Pacific Section AAPG, SPE and SEPM Joint Technical Conference

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Depositional And Diagenetic Microfabrics Of Diatomite And Opal CT Porcelanite Reservoirs, North Midway Sunset Field, CA


Siliceous Monterey mudrocks show complex depositional microfabrics comparable to those documented in argillaceous and carbonate-rich mudrock reservoirs. Cores from three wells in North Midway Sunset Field (NMWSS) have been studied using thin section petrography of oversize, slightly wedged thin sections, Scanning Electron Microscopy (SEM), and X-Ray diffraction (XRD). The wells are located north of Fellows, CA. The use of image enhancement and detailed microscopy reveals fabrics such as millimeter-scale laminae with sharp or scour contacts, graded laminae, and homogeneous intervals. Evidence for biological activity in the diatomites includes burrows, pellets, and possible bioturbation. Volcanic shards occur as lenses and stringers rarely. The NMWSS diatomite fabrics are compared to recent studies of other mudrock reservoirs. Texturally, siliceous microfossils are the principal clay-size grain type in diatomites and show sorting by size, degree of disarticulation, and species. Silt to very fine sand-size diatomite intraclasts or peloidal grains are present. Phyllosilicate and siliciclastic silt grain content varies between laminae, along with dispersed organic matrix content. The relationship between these particles gives clues to the mode of deposition (flocculation, settling, bioturbation, etc.). Early post-depositional structures include fluid escape structures, intraformational folds, microfaults, crackle breccia, and microshear zones filled with highly comminuted diatomite. Structures are easiest to identify when oriented at a high angle to lamination, but also occur parallel to lamination. Burial-diagenetic conversion to Opal-CT is documented using XRD results and SEM images, then correlated back to thin section observations. SEM examples of Opal CT fabrics that overprint primary fabrics are presented for discussion. Documentation of the complex depositional and diagenetic fabrics helps explain the variations in porosity and permeability within the reservoir.