The Fuling Shale Gas Field — A highly productive Silurian gas shale with high thermal maturity and complex evolution history, southeastern Sichuan Basin, China

The organic-rich Lower Silurian Longmaxi and Upper Ordovician Wufeng Shale (LSLUOWS) is one of the most important marine shale gas plays in southern China, with relatively high thermal maturity and complex structural evolution. The delineation of the Jiaoshiba shale gas play has been highly successful for the LSLUOWS in the Fuling area of southeastern Sichuan Basin. The drilling targets the basal part of the LSLUOWS, where the approximately 38–45 m of organic-rich (total organic carbon of greater than 2%) section corresponds to a maturity range around 2.2%–3.0% Ro, with high brittle mineral content (55%–65%). The produced shale gas displays a clear reversal in the stable carbon isotopes. The shale gas play zone is between the overlain mudstone in the Middle-Upper Longmaxi Formation (Fm) and the underlain Upper Ordovician tight limestone in the Jiancaogou Fm. The shale gas reservoir is overpressured, with an average pressure coefficient of 1.55 (relative to hydrostatic pressure). From December 2013 to late 2014, results of production tests determined relatively stable pressure and production curves. Fifty prolific gas wells have been completed to date in the Jiaoshiba Shale Gas Field in Fuling. Preliminary study revealed several characteristics of this structurally complex shale play that are distinctly different from those in the United States, including an anticlinal structure with mild deformation, abundant free gas, and very short gas migration through microfractures, the natural fracture network formed by the two fault systems, and the slip parallel to the layer in the basal LSLUOWS. Movement along strike-slip faults has gone through up into the Quaternary, which is considered to be a significant factor in the establishment and preservation of the overpressured region in the LSLUOWS.

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Caledonian detachment deformation and deposition in the Fuling gas field of the southeastern Sichuan Basin in China: Implications for the Lower Silurian Longmaxi Shale gas exploration and production

Interpretation ◽

10.1190/int-2018-0100.1 ◽

2018 ◽

Vol 6 (4) ◽

pp. SN119-SN132

Author(s):

Dengliang Gao ◽

Taizhong Duan ◽

Zhiguo Wang ◽

Xiaofei Shang

Keyword(s):

Pore Pressure ◽

Shale Gas ◽

Sichuan Basin ◽

Large Scale ◽

Time Structure ◽

Small Scale ◽

Upper Ordovician ◽

Gas Field ◽

Lower Silurian ◽

Southeastern Sichuan Basin

The Fuling gas field in the southeastern Sichuan Basin is the first and the largest shale gas play in China that has been producing primarily from the organic-rich shale in the Upper Ordovician Wufeng Formation and the Lower Silurian Longmaxi Formation. Newly processed 3D seismic data along with well-completion data in the gas field reveal important structural, depositional, and reservoir details in the Lower Paleozoic sedimentary section. Lateral (along-stratal) variations in time structure and bed curvature demonstrate the diversity in faults that can be classified based on their orientation (regional and cross-regional), scale (small, intermediate, and large), and mode (contractional, extensional, and wrench). Vertical (cross-stratal) variations in time structure and bed curvature demonstrate that the deformational intensity increases from the Lower Cambrian to the Upper Ordovician, then decreases from the Upper Ordovician to the Silurian. Seismic isochron and facies analyses indicate that the structural deformation influenced the syntectonic deposition of turbidite sand in a channel complex above the reservoir. The pore pressure, porosity, and gas productivity of the reservoir are the highest in the central portion of the field, where small-scale faults dominate, but drop significantly at the edges of the field, where large-scale lineaments dominate. The relationships suggest that faults and fractures could either reduce or enhance pore pressure, porosity, and gas productivity, depending on their scale. Large-scale faults have the most negative impact on gas enrichment and pressure build-up, leading to reduced pressure, porosity, and productivity; whereas, small-scale ones have the least negative or even positive impact on gas enrichment and pressure build-up, leading to increased pressure, porosity, and productivity. These observations and interpretations offer new insight into the dynamic interplay among tectonic deformation, syn-tectonic sedimentation, and reservoir integrity during the Caledonian (Late Ordovician to Silurian) in the southeastern Sichuan Basin (China).

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Coevolutionary Dynamics of Organic-Inorganic Interactions, Hydrocarbon Generation, and Shale Gas Reservoir Preservation: A Case Study from the Upper Ordovician Wufeng and Lower Silurian Longmaxi Formations, Fuling Shale Gas Field, Eastern Sichuan Basin

Geofluids ◽

10.1155/2020/6672386 ◽

2020 ◽

Vol 2020 ◽

pp. 1-21

Author(s):

Zhijun Jin ◽

Haikuan Nie ◽

Quanyou Liu ◽

Jianhua Zhao ◽

Ruyue Wang ◽

...

Keyword(s):

