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Eastern Section Meeting

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Facies analysis of thin-bedded reservoirs in mixed-influence deltaic systems

Abstract

Despite the historical assumptions that most marine “shelf” mud is deposited by gradual fallout from suspension in quiet water, recent studies of modern muddy shelves show that they are dominated by hyperpycnal fluid mud. Flume work shows that bedload transport is critical in the deposition of mud, and storm-wave aided hyperpycnal flows are common on many modern muddy shelves. We have applied these ideas to the interpretation of mud-rich prodelta deposits of the Cretaceous Western Interior Seaway depositional systems of North America. Thin-bedded heterolithic deposits contain significant sandstone and are volumetrically important zones of bypassed unconventional pay surrounding many conventional reservoirs. The Upper Cretaceous Ferron Sandstone in Utah shows deposition by ignitive turbidity currents, hyperpycnal flows, storm surges, as well as complete bioturbation in areas away from river-influence. Ignitive turbidites show fining upward Bouma sequence. Hyperpycnites show either inversely or normal grading. Storm deposits (tempestites) fine upward and contain hummocky cross stratification (HCS) and wave ripples. Ignitive turbidity currents and hyperpycnal flows indicate fluvial-dominated depositional environments, whereas tempestites are linked to storm-wave dominated environments. Detailed measured sections, as centimeter-scale, allow the relative proportion of sedimentary and biogenic structures generated by each depositional process/event to be calculated. These were measured from a series of stratigraphic sections within a mixed wave and fluvial dominated parasequence, exposed continuously exposed along depositional strike. The parasequence shows strong along-strike variation with a completely wave-influenced shoreface environment in the north, that contains minimal thin-beds, passing abruptly into a fluvial-dominated area with a thick heterolithic prodelta, then to an environment with varying degrees of fluvial and wave influence southward, and back to a wave-dominated environment further to the southeast. The depositional model of the parasequence is therefore interpreted as a storm-dominated symmetric delta with a large river-dominated bayhead delta system. It is thus practical to quantify the relative importance of depositional processes and determine the along-strike variation within thin-bedded heteroliths an ancient delta system using thin-bedded facies analysis, which has implications of along-strike heterogeneity of thin-bedded reservoirs.