The Tectonic Characteristics Of Long Wangmiao Formation In Sichuan And Its Relationship With The Gas-oil Reservoir

Sichuan Basin is located in the west of the Yangtze platform, which is the most important area of the oil and gas exploration in South China. During the Sinian to the middle Triassic, with a set of carbonate sediments. The vertical forms complete sets of source reservoir cap combination, which has favorable petroleum geology conditions. The Sinian and early Paleozoic of Sichuan Basin is one of the important potential exploration target layer in Sichuan Basin. The exploration of Sinian–early Paleozoic in Sichuan Basin is from the mid fifty’s, has a history of nearly 50 years. But now,the focus of gas reservoir is gradually transferring from Sinian to Longwangmiao Formation of Cambrian. Based on the achievements of predecessors, the main idea of this thesis is through studying the tectonic evolution and the modern tectonic characteristics of the paleo-uplift in the central of Sichuan petroliferous basin to analysis the relationship between tectonic characteristics and Cambrian system reservoir. In this thesis, there are two ways were combined to study the tectonic characteristics of the paleo-uplift which are geologic method and geophysical method. The main idea of geologic method is a progresses of combining point, line and plane, about this thesis’ s research contents, the first step is collecting well data of Sichuan petroliferous basin, the wells should all be drilled out Cambrian system or deep layers, and then we should statistic the thickness of every layer. In Sichuan basin there are some areas have no wells that we can not afford to statistic the thickness of layers, so we must applicate some field sections and seismic sections those locate in places where have no wells in Sichuan basin to read the thickness of every layer, after that we can use these well points and the points of both field sections and seismic sections to recover the denudation quantity from Cambrian to Siluric by the way of trend method, and then, the last step is drawing paleo tectonic pictures of the top surface of Cambrian in every ages; Then using 2DMOVE software to draw the tetonic evolution sections. Because of organic combination of geologic method and geophysical method that we can draw many tectonic related pictures of paleo-uplift, therefor we can deeply analysis the ancient tectonic evolution, current tectonic characteristics and the relationship between tectonic characteristics and Cambrian reservoir, after that analysis we find some new geologic cognitions about paleo-uplift in the central of Sichuan basin:(1) The Central uplift belt is between the east of the Longquan mountain and the large areas of Jurassic red strata distribution area of the west of the Huaying mountain, which is in the weakest deformation region of Sichuan Basin, the main performance is the flat fold deformation of the large uplift, no growth of the fault, gentle stratum, structural trend of EW direction. It is in the state of the relative uplift during Palezoic and Mesozoic. The total thickness of the sedimentary cover is only the half of the west and east of Sichuan. The hardening degree of the basal is high and the buried is shallow. The main body of the present tectonic is Weiyuan and Longnvsi structure, the main tectonic line direction is NEE.(2)During the end of sedimentary in the Early Cambrian, the Leshan-Longnvsi paleo uplift is along the axis of NEE, Ya’an-Leshan is a nose like structure, Moxi-Longnvsi is a large trap. From the late Cambrian to late Silurian, Ya’an-Leshan-Moxi-Lonvnvsi unified a giant nose shaped uplift, Moxi-Longnvsi is a complete trap in this giant nose shaped uplift, which is relatively independent, which has a large scale in the Caledonian movement.(3)Through the balanced cross section, based on the principle of conservation of area before and after deformation and the conservation of length of the main geological interface, study on the three frame section, come to the conclusion, Leshan-Longnvsi paleo uplift is growth anticline in the sedimentary period of the Cambrian and Silurian, at the end of the Caledonian movement has a significant tectonic deformation, the uplift of the core, the strata of Cambrian-Silurian was razed to the ground. Weiyuan structure is locataed in the east slope of paleo uplift in Leshan-Longnvsi in the Caledonian-Indosinian stage, and become structural highs after the uplift in the Himalayan. The sedimentary of eastern Qiongzhusi Formation is thickness in Weiyuan structure, which is a favorable hydrocarbon generating area.(4)The axis of paleo uplift, in the tectonic evolution history, conducted a anti clockwise rotation from the NW-SE of the early Paleozoic, the nearly east-west of Late Paleozoci, to the NE-SW of Triassic Paleogene. The area of Ya’an, Leshan, Ziyang, Gaoshiti-Moxi-Longnvsi, Nanchong, Guang’an is the favorable position of oil and gas migration direction, shich is long-term in the high part of structural and adjacent to the ancient depression, Weiyuan structure was located on the high position of Ziyang paleo slope, which was higher than the ancient tectonic location of Ziliujing, Panlongchang, Tiangongtang. It was in the long-term tectonic depressions in South Sichuan area, East Sichuan area and parts of North and northwest of Sichuan Basin. The area of Tongjiang-Kaijiang has been in a relatively high position of the structure which is the favorable area for oil and gas migration direction.(5)In the east of the palaeohigh, including the area of Anyue, Wusheng, Suining in Gaoshiti and Moxi, had been formed ancient gas reservoir in late Silurian. The negative impact of the uplift is weak during the Caledonian movement. It is an oil enrichment gas zone, which is an area of modern structural superposition and is inherited.Although suffering the Himalayan tectonic movement, it still maintined an outlook of a positive uplift. On the other hand, the transformation of this area of the Himalayan period resulting in oil and gas redistribution, is the most favorable exploration blocks.