Influence of Clay Diagenesis on Shale Velocities and Fluid-Pressure

摘要

Fluid pressure in the pore system of sedimentary rocks is anrnimportant parameter to the exploration and production ofrnpetroleum. Compaction disequilibrium is considered by manyrnby many workers to be the dominant pressure-generatingrnmechanism. Clay mineral diagenesis (particularly the smectiternto illite reaction) is recognized as a potential contributor tornpore fluid pressure, but there have been few systematic studiesrnwhich integrate clay reaction extent with pressure and logrnresponses. This study examines samples from two basins withrndifferent shale mineralogy by detailed characterization ofrnmudstone drill cuttings by X-ray diffraction, and comparesrnthese data with pore pressure information, bulk density andrnsonic velocity obtained from standard wireline well logging.rnSamples from Well A, with unreactive illite-smectite (I-S)rnyielded a single stress/density and single stress/velocityrnrelationship. Unreactive I-S also yielded a simplernvelocity/density relationship where samples from a velocityrnreversal zone have the same sonic/density relationship as dornsamples from shallow zones in the well.rnWell B, with a reactive I-S suite, has complexrnstress/velocity, stress/density and velocity/densityrnrelationships. Open system illitization induced a more rapidrnvelocity increase, relative to the rate of bulk density increase.rnIllitization with unloading induced velocity reduction whilernallowing bulk density to increase. The effect of illitization onrndensity/stress relationships appears to be most dramatic duringrnthe latter stages of illitization.