Study On Sequence-paleogeography And Sand Body Distribution Of The Xujiahe Formation Of Upper Triassic In Sichuan Basin

The Xujiahe Formation of Upper Triassic in Sichuan basin is mainly researchobjects of this thesis. First, by incorporating the tectonic evolution of Sichuan basinand Songpan-Ganzi fold belt into Tethyan tectonic domain for integral analysis, theconclusion is drawn that the western Sichuan depression was a intracontinentalforeland basin superimposing on Songpan-Ganzi peripheral foreland basin which hadfolded inversion before the Xujiahe Age of Late Triassic. As result, Sichuan forelandbasin is divided into three evolutionary phases - early peripheral foreland basin stage(T_3m-T_3t), middle intracontinental foreland basin stage (T_3x-J_3p) and late shrinking andextinct stage (K₁-E₂).Second, according to the analysis of base-level cycles, a new correlation betweenXujiahe Formation and Xiangxi Group of Upper Triassic in Sichuan basin issuggested. At the same time, the Xujiahe Formation is divided into five long-termsequence cycles corresponding to the second member to the sixth member of XujiaheFormation, and Each long-term sequence cycle consists of 3-4 middle-term sequencecycles. By choosing the rising and falling base-level phases of each middle- andlong-period sequence cycle as the isochronous mapping unit, the sequencepaleogeography maps of Xujiahe Formation have been drawn in Sichuan basin, whichindicates that the sedimentary systems controlled strictly by tectonic pattern have acertain regularity: from basin margin to center - alluvial fan, fan delta, braidedstream-delta and shallow lake. Therefore, the distribution of sedimentary facies of theXujiahe Formation in Sichuan basin was characterized by "one paleouplifi surroundedby three depressions partially". "one paleouplift" was central Sichuan paleouplifl, and"three depressions" was West Sichuan foredeep depression, Northeast Sichuanfrredeep depression and East Chongqing-Southeast Sichuan craton depression.Finally, by applying the geologic, logging and seismic methods, amutiple-discipline comprehensive research on the distribution of sand bodies ofMember 4 of Xujiahe Formation has been done in Xiaoquan, Xinchang andHexingchang of West Sichuan depression.In addition, on the basis of the study on different subbasins in Sichuan basin, the different filling patterns of foreland basins and craton basins have been drawn.â‘ The two sides of foreland basins take on asymmetric sequence filling patterns. On theside of foreland thrust belt, with intenseness of tectonic activity in orogenic belt, thesedimentary supply and accommodation show a tendency to increase in positiveproportion together. However, the sedimentary supply is (much) more thanaccommodation, and the grain size of the sediments is coarser. In addition, becausethe cutting erosion is strong on the side of foreland thrust belt, lowstand system tractdevelops relatively, but highstand system tract reserves incompletely by virtue of latedenudation. On the side of slope belt, the sequence filling patterns is mainlyinfluenced by the change of accommodation, which is controlled by distal effect ofthe thrusting event in basin margin indirectly. The sedimentary supply varies inreverse proportion to the accommodation. The grain size of the sediments is fine onthe side of slope belt. Because the tectonic activity and cutting erosion are not strongenough, lowstand system tract develops incompletely, but highstand system tractdevelops relatively. The depositional center of foreland basins locates in foredeepdepression closing to the side of foreland thrust belt.â‘¡The sequence filling patternsin two sides of craton basins are similarity and symmetry where lowstand system tractdevelops incompletely, but highstand system tract develops relatively. Thedepositional center of craton basins locates in the middle of craton depression.â‘¢The reservoirs of Sichuan basin formed during the tectonic active period, and thesource-cap rock formed during the tectonic inactive period.