Abstract The facies architecture, sequence-stratigraphic framework, and structural configuration of a petroleum reservoir were delineated by an integrated study. These interpretations formed the basis of three-dimensional (3-D) geologic models that were constructed for estimation of reserves, flow-simulation studies, and field-development planning. The study incorporated 3-D seismic interpretations, well-log correlations, facies and petrophysical analyses of cored intervals, and interpretations derived from outcrop exposures of the reservoir. The reservoir intervals are interpreted to represent a fluvial depositional system that varies systematically along an updip to downdip transect. Proximal (updip) fluvial-facies deposits are inferred to represent amalgamated channel complexes that form widespread sheets. Medial fluvial-facies deposits are interpreted as amalgamated to semiamalgamated braid-bar deposits that are thinner and less laterally persistent. Distal (downdip) fluvial facies are inferred to represent thin yet laterally extensive braid-bar deposits. Object-based modeling techniques were used to model the internal architecture of the reservoir intervals. Proximal channel facies were generated using standard software to populate the zones with channel elements that are clustered to form channel complexes. Medial and distal bar facies, however, required an innovative method that populates the zones with discrete, user-defined, braid-bar elements that are distributed along thalwegs. Clusters of thalwegs form amalgamated to semiamalgamated bar complexes. This capability, referred to as bar-train modeling, results in a better computer-model representation of the fluvial-sandstone bar geometry and spatial distribution. The resulting geologic models provide an improved reservoir characterization of the large-scale and small-scale fluvial architecture for the subsurface reservoir. In particular, the geologic models more accurately describe the complex architecture of the lowstand channel and braid-bar fluvial sandstones as well as the internal architecture of the intervening mudstone deposits.

Abstract Detailed stratal correlations of the Skull Creek, Muddy, Shell Cheek, and Mowry formations in the vicinity ofHilight field in the south-central Powder River basin, Wyoming, reveal that the Muddy Sandstone in this region is an unconformity-bounded stratigraphic unit (depositional sequence). This depositional sequence represents an erosional remnant of what were originally more widespread incised-valley and younger (high- stand) shoreline deposits. The unconformity at the base of the Muddy Sandstone (green sequence boundary) is characterized by incision ofunderlying Skull Creek strata and onlap of overlying Muddy strata. In core, this boundary also corresponds to anabrupt basinward shift in fades (coastal plain on distal offshore deposits) and, locally, rooted horizons. The unconformity at the top of the Muddy Sandstone (red sequence boundary) is characterized by truncation of and incision into underlying Muddy strata, onlap of overlying Shell Creek strata, and angular discordance. In core, this sequence boundary coincides with a marine flooding surface that is locally underlain by root structures. Within the “Muddy” depositional sequence (green to red sequence boundaries) three informal units, nonmarine (oldest), lower paralic, and upper par- alic, can be defined. These units are separated by two regional well-log markers (Ml and M2) that, in core, correspond to bentonites that overlie interpreted marine flooding surfaces. The basal nonmarine unit of the Muddy: (1) onlaps topography carved into the underlying Skull Creek Shale by the green sequence boundary, (2) contains only nonmarine deposits in core, and (3) is overlain by a flooding surface and the M2 marker. This stratigraphic interval is interpreted as the lowstand systems tract (incised-valley fill) of the green sequence. The overlying lower paralic and upper paralic units of the Muddy contain shoreline and coastal-plain deposits in core and are interpreted as the highstand systems tract of the green sequence. Shell Creek strata which onlap and overlie the red sequence boundary are interpreted as the lowstand and transgressive systems tracts of the red depositional sequence. Bentonites and organic-rich shales at the base of the overlying Mowry Shale represent the condensed section of the red depositional sequence. Toward the margins of the Hilight field, the upper paralic, lower paralic, and nonmarine units of the Muddy are progressively truncated by the over- lying red sequence boundary. Thus, both primary depositional and secondary erosional patterns control Muddy thickness variations, spatial distributions, facies patterns, and the hydrocarbon trap at Hilight field.

Abstract: In central Alabama, near the town of Braggs, a complete section across the Cretaceous-Tertiary (K-T) boundary is present within the lower portion of the Clayton Formation. The K-T microfauna and microfloral transition occurs within a 2.5-m (8 ft) section of interbedded sandstones and limestones that directly overlies a sequence boundary, marked by regional truncation of the underlying Prairie Bluff Formation. This sequence boundary is related to a major eustatic fall in the late Maastrichtian (67 Ma). The interbedded sandstones and limestones in the basal Clayton Formation are interpreted as two backstepping marine parasequences deposited on the inner shelf during the subsequent relative rise in sea level. These two backstepping parasequences are overlain, in turn, by 1.5 m (5 ft) of glauconite-rich strata representing a condensed section produced during a period of slow terrigenous deposition, continued par-asequence backstepping, and shoreline retreat. Three small iridium anomalies have been identified at the Braggs locality. These anomalies occur at marine-flooding surfaces, interpreted to be parasequence boundaries, in the uppermost Prairie Bluff and basal Clayton formations. The uppermost of these anomalies also coincides with the base of the well-developed condensed section in the basal Clayton Formation. The concurrence of iridium concentrations with marine-flooding surfaces at Braggs suggests that iridium was present in the open ocean during the latest Maastrichtian through earliest Danian but concentrated only during periods of terrigenous-sediment starvation. Thus, variations in sediment supply and possibly basin location are critical factors controlling iridium enrichment across the K-T boundary.