Characteristics of mud diapirs and mud volcanoes and their relationship to oil and gas migration and accumulation in a marginal basin of the northern South China Sea

Mud diapir and mud volcano have the same formation mechanism and similar development and evolution characteristics, but different geological features and forms. In this study, the development and characteristics of a mud diapir in Yinggehai Basin (northwestern South China Sea) and a mud volcano in Southwest Taiwan Basin (northeastern South China Sea) were investigated. The geochemical characteristics of their associated natural gases were also analyzed. The results indicated that the Yinggehai Basin mud diapir is mainly distributed in the deep and large depression of the central basin and presents five lines in echelon arrangement along a northwest direction. Its development and evolution characteristics are low density and speed, and abnormally high temperature and overpressure. In contrast, the Southwest Taiwan Basin mud volcano is mainly distributed in a deep water slope area of the south sag and Tainan-Kaohsiung. Onshore mud volcanoes are mainly distributed along the deep and large fault zone, and their morphological characteristics are similar to venting volcanoes. The seabed morphology of offshore mud volcano seems similar, but current research is still insufficient. There are many types of associated hydrocarbon gases in mud diapir and mud volcano, but the high maturity coal-type hydrocarbon gas is the main one. The associated non-hydrocarbon gas is rich in CO₂ and N₂, which is mainly derived from a CO₂ source from inorganic crust. The source of both diapir and volcano is thick marine Miocene and Pliocene age mud with hydrocarbon-generating potential. The mature hydrocarbon generation, mud diapir/mud volcano migration and deposition into an appropriate reservoir, as well as diapir-associated structures, constitute a unique oil/gas migration and accumulation system, which thus adds to our understanding of the distribution of such “mud diapir/volcano-type oil/gas reservoir.”