Browsing by Subject "Rocks, Carbonate"

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Characterization, variations, and controls of reef-rimmed carbonate foreslopes
(2008-05) Playton, Ted; Kerans, C. (Charles), 1954-
Allochthonous, seaward-dipping deposits that flank reef-rimmed carbonate platforms (reef-rimmed carbonate foreslopes) display a spectrum of deposit types, seismic-scale stratal architecture, and bed-scale heterogeneity due to diverse sediment sources and resedimentation processes. This variability has resulted in a lack of consistent characterization approaches, and has made the development of predictive models that link carbonate foreslope deposit types to stratal architecture challenging. This study uses data from outcrops, including the Upper Devonian of the Canning Basin, Western Australia and the Upper Permian of the Delaware Basin, West Texas, and examples from literature to provide 1) an approach for characterizing reef-rimmed carbonate foreslopes in terms of deposit types and architecture, 2) conceptual models that outline the variations that exist, and 3) discussion of the intrinsic and extrinsic factors that control the observed variations. The primary depositional elements that construct reef-rimmed carbonate foreslopes can be categorized as 1) debris elements (breccias and blocks) from brittle reef failure, 2) grain-dominated elements (grainstones and rudstones) from offbank transport of sand and gravel, and 3) mud-dominated elements (mud-dominated fabrics) that record relative foreslope quiescence and fine-grained periplatform shedding. The fundamental stratal geometries observed are 1) accretionary margins, characterized by margin-toforeslope interfingering and clinoforms, and 2) escarpment margins, characterized by aggrading-retrograding margin architecture and foreslope onlap. The combinations of element proportions, element distribution, stratal geometry, bed- to bedset-scale architecture, and depositional profile scale that exist in carbonate foreslopes range widely, warranting multiple depositional models. The deposit type and architectural variations observed in detail from Upper Devonian and Upper Permian outcrops of the Canning Basin, Western Australia, and the Delaware Basin, West Texas, respectively, are linked to differing scales of superimposed accommodation change and reef faunal assemblage. Observations from other outcrops and extensive literature review display further controlling factors that affect carbonate foreslope development, such as platform morphology, oceanographic conditions, slope height, tectonic setting, and siliciclastic input, suggesting a multi-variable interplay of controls. These controls dictate the productivity and resedimentation of the contributing sediment factories, and/or influence the development of the carbonate platform system as a whole. Knowledge and classification of carbonate foreslope deposit types, architecture, and controls not only improve understanding of these complex systems, but also allow for the development of predictive relationships for economic purposes.
Multiscale flow and transport in highly heterogeneous carbonates
(2005) Zhang, Liying; Bryant, Steven L.; Jennings, James W.
Vuggy carbonates often exhibit multiscale features that make rock characterization a challenging problem. Moreover, a thorough understanding of the variability and fluid flow in vuggy carbonates is lacking. Classical theories of flow and transport are not applicable for highly heterogeneous carbonates. Therefore, it is necessary to conduct research on this type of carbonate which is found in many petroleum reservoirs and aquifers. A new method was developed in this research to understand the flow and transport properties of a vuggy carbonate sample. This method combines physical experiments, theoretical modeling, numerical simulations, and high resolution visualizations. The study focused on a Cretaceous carbonate sample containing centimeter-scale vugs. This roughly cylindrical sample, 25 cm in diameter by 36 cm in height, was X-ray CT scanned with a resolution of 0.5 mm. Effective permeability and transport properties were determined from experiments on three sub-samples from the 36-cm high rock. Single phase flow and transport through a computational sub-sample was numerically simulated using the parallel subsurface simulator (Parssim). Flow and transport through a touching-vug network, characterized from CT scan data were also numerically simulated. Seven types of pore space were characterized for the computational sub-sample, whose porosity is 21.6%. The total pore volume in this sub-sample is partitioned into 64% of vugs and 36% of matrix porosity. The vug volume is composed of separate-vugs and a touching-vug network, which account for 21% and 43% of the total pore volume respectively. The touching-vug network is divided into a flowing-vug network and deadend-vugs, which account for 22% and 21% of the total pore volume respectively. Study results show that vugs in this sample are interconnected with relatively narrow throats of a few millimeters or less in diameter. Dispersivity in this sample is a non-trivial convolution of at least three phenomena: advection through a flowing-vug network, mass-transfer between the flowing-vug network and dead-end vugs, and transport through the matrix and non-touching vugs. This miscible flooding behavior cannot be simply explained by classical one dimensional convection-dispersion model.