Hydrocarbon Seepage, Gas Hydrates, and Authigenic Carbonate in the Northwestern Gulf of Mexico

ABSTRACT Rapid deposition of organic rich sediments onto thick Jurassic salt deposits created conditions conducive to the formation and entrapment of large volumes of oil and gas as well as active salt diapirism. Resulting hydrocarbon seepage, brine seepage, hydrate formation and decomposition, methane oxidation, oil degradation, and authigenic carbonate precipitation have significantly effected the geology, geochemistry, and morphology of the continental slope of the northern Gulf of Mexico. Regional topography is largely controlled by salt diapirism and slumping, and is characterized by pock marks, mud volcanoes, disrupted sediments, and large accumulations of carbonate. Sediment chemistry is driven by the influx of oil, gas and brine seepage and secondarily altered by the enhanced microbial and benthic biology. INTRODUCTION The active formation of salt diapirs on the Louisiana slope has created a geographically complex region characterized by hummocky topography, large mounds, basins, and troughs1,2. The growth of salt diapirs has also caused extensive faulting, slumping and mass sediment movement 3, 4. Sediments between diapirs are as thick as 30,000 ft. and consist of interbedded shales and sand. The Louisiana slope is an active oil producing province with several large oil fields presently being developed in the Green Canyon lease area and other deep water locations5. Until recently, most reports of oil and gas seepage in the Gulf of Mexico were restricted to continental shelf locations. Seepage of gas is a common occurrence and acoustical methods have been extensively used to locate gas seeps6–8. Many of these seeps are associated with faulting 9- 11. By contrast, Wilson et al. 12 reported that the Gulf of Mexico was not a region of extensive oil seepage because the Gulf coast is young and not extensively deformed by folding12. Although Link13 located numerous onshore oil seeps within 100 mi. of the Texas-Louisiana shelf, it was only recently that offshore liquid petroleum seeps have been directly sampled and characterized13 - 15. Extensive coring on the Louisiana slope indicates that oil and gas seepage is a widespread regional phenomenon on the northern Gulf of Mexico slope16. RESULTS AND DISCUSSION Hydrocarbon Seepage Natural seepage has long been recognized as a source of petroleum to the environment In the Gulf of Mexico region asphalt usage in pre-Colombian pottery suggests that beach tars existed for thousands of years prior to the advent of a petroleum based economy18. Oil seepage has only recently been extensively documented in the offshore regions of the northern Gulf of Mexico. Several cores containing as much as 4% oil, by weight, were recovered on the northwestern continental slope in 1983 by Anderson et al. 14 Subsequently extensive seepage was reported at several locations in the Green Canyon lease area15.