulted in prolonged platform exposure. This erosional event removed any Silurian and Devonian rocks (post Viola Limestone unconformity) that may have been present. Barnett Shale was deposited over the resulting unconformity. Provenance of the terrigenous material that constitutes the Barnett Shale was from Ouchita thrust sheets and the reactivation of older structures such as the Muenster Arch. Post-Barnett deposition continued without interruption as a sequenced of extremely hard and dense limestones were laid down. These limestones have often been confused with the lower part of the overlying Marble Falls Formation, and they have never been formally named in the literature. Since the underlying Barnett is generally assumed to be Late Mississippian Chester in age, the superposed carbonates are often referred to informally as "the Chester Limestones."

Clastic rocks of provenance similar to the Barnett dominate the Pennsylvanian part of the stratigraphic section in the Bend Arch–Fort Worth Basin. With progressive subsidence of the basin during the Pennsylvanian, the western basin hinge line and carbonate shelf, continued migrating west. Deposition of thick basinal clastic rocks of the Atoka, Strawn, and Canyon Formations occurred at this time. These Mid- and Late Pennsylvanian rocks consist mostly of sandstones and conglomerates with fewer and thinner limestone beds.