Journal: |
Acta Geologica Sinica 2018 No.6
clicks:143 |
Title:
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Author:
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FAN Cunhui1, 2, 3, HE Shun1, 3, ZHANG Yu1, 3, QIN Qirong1, 2, 3, * and ZHONG Cheng1, 3 |
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Abstract:
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Shale gas has currently attracted much attention during oil and gas exploration and
development. Fractures in shale have an important influence on the enrichment and preservation of
shale gas. This work studied the developmental period and formation mechanism of tectonic fractures in
the Longmaxi Formation shale in the Dingshan area of southeastern Sichuan Basin based on extensive
observations of outcrops and cores, rock acoustic emission (Kaiser) experiments, homogenization
temperature of fracture fill inclusions, apatite fission track, thermal burial history. The research shows
that the fracture types of the Longmaxi Formation include tectonic fractures, diagenetic fractures and
horizontal slip fractures. The main types are tectonic high-angle shear and horizontal slip fractures, with
small openings, large spacing, low densities, and high degrees of filling. Six dominant directions of the
fractures after correction by plane included NWW, nearly SN, NNW, NEE, nearly EW and NW. The
analysis of field fracture stage and fracture system of the borehole suggests that the fractures in the
Longmaxi Formation could be paired with two sets of plane X-shaped conjugate shear fractures, i.e.,
profile X-shaped conjugate shear fractures and extension fractures. The combination of qualitative
geological analysis and quantitative experimental testing techniques indicates that the tectonic fractures
in the Longmaxi Formation have undergone three periods of tectonic movement, namely mid-late
Yanshanian movement (82–71.1 Ma), late Yanshanian and middle Himalaya movements (71.1–22.3 Ma),
and the late Himalayan movement (22.3–0 Ma). The middle-late period of the Yanshanian movement and
end of the Yanshanian movement-middle period of the Himalayan movement were the main fractureforming periods. The fractures were mostly filled with minerals, such as calcite and siliceous. The
homogenization temperature of fracture fill inclusions was high, and the paleo-stress value was large; the
tectonic movement from the late to present period was mainly a slight transformation and superposition
of existing fractures and tectonic systems. Based on the principle of tectonic analysis and theory of
geomechanics, we clarified the mechanism of the fractures in the Longmaxi Formation, and established
the genetic model of the Longmaxi Formation. The research on the qualitative and quantitative
techniques of the fracture-phase study could be effectively used to analyze the causes of the marine shale
gas fractures in the Sichuan Basin. The research findings and results provide important references and
technical support for further exploration and development of marine shale gas in South China. |
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