Low net to gross fluvial reservoirs : sedimentary architecture and geochemical correlation
Detailed understanding of sediment architecture is crucial for planning well placements and minimizing bypassed oil in low net-to-gross fluvial successions (those with a high proportion of mudstones and siltstones). Field development planning relies greatly on architectural models. These, in turn, are dependent on obtaining an adequate description of the subsurface and accurate interpretation of the fluvial style. But this is difficult due to the limited quantity of data available. Inevitably, therefore, the models are biased by preconceptions and assumptions. For low net-to-gross fluvial succession, the common presumption is that they are the products of high-sinuosity rivers, because of the high proportion of mudstone present. This choice drives the selection of parameters of reservoir modelling. Comprehensive architectural analysis of the well-exposed Wasatch Formation (Late Paleocene to Early Eocene) in the SE part of the Piceance Basin, in western Colorado, USA, shows this presumption to be an unsafe choice. The upper part of this formation (the Shire Member) is almost entirely the product of straight rivers. Evidence of high-sinuosity rivers is distinctly lacking, with just a single example found of point-bar deposits, and even that is laterally restricted and weakly developed. The bulk of the member, which is over 200m in thickness, is composed of mudstones showing pedogenic alteration of a wide range of intensities, and with evidence of both poorly and well drained soils. Isolated within these mudstones are narrow ribbon sandstones, up to 30m wide and 3m thick, interpreted to be the product of low-sinuosity single-thread (anastsomosed) rivers. At intervals through the succession are a small number of sheet sandstones up to 1km wide and 12m thick. These sheets are an amalgamation of downstream-accreting bars, some of them gravely, and channel-fill deposits, and are interpreted to be the product of a low-sinuosity but multi-threaded (braided) river. The predominance in the succession of mudstone, of relatively high cohesive strength, suggests at first sight conditions favourable for the development of high-sinuosity rivers.