Cold seepages: An economic tool for hydrocarbon appraisal

Spontaneous cold fluid seepages are a renowned phenomenon occurring in a wide range of geologic and geodynamic settings, including deep sea fans, rapidly subsiding basins, and compressive tectonic settings (e.g., Dimitrov, 2002; Morley et al., 2011; Oppo et al., 2013, 2014). Cold seepages are marked by various structures, both on land and offshore, such as mud volcanoes (MVs), methane-derived authigenic carbonates (MDACs), and chemosymbiotic communities. Their formation mechanism requires the increase of pore-fluid pressure above the lithostatic gradient with the subsequent upward fluid migration through hydrofracturing or along carrier beds and tectonic discontinuities.

Cold seepages have long been investigated, especially for the information that they may provide for the exploration of hydrocarbons present in different types of reservoirs associated with this phenomenon (e.g., Link, 1952; Heggland, 1998; Abrams, 2005). Although the relation between cold seepages and hydrocarbon reservoirs has not been completely enlightened in numerous settings, as in fold and thrust belts, it is established that the occurrence of gas, frequently associated with oil, is a common characteristic that most of the seepage areas show. In particular, the spontaneous leakage of oil and gas represents a prime indication of hydrocarbons occurrence in the subsurface and valuable source of information on the petroleum system. The associated fluids also provide evidence of the geochemistry of deep-seated hydrocarbons. A useful example of this association is represented by the oil and gas field exploited near MVs along the coast and offshore in the Caspian Sea (e.g., Planke et al., 2003; Davies and Stewart, 2005; Oppo et al., 2014; Oppo and Capozzi, 2016).