--> Depositional Facies, Depositional Processes, and Reservoir Quality of the Miocene Sandstones of the Midway-Sunset Oil Field, San Joaquin Basin, CA, USA

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Depositional Facies, Depositional Processes, and Reservoir Quality of the Miocene Sandstones of the Midway-Sunset Oil Field, San Joaquin Basin, CA, USA


The 125-year old Midway-Sunset Oil Field has produced over 2.5 billion barrels of oil with an estimated 500 million barrels remaining in place as of 2009. A re-assessment of a prolific Miocene reservoir in the field was initiated in 2017. Over 433’ of whole core and 168 side wall cores were taken from a recently drilled well, in a well-developed area of the field, targeting the Miocene X and Y sandstones. These cores were examined to determine depositional facies, depositional processes, and reservoir quality. The X reservoir consists of three depositional facies: (1) Disorganized, conglomeratic, coarse-grained sands, (2) Fine to very coarse-grained sandstones with no clasts, and (3) Sands with few floating or non-floating clasts. The Y reservoir consists of four depositional facies: (1) Massive, structureless sandstones (no clasts present), (2) Very thin beds between 0.1 to 5 inches (laminated sand and diatomite), (3) Conglomeratic and coarse sands, (4) Silty/Diatomaceous and bioturbated mudstones. Interpreted depositional processes are Sandy Debris (conglomerates, sands with floating clasts), Turbidites (massive, fine grained, normal grading sands), Slump (diatomites with associated sand injections), Bottom-Current-Reworked (rhythmic occurrence of thin bedded sands with sharp contacts), pelagic and hemipelagic (silty/diatomaceous, bioturbated mudstones). The sandy debrites and turbidites have the best reservoir qualities; oil saturation is over 60%, effective porosity is as high as 43%, and permeability over 10D. This is possibly caused by grain-size distribution and sorting directly influenced by depositional processes. Clasts present in sand matrix are majorly granitic, diatomitic, and carbonaceous; they act as baffles and directly influence pore volume. Some of the clasts are fractured and have been healed by carbonate cement. In other cases, carbonate cement has been dissolved and hydrocarbon has filled fractures. Visual estimation of clast percentages shows that clast percentage in the X reservoir forms over 70% of the total reservoir and less than 20% in the Y sands. In general, these sandstones are unconsolidated and are held together by heavy oil except in cemented zones. The X and Y sandstones are interpreted to be deposited in a deep-water, submarine fan complex. A thorough understanding of depositional environment is important when managing a reservoir because the internal architecture (barriers, baffles, flow units), vertical, and lateral continuity of sand packages determine the efficiency of steam cycling and/or steam flooding (EOR techniques) in heavy oil reservoirs. As a rule of thumb, the depositional environment should be considered as very important for successful subsurface heat management in heavy oil reservoirs.