Gas venting and subsurface charge in the Green Canyon area, Gulf of Mexico continental slope: evidence of a deep bacterial methane source?

Questions as to the role of modern carbon in methanogenesis and the maximum depth of methane sources in the Gulf of Mexico continental slope remain unanswered. A research submersible was used to sample mixed bacterial and thermal gas (delta(13)C of methane = -62.8parts per thousand, deltaD = -176parts per thousand) venting to the water column from the Gulf slope in Green Canyon (GC) 286. The Delta(14)C value of the methane (-998parts per thousand) is consistent with fossil carbon. Another gas vent on GC 185 is 100% methane (delta(13)C= -62.9parts per thousand, deltaD= -155parts per thousand) and may be from a bacterial source. The Delta(14)C (-997parts per thousand) of this bacterial methane is also consistent with fossil carbon. Fossil bacterial methane and thermal hydrocarbons are present in Pliocene to Pleistocene reservoirs (similar to3509-4184 m) of Genesis Field (GC 205, 161, 160). Oil in these reservoirs is biodegraded but gas is not, suggesting that gas charge to reservoirs continues presently at 3-4 km depth. Mixed thermal and bacterial methane may charge the deep reservoirs, and fossil methane from depth may ultimately vent on the sea floor at GC 286 and GC 185. Results of this study of Green Canyon suggest that bacterial methane in gas vents and in reservoirs is from deep fossil sources. (C) 2003 Elsevier Ltd. All rights reserved.