Seismic reflection characteristics and controlling factors of shallow gas in the alongshore mud clinoform of the East China Sea

Abstract

This study aims to investigate seismic reflection characteristics and controlling factors of gas within the East China Sea clinoform based on seismic profiles and borehole core data, including organic geochemistry and lithology. The seismic profiles reveal distinct gas-related seismic reflection characteristics along the coast, extending from the Yangtze Estuary southward to the elongated inner shelf. In the northern region, specifically the northern part off the Yangtze Estuary, widespread acoustic blanking is topped by a shallow gas front reflector parallel to seabed. In the middle region, spanning the southern part off the Yangtze Estuary and the adjacent northern inner shelf, widespread acoustic blanking is topped by a gas front reflector following the clinoform stratigraphy. In the southern region, off the Oujiang Estuary, there are widespread acoustic turbidities that are topped by a severely undulated gas front reflector. Based on the combined seismic-core data, it is suggested that the seismic reflection in the northern region, characterized by the gas front reflector parallel to the seabed, is controlled by the combination of anaerobic oxidation of methane and active gas diffusion triggered by seabed erosion. In the middle region, the obstructive effects of clinoform stratification on gas diffusion contribute to the seismic reflection characterized by the gas front reflector following the clinoform stratigraphy. In the southern region, the seismic reflection of low-concentration gas characterized by widespread acoustic turbidities is primarily influenced by the interplay between abundant gas source supplies due to the clinoform depocenter and elevated hydrostatic pressure in deeper water conditions. Furthermore, local gas seepage is observed in the northern region, likely due to seabed erosion and lower hydrostatic pressure in coastal zones. Additionally, the coastal zones and seaward slope break sites along the rest of the clinoform sections are also identified as having higher potentials for gas seepage.