Multiple Sources of Hydrocarbon Gases in Mud Volcanoes of the Gulf of Cadiz (SW Iberian Margin) – Insights from a Large Organic Geochemistry Data Set
Marianne Nuzzo, C. Hensen, M. Schmidt, F. Scholz, P. Terrinha, and L. M. Pinheiro
Active submarine mud volcanoes in the Gulf of Cadiz are deep-rooted structures generally located at the intersection of strike-slip and thrust faults. The Gulf of Cadiz mud volcanoes mediate the transport of water, gas and sediment from buried strata to the seafloor and hence provide insights on sources and transport mechanisms. In this region, the hydrocarbon gases have predominantly a thermogenic origin. However, the gas composition is largely variable at mud volcanoes across the Gulf of Cadiz. This stems not only from the variability of organic matter sources and thermal maturation state, but also from complex migration and mixing processes.
In this tectonically complex and active hydrocarbon province, seismic imaging is often impeded by the widespread presence of a seismically chaotic facies body. Hence “geochemical mapping” constitutes an essential tool to infer on the presence of gas reservoirs. The comparative interpretation of the chemistry of fluid, gas and sedimentary organic matter provide clues on the complex tectonic control exerted on transport and degradation of deep thermogenic gases.
Here we present organic geochemistry data from deep-ocean mud volcanoes located west of the Gulf of Cadiz accretionary wedge and discovered during the Transflux research cruise onboard the German RV-Meteor in 2012. These novel results provide essential indications on the relation between tectonic activity, fluid circulation and hydrocarbon gases transport and recycling in deep sediments. They are interpreted in the context of a large dataset acquired at twelve mud volcanoes all across the Gulf of Cadiz accretionary wedge since 2005, and shed a new light on our understanding of hydrocarbon gas provenances in the broad regional scale. Different provinces can be distinguished, and the importance of secondary gas formation in active tectonic settings highlighted. The relevance for unconventional gas (re)sources is discussed.
AAPG Search and Discovery Article #90161©2013 AAPG European Regional Conference, Barcelona, Spain, 8-10 April 2013