Methods of phase division and distribution prediction of volcanic rocks in the Anda Sag, Xujiaweizi Fault Depression, Songliao Basin

The Anda Sag, the northern part of the Xujiaweizi Fault Depression in the Songliao Basin, contains high-quality source rocks and tight, lithologic gas deposit-dominated volcanic reservoirs of the Lower Cretaceous Yingcheng Formation. This area is known to have a variety of volcanic edifices that are characterized by multi-phase eruption and superimposed distribution. Currently, the sag has been highly explored, and drilling of targets in the critical crater areas with reservoirs relatively developed has basically finished. So, additional targets will be defined. Thus, a criterion for dividing the volcanic eruption phases was established based on core, well log and seismic response marks. Through a well-seismic skeleton section analysis, it was believed that three volcanic eruption phases (I, II and III) occurred in the Anda Sag. Finally, the volcanic edifice and facies belt distribution of each phase were predicted by using seismic coherence body and trend-surface analysis technologies, with over 30 new volcanic crater targets identified. The predicted results show that the distribution of lithofacies and lithologies determines the framework of better volcanic reservoirs in the west and south than that in the east and north respectively. Lithological-structural gas reservoirs are dominant in the west, while tight lithologic gas reservoirs are dominant in the east. Based on the study results, the favorable exploration area is finalized as 950 km2, a potential of 100 billion m3 gas resources has been implemented.