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Characterization Of Low-Detritus Quartz-Phase Porcelanite Within 29R And Gunslinger Shale Reservoirs, Elk Hills Field, California


The 29R Shale Reservoir (29R) and Gunslinger Shale Reservoir (GS) are located on adjacent north-west trending structures within the Elk Hills Field, and are operated by California Resources Elk Hills, LLC. The 29R Structure is shallower than the Gunslinger Structure, with crestal depths of approximately −3600ft TVDSS and −6100ft TVDSS respectively. This study 1) identified and described GS and 29R lithologies; 2) identified differences in reservoir and rock properties within the low-detritus quartz-phase porcelanite (LDQP) of each structure; 3) identified potential lateral and vertical variability in lithology, reservoir properties, and rock properties. Cores from six wells were analyzed to determine and classify lithology. On the GS structure, samples were collected from Monterey Formation members A, B, C, D, and PG Shale, and on the 29R structure, samples were collected from members A and B Shale. The following data were analyzed: optical petrography, photomicrograph, x-ray diffraction (XRD), mercury injection capillary pressure, porosity, permeability, oil and water saturation. This study recognizes LDQP as the primary reservoir rock within 29R, while the primary reservoir rocks within GS are LDQP and quartz-phase chert (QPC). The LDQP is primarily a matrix reservoir, containing greater than 75% silica, minor to abundant microporosity, and variable fracturing. The QPC is a fractured reservoir, containing greater than 85% quartz, little to no visible microporosity, and common extensive fracturing. There is a continuum of reservoir rock types between the LDQP and QPC, with varying amounts of silica, detritus, microporosity, and fracturing. XRD data collected on LDQP samples indicate negligible compositional differences between 29R and GS; however, 29R has higher permeability and porosity. Additionally, capillary pressure data indicate an average oil entry point height of 284 feet (height above free water) for GS and 185 feet for 29R. This study indicates that compaction and diagenesis, as a result of burial, are the driving forces that contribute to differing rock and reservoir properties in 29R and GS.