Miocene Depositional Systems and Hydrocarbon Resources: the Texas Coastal Plain

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DuBar, Jules R.

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The Miocene Major Stratigraphic Unit (MSU) of the Texas Coastal Plain is an offlapping sedimentary sequence deposited over the continental platform constructed by Late Oligocene Frio progradation. These Miocene deposits have yielded nearly 3 billion bbl of oil and 7 trillion ft3 of gas. Correlations with mammalian and foraminifer zones indicate that the Miocene MSU and its updip equivalents, the Oakville and Lagarto Formations (Fleming Group), range in age from Early to Late Medial Miocene (22.5 to 12 m.y.a.). Downdip, the upper Heterostegina and Discorbis Zones of the Anahuac shale wedge are included as a part of the Miocene MSU.

Regional subsurface study indicates division of the Miocene MSU into six principal depositional systems: the Cypress fluvial system and San Jacinto delta system, developed in the Houston Embayment; the Moulton streamplain and Indianola barrier-strandplain-lagoon system across the San Marcos Arch; and the Santa Cruz fluvial system and Rosita delta system located in the Rio Grande Embayment. Integration of regional studies of depositional systems, their facies suites, structural styles, and character of produced hydrocarbons permitted delineation of 10 hydrocarbon production-exploration plays. The play forms the basic analytical unit in characterization of hydrocarbon production histories, predictions of future discoveries, and the placement of potential resources in a geographic-stratigraphic context.

The Miocene MSU lacks entirely, or contains only negligible volumes of thermally mature source rocks. Thus, all Miocene MSU hydrocarbons are exotic, having been derived either by upward migration from older formations, or by lateral, updip migration from offshore marine Miocene units. Based on three historical evaluation methods employed in this study, the Miocene MSU contains between 250 and 1360 million boe of undiscovered, conventionally producible hydrocarbons.


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