Lower Miocene (Fleming) Depositional Episode of the Texas Coastal Plain and Continental Shelf: Structural Framework, Facies, and Hydrocarbon Resources

The Fleming Group and its basinward equivalents constitute the stratigraphic record of one of the major Cenozoic depositional episodes of the northern Gulf Coast Basin. The depositional sequence representing the episode is bounded above by the Amphistegina B shale and below by the Anahuac shale. Initially, lower Miocene (Oakville) progradation advanced across the broad submerged shelf platform constructed during earlier Frio deposition. When outbuilding reached the Frio paleocontinental margin, the rate slowed as large-scale growth faulting created a narrow lower Miocene expansion zone. The later portion of the lower Miocene episode, generally equivalent to the Lagarto Formation, was characterized by long-term shoreline stability and retreat punctuated by local, temporary progradation.In South Texas, the lower Miocene depositional framework includes the Santa Cruz fluvial system and the North Padre delta system. The bed-load fluvial complex fed a wave-dominated delta, constructing a broadly convex deltaic headland across the foundered Frio Norias delta system. Extensive wave reworking and longshore transport of sand and mud nourished a broad barrier island/ lagoon and strandplain complex that extended along the central and much of the northeastern Texas coast. This well-known barrier/strandplain system was bounded updip by a coastal plain traversed by numerous, small, intrabasinal streams. Near the present Sabine River, westernmost deposits of a continental-scale mixed-load fluvial and equivalent delta system extend beneath the Texas Coastal Plain and shelf from the Miocene depocenter in Louisiana. Here, the early phase of lower Miocene progradation was also complicated by the incision and filling of numerous submarine gorges.Lower Miocene reservoirs have produced nearly 4 billion barrels of oil equivalent of petroleum from nine identified plays in the Texas Coastal Plain and shelf. The most prolific play, the Houston Embayment salt domes, accounts for nearly all the oil and more than two-thirds of the total production from deposits of the episode. Four offshore plays offer the greatest area for discovery of substantial new reserves, primarily of gas. To date, however, the yield per volume of reservoir sandstone for Miocene plays remains low relative to more prolific units, such as the Frio Formation.