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The AAPG/Datapages Combined Publications Database

GCAGS Transactions

Abstract


Gulf Coast Association of Geological Societies Transactions
Vol. 48 (1998), Pages 139-150

Mineralogical and Fabric Changes of Shale During Burial Diagenesis and Their Effects on Petrophysical Properties

Jin-wook Kim (1), William R. Bryant (2), Joel S. Watkins (1), Thomas T. Tieh (1)

ABSTRACT

Shale samples from wells located offshore Louisiana, Gulf of Mexico have been examined by transmission electron microscopy (TEM), scanning electron microscopy (SEM), and x-ray powder diffraction (XRD) to study the relationship between clay mineralogical and fabric changes during burial diagenesis, and the effects of these changes on petrophysical properties, For electron microscopic observations a new sample preparation method allows elucidation of the details of clay fabric in the sample. Neutron-density logs from the wells studied show an abrupt density decrease and porosity increase at an approximate sub-bottom depth of 2200 m, which marks the beginning of the geopressured zone, Conductivity and sonic logs also show features typical of a geopressured zone at the same depth, Systematic changes in both clay mineralogy and clay fabric are the primary causes for the well log responses, Above 2200-m depth the shales are smectite rich, generally lack particle orientation, and contain appreciable pores and crystal dislocations, features indicative of modest local permeability, From 2200-m depth downward, with increasing burial, the shales become more illite rich, better laminated, and less porous. The fabric changes and concomitant growth of illite crystals render a significant decrease in ermeability and a rise in the fluid pressure gradient.


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