Summary

Description

Version 1 of the Miocene deals almost entirely with southern Louisiana primarily because of the availability of data, especially the biostratigraphy. Publicly available data have been used wherever possible, however, in the case of proprietary data such as the Tenroc Regional Geologic Database, only derivative products are made available so that, for example, the isopach or structure contour maps show the locations of wells used to construct the contours but do not reveal the precise depths of the micropaleontologic identifications in that well. In addition to the data specific to southern Louisiana, a number of regional geologic coverages that will be applicable to all versions are also included for reference. The Miocene is the most important producing interval in the Gulf of Mexico Basin. Nehring (1991) reported that as of 1987, known recovery from the Miocene was nearly 150 trillion cubic feet (TCF) of natural gas, 19 billion barrels of crude oil, and 6 billion barrels of natural gas liquids (NGL) for nearly 49 billion barrels of oil equivalent (BOE). He also noted that only seven other provinces worldwide contain more petroleum and none contained more in large (200-500 million BOE) fields. In its 1995 National Assessment of Oil and Gas Resources, the USGS reported the onshore and state waters component of these production totals to be approximately 37.1 TCF of natural gas, 2.5 billion barrels of oil, and 851 million barrels of NGL (Schenk and Viger, 1995). Known recovery from the Miocene in the northern Gulf of Mexico Basin increases upward and eastward from the Lower to the Upper Miocene (Nehring, 1991). Along the northern Gulf of Mexico Basin the Miocene comprises a series of thick off-lapping sequences of terrigenous clastics dominated by several long-lived deltaic systems and overlain by transgressive shale tongues. The total updip thickness is approximately 3,900 ft (1,200 m) in onshore Texas and 7,800 ft (2,400 m) in Louisiana (Galloway and others, 1991). It thickens to more than 25,000 ft (7,600 m) in offshore Louisiana (Meyerhoff, 1968). Galloway, and others (1991) attributed the tremendous downdip thickening of the Miocene to deposition on and basinward of the underlying unstable Frio Formation and thick salt. The rapid deltaic deposition triggered both growth faulting and movement of salt out of withdrawal basins into nearby diapirs thus providing additional accommodation space and the accumulation of thick, highly expanded sections of sandstone and shale. Eastward thickening is the result of the shift of feeder systems from west to east from early to late Miocene so that the late Miocene deltas occupied the position of the present-day Mississippi Delta (Galloway and others, 1991).