ABSTRACT: Reservoir Characterization and Diagenesis of the Oligocene 64-Zone Sandstone, North Belridge Field, Kern County, California

Thomas R. Taylor, C. H. Soule

The Oligocene (Zemmorian) 64-Zone sandstone is an important reservoir in North Belridge field, Kern County, California. After more than 50 yr of production, a detailed study of the reservoir rock was conducted in order to evaluate it as a potential enhanced oil recovery target. The 64-Zone sandstone ranges from approximately 275 to 450 ft in thickness and is interpreted as a submarine fan deposit. The section is shale free and consists of amalgamated beds of fine- to coarse-grained arkosic sandstone. It is characterized by a basal section composed of fine-grained, burrowed, clay-rich sandstone overlain by a thick section of medium- to coarse-grained, clean sandstone. In general, the sandstone becomes coarser grained toward the top of the section.

The reservoir quality of the 64-Zone has been significantly affected by diagenesis. The intergranular volume of the sandstones averages 20%, indicating that compaction has significantly reduced intergranular porosity. An early stage of calcite cement, along with concretionary dolomite, formed during shallow burial (< 100 m) under the influence of bacterial sulfate reduction (^dgr¹³C = -13 to -19 ^pmil). Quartz is the most abundant cement, averaging 6.2%. Secondary porosity formed by dissolution of feldspar averages 2.7%, approximately one-quarter of the total pore volume. Late-stage calcite cements (^dgr¹⁸O = -9 to -13 ^pmil) occur in and adjacent to fractures and formed after significant burial (> 70°C). Dissolution of carbonate cements is common and ontributes to reservoir porosity.

Despite the effects of diagenesis, reservoir quality of the 64-Zone still reflects the change in grain size associated with depositional processes. The reservoir can be modeled as a system containing three layers of variable thickness (75-125 ft). The fine-grained, lower part of the section has 10-15% porosity and moderate permeability (2-50 md). The relatively coarse-grained, upper portion of the section has 13-18% porosity and higher permeability (10-300 md). The layered nature of the 64-Zone reservoir must be taken into account in enhanced oil recovery planning and designed in order to avoid breakthrough in the high-permeability layer and promote recovery from the low-permeability layers.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990