OCS Study MMS 95 - Data Center

tectonic fracturing, are relatively common (Hovland and Judd 1988) .In the Gulf ofMexico, the source layers are Cretaceous carbonate sediments and Cenozoicterrigenous sediments, both of which have layers containing abundant organic matterthat can form hydrocarbons .Early in its history, during the Jurassic Period, the Gulf of Mexico was ashallow, periodically desiccated basin in which an extensive evaporite layer (theLouann Salt), up to several kilometers in thickness, was deposited (Buffler et al .1980 ; Worrall and Snelson 1989). Many additional kilometers of sediment, firstcarbonate and then terrigenous were piled atop the evaporites, as the basin marginsaged and subsided (Worrall and Snelson 1989 ; Wu et al . 1990a) . The organic materialin these sediments was compressed and heated to form gas and oil .Salt is the mainconstituent of the evaporates .Because it is nearly incompressible, its densitychanges little with burial whereas the overlying sediments become denser throughcompaction .time scales .Moreover, under differential stress, salt deforms plastically on geologicThe end result is that with continuing burial, the Louann Salt becamemobile beneath the Gulf of Mexico shelf and slope during the Cenozoic Era.The mobile salt has greatly affected the physiography of the northern Gulf ofMexico (Jackson and Seni 1983 ; Jackson and Galloway 1984 ; Jackson and Talbot1986 ; Wu et al . 1990b ; Simmons 1991) . It has moved vertically to form diapirs andridges, only some of which have reached the surface .salt nappes and intraslope salt-withdrawal basins .It has moved laterally to formOf greatest significance forchemosynthetic ecosystems, the movements have caused faulting, which is activetoday at many locations .The faults are typically tensional, listric, growth faults,often with parasitic antithetic faults (Worrall and Snelson 1989 ;Seni and Jackson1992) . These faults frequently tap reservoirs of hydrocarbons deep within thesediment column .Not only to they break the reservoir seal, but they also provide aconduit for upward hydrocarbon migration .Thus, hydrocarbon seeps are associated3-4