Analysis of Developmental Characteristics and Dominant Factors of Pore-Fracture Systems in Lower Cambrian Marine Shale Reservoirs: A Case Study of the Niutitang Formation, Fenggang Block, Southern China

Due to breakthroughs in the Lower Silurian Longmaxi Formation in the Sichuan Basin and multiple strata around the basin, the northern part of Guizhou adjacent to the Sichuan Basin has become a key area for shale gas exploration. Compared with the Longmaxi Formation, the Niutitang Formation displays greater TOC (total organic carbon) content, depositional thickness and distribution area, but the details remain undetermined. In the study area, the Lower Cambrian Niutitang Formation typically has high TOC content, maturity and brittle mineral content. The study area has experienced multiple periods of tectonic movement, which have great influence on the fracture and pore characteristics. The fractures are mainly structural fractures and have obvious zoning. The primary types of pores are intraparticle pores, organic matter pores, and interparticle pores. Further, macropores and mesopores less than 50 nm contribute most of the pore volume, while pores less than 2 nm contribute most of the specific surface area. Many factors affect the pore-fracture system, such as tectonism, TOC content, mineral composition, and sedimentary environment. Tectonic movements produce fractures based on the changing stress field, but the degree of fracture development does not agree well with the degree of pore development. The TOC content has good positive correlations with the development of fractures and micropores, especially for nanoporosity, while clay minerals show a negative correlation with the development of fractures but a strong positive correlation with the growth of micropores. Quartz displays a positive correlation with the development of fractures but no good correlation with pore development. Finally, the lithofacies, lithologies and mineral compositions under the control of sedimentary environments are internal factors that can impact the development of pore-fracture systems.

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Prediction of Fracture Toughness in the Shale Formation Based on Well Logging and Seismic Data: A Case Study of the Lower Silurian Longmaxi Formation in the Sichuan Basin, China

Geofluids ◽

10.1155/2020/8896352 ◽

2020 ◽

Vol 2020 ◽

pp. 1-13 ◽

Cited By ~ 1

Author(s):

Kaiyuan Liu ◽

Jian Xiong ◽

Xi Zhang ◽

Xiao Fan ◽

Le Li

Keyword(s):

Fracture Toughness ◽

Spatial Distribution ◽

Sichuan Basin ◽

Well Logging ◽

Distribution Area ◽

Distribution Characteristics ◽

Longmaxi Formation ◽

Lower Silurian ◽

Shale Formation ◽

The Sichuan Basin

The rock physics experiments and fracture toughness tests of shales from the Lower Silurian Longmaxi Formation in the Sichuan Basin in China were carried out. Based on this, the calculation model of the fracture toughness was constructed, thus, the single well evaluation of the fracture toughness in shale formation would be obtained based on the well logging data, which can be used to summarize the spatial distribution characteristics of the fracture toughness in the shale formation. However, it is difficult to obtain transverse distribution characteristics of fracture toughness in shale formation based solely on the well logging data. Therefore, in order to investigate the spatial distribution of the fracture toughness, jointing well logging and seismic method could be adopted to quantitatively predict the fracture toughness in shale formation. The results show that fracture toughness of shales is sensitive to acoustic interval transit time and wave impedance. The prediction model of the fracture toughness of shales was constructed, which had a good prediction effect. The fracture toughness values of shales from the Upper Silurian Wufeng-Longmaxi Formation were larger, whereas those of shales from the Lower Silurian Wufeng-Longmaxi Formation were lower. The fracture toughness is mainly distributed in strips along the vertical direction while the distribution area is continuous in the lateral direction, indicating that it has obvious stratification characteristics.

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Porosity evolution mechanisms of marine shales at over-maturity stage: Insight from comparable analysis between Lower Cambrian and Lower Silurian inside and at the margin of the Sichuan Basin, South China

Interpretation ◽

10.1190/int-2017-0221.1 ◽

2018 ◽

Vol 6 (3) ◽

pp. T739-T757 ◽

Cited By ~ 3

Author(s):

Xin Li ◽

Zhenxue Jiang ◽

Yan Song ◽

Gangyi Zhai ◽

Shujing Bao ◽

...

Keyword(s):

Continental Shelf ◽

Clay Minerals ◽

South China ◽

Lower Cambrian ◽

Sichuan Basin ◽

Gaseous Hydrocarbon ◽

Longmaxi Formation ◽

Porosity Evolution ◽

Lower Silurian ◽

The Sichuan Basin

To figure out porosity evolution mechanisms of marine shales inside and at the margin of the Sichuan Basin, South China, we measured samples selected from Lower Cambrian Qiongzhusi shales and Lower Silurian Longmaxi shales by a combination of X-ray diffraction, geochemistry measurement, focused ion beam milling and scanning electron microscopy imaging, and [Formula: see text] adsorption. It was suggested that shales of the upper Qiongzhusi Formation (a marine shallow continental-shelf setting) and the Longmaxi Formation (a marine abyssal continental-shelf setting) possessed larger pore volumes and larger pore surface areas than those of the lower Qiongzhusi Formation (a marine abyssal continental-shelf setting). Pores, in terms of pores of mineral frameworks, pores associated with clay minerals, dissolved pores, and organic matter (OM)-hosted pores, were all observed in the upper Qiongzhusi Formation and Longmaxi Formation, whereas none of the four types of pores developed well in the bottom of the Qiongzhusi Formation. Moreover, migrated OM is superior to depositional OM in the contribution of the pore space, in terms of pore productivity and pore protection. Good sealing abilities of the upper Qiongzhusi Formation and Longmaxi Formation allow more migrated OM and gaseous hydrocarbon retention than the lower Qiongzhusi Formation with an unconformity beneath acting as a channel of liquid and gaseous hydrocarbon migration, which bring about better pore properties. Finally, through the above analysis, the pore evolution mode has been established to gain insights for mechanisms of destruction, formation, and preservation of pores ranging from original sedimentary to metamorphic stage ([Formula: see text]–[Formula: see text]). Mechanisms of pore destruction contain mechanical compaction, chemical cementation, and OM carbonation. Mechanisms of pore generation comprise thermal pyrolysis of OM, transformation of clay minerals, and dissolution of soluble minerals. Mechanisms of pore preservation include mechanical stability of rigid grains, chemical stability of hydrophobic OM, and gas supporting through overpressure.

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