The Formation Mechanism Of Gas Pools In The Forth Member Of Upper Triassic Xujiahe Formation In Central Sichuan Basin

Taking the gas pools in 4th member of Upper Triassic Xujiahe formation in the central Sichuan basin as target, this dissertation researches the palaeo-structures, sedimentary facies (sedimentary subfacies, sedimentary microfacies), reservoirs, hydro-chemical fields, hydrodynamic fields, source rocks and the distribution characteristic of oil and gas, and reveals the formation mechanism of the gas pools in the 4th member of Xujiahe formation, then sets up gas accumulation model, confirms the beneficial prospecting targets and acquires results as follows:The sedimentary facies in the 4^th member of Xujiahe formation are northern and southern zoned, and the zonality of sedimentary facies leads to various accumulation factors in this section. This study fixes the source area of Xujiahe formation in central Sichuan basin, sets up the sedimentary model and reconstitutes the space distribution pattern of sedimentary facies, subfacies and microfacies in each section of it. The facies of the north and northeast of the lower 4^th member of Xujiahe formation are deltaic plain and delta front, the rests are sandbars, the south may be river mouth bars and mere sandbars, the north of the upper 4^th member are fluvial facies, deltaic plain facies and deltaic front facies, the northeast is deltaic front facies, the rests are mere and mere sandbar facies. The compositional maturity of the reservoir is from moderate to poor, and the textural maturity is from moderate to good. It is the type of low porosity and permeability, and secondary pores dominant. Sedimentary microfacies control rock types and reservoir physical properties, and the reservoir differentiates from north to south. This dissertation also researches the petrology characteristic and diagenesis of reservoirs in the 4th member: the compositional maturity of the sandstone is moderate to poor and the textural maturity is moderate to good; the destructive diagenesis includes: mechanical compaction, pressure solution, cementation, secondary growth, replacement, formation of authigenes; the constructive diagenesis includes: chlorite cements cycle in the eogenetic stage, denudation of clastic and interstitial matters. Then we classify the diagenesis sequence and form the diagenesis model of pore evolution of reservoirs sandstones in Xujiahe formation.The characteristics of pore development and its influencing factors are as follows: (1) main pore types include: inter-granular pores, inner-granular pores, matrix pores, grain boundary pores, break splits; secondary pores are important, and they may improve porosity about 3%-5%, usually formed by denudation of feldspars, also matrix and detritus; (2) the control factors include: compaction and cementation decline porosity, chlorite cycles could protect pores, denudation increases porosity, the porosity of grit-medium sandstone is higher than that of medium-post sandstone, and sedimentary microfacies control rock type sand reservoir physical properties. Type I reservoirs in the 4^th member are apparently in a particular depth; There are three subfacies (brink-mere, slope in pre-delta, deltaic plain above water ), and microfacies such as bars in mere, branch channels underwater, river mouth bars, branch channels above water, point bars. Reservoirs are mainly developed in microfacies above, the frequency of developing type I reservoir in pre-deltaic slope and deltaic plain above water is higher than in brink-mere subfacies. The 4^th member formation mainly develops type III reservoir. Type I mainly distribute at Bajiaochang-Jinhua-Qiulin region, then distribute at Chongxi, Moxi, Suinan. The plane dominant facies in Bajiaochang-Jinhua-Qiulin region are point bar microfacies in deltaic plain above water, river mouth bar in pre-deltaic slope, and branch channels underwater. That in Moxi, Suinan, Guang'an, Tongnan is sand bar in brink-mere subfacies.Formation water originates from sedimentary water, pressure elucidating water, surface water, atmospheric water and condensation water. Among them, pressure elucidating water has the most volume, has had the longest influence and is most related to oil and gas. It is controlled mainly by the third hydro geologic cycle. In the region of interest there exists two independent hydrochemistry fields at the north and south, and the water alternation condition of the one in the north is better, with little hydrochemistry difference, low ion concentration, and much concordance in many coefficients. However, at the one in the south, groundwater flow is complicated, the relation of different structural belts is poor, convection of hydrochemistry elements is difficult, and the water concentration is high. There are five concourse models in the reservoir subsurface water: low potential-tinajita type (trap type) , potential gradient change type(platform edge type), water potential step type, half affluxion type(winnowing fan type), saddle current type, and transition or mixed type. According to research, potential gradient change type low potential-tinajita type and water potential step type play essential roles in gas accumulation in 4th section Xujiahe formation in central Sichuan basin. This region which is almost at the slope or high localization of Sichuan basin is the migration target of source hydrocarbon generated by Zitong and Chengdu depression. Fault systems are very developed in the northeast of central Sichuan basin; they improve the reservoirs with low porosity and permeability of 4th member of Xujiahe formation. Bajiaochang-Jinhua-Qiulin structure high situs always exists and is close to Zitong depression, gas resource is relatively abundant so it is favorable for gas accumulation. Nanchong-Chongxi structure, which is at the slope of paleo-structure, easily catches and accumulates the resident hydrocarbon. Structures such as Penglaizheng, Guihuachang, Suining, Moxi, Longnusi, Guang'an at the south are also favorable for hydrocarbon accumulation. Since Tertiary, tectonics movement in Central Sichuan is mainly divergence ascend, it redistributed gas pools, which complicated the oil-gas-water distribution.Reservoir in 4th member of Xujiahe formation is low in porosity and permeability, in which displacement pressure is high, grain is poorly sorted, the pore throat radius is small, and gas filling is not sufficient.Gas and water layers are centralized in the middle and lower section of the 4th member of Xujiahe formation, gas and water distribution is rigorously controlled by lithology, and the structure fluctuation is just supplementary to it. In a definite storage space, if reservoir condition is feasible, formation water permeates to the lower location of the structure, while hydrocarbon accumulates at a higher location, forming pools. On the contrary, distribution of gas and oil is complicated. Reservoir physical property gets poor at the slope, the phenomenon of gas at bottom and water at top appears and then irregular gas accumulation forms. At the lower location of the structure, it develops lithological traps or traps sealed by faults, even leads to the phenomenon of gas at bottom and water at top during different layers.The formation mechanism of the gas pools in the 4^th member of Upper Triassic Xujiahe formation in central Sichuan basin is mainly controlled by sedimentary facies and lithology. Its characteristics include low porosity and permeability of reservoir rock, insufficient petroleum filling, and abnormal distribution of water and gas.