Fluviodeltaic stratigraphic architecture and its impact on fluid flow have been characterized using a high-resolution, three-dimensional, reservoir-scale model of an outcrop analog from the Upper Cretaceous Ferron Sandstone Member of central Utah. The model contains two parasequence sets (delta complexes), each with five or six parasequences, separated by an interval of coastal plain strata. Each parasequence contains one or two laterally offset teardrop-shaped delta lobes that are 6 to 12 km (4–7 mi) long, 3 to 9 km (2–6 mi) wide, 5 to 29 m (16–95 ft) thick, and have aspect ratios (width/length) of 0.4 to 0.8. Delta lobes have a wide range of azimuthal orientations (120) around an overall east-northeastward progradation direction. In plan view, delta lobes in successive parasequences exhibit large (as much as 91) clockwise and counterclockwise rotations in progradation direction, which are attributed to autogenic lobe switching. In cross-sectional view, parasequence stacking is strongly progradational, but a small component of aggradation or downstepping between parasequences reflects relative sea level fluctuations. We use flow simulations to characterize the impact of this heterogeneity on production in terms of the sweep efficiency, which is controlled by (1) the continuity, orientation, and permeability of channel-fill sand bodies; (2) the vertical permeability of distal delta-front heteroliths; (3) the direction of sweep relative to the orientation of channel-fill and delta-lobe sand bodies; and (4) well spacing. Distributary channel-fill sand bodies terminate at the apex of genetically related delta lobes and provide limited sand body connectivity. In contrast, fluvial channel-fill sand bodies cut into, and connect, multiple delta-lobe sand bodies. Low, but non-zero, vertical permeability within distal delta-front heteroliths also provides connectivity between successive delta-lobe sand bodies.
Abstract Feldspar minerals near the rock‐saprolite interface weathered directly to gibbsite, tubular halloysite, and amorphous aluminosilicate minerals. The gibbsite precipitated as aggregates of tiny plates deep in the profile. This mineral then resilicated into a tabular halloysite pseudomorphic after the gibbsite. There was very little evidence of amorphous materials throughout the profile but apparently amorphous spheres formed on the surface of feldspars and quartz in the weathering rock. These amorphous spheres seemed to radially crystallize into tube‐shaped minerals, presumably halloysite. Halloysite in the deep saprolite later recrystallized, via a randomly‐interstratified phase, to plate‐shaped kaolin minerals. Morphology of the 1:1 layer aluminosilicates did not provide evidence for degree of crystallinity as determined by X‐ray diffraction; poorly crystalline plate‐shaped and more highly crystalline tube‐shaped kaolin minerals were found in the same profile.
Abstract Differential thermal analysis (DTA) is combined with selective dissolution (hot 0.5 N alkali solution) to provide a quantitative estimate of amorphous inorganic material in deferrated soil clays consisting predominantly of 1:1 type layer silicate minerals, gibbsite, and amorphous material. Data from DTA are used to estimate gibbsite and 1:1 type layer silicate minerals in the original and alkali‐treated clay. Correction is then made to the weight loss due to alkali treatment to estimate amorphous inorganic material solubilized in boiling 0.5 N NaOH or KOH. Qualitative support for conclusions reached is provided by x‐ray diffraction and infrared spectroscopic studies of the original and treated samples.
Geologic models of hydrocarbon reservoirs are routinely used in the petroleum industry for volumetric calculation, well planning, and flow-performance prediction. A geologic model is a computer-based representation of the reservoir's architecture and properties. Properties such as porosity and lithofacies are estimated at each cell in a geologic model using various estimation methods; the most common are geostatistical simulation, such as sequential-Gaussian simulation and sequential-indicator simulation (Goovaerts, 1997), which take into account the spatial continuity of the property and also honor its heterogeneity.
Multiple techniques are available to construct three-dimensional reservoir models. This study uses comparative analysis to test the impact of applying four commonly used stochastic modeling techniques to capture geologic heterogeneity and fluid-flow behavior in fluvial-dominated deltaic reservoirs of complex facies architecture: (1) sequential indicator simulation; (2) object-based modeling; (3) multiple-point statistics (MPS); and (4) spectral component geologic modeling. A reference for comparison is provided by a high-resolution model of an outcrop analog that captures facies architecture at the scale of parasequences, delta lobes, and facies-association belts. A sparse, pseudosubsurface data set extracted from the reference model is used to condition models constructed using each stochastic reservoir modeling technique. Models constructed using all four algorithms fail to match the facies-association proportions of the reference model because they are conditioned to well data that sample a small, unrepresentative volume of the reservoir. Simulated sweep efficiency is determined by the degree to which the modeling algorithms reproduce two aspects of facies architecture that control sand-body connectivity: (1) the abundance, continuity, and orientation of channelized fluvial sand bodies; and (2) the lateral continuity of barriers to vertical flow associated with flooding surfaces. The MPS algorithm performs best in this regard. However, the static and dynamic performance of the models (as measured against facies-association proportions, facies architecture, and recovery factor of the reference model) is more dependent on the quality and quantity of conditioning data and on the interpreted geologic scenario(s) implicit in the models than on the choice of modeling technique.
Abstract A lithologically continuous vertical rock‐saprolite‐soil sequence was studied in order to determine its mineralogical characteristics and transformations. The soil was a Typic Hapludult from the North Carolina Piedmont formed on granitic gneiss. The profile was sampled in 19 horizons, from the surface to slightly altered rock, on the basis of morphology and depth. Though weathering intensity was generally antipathetic with depth, this was not always the case, especially in the deep saprolite where two horizontal weathering fronts moved in opposite directions from a single fracture plane. The soil material differed morphologically from the saprolite by having soil structure and clay skins. Elemental analysis of the samples revealed a release of Ca and Na from the altering rock, and a subsequent decrease in abrasion pH, presumably due to rapid plagioclase alteration. Aluminum seemed to precipitate initially as an Al‐rich clay mineral in the altering rock. However, this clay was rapidly resilicated coincident with a decrease in abrasion pH. The most rapid changes occurred in the deepest saprolite horizons.
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