Log-based identification of gas-bearing shales in the Longmaxi Formation of the Laifeng area:A case study of well WY1
Except for the Sichuan Basin,the marine shales in the Longmaxi Formation in Hubei show great potential for shale gas ex-ploration.Logging technology is highly mature in the exploration and development of hydrocarbons.This study systematically analyzed the log response characteristics of gas-bearing shales in the Longmaxi Formation based on logs obtained from well WY1 in the Laifeng area.As indicated by the analysis results,the gas-bearing shales of the Longmaxi Formation in well WY1 are characterized by high nat-ural-gamma-ray values,high contents of uranium,thorium,and compensated neutrons,high interval transit time,and low density.Their deep and shallow lateral resistivity is lower than that of the underlying Baota Formation carbonate rocks but is higher than that of the sandy shales.Moreover,the positive differences indicate that high-angle fractures occur in the shales.The superimposition of log curves and the analysis of cross plots for gas-bearing shales reveal that ① the superimposed log curves of both natural gamma ray-lithol-ogic density and interval transit time-lithologic density show significant positive differences,while those of both natural gamma ray-com-pensated neutrons and interval transit time-compensated neutrons show significant negative differences;② the superimposed log curves of interval transit time-lithologic density indicate the gas-bearing intervals the most accurately;③ the organic-rich gas-bearing shales can be identified the most effectively using the log cross plots of interval transit time-compensated neutrons,compensated neutrons-natu-ral gamma ray,and deep lateral resistivity-compensated neutrons.
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