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Version 1 Introduction
DS 90-A, Version 1.0—Miocene of Southern Louisiana

A. Curtis Huffman, Jr., Scott A. Kinney, Laura R.H. Biewick, Heather R. Mitchell, and Gregory L. Gunther 

Version 1.0 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 in the Louisiana State database used to construct the contours but do not reveal the precise depths of the micropaleontologic identifications. 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 a total of 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).

Details of the Cenozoic sediment dispersal axes were documented and correlated with source areas by Galloway and others (2000). They noted that the early Miocene sediment influx exhibited a shift to the central Gulf fluvial axes that dominate the late Neogene due to a shift of source areas as a result of uplift of the Edwards Plateau and uparching across the western interior of North America at the onset of Basin and Range extension. During the Middle Miocene, unroofing of the Edwards Plateau supplied carbonate-rich sediment directly into the Corsair delta and adjacent northwest Gulf (Morton and others, 1988). The Central and East Mississippi dispersal axes produced a composite delta system that dominated the central Gulf margin (Galloway and others, 2000). Late Miocene rejuvenation of the southern Appalachians, Nashville dome, and continental interior provided sandy sediment to the Central and East Mississippi axes (Boettcher and Milliken, 1994; Galloway and others, 2000) and firmly established the late Neogene depositional pattern.

Recent sequence stratigraphic studies of several offshore Miocene oil and gas fields demonstrate a general correspondence with the global cycles and sequence boundaries of Haq and others (1988) and explain most sequence differences, especially higher frequency cycles, by basin-specific high sediment flux in the vicinity of major sediment dispersal axes (Wagner and others, 1994; Hentz and Zeng, 2003). Wagner and others (1994) used both 2D and 3D seismic data to map sequence boundaries and interpret paleogeography in the vicinity of the Lower Miocene shelf break just offshore from Cameron Parish. Hentz and Zeng (2003) identified the low stand systems tract of some third-order Middle Miocene sequences as being particularly prolific oil and gas producers although the entire section produced hydrocarbons. More specifically, they noted that within the Starfak and Tiger Shoal fields reserves were highly concentrated where the reservoir-scale fourth-order systems tracts stack to form third-order lowstand systems tracts, which compose approximately 30 –50% of the succession. They also noted that although a dominant structural–trapping component is present in the fields, the thick sealing shales of the third-order slope fans and third-order transgressive and highstand systems tracts minimize the risk of cross-fault juxtaposition of lowstand reservoir sandstones against third-order highstand sandstones that can act as points of leakage.