Multiscale Seabed Seepage Manifestations in the South Caspian Basin, Azerbaijan

Abstract

Abstract Multiscale seabed seepage manifestations are identified on high-resolution geophysical datasets in an area of the South Caspian Basin and compared with seepage features at the Dashgil mud volcano, onshore Azerbaijan. Features identified range from small-scale gas seepage mounds to large-scale mud volcanoes and collapse craters. The size and distribution of seepage features reflect the size and activity of underlying subseabed fluid chimneys. Important differences between seabed and land seepage features include the occurrence of authigenic carbonate in the subsea equivalent of cinder mounds and the occurrence of pockmarks instead of salses in areas of lighter-phase fluid escape. Large collapse craters (calderas) that exhibit spatial phase segregation of fluids (and therefore types of seabed seepage feature) occur without or coevally-with mud volcano pioneer cones. A new model of initial caldera formation is proposed that does not require the collapse of a shallow mud chamber beneath the caldera. Rather, thicker brecciated mud (upwelled through pipes and diatremes) interacts with faults and fractures which breach lighter-phase hydrocarbons and brines trapped in intermediate reservoirs. This may mean that thicker, gas-charged mud breccia extrusions may be spatially separated from thinner, brine/hydrocarbon-rich mud flow expulsions. As the mud feeding the seabed features is sourced from a single feeder complex at depth, expulsion and depositional features may become spatially separated from collapse features. Storage and lateral migration of lighter-phase fluids in intermediate reservoirs explains how spatially separated seepage features exhibiting different erupting fluids can coexist in broader chimney upwelling zones, despite a common source of mud at depth. Present-day seabed seepage features observed within the study area may be thought of illustrating key stages in mud volcano evolution, from early stage caldera formation to late stage edifices whose original calderas are long since buried. Introduction The seabed expression of fluid seepage can take many forms such as piercement structures (e.g. mud volcanoes), expulsion features (e.g. pockmarks) and authigenic features (e.g. carbonate mounds). Seepage features are readily identifiable on geophysical survey data (e.g. bathymetric terrain models, seismic and sonar images) and often form major morphological zones on integrated interpretations of the shallow geological section. The type, size, distribution and activity of seepage features can provide important insights into deeper geological processes (e.g. Planke et al. 2003; Stewart and Davies 2006; Roberts et al. 2011). Further, an understanding of the interaction of deeper geological processes with seepage features can help in assessing risk to seabed and subseabed facilities and wells, for example that posed by a major mud volcano eruptive event. The South Caspian Basin is characterized by a series of anticlinal folds relating to tectonic compression that still continues today at horizontal motion rates of several centimeters per year (Philip et al. 1989; Masson et al. 2006). Rapid sedimentation has led to a thick sequence (up to 20 km) of sedimentary deposits that contain a sequence of overpressured shale beds (The Maykop shale) which provide the primary source for hydrocarbon generation in the region (e.g. Jackson et al. 2002). Rapid sedimentation and tectonic compression has led to the migration of shales and associated hydrocarbons that have resulted in some of the best examples in the world of seabed and land fluid seepage.