Characteristics And Genetic Mechanism Of Tight Reservoir In The Second Member Of The Upper Triassic Xujiahe Formation In The Middle Part Of Western Sichuan Depression

The sandstone of Xujiahe formation in the middle part of western Sichuan depression is not only the important producing layer and gas reservoir, but also the one of the most deeply buried and tightest clastic reservoir. As one of the most important formation of Xujiahe group has the general complex change in the diagenetic stage because of the large burial depth and geological features, which results in the densification and the extremely strong heterogeneity of reservoir,and the mechanism of reservoir formation is also not clear. These factors above restrict the oil and gas exploitation. This article guided in the theories of sedimentology, petroleum geology, reservoir geology, based on analysis of the core observation, thin section, physical property, scanning electron microscope, X ray diffraction, carbon and oxygen isotope and temperature measuring of fluid inclusion, has the analysis and discussion to the reservoir characteristic and the controlling factors of densifying.The article has determined the sandstone’s type and distribution of the second member and fourth member of the Xujiahe(Upper Triassic) Formation(hereafter called T3x2 and T3x4) in the target area by pointing four thousand data collected about sandstone components in the QFR sandstone-classification triangle, using the sandstone-classification standard of junbao Liu(1980);The sandstone of the T3x2 and T3x4 in the target area is composited with lithic sandstone mainly, followed by feldsparlithic sandstone and lithic-quartz sandstone, performing as this characteristics poor feldspar-rich lithic, which was particularly prominent in the sandstone of T3x4.Through the analysis of sandstone composition and structural features, we can find that the matrix content in the sandstone of T3x2 and T3x4 in the target area is very low, interstitial material is composed of cement mainly, carbonate cements is a major cement and some siliceous cement and clay cement also exist; carbonate cements of sandstone of T3x2 is dolomite cement-based, carbonate cements of sandstone of T3x4 is dolomite cementoriented, sandstone of T3x2 contains higher quantities of Siliceous cement than that of sandstone of T3x4, sandstone of T3x4 contains higher quantities of illite than that of sandstone of T T3x2, Kaolinite of sandstone of T3x2 is rare, Kaolinite in the top layer of T3x4 is rich; The sandstone of T3x2 and T3x4 has a Medium structure-maturity, the grain size of sandstone of T3x2 and T3x4 is medium-fine dominated, sorting is good mainly, rounding is subangular, the way of sandstone cemented of T3x2 is pressure and pore-pressure oriented, while T3x4 is pore based.We research on diagenesis’ s characteristic by analyzing the core observation, thin section, physical property, scanning electron microscope, X ray diffraction, carbon and oxygen isotope and temperature measuring of fluid inclusion: diagenesis of sandstone of T3x2 and T3x4 is based on compaction and pressolution, carbonate cementation, siliceous cementation, clay mineral cementation and dissolution. The cause of carbonate minerals in the T3x2 related to biogenic gas, the cause of carbonate minerals in the T3x4 related to organic acids; The main source of siliceous cement is extraneous siliceous, quartz detrital, pressolution and solution from aluminosilicate such as feldspar, pressolution of T3x2 provided more extraneous siliceous. The main source of clay minerals are dissolved feldspar, kaolinite of T3x2 transform to illite basically, kaolinite of T3x4 is rich and feldspar is poor; The object of dissolution is feldspar and lithic contained feldspar, dissolution medium is organic acids mainly, dissolution is more developed in the T3x4.Based on the analysis of the types and mechanism of diagenesis, we studied on the diagenetic stage and sequences: the sandstone of T3x2 and T3x4 in the target area is in the late diagenetic stage mainly, some is in the mesogenetic A~B stage, the whole sequences can be show as: early diagenesis A stage, early diagenesis B stage, mesogenetic A stage, mesogenetic B stage and the late diagenetic stage. Overall the diagenetic sequences is as follows: Even early calcite cement growth â†'authigenic chlorite â†' quartz I Increasing â†'dissolution of feldspar and lithic, kaolinite precipitation â†' quartz II period increasing, intergranular authigenic quartz â†' authigenic illite growth â†'dissolution â†'late carbonate cementâ†' late dissolution.This article researched on the types of pore and characteristics of pore and petrophysical properties through several test methods such as slice observation and mercury-injection test. The main types of the sandstone of T3x2 and T3x4 in the target area are residual intergranular pore, intergranular dissolved pore, intragranular pore and cracks, secondary porosity are the main reservoir space. The characteristics of the sandstone of T3x2 and T3x4 in the target area are as follows: bad pore configuration m strong heterogeneitym, capillary pressure curve’s platform is not obvious, slightly rough skewness. The porosity and permeability of the sandstone of T3x2 and T3x4 is very low, the reservoir properties is low, and permeability is low- ultra-low, the properties of T3x2 is lower than T3x4, and the permeability of T3x2 is higher than T3x4. The relationship between porosity and permeability of T3x4 is positively correlated, showing that permeability’s major controlling factors is porosity; the relationship between porosity and permeability of T3x2 is not very good, because of a large number of crack.Based on the achievement about the characteristics of petrology, diagenesis and pore, we can continue to study the major factors of leading the sandstone of T3x2 and T3x4 in the target area densifyed. Compaction made pore loss is about 70%, Quartz particles, or early chlorite growth played a important role of resisting compaction; The chlorite of T3x2 had very good protection effect on primary pore, while the chlorite of T3x4 had no function; The illite and carbonate cement of T3x2 and T3x4 had destructive effect on porosity and permeability; Overall, cement reinforced the progress of the sandstone densifying; Dissolution and fracture can greatly improve the reservoir sandstone physical property, the dissolution intensity of the sandstone of T3x2 and T3x4 in the target area is weak, and some dissolution holes have mostly been cement filling, so dissolution effect on the improvement of the permeability sandstone ability is limited.