The Formation Of Western Sichuan Foreland Basin And Its Significance In Oil-gas Exploration During Late Triassic

The western Sichuan Basin refers to an wide area of about 4×10⁴km² located to the west of Nanjiang-Santai-Lezhi and to the east of Anxian-Dujiangyan-Shuangshi fault. For a long time it is being argued in its nature, development, evolution and formation mechanisms. This paper analyzed the formation of western Sichuan foreland basin during late Triassic by analyzing two processes including the marine carbonate rocks to marine clastic rocks and the marine clastic rocks to nonmarine clastic rocks. We study from the angle of sedimentology and based on studies of 3 ourcrops in the front of Longmen mountain and studies of Chuanfeng 563 and Fenggu 21 dring core section in combination with systematically analysis of detrital zircons derive from upper Triassic in western Sichuan foreland basin using LA-ICPMAS U-Pb dating. As a result we fully understand the formation of western Sichuan foreland basin during late Triassic.T_3m-T_3t, the marine carbonate rocks transferred into marine clastic rocks in western Sichuan foreland basin. There were quite different between the north and the south of western Sichuan basin. In the north, the clastics interfered with the growth of reefs. In the south, a great quantity of black mudstone deposited. The north process was extensional while the south process was compressive. The sediments in south of western Sichuan foreland basin were marine-nonmarine transition facies at the beginning of T_3m and had completely transferred into marine clastic deposits by T₃t. However, the carbonate rocks could still be seen in some lower position of T₃t in the middle-north of western Sichuan basin. It presented a palaeogeography that had northern and southern uplift, north-central, southern-central depression. The north and south provenance progressing into basin, which showed the end of regional extension and the beginning of the foreland basin stage in the south of western Sichuan as the compression of northeast-southwest. Up until Norian, the extension totally ended and the western Sichuan basin started its new stage as a whole foreland basin.T₃x₂,the north part of Longmen mountain which located on the northern part of western Sichuan basin uplifed strongly, which resulted the north part of western Sichuan foreland basin once been exposure. The center of subsidence was at Jiangyou-Mianzhu and there were mainly deposited sandstones. The northern part is mainly channel and tidal influenced delta.T₃x₃, the Longmen mountain is relatively equability and deposited a lot of mud rock. The center of subsidence move southward to Mianzhu-Djiangyan.At the end of T₃x₃, Longmen mountain uplifted strongly. It regarded as the largest scale of uplifting, so called"Anxian movement". Songpan-ganzi folded back heavily. As a result, there were fan deltas and basal conglomerate deposited around Guangyuan and Jiangyou. However, the study of detrital zircons shows that the major provenance of western Sichuan foreland basin were still Qinling mountain, which means that the"Anxian movement"effected limited.After uplifting during T₃x₃, the Longmen mountain had already grown above the water face. A great quantities of deposits came from Sonpan-ganzi fold belt and Longmen mountain were carried into western Sichuan foreland basin. At that moment, the subsidence center were mainly located at Pengzhou, south part of western Sichuan basin. AS the uplift of northern part of Longmen mountain slowered down and the compaction rate equaled the subduction rate, there were no deposit in Guangyuan. As a matter of the fact that the middle-south part of Longmen mountain were still mainly beneath the water surface and the subduction rate were still faster than compaction rate. Therefore, deposit went on there.T₃x₅, the subduction center moved further southward to Pengzhou-Dayi. When south part of the Longmen mountain uplifted more and more clastics deposit flew into the southern part of western Sichuan foreland basin, the middle-south of the Foreland basin had already begun to regression. At last, as the same time as extensive orogenity of southern part of Longmen mountain and the closesure of southern western Sichuan ocean trough, the transitional facies replacemented rapidly by nonmarine lacustrine facies, which marked the end of formation of western Sichuan foreland basin during Triassic.The middle-north part of Longmen mountain uplifted earlier, therefore the middle-north part of western Sichuan basin located at the higher part and had coarser sediments, which is good for forming ordinary reservoirs. The south part of western Sichuan basin grown as a foreland basin earlier than the middle-north part and had experienced long term sinking at high speed, therefore it had thinner deposits, preserved faster and rich in organic matrix, which was best for generating hydrocarbon and was keen to forming deep basin gas in large area during early stage.T₃x₄- T₃x₅ the western Sichuan foreland basin experienced diachronic marine regression from northeast to southwest, which was caused by compaction of clastic compaction. There is a paraconformity lies in the upper part of T₃x₄- T₃x₅. This interface had long been exposured and leached by fresh water and is a worthy place to study in future.