Organic matter is the material basis of hydrocarbon generation and the abundance of organic matter is a main factor of regional selection and evaluation in shale gas. Also the enrichment is influenced by sedimentary environments. Thus, it is important for the study on the geological factors controlling organic matter enrichment and further to provide scientific basis of regional selection and evaluation by organic matter enrichment area with analysis of the factors. In this paper, the Upper Ordovician-Lower Silurian shale from representative wells in the Upper-Lower Yangtze area is selected as the research object. The goal of this study is to quantitatively calculate the excess siliceous mineral content in shale siliceous minerals and determine the origin of excess silicon based on Al, Fe, and Mn elements; as well as to analyze the sedimentary organic matter enrichment mechanism based on the water body redox environment and bio-productivity. The results show that excess silicon from the Upper Ordovician-Lower Silurian shale in the Upper Yangtze area is biogenic and deposited in closed water bodies. On the one hand, the upper water body contains oxygen, which leads to higher bio-productivity. On the other hand, the lower water body has strong reducibility, which is conducive to sedimentary organic matter preservation. However, the excess silicon in the Upper Ordovician-Lower Silurian shale of the Lower Yangtze area is derived from hydrothermal solution. Hydrothermal activity can enhance the bottom water reducibility, and its nutrient elements can improve bio-productivity and enrich sedimentary organic matter. Therefore, the organic matter enrichment, which depends on the biological productivity and redox conditions, is controlled by the water closure in the Upper Yangtze and hydrothermal activities in the Lower
As a result of complex tectonic background, shale gas in China exhibits differential enrichment. Choosing a favorable exploration target accurately is a crucial problem to be solved. In this study, the tests show that there is a superior transportation pathway within shale layer. Gas in the shale layer percolates much more in the direction parallel to the plane. Therefore, the accumulation pattern of shale gas indicates a complex tectonic background. Gas in the lower part of the structure diffuses and percolates in the vertical direction into the surrounding rock. Most gas percolates towards the high part of the structure in the direction parallel to the plane. When the shale was exposed, gas percolated along the parallel direction into the air. In the case of fracture development, if there is a reverse fault, gas would be enriched in the footwall. However, if there is an unsealed fault, it would become a pathway for gas migration. The above accumulation pattern was proved in several Areas. Also, this research presented a basis of evaluation units division. According to the buried depth, fractures, and structural position, Xiuwu Basin was divided into five evaluation units and Unit A3 is the most favorable exploration target.
Summary
This paper presents a case study of fault reactivation and induced seismicity during multistage hydraulic fracturing in Sichuan Basin, China. The field microseismicity data delineate a fault activated near the toe of the horizontal well. The spatio-temporal characteristics of the microseismicity indicate that the seismic activity on the fault during the first three stages is directly related to the fluid injection, while after Stage 3, the seismic activity is possibly due to the relaxation of the fault. The fault-related events have larger magnitudes and different frequency-magnitude characteristics compared to the fracturing-related events. Three-dimensional (3D) fully coupled distinct element geomechanical modeling for the first two hydraulic fracturing stages and a shut-in stage between them is performed. The modeling result generates features of microseismicity similar to that of the field data. The energy budget analysis indicates that the aseismic deformation consumes a major part of the energy. The simulated fault shear displacement is also consistent with the casing deformation measured in the field. The model is also used to investigate the impact of possible operational changes on expected seismic responses. The results show that lower injection rate and lower fluid viscosity would be helpful in reducing casing deformation but not in mitigating seismicity. Decreasing the total fluid injection volume is an effective way to mitigate the seismicity, but it may hinder the stimulation of the reservoir formation and the production of the well.
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