A Study Of Shale Gas Preservation Conditions For The Longmaxi Formation In The Fuling-Yanhe District

As one of the black shale developed formations, the Lower Silurian Longmaxi Formation of the Fuling-Yanhe district of southeast Sichuan Basin is the most advantageous area for shale gas exploration in the Upper-Middle Yangtze Platform. This area has experienced cycles and stages of structural overlap and rework, the paper studied the shale gas preservation conditions by material basis, tectonism, evolutionary history and the characteristics of the fluid through collecting previous information, field investigation, sampling research. Getting the following understading:The shale of the Longmaxi Formation in the study area has the excellent material basis. It belongs to shallow shelf-deep water shelf deposition that provide good sedimentary environment for shale.The organic matter type of the Longmaxi Formation shale belongs to the II kerogen, the thickness of the rich organic shale is generally greater than 30 meters, its Total Organic Carbon(TOC) content range is 0.1%~6.65%, on average 2.3%, its maturity of organic matter(Ro) range is 1.36%~2.97%, on average 2.22%, the thermal evolution degree is high-mature; The shale has low porosity and low permeability; The sample of low-Longmaxi Formation show that the quartz content is high-middle, clay minerals content is higher than the sample of the Wufeng Formation, the carbonate content is low; The mineral composition of the shale in upper-Longmaxi Formation is mainly clay minerals, the brittle quartz and feldspar content is low.This area has experienced cycles and stages of structural overlap and rework, the sctructural deformation styles is different. There were four important tectonic movements in the area, characterized by NW-SE, E-W, NE-SW and NWW-SEE trending compressive thrust movements.This article evaluate the shale gas storage conditions qualitatively and semi-quantitatively and filters the preferred regions of shale gas preservation conditions by five index of denude, transformation and fracture, dip, altitude.The fluid inclusion show that there were four times large scale oil and filling by tectonic movements. By studying carbon, oxygen and strontium isotope of the shale show that the characteristics of fluid and surrounding rock are almost similar, the connectivity of formations is bad, the fluid migration happened in the strong transformation zone or near the fault zone. The spring and hydrothermal mineral always be found in big faults, using the hot spring circulation depth speculate the depth storage show that the depth achieve the Sinian System near the fault zone, the shale gas preservation conditions is bad.The paper study the fluid inclusion,carbon,oxygen and strontium isotope of the samples and the spring and hydrothermal mineral, and combine the structure-preservation optimal selection to evaluate the shale gas storage conditions of the Longmaxi Formation in Fuling-Yanhe zone comprehensively. The shale gas preservation conditions of the Longmaxi Formation is the most efficient in Fuling zone and followed by Wulong, Pengshui fault-fold belt, Daozhen syncline has good preservation conditions in Zhengan rewordly fault-fold belt, the preservation conditions of shale gas are damaged in the Wuchuan and Yanhe rewordly fault-fold belt. The preservation conditions of shale gas gradually become worse from the northwest to southeast.