Characteristics and hydrocarbon accumulation models of Mesozoic volcanic reservoirs in the Dawa area of Liaohe depression

The Dawa area is adjacent to the Qingshui large oil-generating sag, where the composite hydrocarbon migration system constituted by multi-stage faults and unconformities provides a favorable condition for hydrocarbon accumulation. However, the volcanic lithology and lithofacies are complex with changeable eruption environment in this area. In order to clarify the reservoir characteristics and the main controlling factors of hydrocarbon accumulation, based on 107.73 m core, 270 thin sections and logging data from 15 wells, the volcanic lithology and lithofacies are classified. It is suggested that the Mz-ⅠMember was formed in underwater eruption environment with overflow hyaloclastite subfacies basaltic volcanic breccia and overflow lava subfacies basalt. The Mz-Ⅱ Member was developed in abovewater eruption environment with explosive tuff subfacies rhyolitic tuff and overflow lava subfacies andesite. Among the volcanic lithofacies, the basaltic volcanic breccia shows the best physical property with porosity of 19.3% and permeability of 6.71×10^-3μm², and the basalt shows the worst physical property with porosity of 9.2% and permeability of 0.23×10^-3μm². Based on the logging and seismic data, it is recognized that the Dawa area possesses a favorable source-reservoir matching and composite hydrocarbon migration system constituted by multi-stage faults and unconformities. The hydrocarbon reservoir distribution is determined by production test data to definite the main controlling factors of hydrocarbon accumulation and establish accumulation models. The study shows that the reservoir quality is controlled by lithology, lithofacies, volcanic eruption environment and tectonic movement. The hydrocarbon accumulation is mainly controlled by source-reservoir configuration and migration system. The reservoir type is structural-lithologic composite reservoir. The favorable exploration target area is on the west side of the study area which develops favorable lithology and lithofacies and locates near oil source and Dawa fault. The west side of the study area and localities near oil source and Dawa fault are the priority prospects for exploration, where favorable lithology and lithofacies are developed.