Gulf of Cadiz (western Spain): characterized by a complex petroleum system

Somoza, Luis, ITGE: Geological Survey of Spain, Madrid, Spain; B. M. Battista, Consultant, , Bay Saint Louis, MS ; Joan Gardner, and Allen Lowrie, Consultant, Picayune, MS

Abstract

The petroleum system is that series of interrelated processes by which hydrocarbons are generated and migrate to reservoirs from which they can be extracted commercially. The complex geology of the Gulf of Cadiz provides three separate heat sources for thermal maturation. Energetic downslope migration has created reservoirs and traps result from both structure and stratigraphy.

Westward extension by continental margin collapse of the Alboran Sea platelet in the early Neogene initiated large slump-block movements (originating at depths from 20 to 25 km and temperatures from 450° to 600°C) along the Iberic and Moroccan continental margins. Extensive allochthonous salt may underlie the slump-blocks and could serve as a lubricant for gravity-driven translation; overall movement may exceed 400 km. Mobile, Late Cretaceous-Paleogene salt (salt is from two to three times as heat conductive as a sandy clay sediment), deposited in a paleobasin having a western limit of 10°W to 12°W, appears to have become diapiric. The migrating blocks, operating as thin-skinned tectonics, and associated down-dip debris flows may be trapping mechanisms for hydrocarbons. Migrating debris covers an area as great as 90,000 km² and potential reserves rival the Gulf of Mexico subsalt play. Hot, upthrust basement blocks and normal sedimentary compaction thus warm the petroleum system from heat transported along the salt wedges/layers and diapirs. In their often interrupted upward migration, hydrocarbons may be stored beneath the thrust crustal blocks.

The eastern terminus of the Azores triple junction is well marked by shallow earthquakes (<45 km and many <30 km deep). The junction passes beneath the Gulf of Cadiz and its continental margin. Major igneous extrusions, resulting in seamount growth especially during the Upper Neogene, characterize much of the bathymetrically complex Azores-Gibraltar deformation zone. The prograding Iberic and Moroccan continental rise and slope bury the tectonically active and potentially warm oceanic crust. In summary, tectonics as denoted by earthquakes within the crust (oceanic, transitional, and continental) means that heat is being generated and is migrating upward. The specifics of these crustal tectonics have not been deciphered; deeper seismic penetration is needed.

This complex geologic evolution apparently began as the Gibraltar Arc moved westward in the Neogene and caused an overthrust that formed an accretionary wedge onto the Gulf of Cadiz. The westward Gibraltar Arc migration of continental margins initiated farther westward migration of gravity-driven continental and salt-floored blocks. Ultimate emplacement was over oceanic crust. As the continental blocks migrated, normal listric (???) faulting along the present continental shelf of the Gulf of Cadiz developed, thus accommodating upper Miocene (past 19 Ma) tectonics and deposition. Multiple stacking of thrusting wedges occurred at the foot of the rotated continental blocks. Lateral migration can generate compression zones that, in turn, can provide the extensive fracturing, faulting, and jointing suitable for further petroleum storage. This is the first proposed ocean-basin hydrocarbon province based on long-distance allochthonous mass-wasting migration.

Petroleum exploration in the Gulf of Cadiz began in the early 1970’s. By 1999, 26 commercial wells were drilled. Several 2-D multichannel seismic grids at spacings of approximately 14 and 6.5 km were shot across the continental shelf's upper and middle slopes in water depths to 1000 m. Recent drilling by Repsol (1996-1998) on the continental shelf off the city of Cadiz has produced natural gas, some biogenic. The gas is being transported to nearby Cadiz.

The geothermal gradient measured in these few wells on the central and inner shelf is 2.4°C/100 m. If that value should characterize the Gulf of Cadiz and the adjacent continental margin, then it is likely—given observed sediment thicknesses—that the area might not be commercially petroliferous. There are three heat sources: normal compaction of sediment; deep-seated upthrusted continental blocks from the east; and continuing basement tectonics. The occurrence of salt provides an active conduit through which to transport heat.

We present geologic arguments that the regional geothermal gradient, especially along the shelf break and continental slope, may be appreciably higher—locally up to 3.1°C/100 m, which makes the Gulf of Cadiz a strong contender as a future major hydrocarbon province. At present, some 180,000 to 360,000 km³ of sediment with thicknesses of 2 to 4 km have never been explored by drilling.