Shale Gas ◽

Sichuan Basin ◽

Hydrocarbon Generation ◽

Upper Ordovician ◽

Gas Field ◽

Pressure System ◽

Lower Silurian ◽

Eastern Sichuan Basin ◽

Eastern Sichuan

Shale gas deposits are self-sourced, self-accumulating, and self-preserving in the Upper Ordovician Wufeng Formation and Lower Silurian Longmaxi Formation of the Fuling Shale Gas Field in the eastern Sichuan Basin. They were both seemingly mixed by secondary oil cracking and kerogen cracking gases during the high maturation window. The reservoir space primarily consists of mineral pores and organic matter (OM) pores, and the shale gas was mainly trapped by a high-pressure system. In this study, the Fuling O3w-S1l Shale Gas Field in the eastern Sichuan Basin was used as a case study to discuss the coevolutionary process and organic-inorganic interactions of hydrocarbon generation, accumulation, and preservation. The results indicate that the processes and mechanisms of organic-inorganic interactions and coevolution of hydrocarbon generation and reservoir preservation are quite different among the shale graptolite zones (GZ) with respect to hydrocarbon generation, types and characteristics of shale gas reservoirs, seal characteristics, and their spatiotemporal relations. In the WF2-LM4 GZ, the favorable OM, biogenic authigenic quartz and organic-inorganic interactions are highly coupled, leading to the high level of coevolution demonstrated within the field, as well as to the favorable conditions for shale gas accumulation. Conversely, the overlying LM5-LM8 GZ seemingly exhibits early densification and late charge and has a reverse mode of reservoir development (i.e., low degree of coevolution). These two coevolutionary processes were conducive to the development of a high degree of spatiotemporal matching between the reservoir (i.e., WF2-LM4 GZ) and the seal (i.e., LM5-LM8 GZ). This is due to underlying differences in their coevolutionary histories. The synthetic work presented here on the coevolutionary processes and mechanisms of formation for organic-inorganic interactions and hydrocarbon generation and reservoir preservation reveals insights into the driving mechanisms of shale gas enrichment, providing a basis for effectively predicting favorable enrichment intervals for shale gas worldwide.

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The Genetic Mechanism and Evolution Process of Overpressure in the Upper Ordovician–Lower Silurian Black Shale Formation in the Southern Sichuan Basin

Minerals ◽

10.3390/min10030238 ◽

2020 ◽

Vol 10 (3) ◽

pp. 238

Author(s):

Xiaoqi Wang ◽

Yanming Zhu

Keyword(s):

Shale Gas ◽

Southern China ◽

Pressure Coefficient ◽

Fluid Pressure ◽

Initial Pressure ◽

Gas Production ◽

Controlling Factors ◽

Formation Mechanisms ◽

Hydrocarbon Generation ◽

Upper Ordovician

The overpressure phenomenon is a widespread occurrence in unconventional shale gas reservoirs. Multiple overpressure shale gas fields were discovered in southern China, and there is no doubt that the gas production per well increases with increasing pressure coefficient (the ratio of the reservoir fluid pressure to the corresponding normal hydrostatic pressure). Thus, successful evaluation and production strategies of organic-rich shale deposits require an understanding of the evolution of the pressure coefficient and its controlling factors in these deposits. In this paper, drilling engineering data of a typical well were collected, and clay mineral tests and overburden diffusion coefficient experiments were conducted. Based on multiphysics simulations, this paper analyzes the overpressure characteristics and formation mechanisms of overpressure in the Longmaxi Formation shale, as well as its geological evolution and controlling factors. The results show that the large amount of shale gas is the cause of overpressure formation rather than disequilibrium compaction. The simulation results show that pressure coefficients of the typical well range from 0.84 to 1.49. The current pressure coefficient increases with increasing pressure coefficient after the last hydrocarbon generation. A large initial pressure coefficient (>1.9), short lifting time (<100 Ma) and small lifting amplitude (<4000 m) are favorable for the preservation of shale gas.

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Difference Analysis of Organic Matter Enrichment Mechanisms in Upper Ordovician-Lower Silurian Shale from the Yangtze Region of Southern China and Its Geological Significance in Shale Gas Exploration

Geofluids ◽

10.1155/2019/9524507 ◽

2019 ◽

Vol 2019 ◽

pp. 1-14 ◽

Cited By ~ 1

Author(s):

Ming Wen ◽

Zhenxue Jiang ◽

Kun Zhang ◽

Yan Song ◽

Shu Jiang ◽

...

Keyword(s):

Organic Matter ◽

Shale Gas ◽

Southern China ◽

Sedimentary Organic Matter ◽

Gas Content ◽

Silicon Isotope ◽

Upper Ordovician ◽

Lower Silurian ◽

Excess Silicon ◽

Lower Yangtze

The upper Ordovician-lower Silurian shale has always been the main target of marine shale gas exploration in southern China. However, the shale gas content varies greatly across different regions. The organic matter content is one of the most important factors in determining gas content; therefore, determining the enrichment mechanisms of organic matter is an important problem that needs to be solved urgently. In this paper, upper Ordovician-lower Silurian shale samples from the X-1 and Y-1 wells that are located in the southern Sichuan area of the upper Yangtze region and the northwestern Jiangxi area of the lower Yangtze region, respectively, are selected for analysis. Based on the core sample description, well logging data analysis, mineral and elemental composition analysis, silicon isotope analysis, and TOC (total organic carbon) content analysis, the upper Ordovician-lower Silurian shale is studied to quantitatively calculate its content of excess silicon. Subsequently, the results of elemental analysis and silicon isotope analysis are used to determine the origin of excess silicon. Finally, we used U/Th to determine the characteristics of the redox environment and the relationship between excess barium and TOC content to judge paleoproductivity and further studied the mechanism underlying sedimentary organic matter enrichment in the study area. The results show that the excess silicon from the upper Ordovician-lower Silurian shale in the upper Yangtze area is derived from biogenesis. The sedimentary water body is divided into an oxygen-rich upper water layer that has higher paleoproductivity and a strongly reducing lower water that is conducive to the preservation of sedimentary organic matter. Thus, for the upper Ordovician-lower Silurian shale in the upper Yangtze region, exploration should be conducted in the center of the blocks with high TOC contents and strongly reducing water body. However, the excess silicon in the upper Ordovician-lower Silurian shale of the lower Yangtze area originates from hydrothermal activity that can enhance the reducibility of the bottom water and carry nutrients from the crust to improve paleoproductivity and enrich sedimentary organic matter. Therefore, for the upper Ordovician-lower Silurian shale in the lower Yangtze region, exploration should be conducted in the blocks near the junction of the two plates where hydrothermal activity was active.

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