Pacific Section AAPG, SPE and SEPM Joint Technical Conference

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Sedimentology And Stratigraphy Of Diatomaceous Sediments In The Casmalia Hills And Orcutt Oil Fields In The Santa Maria Basin, California


The upper portion of the California Neogene siliceous succession (e.g., Belridge and Sisquoc formations, etc.) is gathering increased importance as potential unconventional reservoirs because of its highly porous diatomaceous composition. In contrast to the general conception of diatomites as being rather homogeneous hemipelagic sediments, these strata were locally deposited in dynamic settings that underwent significant tectonic and eustatic change, leading to a variety of depositional environments and depositional mechanisms. Two industry acquired cores from the Casmalia Hills (Stokes A-30804) and Orcutt (Newlove 76-RD1) oil fields were used to better understand the complex diatomaceous sediments of the upper Monterey and Sisquoc Formations within the Santa Maria Basin. Understanding the relationship between depositional environment, lithology, and sedimentary features of these diatomaceous sediments is critical to further develop and exploit these unconventional diatomite reservoirs. The stratigraphy and sedimentology of Newlove 76-RD1 and Stokes A-30804 were analyzed using a combination of observational data, laboratory data (x-ray diffraction and scanning electron microscope), and petrophysical data (gamma ray logs). These data helped identify the location and abundance of different lithologies, sedimentary features, and chemical/compositional trends. Lithologies within Newlove 76-RD1 and Stokes A-30804 chiefly range from diatomite to diatomaceous mudstone with varied detrital content, total silicates, and the presence or absence of laminations (discontinuous to continuous). Sedimentary features include slump folds, discontinuities, truncation surfaces, faults, fossils, burrows, scour surfaces, hardgrounds, debris flow deposits, intraformational breccia, and sandstone/siltstone laminations. Furthermore, both cores are subdivided into four distinct lithostratigraphic units that reflect changes within their depositional environments. Prominent variations of depositional environment include oscillations of low to high oxygen conditions and variable levels of energy influencing the influx of detritus, total silicates, and debris flow deposits. Debris flow deposits are highly saturated with oil, suggesting they may have played a role in oil migration into these diatomaceous sediments. The opal-A to opal-CT transition zone depth varied between the two wells suggesting differences in burial and uplift history.