Structure, burial, and gas accumulation mechanisms of lower Silurian Longmaxi Formation shale gas reservoirs in the Sichuan Basin (China) and its periphery

Paleozoic marine shale gas resources in Southern China present broad prospects for exploration and development. However, previous research has mostly focused on the shale in the Sichuan Basin. The research target of this study is expanded to the Lower Silurian Longmaxi shale outside the Sichuan Basin. A prediction scheme of shale gas reservoirs through the frequency-dependent seismic attribute technology is developed to reduce drilling risks of shale gas related to complex geological structure and low exploration level. Extracting frequency-dependent seismic attribute is inseparable from spectral decomposition technology, whereby the matching pursuit algorithm is commonly used. However, frequency interference in MP results in an erroneous time-frequency (TF) spectrum and affects the accuracy of seismic attribute. Firstly, a novel spectral decomposition technology is proposed to minimize the effect of frequency interference by integrating the MP and the ensemble empirical mode decomposition (EEMD). Synthetic and real data tests indicate that the proposed spectral decomposition technology provides a TF spectrum with higher accuracy and resolution than traditional MP. Then, a seismic fluid mobility attribute, extracted from the post-stack seismic data through the proposed spectral decomposition technology, is applied to characterize the shale reservoirs. The application result indicates that the seismic fluid mobility attribute can describe the spatial distribution of shale gas reservoirs well without well control. Based on the seismic fluid mobility attribute section, we have learned that the shale gas enrich areas are located near the bottom of the Longmaxi Formation. The inverted velocity data are also introduced to further verify the reliability of seismic fluid mobility. Finally, the thickness map of gas-bearing shale reservoirs in the Longmaxi Formation is obtained by combining the seismic fluid mobility attribute with the inverted velocity data, and two favorable exploration areas are suggested by analyzing the thickness, structure, and burial depth. The present work can not only be used to evaluate shale gas resources in the early stage of exploration, but also help to design the landing point and trajectory of directional drilling in the development stage.

Download Full-text

Mineralogy and fracture development characteristics of marine shale-gas reservoirs: A case study of Lower Silurian strata in southeastern margin of Sichuan Basin, China

Journal of Central South University ◽

10.1007/s11771-015-2704-6 ◽

2015 ◽

Vol 22 (5) ◽

pp. 1847-1858 ◽

Cited By ~ 5

Author(s):

Ling Guo ◽

Zai-xing Jiang ◽

Feng Guo

Keyword(s):

Shale Gas ◽

Sichuan Basin ◽

Gas Reservoirs ◽

Lower Silurian ◽

Marine Shale ◽

Shale Gas Reservoirs ◽

Fracture Development ◽

Southeastern Margin

Download Full-text

Laminae characteristics and influence on shale gas reservoir quality of lower Silurian Longmaxi Formation in the Jiaoshiba area of the Sichuan Basin, China

Marine and Petroleum Geology ◽

10.1016/j.marpetgeo.2019.06.022 ◽

2019 ◽

Vol 109 ◽

pp. 839-851

Author(s):

Chao Wang ◽

Boqiao Zhang ◽

Qinhong Hu ◽

Zhiguo Shu ◽

Mengdi Sun ◽

...

Keyword(s):

Shale Gas ◽

Sichuan Basin ◽

Reservoir Quality ◽

Gas Reservoir ◽

Longmaxi Formation ◽

Lower Silurian ◽

Shale Gas Reservoir ◽

The Sichuan Basin

Download Full-text

Factors Affecting Shale Gas Chemistry and Stable Isotope and Noble Gas Isotope Composition and Distribution: A Case Study of Lower Silurian Longmaxi Shale Gas, Sichuan Basin

Energies ◽

10.3390/en13225981 ◽

2020 ◽

Vol 13 (22) ◽

pp. 5981

Author(s):

Chunhui Cao ◽

Liwu Li ◽

Yuhu Liu ◽

Li Du ◽

Zhongping Li ◽

...

Keyword(s):

Natural Gas ◽

Isotopic Composition ◽

Shale Gas ◽

Sichuan Basin ◽

Noble Gas ◽

Longmaxi Formation ◽

Lower Silurian ◽

The Sichuan Basin ◽

Gas Fields ◽

Cracking Gas

The Weiyuan (WY) and Changning (CN) fields are the largest shale gas fields in the Sichuan Basin. Though the shale gases in both fields are sourced from the Longmaxi Formation, this study found notable differences between them in molecular composition, carbon isotopic composition, and noble gas abundance and isotopic composition. CO2 (av. 0.52%) and N2 (av. 0.94%) were higher in Weiyuan than in Changning by an average of 0.45% and 0.70%, respectively. The δ13C1 (−26.9% to −29.7%) and δ13C2 (−32.0% to −34.9%) ratios in the Changning shale gases were about 8% and 6% heavier than those in Weiyuan, respectively. Both shale gases had similar 3He/4He ratios but different 40Ar/36Ar ratios. These geochemical differences indicated complex geological conditions and shed light on the evolution of the Lonmaxi shale gas in the Sichuan Basin. In this study, we highlight the possible impacts on the geochemical characteristics of gas due to tectonic activity, thermal evolution, and migration. By combining previous gas geochemical data and the geological background of these natural gas fields, we concluded that four factors account for the differences in the Longmaxi Formation shale gas in the Sichuan Basin: a) A different ratio of oil cracking gas and kerogen cracking gas mixed in the closed system at the high over-mature stage. b) The Longmaxi shales in WY and CN have had differential geothermal histories, especially in terms of the effects from the Emeishan Large Igneous Province (LIP), which have led to the discrepancy in evolution of the shales in the two areas. c) The heterogeneity of the Lower Silurian Longmaxi shales is another important factor, according to the noble gas data. d) Although shale gas is generated in closed systems, natural gas loss throughout geological history cannot be avoided, which also accounts for gas geochemical differences. This research offers some useful information regarding the theory of shale gas generation and evolution.

Download Full-text