The Holocene gas system of the Ría de Vigo (NW Spain): Factors controlling the location of gas accumulations, seeps and pockmarks

Abstract

Abstract High-resolution seismic profiles, cored sediment (gravity cores and vibro-cores) and radiocarbon data from suitable shell material were used to establish the stratigraphic context of gassy sediments in the Ria de Vigo. Mapping of seeps and pockmarks were compared with the stratigraphy of gassy sediment to ascertain the relationships. In the Ria de Vigo three Holocene seismic units were identified: the first two (Uh 1 and Uh 2 ) correspond to the transgressive systems tract (TST) and the third (Uh 3 ) corresponds to the highstand systems tract (HST). The presence of acoustic turbidity within the three seismic units indicates that generation of gas took place during the relative sea-level rise that occurred after the Younger Dryas event, i.e., during the TST and mainly during the HST. The relative sea-level variations are shown to be a major factor controlling the type (fine- vs. coarse-grained) and distribution of facies (lateral and vertical) that generate and/or allow gas accumulation or escape. Acoustic turbidity due to presence of free gas was recognized in the three Holocene units with a total of 12 acoustic turbidity fields mapped in the Ria de Vigo. No significant variations in the spatial distribution or extent of the gas fields or in their vertical location (below the present sea bed) have been observed during the seismic surveys carried out in the ria during the last decade. Furthermore, gas escape from the sediment and active pockmark has been observed. Therefore, we conclude that gas escape (seeps, pockmark generation and maintenance) and methane formation are in equilibrium. The combined analysis of high resolution seismic profiles, sediment temperature, pore water salinity and hydrostatic pressure revealed: (1) a direct relationship between depth of free gas and hydrostatic pressure; and (2) a direct link between sediment temperature and number of acoustic plumes from seabed to water column; more acoustic plumes were detected in summer when the temperature of the sediment was higher than in winter, indicating a seasonal variation of seeps within the ria. Salinity showed no significant differences between sea bottom water and sediment pore water and was therefore not a major factor controlling gas-front variations.