Trinidad Mud Volcanoes: The Origin of the Gas
Bubbling gases from the mud volcanoes of Trinidad and gases associated with oil in deeper reservoirs were sampled and analyzed to understand their possible relationships. Numerous geochemical analyses were performed on the gas samples. The chemical concentrations of organic compounds, CO2, and noble gases (from He to Xe) were measured, and isotopic signatures (δ13C from C1 to C5,CO2, and noble gases) were also determined. In the southern part of the island, our data show a typical thermogenic origin for the gases from oil reservoirs. However, the gases from the mud volcanoes, all located in the Southern Range of Trinidad, exhibit intermediate values between thermogenic and bacterial signatures. The hypothesis of simple mixing between these two end members can be discarded because of an incompatible δ13C for the bacterial end member (ranging from −52 to −33‰). We thus conclude that most hydrocarbon gases found in the mud volcanoes are purely thermogenic gases.
The isotopic δ13C of CO2 versus CO2/C1 elemental ratios from the oil reservoir gases and the mud volcano gases follow a global trend. We therefore conclude that all gases come from the same source. This is corroborated by the isotopic composition of the hydrocarbons and of CO2 and by the radiogenic fraction of the associated noble gases.
However, mud volcano gases show an extreme dryness, which cannot be directly related to gas generation but instead to postgenetic processes. A process of migration involving local process of dissolution and diffusion in water and preferential adsorption of the larger organic molecules onto the solid mud particles should be involved. Finally, mud volcano gases have never been trapped but are permanently expelled to the atmosphere and thus have a lower time of residence than oil reservoir gases as indicated by noble gas data.
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The phenomenon of rocks moving under their own means has always fascinated both scientists and nonscientists alike. The 2006 AAPG Hedberg Conference on Mobile Shale Basins was held in response to a need to gather industry and academic communities in a common forum to address the very existence of mobile shales. Stimulating and informative discussions at that Conference led to this special volume on shale tectonics. AAPG Memoir 93 documents shale tectonics from a variety of basins around the world, including the southern Beaufort Sea; the Krishna-Godavari Basin, India; eastern offshore Trinidad; offshore Brunei; and along the westernmost portion of the Mediterranean Sea. The book also provides information on the petrographic framework, behavior, geometries, and geodynamic models of shales. Publication of this Memoir coincides with a growing interest in shales as hydrocarbon reservoirs, and will add to the body of literature that significantly addresses both extrusive and intrusive shales.