ABSTRACT

The Fleming Group and its basinward equivalents constitute deposits of a major Cenozoic depositional episode of the northern Gulf Coast Basin. The facies complex is bounded above by the Amphistegina B Shale and below by the Anahuac Shale. Initially, lower Miocene (Oakville) progradation built across the broad, submerged shelf platform constructed during the earlier Frio depositional episode. At the Frio paleo-continental margin, the rate of outbuilding slowed as large-scale growth faulting created a narrow lower Miocene expansion zone. The later part of the episode was characterized by a stable to retreating shoreline and consequent aggradational to retrogradational deposition (Lagarto Formation).

The lower Miocene depositional framework includes, in South Texas, 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/lagoon and strandplain complex that extended along the central and much of the northeastern Texas coast. This well-known Matagorda 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 initial phase of early 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 hydrocarbons from nine identified plays in the Texas Coastal Plain and shelf. The most prolific play, the salt domes of the Houston Embayment, accounts for nearly all the oil and more than two-thirds of the total production from the sequence. Four offshore plays offer greatest room 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.