Structural Fracture And Its Development Regularity In The 3rd Member Of Xujiahe Formation Within Dayi Structure, Western Sichuan Depression

The 3rd Member of Xujiahe Formation in the Dayi structure is a favorable exploration area for deep-seated oil gas in western Sichuan Depression. The reservoir in the 3rd member of Xujiahe Formation is composed mainly of fine-grained quartz sandstone with very low porosity and permeability, which is a typical low-permeability sandstone reservoir. The fracture has an important impact on the reservoir and its infiltration capacity . The fracture is main channel for the seepage and migration of oil gas, especially for the low-permeability sandstone reservoir. Moreover, they play an important role in the formation and distribution of oil gas. Therefore, the research of structural fracture in the low-permeability sandstone reservoir of the area has very important significance.This thesis takes the low-permeability sandstone reservoir in the 3rd Member of Xujiahe Formation in Dayi structure as the main research object. Based on the core observations and logging image data, together with the regional geology, we described and analyzed the characteristics and effect factors of the structural fractures in the reservoir. By means of the prediction of fracture based on numerical simulation of tectonic stress field and make use of the rock mechanical parameter and rock rupture criteria, we predicted the development regularity of structural fractures in the reservoir and comprehensively evaluated the development of structural fractures. Through the comprehensive analysis of the area, we get following conclusions: in this area, the most widely distributed low-angle fracture and followed by vertical fracture, while development of high-angle fracture at least. The fracture trend is mainly nearly EW, NEE-SWW,NW-SE and nearly SN, which tend to mainly N, NNW, SW and E. Formation of structural fractures in 3rd Member of Xujiahe Formation is affected by Late-Indosinian, Yanshannian and Himalayan movement, while structural fractures in this reservoir is mainly formed during Yanshannian and Himalayan period. It was tectonics stress field, tectonic position, mechanical properties of rocks and strata thickness that affect formation of fractures. Modeling of tectonic stress field illustrates that the orientation of maximum principle stress is NWW-SSE and shear stress values locates between 25~40 Mpa in late K1, and NW-SE and 12-16 Mpa in nowadays. The value of shear stress is apparently larger in plunging zone in Dayi structure and is smaller in the middle, in part of which the shear stress value becomes larger for the regional faults affection. The value of shear stress expresses gradient changes near the main fault and increase apparently in conjuncture of faults. The result indicates that there are four levels of developed structural fractures. It is better in Dayi 1st well, Dayi 3rd well, Dayi 7th well and Dayi structure, well in Dayi 6th well and Dayi 102rd well, fair in Dayi 4th well, west plunging zone and north and south shoulder zones, worse in Dayi 2rd well, Dayi 5th well, Dayi 101st well and east plunging zone. Through analyzing, we think that the present tectonic stress field play an important role in the validity of structural fractures in this area.