Bradford E. Prather

Classification, Lithologic Calibration, and Stratigraphic Succession of Seismic Facies of Intraslope Basins, Deep-Water Gulf of Mexico

Seismic facies in Gulf of Mexico intraslope basins reflect the interplay of a variety of deep-water depositional processes and the evolution of accommodation space on the slope. This interplay of processes results in a transition from an early, sand-prone ponded basin-fill succession (ponded facies assemblage) to a later shale-prone, slope-bypass succession (bypass facies assemblage). Convergent-baselapping facies in combination with localized chaotic and draping facies dominate the ponded facies assemblage. Stratigraphic relationships among these three units illustrate how fill-and-spill depositional processes occur within ponded-basin accommodation space. Convergent-thinning facies with widespread chaotic and draping facies dominate the bypass facies assemblage. These units represent filling of different types of slope accommodation space. The transition from ponded to bypass facies assemblages can be sharp or gradational over hundreds of meters. Transitions occured across the central Gulf of Mexico during the late Pliocene between 2.0 and 1.8 Ma, and in the early Pleistocene between 1.2 and 1.0 Ma. Nearly synchronous transitions throughout basins in the upper to middle slope suggest that increased sediment supply, resulting from a second-order sea level fall, and capture of large drainage areas by the Mississippi River during the Pleistocene are the primary controls on development of this large-scale stratigraphic architecture.

Calibration and visualization of depositional process models for above-grade slopes: a case study from the Gulf of Mexico

Abstract Mobile substrates with relatively large amounts of ponded-basin and healed-slope accommodation space across the mid-slope characterize above-grade slopes. There are many above-grade slopes around the world but the Gulf of Mexico (GOM) is arguably the best calibrated one. Because of the unique geologic processes attributed to the GOM, however, such as high rates of intraslope basin subsidence (locally >10 km/Ma) and meltwater spikes, the transfer of depositional process concepts to other deep-water provinces may not seem intuitively obvious. Ponded-basin ‘fill-and-spill’ processes dominate early phases of deposition on above-grade slopes and precede progradation of unconfined slopes. A stepped-equilibrium profile across a series of ponded basin forms a base for late unconfined slope progradation. Deposition in the unconfined slope occurs in both slope and healed-slope accommodation space. These spaces are filled as progradational delta fronts build beyond critical angles, and later collapsed. Sediments tend to be sand lean because the slope angles are too high to allow turbidite fan deposition. Sands are generally associated with channels with some localized sheet sands. This paper describes the use of a proprietary stratigraphic forward modeling application (STRATAGEM) to visualize and distill a GOM-based depositional process model potentially transferable to other slope and base-of-slope systems. Forward models show that the introduction of meltwater spikes do not radically change the depositional architecture of the slope but they do force gravity flow deposition to continue into eustatic sea level rises. These models also show that rapid salt withdrawal beneath intraslopes basins controls the distribution of accommodation space across the slope and thus its stratigraphic architecture.

Sublacustrine depositional fans in southwest Melas Chasma

Two depositional fan complexes have been identified on the floor of southwest Melas Chasma. The western fan complex is located near the center of an enclosed basin in southwest Melas Chasma and is composed of multiple lobes with dendritic finger-like terminations. These fans are very flat and have a morphology unlike any other fan that has been previously identified on Mars. On the basis of the morphologic similarity of the western fan complex to the Mississippi submarine fan complex, we suggest that it may be a deep subaqueous fan depositional system. There are numerous channels on the surface of the western fan complex, and measurements of channel length, width, and sinuosity are consistent with channels observed on terrestrial submarine fans. The eastern Melas depositional fans are less well preserved and may be of deltaic or sublacustrine origin. Recognition of the fans supports earlier suggestions for the presence of a former lake in Melas Chasma and indicates that a significant body of water was present and stable at the surface of Mars for at least 10^2 to 10^4 years.

