A New Major Oil/Gas Play ("Sub-Salt") on the Texas/Louisiana Gulf Coast Shelf
Authors: Pratsch J.E. 
Published: October 2008

At present most oil and gas reserves in the offshore Gulf of Mexico region, both on the shelf and in deeper waters (Figs. 1,2), are contained in Miocene, Pliocene and Pleistocene sands (Fig.3). Traps are mainly anticlines or fault closures, growth fault systems and salt tectonic features. Stratigraphic traps are rare, stratigraphic trap parameters common. Seals are formed by interbedded shales or, on salttectonic features, by salt. Oil and gas clearly migrated late, some today. In US Federal waters at least the 782 proved oil and gas fields studied by the MINERALS MANAGEMENT SERVICE occur; they contain combined ultimate reserves of 10,64 billion barrels oil and 129.9 trillion cubic feet of gas (US DEPARTMENT OF THE INTERIOR MMS 1991). However, daily oil and gas production are decreasing (gas production today more foreconomic reasons), new reserves decrease. Thus, many see the offshore Gulf of Mexico, especially the Texas-Louisiana shelf, as a mature to over-mature "dying" exploration area. s this correct? No! Why? Because even in 1992 we do not yet understand fully the underlying petroleum systems of the region; and whenever such fundamental questions about the origin of discovered or undiscovered oil and gas exist in a basin, the chance to discover additional major new reserves are present. Recent efforts to search for major new oil and gas reserves in Gulf Coast deep-waters face the additional burden of high-cost new technology in a diverless environment. In fact, the economic incentives to search for new reserves in shallow waters on the TexasLouisiana shelf are so great that certain higher exploration risks appear acceptable.

What then is the origin of all discovered oil and gas on the shelf of Texas and Louisiana? The most commonly heard opinion is that the next syncline contains the critical generative source beds, or that down-dip shale sections generate oil, while gas may be shallow-biogenic or just of unknown origin. "Why bother with geochemistry when oil and gas are produced."? Well, the time to bother with geochemistry definitely has arrived because the new reserves we need are missing.

The US Gulf Coast region is part of a major passive continental margin of post Pennsylvanian age (MILLEKEN 1988; ARBENZ 1989). From mid-Cretaceous time onwards clastics have been the sole dominant lithology (WINKER 1982). Sands were transported from the north from the rising North American craton and deposited into a pre-existing regional low areas. Lateral and vertical motions of Jurassic Louann Fm salt added to the mobile late-Jurassic sediment base(MARTIN 1984; WORRAL and SNELSON 1989). Rivers shifted laterally through geologic times as did the resultant Tertiary, especially Neogene, depocenters (FigA). Deltas, growth fault systems and salt tectonic features developed -- all this over a mobile salt base and over a rifted Permian to ? Jurassic margin. In cross section view in the Tertiary sequence younger sediments overlap older units from north to south and become progressively shaly from north to south (DOW 1984; SASSEN 1990) (Fig.5).