Comparative study on volcanic hydrocarbon accumulations in western and eastern China and its significance

Volcanic hydrocarbon accumulations in eastern and western China have both generality and discrepancy. The common features are as follows: their source-reservoir-caprock association is a near-source type; their reservoir properties are little affected by burial depth and possess strong heterogeneity; the main source rocks are coal measures mudstone and usually have a high geothermal evolution degree. The differences between the two areas are: volcanic rocks in the eastern area are mainly formed in Mesozoic-Cenozoic intracontinental rifts; those in the western area are in Paleozoic intracontinental rifts and island-arc. Eastern volcanic activity is characterized by central eruption controlled by deep-seated faulting movement, which produced mainly intermediate-acid volcanic rocks; western volcanic activity possesses more eruption types and periods, which produced mainly intermediate-basic volcanic rocks. The reservoir space of volcanic reservoirs is primary pores and fractures in the east, while secondary dissolved pores and fractures in the west. In terms of reservoir types and controlling factors, the eastern reservoirs are mainly lithologic and structural-lithologic, with hydrocarbon accumulations controlled by effective source kitchens, deep-seated faults, and volcanic eruption structures, and the in-situ features are obvious; the western reservoirs are mainly stratigraphic unconformity reservoirs, with hydrocarbon accumulations controlled by unconformities, source-reservoir-caprock associations and large faults, and both heterotopic and in-situ features exist.