Pore Characteristics of Over‐Mature to Semi‐Graphitised Marine Shale: A Case Study of the Longmaxi Formation, Upper Yangtze Area

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Abstract
With ongoing exploration of deep to ultra‐deep organic‐rich shales in the Sichuan Basin, understanding pore evolution in highly mature shales is crucial for improving gas recovery. This study addresses the limited research on pore origins and development in over‐mature to semi‐graphitised marine shales. The first sub‐member of the Longyi Member, Longmaxi Formation in the Luzhou Block (Upper Yangtze area), was investigated using data from wells L1 and L2. Methods included organic geochemistry, CO 2 and N 2 adsorption, high‐pressure mercury intrusion, focused ion beam scanning electron microscopy (FIB‐SEM), and nuclear magnetic resonance (NMR). Results indicate that pyrobitumen pores are the dominant storage space, contributing 59.95%–62.00% of total porosity. With increasing thermal maturity, pore volume and connectivity decline, especially in micropores and macropores (25.02%–52.86%), compared to mesopores (8.37%–9.14%). Connectivity loss is most pronounced in the 100–1000 nm range (33.46%–62.09%). During the mature to high maturity stages, retained hydrocarbons fill interparticle spaces and transform into pyrobitumen under compaction, reducing surface porosity. In the over‐mature stage, gas‐filled pyrobitumen pores are preserved due to pressure balance, increasing surface porosity. At the semi‐graphitised stage, compaction and carbonisation of organic matter significantly reduce pore volume and surface porosity. This study reveals the pyrobitumen‐dominated pore evolution mechanism in thermally advanced shales, bridging a gap in the understanding of deep marine shale reservoirs and offering theoretical support for efficient shale gas development in China.
Funding Information
  • Natural Science Foundation of Sichuan Province
  • National Natural Science Foundation of China

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