Upper Miocene paleo-pockmarks and their correlation to methane-derived authigenic carbonates through 3D seismic data in External Western Patras Gulf, Greece

This study focuses on the 3D seismic investigation of high-amplitude elliptical reflections (HAER) within Miocene stratigraphic interval, in Western Patras Gulf, in a sedimentary basin that is affected by salt tectonics.

Miocene basins across Western Greece have been attributed to the formation of foreland and piggy-back basins of a westward advancing fold-and-thrust belt. The base and top of the Miocene basin in the study area are marked by two regional unconformities. The lower unconformity has formed during Burdigalian, following uplift related to an early compressional phase. The upper unconformity is related to the sea-level fall during the Messinian Salinity Crisis (MSC). A salt diapiric wall of NW-SE orientation along the eastern side of the basin is interpreted of Triassic age. The different deformation style between the underlying Miocene and the overlying Pliocene – Quaternary strata indicates that the salt wall went through at least two stages of re-activation, one during late Miocene and another one during Pleistocene. Seismic stratigraphy and neighboring outcrop data onshore Kephalonia Island, reveal a basin infill ranging from fluvial to lagoonal and progradational deposits to more hemipelagic mud-dominated deposits towards the top.

HAER are structures of circular to elliptical shape, that appear as patches of high amplitude anomalies at the upper Miocene stratigraphic level. Due to their seismic signal, indicative of hard lithologies, they are interpreted as methane-derived authigenic carbonates (MDAC), precipitated on top of paleo-pockmarks. Our interpretation infers that those paleo-pockmarks develop through the gas escape along a fault network associated with a late Miocene diapiric re-activation.  The presence of those paleo-pockmarks, combined with the underlying Mesozoic sequence, raises two major questions: 1) the origin of the paleo-pockmarks is thermogenic or biogenic, and 2) is it possible for the Miocene subsidence to result in thermal maturation of Mesozoic source rocks in the area?

A preliminary thermal maturity modeling indicates that there is a late kick during Neogene, and thus, a thermogenic origin for the paleo-pockmarks seems reasonable. This is also supported by multiple present-day oil seeps and gas-escape structures along Western Greece. The absence of paleo-pockmarks within the Pliocene – Quaternary section is attributed to the extensive erosion during MSC and the reduction of Pliocene - Quaternary sedimentation rates.