Shallow gas accumulation in sediments from the North Terminal of the Port of Aveiro, Ria de Aveiro, Portugal

Abstract

In this work, we report evidence of shallow gas accumulations in sediments and escape in the water column, discovered and mapped in the North Terminal of the Port of Aveiro, using high resolution seismic data (Chirp Sonar). This evidence includes: Acoustic Turbidity in the subsurface and Acoustic Plumes in the water. Two high resolution seismic and sediment sampling campaigns were carried out in this area, separated by 5 years, to analyze gas field evolution. In the first campaign (2013) a gas field with an area of 0.22Km² was mapped. In the second campaign (2018) the same field showed a total area of approximately 0.34 km², suggesting an increase in the area of gas accumulations. The seismic stratigraphic analysis of the high-resolution seismic profiles showed three main seismo-stratigraphic units: Unit U1, consisting of marls, clays and limestones from the Upper Cretaceous, and Units U2 and U3, composed of sediments (sands and clays) from the Quaternary (Plio-Pleistocene and Holocene). The gas accumulations were found in the Quaternary sediments. The gas is inferred to be of biogenic origin, resulting from the degradation of the accumulations of organic matter deposited by river channels from the sea regression after the last glacial maximum.