Reservoir Characterization of Upper Miocene Western 31S, Main Body B, and B Shale Sands, Elk Hills Field, California
P. F. Cowell
Upper Miocene sands are the primary reservoirs undergoing peripheral water flood at the doubly plunging 31S anticline of Elk Hills Field, San Joaquin Basin, California. Refinement of correlations and mapping in combination with a new petrophysical model results in improved characterization and understanding of the reservoirs. These thinly bedded arkosic turbidite sands were deposited as complete, incomplete, and amalgamated Bouma sequences. Five reservoirs separated by unconformities are recognized in ascending order: Lower Western 31S, Upper Western 31S, Lower Main Body B, Upper Main Body B, and B Shale sands.Within the five reservoirs, widespread shales or shaly zones define 28 correlative layers. Small- to large-scale sand distribution patterns are a result of syndepositional structural growth and offset stacking. Each of the reservoirs and their correlative layers exhibit wedge geometry. Up-dip terminations are of two types: 1) gradual interval thinning and decreased sand content terminating at a pinchout, and 2) onlap against positive areas resulting in abrupt termination due to impingement against the high. These mapped wedge geometries have been confirmed with recently acquired 3D seismic data.
Cores from 28 wells were used to develop a new petrophysical model, which was applied to 400 wells with modern log suites (of 725 total wells on the structure). Re-evaluation of the correlations utilizing effective porosity, water saturation, clay volume and permeability curves combined with dipmeter data from 300 wells results in better understanding of sand distribution and reservoir quality for each layer. Previous geologic models contained inconsistent, anomalous oil/water contacts. The new geologic and petrophysical models combined with an oil/water contact redetermination establishes consistent down-dip reservoir boundaries. This new characterization and improved understanding of the reservoirs has assisted water flood optimization, infill drilling, and bypassed oil development. It is also being used as the geologic input for 3D modeling and reservoir simulation.
AAPG Search and Discovery Article #90904©2001 AAPG Pacific Section Meeting, Universal City, California