Sequence Stratigraphy And Depositional Systems In The Xujiahe Formation, Upper Triassic System, Western Sichuan Basin

Based on terrestrial sequence stratigraphy, sedimentology and data of log, seism, rock and mineral experiment and geochemistry, this paper has delta with the sequence stratigraphical framework, sedimentary systems, accommodation transformational system and reservoir models of the exploration of subtle traps in the western Sichuan basin.Xujiahe Formation consists of 4 sequences . The paper emphasizes more sequence I and sequence II. Based on microfacies of cores and sand body types as well as facies analysis, the main types of sedimentary facies in the Upper Triassic sequence of western Sichuan Basin have been identified. The delta and fan delta facies dominate reservoirs. With correlation of sedimentary provenance, sand body geometry and environmental parameter, the depositional systems and sedimentary facies sequence are described in detail.During the evolution of a basin, the changes of accommodation in different areas are not always consistent. There is a deposition梕rosion neutral spot in a basin. On one side of the spot the accommodation increases and on the other side it decreases. Based on this phenomenon, the concept and research methods of accommodation transformational system are put forward.The accommodation transformational system is composed of accommodation transformational system and accommodation transformational Body, which includes Increasing Body, and Decreasing Body. Accommodation transformational system can be used to resolve the disunities during the developing of a basin. The accommodation transformational systems in the foreland basin can be divided into 7 orders. On the plane, the accommodation transformational system can be divided into 2 parts, i.e., increasing belt and decreasing belt. The accommodation transformational systems in Longmenshan foreland basin have been studied in terms of spot, line surface and body. Accommodation transformational system of the Xujiahe Formation is analyzed more carefully and it consists of a unified increasing body and a disunited body. On the plane, there are some relativities between accommodation transformational system and tectonics, sediments, sedimentary facies, paleo梬ater depth, and the types and distribution of sandbodies.The transformational belts in a accommodation transformational system has a close relation to the oil and gas occurrence in Longmenshan foreland basin. The formation of the gas pools in Xujiahe Formation was controlled by sequence stratigraphic framework. Through the analysis of sedimentary facies , the distribution of sandstone and the tectonic evolution in the Xujiahe Formation, the author point out that tectonic fracture and lithology complex trap types control the reservoir formation.