Potential Hydrocarbon Plays of the Gulf of Mexico

Abstract

Abstract Due to a complex geologic evolution, the Gulf of Cadiz hydrocarbon province contains a complex petroleum system. North Atlantic rifting caused deposition of sedimentary sequences that show a progression of facies from riverine to lacustrine to evaporites to marine sediments. Concomitant tectonically induced basin subsidence occurred as rifting progressed. The Alboran Domain, underlying the present Alboran Sea, began moving westward from western Mediterranean in the Miocene as result of the northwesterly compression between the Iberian and African plates forming the Gibraltar Arc. Westward crustal motions caused serial events: upper slopeover steepening triggered major mass-wastings to descent some 500-600 km downslope forming the accretionary wedge of the Gibraltar Arc. Simultaneously, subshelf basins developed and were infilled with material from the nearby shelf and Guadalquivir Basin-river system. Across the continental margin, extension occurred over the upper slope and compression along the lower slope. 1.0 Introduction The Gulf of Cadiz continental margin has been proposed as a potential world-class petroleum province1, 2. The margin is a known hydrocarbon producer with largely biogenic natural gas extracted under the continental shelf1 Abundant evidence exists for pockmarks, i.e., mud volcanoes with natural gases sampled from them (research conducted by Joan M. Gardner and Luis Somoza, 2000 and 2001), along the upper and middle continental margins off the Gulf of Cadiz and Morocco). Thus, these margins are undeniably petroliferous. A complex tectonic history beginning with the Triassic-Jurassic North Atlantic rifting to the present3 has resulted in a complex petroleum system,1 based on sizable seismic data sets, including high and low frequency lines. Figure 1 is the location of industry-quality data. These lines were collected in he 1970s and 1980s by industry for hydrocarbon exploration. The data set spans the continental shelf, the upper slope, and the down-to-middle slope in water depths up to 1000 m. The overriding geologic problem is that the measured geothermal gradient along the shelf (2.4°/100 m) is too low to cause a hydrocarbon column that has been reported to be some 2.0 to 2.5 s two-way travel time (t-wtt) thick1 (also see Figs. 2, 3, and 4), with sediments extending to depths of 3-3.5 s, apparently interspaced between rising (from east to west) thrusts of continental rocks. The actual sediment thickness may reach 4 to 6 km, possibly even 7 km at maximum depths beneath the thrusts1,2(Fig. 2). Figure 3 shows that with basement thrusting from east to west, the thrusts may override thin (< 100-200 m thick) sediment wedges (Fig. 3a) and that advancing thrusts impact the sediments, initiating fracturing deformation of the thrusts (Fig. 3b). Continued thrusting heavily fractures the sediments, shattering them; fractures possibly radiate from the advancing thrusts for a distance equal to the thrust diameter (Fig. 3c).