Study On The Sequence Stratigraphy And Sedimentary System Of Triassic Xujiahe Formation Fifth Segment Inxinchang Area,Western Sichuan Depression

According to the theory of high-resolution sequence stratigraphy, sedimentology and other theories, and combined with regional geology, logging data, seismic, well cores as well as outcrop data, single well logging and seismic sequence of the Triassic Xujiahe Formation Fifth Segment in Xinchang area are analyzed, this thesis establishes the high-resolution isochronous stratigraphic framework, and within the sequence stratigraphic framework the distribution characteristics of sandbody and the distribution rules of sedimentary system are studied. Finally this thesis sets up the sedimentary model, and then predicts the advantageous reservoir face belts, in order to provide theoretical basis for reservoir exploration and development of the Triassic Xujiahe Formation Fifth Segment in Xinchang area.The Triassic Xujiahe Formation Fifth Segment was segmented into a long-term cycle, 3 middle-term cycles and 9 short-term cycles. The 3 middle-term cyclic sequences have good comparison relationship with the upper, middle and lower segments,the 9 short-term cyclic sequences have fine parallelism with 9 sand units.The base-level cycle sequence structure in the study area can be divided into 3 types, namely the upward "deeper" asymmetric cycles(A), the upward " shallower" asymmetric cycles(B) and symmetry turning upward "deeper" then "shallow"(C). Research shows that the short-term cycle structure presents diversity characteristics, 3 types of cycle structure are all developed, among of which A type sequence structure often develop in the lower segment, the upper segment is given priority to B type sequence structure, while the C type tends to develop in the middle segment. The middle-term cycle structure develops C1 and C2 type sequence structure, while the long-term cycle structure only develops C1 type sequence structure, namely the sedimentary thickness of ascending half cycle is thicker than the descending half cycle, which reflects a slowly lake invasion to rapid lake retreat process.On the basis of well logging data, combined with well cores and outcrop data, this study analyzes on sedimentary face-marks, logging facies and single well facies. It comes to the conclusion that the Xujiahe Formation Fifth Segment in Xinchang area develops a shallow lake-delta sedimentary system, 2 kinds of subfacies(delta front and shore-shallow lake subfacies), and can be further devided into 6 kinds of microfacies, namely underwater distributary channel, distributary bay, mouth bar and shore-shallow lake mud and sand-mud flat, beach dam.In this paper, the mid-term cyclic sequence is choosed as isochronous stratigraphic unit to carry out the research on plane distribution characteristics of sedimentary facies, the upper and lower segments are given priority to delta front deposition, while the middle segment tends to develop shore- shallow lake face. It summed that the sedimentation model is delta-lake, the north and west of the study area are close to provenance, where often deposits channel sand. As the water deepens to south, the south of the study area mainly develops shore-shallow lake. Based on the analysis of sedimentary facies and high productive lithologic combination, it comes to the results that the most advantageous face zones for gas reservoir development is channel.