--> --> Mechanisms for Formation of “Isolated Shelf Sand Reservoirs”: Synthesis of Insights Gained From 3-D Seismic, Outcrops & Core

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Mechanisms for Formation of “Isolated Shelf Sand Reservoirs”: Synthesis of Insights Gained From 3-D Seismic, Outcrops & Core


Sedimentologic interpretations of sandstone reservoirs isolated within shelfal mudstone have been highly debated for more than 50 years, with little systematic consensus having been achieved for sandstone reservoirs that may individually contain >300MMBO. Like blind persons touching the elephant, each may accurately describe their specific point of observation and reach different interpretations because the individual pieces function differently and because the whole is ultimately more complex than the parts. The conceptual framework derived by JB Anderson and his students from modern and recent shelfal deposits of the Gulf of Mexico is consistent with observations in ancient isolated sandstone reservoirs. Integrated data gleaned from high-resolution 3D seismic in three formations and outcrops and/or core studies from nine different formations document a diverse spectrum of isolated shelfal sandstone reservoirs that can be categorized into three bins, each linked to relative sea-level fall and subsequent, variably intensive transgressive erosion and marine reworking: 1. Minimally to completely reworked lowstand deltaic, shoreline and fluvial deposits; 2. Tectonically controlled, locally preserved remnants beneath surfaces of transgressive erosion (special cases include growth faults and delta-front slumps); and 3. Preserved remnants of deeply cut thalwegs of either tidal inlets, distributaries or incised valleys below transgressive surfaces of erosion. Each has distinctive signatures, but they require detailed, integrated data sets and rigorous testing of multiple hypothetical alternatives. This range of deposit types can be documented in the Upper Mississippian/Lower Pennsylvanian Springer Formation in central Oklahoma, the Mid-Cretaceous Frontier Formation in Wyoming, the age-equivalent Cardium Formation in Alberta and in numerous isolated Upper Cretaceous sandstones in Wyoming, Colorado and New Mexico, including the Shannon, Sussex, Tapers Ranch, O'Brien Springs, Kremmling, Muddy Buttes, Gunsight Pass, Terry, Hygiene, Tocito and others. Commonalities include depositional settings with generally low accommodation (related to regional tectonics), a preceding drop in relative sea level with subsequent transgressive erosion and intermittent exposure to currents accentuated by storms, wave effect and/or tides. Syn-sedimentary tectonic movement may produce preserved reservoir sandstone morphologies very different from the original depositional pattern.