Seismic interpretation and geological evaluation of hydrocarbon source rocks in volcanic-rich continental lacustrine rift basins: A case study of the Lower Cretaceous Yingcheng Formation from the Changling Fault Depression in the Songliao Basin, NE China

The Lower Cretaceous Yingcheng Formation in the Changling Fault Depression in the Songliao Basin is composed of volcanic-rich continental lacustrine rift basin sediments. Its complex stratigraphic structure and the difficulty in predicting the hydrocarbon source rock distribution restrict the exploration and evaluation of deep hydrocarbons in this region. This paper uses seismic stratigraphy to divide the strata and interpret dark mudstone layers based on logging and seismic. These data are combined with geochemical test data from hydrocarbon source rocks to assess its quality and predict its distribution. This study indicates that the three types of unconformities identified divide the Yingcheng Formation into four sedimentary sequences. In the Yingcheng Formation depositional period, the number of grabens or half-grabens associated with the Changling Fault Depression decreased, the size of the rift basins increased, and the depositional center migrated to the north and west. During the depositional period of the First and Second Members, volcanic rocks were widely developed, and mud sediments were deposited between several small grabens or half-grabens. During the depositional period of the Third and fourth Members, the study area became a unified lacustrine rift basin with clearly defined geological signatures developed in each sedimentary sequence, which can be divided into three systems tracts. Mudstones in the Lake Transgressive Systems Tract are laterally continuous and widely distributed. The organic matter in these mudstones, with fair to good abundance levels, is predominantly type II2 and type III kerogen and all have reached mature to highly mature stage of thermal evolution, which are favorable hydrocarbon source rock layers. In plan view, the favorable hydrocarbon source rock zones are mostly formed in the Changling Sag, Dongling Structural Zone and Longfengshan Sag. This study offers guidance and direction for deep hydrocarbon exploration in the Changling Fault Depression, and also provides a reference for methods and ideas to predict hydrocarbon source rock distribution in other lacustrine rift basins.