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Detailed Reservoir Architecture Of The Kern River Formation: Examples From Outcrop, Core, And Well Logs


The Kern River Field is one of the largest fields in the state of California and the US and one of the world's largest thermally enhanced recovery projects. Located northeast of Bakersfield, California in the San Joaquin Basin, production is primarily from the Late Miocene to Pliocene Kern River Formation, a ∼1000 ft thick succession of sandstones and mudstones deposited in a braided fluvial environment. The field was discovered in 1899 and more than 18,000 wells have been drilled resulting in, arguably, the best subsurface dataset for a fluvial reservoir in the world. The Kern River dataset includes over 12,000 wells with traditional open-hole log suites, 650 observation wells that monitor temperature and saturation, over 300 wells with conventional core, as well as outcrops of several producing intervals. This dataset offers the unique opportunity to evaluate the detailed reservoir architecture of one of the world's largest fluvial reservoirs. The Kern River Formation depositional environment and stratigraphic architecture is interpreted from numerous direct and indirect measurements. Resistivity logs are used to differentiate reservoir from non-reservoir facies, as a result of the fresh nature of the formation water. Core studies indicate a correlation between higher permeability reservoir and higher resistivity intervals. The producing zones are also observable as outcrops within the field boundary and as a more extensive road cut east of the field allowing for detailed reservoir architectural analysis. Reservoir zones, as observed in outcrop and core, consist of fining upward sequences of conglomerate or pebbly sandstone at the base fining into trough cross-stratified sandstone. Individual sequences are on the order of 10-15 feet thick, but may be amalgamated and form sandstone bodies >100 feet thick. Non-reservoir intervals consist of siltstone clast conglomerates and massive siltstone to interbedded siltstones and sandstones with few mudstones. Grain-size, primary structures, and channel belt architecture observed in roadcut support an interpretation of a low sinuosity fluvial channel and overbank environment. The variability of elements within fluvial systems pose challenges for subsurface interpretation. A 4D earth model constructed from the subsurface dataset is utilized to examine the subsurface reservoir geometry. Depositional elements such as channel belt geometries are identified by the lateral association of axis to off-axis resistivity shapes. Time-slicing through the model recreates the avulsion history of the channel belt complex and visualizes the lateral continuity of the reservoir. Although the full field model covers an area of 15 square miles, it offers one of the best examples of a high-resolution subsurface image of the stratigraphic architecture of a fluvial system.