Abstract: Mudstone Diagenesis and Its Effect on Petrophysical Properties

KIM, JIN-WOOK, THOMAS T. TIEH, JOEL S. WATKINS, Department of Geology & Geophysics, Texas A&M University, College Station, TX;
WILLIAM R. BRYANT
Department of Oceanography, Texas A&M University, College Station, TX

Mudstone samples from wells located off-shore Louisiana, Gulf of Mexico have been examined by Transmission Electron

Microscopy (TEM) and Scanning Electron Microscopy (SEM) to detail the relationship between clay mineralogical and fabric changes during diagenesis, and the effects of these changes on petrophysical properties. A new sample preparation method allows elucidation of the geometry and distribution of pores in the sample. Well log data show an abrupt density decrease and porosity increase at an approximate depth of 2200 m, which marks the beginning of the geopressured zone. This zone also corresponds with the smectite-to-illite transformation in the mudstone. Above this depth the mudstones are smectite rich, generally lack particle orientation, and contain appreciable pores and crystal dislocations, features indicative of modest local permeability. From this depth downward, with increasing burial, the mudstones become more illite rich, better laminated, and less porous. These fabric changes and concomitant growth of illite crystals cause significant decrease in permeability and a rise in fluid pressure gradient.

AAPG Search and Discovery Article #90932©1998 GCAGS/GCS-SEPM Meeting, Corpus Christi, Texas