Discovery of Hesigewula Sag on the western margin of Da Hinggan Mountains in China and its significance in petroleum geology

With Songliao Basin in northeast China entering the middle-late stage of the exploration and development of oil and gas resources, it is urgent to open up new areas for oil and gas exploration on the periphery of the basin. Five sedimentary sags have been discovered in the western margin of the Da Hinggan Mountains through high-precision gravity, magnetic, and electrical prospecting engineering. According to the conditions of the 5 sags such as buried depth, area, and the scale of source rock, Hesigewula Sag was preferentially selected to deploy two-dimensional seismic prospecting and drilling. As a result, the stratigraphic distribution and structure of Hesigewula Sag were preliminarily ascertained, the main sedimentary facies developed in Aershan Formation-Tengger Formation of the Lower Cretaceous was summarized, and the assessment parameters of source rock of Hesigewula Sag were obtained. According to the research results, the Hesigewula Sag can be divided into 3 second-order tectonic units including the western bulge, the central sag, and the eastern bulge respectively, and the sag was formed mainly subject to the west fractures. Three sedimentary facies including fan delta facies, braided river delta facies, and lacustrine facies have been identified in the Aershan Formation-Tengger Formation of the sag. The source rock in the first member and second member of Tengger Formation in the sag features high abundance of organic matter, meeting the standards of good source rocks. The kerogen of the source rock is mainly of type II₁ – II₂. The organic matter in the source rock is characterized by low maturity and it entered the evolution stage of low maturity from the second member of Tengger Formation. Furthermore, Hesigewula Sag bears a strong similarity to other petroliferous sags in Erlian Basin in structure, sedimentation, source rock, and other characteristics. Therefore, it can be preliminarily determined that Hesigewula Sag boasts great potential for oil and gas resources.