Abstract:

Fuyu oil layer of Sanzhao Depression is one of the important oil-gas exploration areas in the north of Songliao Basin. Fuyu oil layer mainly is a set of river-shallow water delta deposition. Because single layer thickness of channel sand body is thin and its lateral change is quick, the study of sedimentary microfacies is necessary to conduct, which guide the exploration of stratigraphic-lithologic oil and gas reservoirs. Taking Zhao 35 block as an example, applying the high-resolution sequence stratigraphy principle, sequence boundary is identified and sequence stratigraphic is divided. Taking short-term base level cycles as mapping units, sedimentary facies distribution maps within the framework of the sequence stratigraphic are drawn, the vertical evolution laws are analyzed and sedimentary microfacies model is established. The results show that Quan 4^th Member is a long rising half cycle, which can be divided into 7 short-term base-level cycles (SSC1-SSC7). The study area mainly develops delta plain and delta front subfacies, which can be further classified into (underwater)distributary channel, (underwater) crevasse splay, natural levee and so on. SSC1 and SSC2 periods of 7 short-term base-level cycles are unsymmetry cycles dominated by ascending half cycles because of slowly rising base level. SSC3 and SSC4 periods are unsymmetry cycles dominated by ascending half cycles and symmetry cycles because of quickly rising base level. SSC5 to SSC7 periods are unsymmetry cycles dominated by ascending half cycles and base level rises to the highest. Sedimentary facies and its distribution are mainly controlled by the change of base level. The study area experiences the evolution of delta plain subfacies—plain, front subfacies transition—delta front subfacies—local shallow lake facies. In vertical direction, Quan 4th Member exhibits a lake transgression process, whose sedimentary water changed from shallow to deep.

Key words: high-resolution sequence stratigraphy, sedimentary facies evolution, base level cycle, Quan 4^th Member, Block Zhao 35