Turbidite Channel Architecture: Recognizing and Quantifying the Distribution of Channel-base Drapes Using Core and Dipmeter Data

Field and simulation studies indicate that channel architecture and the presence of channel-base drapes (CBDs) can have a significant impact on oil recovery and represent key uncertainties in the understanding of a turbidite channel reservoir. Accordingly, understanding the frequency and distribution of CBDs provides valuable insights into reservoir performance. Core and dipmeter data contain information that can be used to recognize channel-base disconformities and associated CBDs. By comparing the observed number of channel-base disconformities to the observed number of disconformities overlain by mudstone, a statistical assessment of their frequency and distribution can be made. In a spatial sense, the fraction observed in the wells represents the average percentage of the channel elements within the reservoir that are overlain by a drape.

Petroleum geology of the Upper Jurassic and Lower Cretaceous, Baltimore Canyon Trough, western North Atlantic Ocean

Numerous hydrocarbon shows, including a noncommercial gas and gas-condensate accumulation, occur in the Baltimore Canyon Trough within sandstone units deposited in prograding coastal-plain and transitional-marine environments located updip of an Oxfordian/Kimmeridgian carbonate shelf edge. The coastal-plain and transitional-marine facies are overlain by a fine-grained deltaic complex dominated by delta-plain shales which collectively form a regionally extensive top seal unit. This deltaic complex prograded into a back-reef lagoon during aggradation of lower Kimmeridgian through Berriasian shelf-margin carbonates. Wells drilled seaward of the continental shelf edge (>1500 m water depth) tested large structural/stratigraphic closures along the downdip termination of the Upper Jurassic/Lower Cretaceous carbonate shelf edge but encountered no significant hydrocarbon shows. Reservoir rocks in these wells consist of (1) oolite grainstone, which was deposited within a shoal-water complex located at the Aptian shelf edge, and (2) coral-stromatoporoid grainstone and boundstone, which formed an aggraded shelf-margin complex located at the late Kimmeridgian through Berriasian shelf edge. Structural closures having reservoir and top seals are present in both updip and downdip trends. Hydrocarbon shows in wells along the shelf interior trend indicate the presence of mature source beds, at least locally. The absence of hydrocarbon shows in downdip carbonate reservoirs and around the Schlee Dome, however, suggests charge/migration mechanisms within the fetch areas of these objectives have failed. Failure of charge can be due to (1) absence of mature source rocks, (2) absence of migration pathways from source rocks to reservoirs, and/or (3) absence of top seals at the time of hydrocarbon migration. Continued development of play concepts in the Baltimore Canyon Trough, therefore, requires identification and mapping of potential source-rock intervals and construction of hydr carbon expulsion models to time hydrocarbon generation relative to trap formation.

An Upper Pennsylvanian desert Paleosol in the D-zone of the Lansing-Kansas City Groups, Hitchcock County, Nebraska

ABSTRACT This study suggests that the upper shale (regressive shale) in the Upper Pennsylvanian (Missourian), Lansing-Kansas City Groups D-zone from Hitchcock County, Nebraska, is a paleosol. Pedogenesis of this unit occurred during prolonged subaerial exposure on a semiarid to arid alluvial flood plain. Pedogenesis occurred during maximum retreat of the Late Pennsylvanian sea during the D-zone sedimentary cycle. Initially, clays in this unit were deposited in marine environments but as eustatic sea level continued to drop, deposition became dominated by continental alluvial processes. Characteristics which distinguish the upper shale as a paleosol are the presence of (1) a basal subsoil caliche (C-soil horizon) characterized by weathered limestone regolith clasts, soil nodules, thin-laminated calcrete beds with pseudo-fenestral fabric, pelleted and clotty calcareous and dolomitic caliche, weathered clays, circumgranular cracks, and root casts; and (2) an upper unit (B-soil horizon) characterized by authigenic hematite, intensely weathered clays, soil nodules, slickensides, and gypsum root casts.

