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The Silurian LongMaXi gas shale in the Sichuan basin is an emerging play in China. It experienced multiple major tectonic evolutions with uplifting and burying sequences in its geological time. As expected, this shale could develop tremendous small-scale discrete natural fractures (DNF) like many other notable North American gas shale plays. But only sparse DNFs were observed from several borehole images in a shale gas field. This paper presents a case study for characterizing complex multiscale natural fracture systems of the gas shale.In the study, natural fracture systems were classified as four types: large-scale faults, medium-scale fracture systems (including microfaults and crushed zones or fracture corridors), small-scale DNFs, and micro-scale fissures per their correspondingly scale-associated geological, geophysical, and petrophyscial features and responses. An optimized ant-tracking approach was implemented to track seismic abnormities and discontinuities as indicators of subseismic medium-scale fractures. Key components of this approach include optimized seismic gathers' angle-range, optimal scanning directions and angles, parameter adjustment criteria, and a comprehensive QC process. The QC process, which considers regional geological and tectonic evolution, fracture mechanical mechanism under strike-slip stress regime, and observations from all available resources, was implemented on each identified medium-scale fracture zone to ensure its reliability.This paper has more focuses on characterizing the medium-scale fracture system because of which can be a unique feature of LongMaXi gas shale compared to its North American counterparts. Characterizing of this medium-scale fracture system considers factors including fracture development mechanism and fracture pattern under strike-slip condition, structure control and curvature tendency, and intensity and amplitude of seismic discontinuity and abnormities. The study shows such medium-scale fractures are widely distributed across the study field. Complexities encountered while drilling and stimulation such as heavy mud loss or screen-out due to unexpected high leak-off rate had strong correlation with them. Two principle orientations of the medium-scale fractures were identified and further validated by outcrop observations and microseismic events observed from hydraulic fracturing monitoring. The study suggests that such medium-scale fractures may be mainly controlled by two major tectonic events during Himalayan movement because their principle orientations are almost exactly following the directions of two major tectonic movements. In combination with all types of natural fractures as defined, 3D multiscale natural fracture model and Geomechanical Earth model were established, which are two keystones for drilling quality and completion quality for further applications.These medium-scale fractured or crushed zones along with large-scale faults and small-scale DNFs across this shale gas field could significantly impact on efficiency and effectiveness of...

As an important pilot target for shale gas exploration and development in China, the Longmaxi Formation shale in the Dianqianbei Area is characterized by high content of nitrogen, which severely increases exploration risk. Accordingly, this study explores the genesis of shale gas reservoir and the mechanism of nitrogen enrichment through investigating shale gas compositions, isotope features, and geochemical characteristics of associated gases. The high-nitrogen shale gas reservoir in the Longmaxi Formation is demonstrated to be a typical dry gas reservoir. Specifically, the alkane carbon isotope reversal is ascribed to the secondary cracking of crude oil and the Rayleigh fractionation induced by the basalt mantle plume. Such a thermogenic oil-type gas reservoir is composed of both oil-cracking gas and kerogen-cracking gas. The normally high nitrogen content (18.05%–40.92%) is attributed to organic matter cracking and thermal ammoniation in the high-maturity stage. Specifically, the high heat flow effect of the Emeishan mantle plume exacerbates the thermal cracking of organic matter in the Longmaxi Formation shale, accompanied by nitrogen generation. In comparison, the abnormally high nitrogen content (86.79%–98.54%) is ascribed to the communication between the atmosphere and deep underground fluids by deep faults, which results in hydrocarbon loss and nitrogen intrusion, acting as the key factor for deconstruction of the primary shale gas reservoir. Results of this study not only enrich research on genetic mechanism of high-maturity N2 shale gas reservoirs, but also provide theoretical guidance for subsequent gas reservoir resource evaluation and well-drilling deployment in this area.