Abstract

Voluminous diapiric intrusions are present in 3D seismic data from the Liwan gas field area in the Baiyun sag of the Pearl River Mouth Basin, South China Sea. These diapirs are subdivided into magmatic diapirs and mud diapirs according to their seismic signatures and formation mechanisms. Saucer- and mound-shaped reflectors with anomalously high amplitudes distributed throughout the Early Miocene strata are interpreted as sills and laccoliths and are typified as magmatic diapirs. Mud diapirs in the study area can be divided into two types: wavy reflectors occurring at the basement of the sag caused by the gravitational instability of plastically deformed mudstones and columnar upheaval anomalies rooted in Paleogene rift sequences. Gas chimneys are characterized by vertical weak/blank zones accompanied by pull-down effects. Vertical gas chimneys and mud diapirs pierce the Middle Miocene strata. The vast majority of them are triggered by normal faulting; however, no deep-seated faults can be detected around the peripheries of igneous intrusions. Migration forces and pathways are represented by vertical mud diapirs and gas chimneys and thus control the distribution of shallow gases with negative-amplitude anomalies as well as gas hydrates.

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