GENETIC RESEARCH OF RESERVOIRS AT DELTA FRONT IN DEPRESSED LACUSTRINE BASINS

During the Late Triassic,the Ordos Basin is an asymmetry depressed lacustrine basin with gentle slope in the east and steep slope in the west.In the Yanchang Formation,oil is mainly distributed at the gentle slope in the east.With the short-term base-level cycle theory of high-resolution sequence stratigraphy,it is analized in this paper the reservoir sandstones deposited at delta front in the Chang VI oil formation in Jiyuan and Anbian,the Ordos Basin.According to genetic difference,reservoir sandstones may be divided into three types and seven subtypes,evidently influenced by the short-term base-level up and down changes and the relation between accommodations and sediment alimentations.As the base-level goes up and down,regular changes of short-term base-level cycle affect the distribution of different genetic sandstone bodies.And the distribution model of different genetic sandstones deposited at delta front in depressed lacustrine basins is built.The delta front in this area can be divided into three parts: platform delta front,sloping delta front and basin delta front by the bounds of slope break.Reservoir sandstones are genetically different at different parts. Truncated and integrated channel sandstone bodies are found at platform delta front.Downside deformity recombination bar sandstone bodies are found at sloping delta front.Integral recombination bar,upside deformity recombination bar,isolated mouth bar and reiterative mouth bar sandstone bodies are found at basin delta front.The compartment scheme of delta front and the discription of reservoir bodies can be helpful for high quality reservoir identification and prediction,and it can also serve to subdivide flow units during oil exploitation.