Simplified Modeling of Turbidite Channel Reservoirs

This paper (SPE 114854) was accepted for presentation at the SPE Annual Technical Conference and Exhibition, Denver, 21–24 September 2008, and revised for publication. Original manuscript received for review 7 July 2008. Revised manuscript received for review 19 January 2009. Paper peer approved 21 January 2009. Summary Effective properties can represent fine-scale geologic heterogeneities in simple full-field reservoir models without having to model them explicitly. A comprehensive simulation study tests the sensitivity of dynamic connectivity in turbidite channel reservoirs to a large number of stratigraphic and engineering parameters. Simulations performed using geologically realistic sector models at multiple levels of stratigraphic resolution show that dynamic connectivity is governed by large-scale architectural parameters, such as meander-belt size, net-to-gross ratio, degree of depositionalstorey amalgamation, and stratigraphic parameters that describe the shale architecture at multiple scales (e.g., shale-drape coverage and shale-drape frequency of occurrence). We demonstrate how to rapidly generate effective properties at multiple geologic scales, incorporating the effect of channel architecture and reservoir connectivity into simple dynamic models. Use of simple dynamic models in conjunction with effective properties, principally geologically based pseudorelative permeabilities, accelerates the simulation workflow significantly. We show that a statistical distribution of the recovery factor (RF) can be produced within hours instead of days by the combined use of Monte Carlo simulation and a simple dynamic model with effective properties. Recovery factors estimated with our simplified modeling method agree well with observed RF distributions of turbidite channel reservoirs with significant production history.

Control on Reservoir Distribution and Quality in Regressive Member of an Upper Pennsylvanian Cyclothem: ABSTRACT

Isopach maps and diagenetic features may be used to predict the distribution of reservoir-quality rock in the D-zone cyclothem of the Lansing-Kansas City Groups in southwestern Nebraska. The D-zone cyclothem was deposited during one major oscillation of the epeiric sea in Late Pennsylvanian (Missourian). This cyclothem records a transgression of sea level followed by a major regression. During the regressive phase there was a brief sea level transgression. The D-zone cyclothem consists of the four basic lithofacies common to most cyclic deposits of this age in northwestern Kansas and southwestern Nebraska: (1) a thin lower carbonate unit deposited in a shallow-marine environment; (2) a laterally extensive lower shale unit of marine origin resulting from a terrigenous influx from the north; (3) a complex upper carbonate unit deposited in shoaling water during waning terrigenous influx; and (4) an upper shale unit deposited in tidal flat to nonmarine environments. Core data and an isopach map of the upper shale unit suggest that several shoal areas existed in Hitchcock County during part of the Missourian. Pellet, ooid grainstone deposition was localized on these bathymetric highs. The bathymetric highs may have been formed by (1) differential compaction of the upper shale unit of the underlying E-zone over erosional topography, or (2) movement on the ancestral Las Animas arch. The presence of equant-calcite fringing cements in pores of the grain-supported rock indicate early diagenesis in a freshwater phreatic zone formed during initial subaerial exposure. Limpid dolomite rhombs intergrown with the early calcite cements and replacing the edges of some framework grains suggest cementation in a mixing zone. The highest stratigraphic occurrence of dolomite, if plotted on a cross section, forms a line which transects facies boundaries and may represent either the position of the mixing zone or an early paleowater table. The majority of the leached porosity in the grain-supported rock occurs above this line. Dolomitization of underlying carbonate facies probably occurred contemporaneously as the mixing zone migrated through the porous mud-supported sediments. Fu ther enhancement of porosity may have occurred in a vadose zone above a later paleowater table. The position of this paleowater table is indicated by the distribution of skeletal fragments replaced by red silica, dissolution cracks infiltered with nonmarine clay, and authigenic gypsum. These features formed during a later stage of diagenesis which took place contemporaneously with soil formation and calichification in the upper shale in a semiarid or arid environment. Conclusions: (1) paleobathymetery is reflected in an isopach map of the upper shale unit; (2) distribution of grain-supported rock is controlled in part by formation of bathymetric highs while underlying shales compacted around preexisting topographic highs; (3) enhancement of porosity by dissolution in the grain-supported rocks occurred in the freshwater phreatic and vadose zones; (4) recognition of diagenetic features associated with formation of paleowater tables may be used to predict the distribution of porosity in these grainstones. End_of_Article - Last_Page 1326------------