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Abstract
Deltaic successions and isolated shallow marine sandstones have proven to host economically significant hydrocarbon resources throughout the Cretaceous stratigraphic succession of western North America. Therefore, developing robust interpretations incorporating detailed facies and sequence stratigraphic analyses are critical in understanding their origins. This volume examines the allogenic forcing mechanisms controlling stratigraphic architectures and the hydrocarbon generation potential of the middle-upper Turonian Frontier Formation, Uinta, Piceance, and Green River Basins, Utah, Colorado, and Wyoming.
The first study (Chapter 2) provides a regional synthesis of variations in sediment dispersal and planform geometry. For this analysis, 54 outcrop sections and 410 drillhole locations with wireline log suites generated regional cross-sections, isochore maps, and facies reconstruction maps. From this, five forced and normal regressive deltaic (Intervals 1-4) and shoreline (Interval 5) sequences are defined. Together, they record a change through time from north-south elongate planforms controlled by foreland basin structure (Intervals 1-3) to eastward prograding, broadly lobate geometries influenced by west-east basement lineaments associated with Laramide Foreland buttresses (Intervals 4-5).
Chapter 3 re-defines the origins of the Ashley Valley Member (Interval 5) that was previously interpreted as an ‘offshore bar’. Facies and sequence stratigraphic analyses and regional sandstone isochore thickness maps constructed from outcrop and 88 subsurface wireline log datasets reveal three sharp-based, northwest-southeast elongate, asymmetric sandstone bodies with basinward offlapping stratal stacking patterns. These relationships allow for an alternative explanation, according to which the Ashley Valley Member is the product of multiple tectonically-forced, high-frequency base-level forced regressions and lowstand normal regressions superimposed upon a low-frequency global eustatic regression.
Chapter 4 presents petrographic and isotopic analyses of eight anomalous carbonate lens horizons within the Frontier Formation distal delta front. A number of diagenetic phases are identified and investigated. Fibrous botryoids and fan-shaped calcite mineralogies with low δ13C isotope values precipitated from biogenic methane within the shallow sediment column during synsedimentary diagenesis. Blocky calcite with low δ18 O isotope values associated with hydrocarbon residue, cone-in-cone structures, and calcite ‘beef’ veins indicate deep burial hydrocarbon generation. Along with source quality data, these features suggest that the Frontier Formation has potential to be a significant total petroleum system.
