PARASEQUENCES OF THE TERRESTRIAL TULARE FORMATION, SOUTH BELRIDGE FIELD, KERN COUNTY, CA

This study demonstrates the value of mapping apparent parasequences as a sequence stratigraphic approach to reservoir characterization in terrestrial sediments. The effective depositional controls on fluid flow and reservoir continuity are demonstrated by close well spacing in the data-rich steamflood of the Pliocene-Pleistocene Tulare reservoir sands at South Belridge field, San Joaquin Valley, central California.

The original geologic model based primarily on lithofacies log correlation evolved to the present reservoir model of fluid compartments defined by parasequences. Whereas the original geologic model subdivided the Tulare reservoir succession into five mappable zones, the present parasequence model describes many more reservoir zones, each corroborated by fluid flow using injected steam as a tracer. Water encroachment during production provides a second dynamic tracer of fluid flow that demonstrates reservoir sand continuity between aquifer and the oil leg.

We map parasequence boundaries at widespread chronostratigraphic horizons that are characterized by a reduction of grain size that can be mapped laterally for miles. Parasequence boundaries are characterized by a combination of caliche and mud deposits indicative of soil horizons or lacustrine shales indicative of lake flooding events. Sand deposits in this depositional setting locally punctuate these parasequence horizons. The stacking patterns of fluvial sands within the parasequence boundaries were dominated by autocyclic channel switching and overbank muds that can be mapped over limited distance. This emphasizes the importance of distinguishing overbank deposits from lacustrine shales and soil horizons. This new parasequence reservoir model serves as a basis for improved surveillance and benefits oil recovery.