BARRETT, MARY L., Department of Geology, Centenary College, Shreveport, LA, and RICHARD W. MATHIAS, Skip's XRD Lab, Edgewater, CO

ABSTRACT: Production-Induced Diagenesis During Thermal Heavy Oil Recovery: Grain Size as a Predictor of Alteration

Terrigenous clastic alteration during thermal heavy oil recovery processes has been studied in whole core and laboratory tests from four California fields. Sediments were subjected to steam/hot water processes (up to 2000 C) and/or in-situ combustion processes (up to 500+ C). Original and altered mineral compositions do not fully describe reservoir changes. Fabric and permeability changes can be understood in a predictive sense by characterizing original grain size and sorting. Sand-size constituents are plagioclase feldspar and quartz, with lesser amounts of mica, volcanic rock fragments, and volcanic glass. Original matrix is dominated by smectite, biogenic opal, zeolites, and mica/illite. During hot water/steam reactions, primarily the fine-grained matrix is altered. Sediments with finer grain sizes and/or increased original matrix have the highest degree of permeability decrease as compared to coarser material. This pronounced permeability decrease is due to: 1) additional smectite andzeolite growth at the expense of other primary matrix components; and 2) the dispersion, migration, and pore-throat blockage by the matrix. Finer-grained sediments, characterized by smaller pore throats, are subjected to extensive blockage during fines movement.

Sediment reaction during in-situ combustion follows the alteration pattern above. Fine-grain matrix continues to alter, with some smectite transformed to illite. Calcite/oil reaction rims occur in the burn stage. The overall fabric is a lightly consolidated sandstone. Only in the near-wellbore area of an in-situ burn injector have partially-melted fabrics been found.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.