Stratigraphic sequences and sedimentary evolution in the Yaojia and Nenjiang Formations, Songliao Basin

The Yaojia and Nenjiang Formations in the Songliao Basin consist of a succession of transgressive-regressive depositional cycles deposited during the Late Cretaceous, which may be subdivide into two second-order cycles and six third-order cycles, with a time duration of about 4 Ma. The oil shales at the base of the second member of the Nenjiang Formation deposited during the maximum flooding period are interpreted as both the type boundary for stratigraphic correlation and important reservoir and cap rocks. The first member of the Yaojia and Nenjiang Formations may be assigned to the retrogradational delta deposits buried during the transgressions, while the third member of the Nenjiang Formation is built up of mixing lake and delta deposits formed during the regressions. The large-sized gravity flow channels and terminal turbidite fans appeared during the trangressions, whereas the fluxoturbidite fans occurred during the regressions in the Nenjiang Formation. The Upper Cretaceous Yaojia and Nenjiang Formations are believed to be the key hydrocarbon reservoirs in the Songliao Basin.