Characteristics And Geological Control On The Development Of Shale Reservoir Space At The Lower Segment Of Longmaxi Formation In Southeast Sichuan, China

Multidisciplinary of sedimentary geology, reservoir geology, unconventional oil and gas geology, organic geochemistry have been integrateed as a means in the paper, combined with the latest theories of basic study on shale gas reservoir by domestic and foreign scholars. After Qijiang and Fuling are selected as an object in Southeast Sichuan, and outcrop samples and evaluation data of drilling core are selected as the basis, a discussion of reservoir space of shale rich in organic matter and its key controlling factors under marine shelf sedimentary of deep water in early Longmaxi stage is performed. The main research results are listed as follows:(1)The main minerals of outcrop samples in Guanyinqiao section include clay and quartz with an average of 44.5% and 38.7%, secondly plagioclase, K-feldspar and calcite, dolomite, with an average of 6.6%, 1.9%, 7.4%, 1.5% respectively, while clay minerals mainly contain illite and nearly without montmorillonite. Vertically content of clay mineral generally show an increasing trend while content of carbonate shows a descent first and then a increased afterward, which is consistent with change laws of mineral content in well XX-1 of Jiaoye, Fuling. Chemical experiments show that TOC value with 0.3~5.44%,with a clear decreasing trend bottom-up, and type of organic matter is mainly type I in an overmature stage with Ro 2.9%~3.58%. Main statistical distribution range of TOC is 2%~5%, with an average of 3.95% at layer segment rich in organic matter in well XX-1 of Jiaoye; Range of Equivalent reflectance is 2.39%~3.22%.(2)By means of combining high resolution scanning electron microscope with FIB-SEM, types of shale reservoir space is divided into four categories from the view of the pore carrier, such as I-O/P pore,Ⅱ-C pore, III B pore respectively with organic matter-pyrite, clay mineral and brittle mineral as the carrier, and category of pores and cracks between groups.By the low temperature nitrogen adsorption experiment show the average pore diameter, the total pore volume and BET surface area of shale with an average of 5.88 nm, 0.02132ml/g, 14.2m2/g respectively; distribution range of shale pore is 0~120nm, and distribution curve show double humps, with mesoporous of 10~20nm developed and very concentrated; adsorption isotherm is similar to type IV, and desorption loop type is between H3 and H4, reflecting that shale pore type is mainly containing pore shape of wedge/“V” with one or both ends and pore shape similar to ink bottle with narrow neck and wide mouth; shale pore has obvious fractal characteristics, and the relationship between the fractal value diameter of shale pore and average pore diameter, BJH total pore volume is highly negative correlation, while the relationship between the pore fractal value diameter and the specific surface area is weak negative correlation, and the depth has certain positive correlation with fractal dimension value.(3)The key control factors on the development of shale reservoir space are investigated at the lower segment of Longmaxi formation, including from a macro perspective, such as good marine sedimentary environment in deep-water shelf, stable tectonic subsidence history, and diagenesis including dissolution, transformation of clay minerals, hydrocarbon generation; from a microcosmic perspective,it is due to the different effects of different components on the pore development, and the organic matter abundance and maturity.