Sealing Capacity and Petrographic Characteristics of Eocene Marine Mudrocks, South-Central Pyrenean Foreland Basin, Spain
Frank G. Ethridge, Sally J. Sutton, William C. Dawson, William R. Almon, Jarrad G. Berg, 2004. "Sealing Capacity and Petrographic Characteristics of Eocene Marine Mudrocks, South-Central Pyrenean Foreland Basin, Spain", Depositional Processes and Reservoir Characteristics of Siltstones, Mudstones and Shales, Erik D. Scott, Arnold H. Bouma
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The goal of this research is to develop a predictive model for use in hydrocarbon exploration and risk analysis/mitigation that permits estimation of the sealing capacity of marine mudrocks. Research to date has concentrated on Cretaceous shales of the Western Interior foreland basin in Colorado and Wyoming and Eocene shales in the Ventura basin, California. This research focuses on Eocene rocks of the central Pyrenean foreland basin, Spain. Outcrop mudrock samples were collected from areas around Ainsa, Broto, Undues, and Anso in northeastern Spain. Geologically the areas sampled include the Ainsa basin slope, Broto base of slope, and the Jaca outer fan and basin plain in the south-central Pyrenees. Extensive research to define stratigraphic relationships, depositional environments, basin type, and sandstone body geometry have been undertaken, however the nature and sealing capacity of included mudrocks have been largely ignored. The Eocene Ainsa basin, from which most of our samples were taken, originated as a foredeep ahead of a thrust ramp and evolved into a piggy back setting as the thrust migrated basinward. This foredeep was filled with up to 4000m of slope sandstones and mudrocks, had a source area to the southeast and a basin plain (the Jaca basin) to the northeast.
Sealing capacity of these mudrocks was determined by mercury injection-capillary pressure (MICP) measurements conducted by Poro-Technology on 43 samples representing a range of slope, base of slope, and basin floor environments. Capillary pressure curves generated during mercury injection have been used to evaluate sealing capacity by equating it to the pressure required to achieve 10%mercury saturation. Pore throat diameter was determined from data generated during the MICP analyses. Bioturbation was characterized on a qualitative scale of 0 to 6. Mean quartz grain size was determined by measuring the apparent long axes of thirty quartz grains per sample. Total organic carbon (TOC) and CaCO3 were determined by analytical techniques in the soils laboratory at Colorado State University
Samples analyzed range from laminated and bioturbated, calcareous mudstones to silty biomicrites and calcareous siltstones. Some samples contain thin, normally graded, laminations of silt-size quartz grains. MICP values at 10%saturation range from 500 PSIA to more than 60,000 PSIA. Despite the high degree of variability in samples from a single outcrop and from each sampled area there is a general correspondence with geographic and sequence stratigraphic setting. Furthermore, significant correlations exist between MICP and sample porosity, pore aperture diameter, standard deviation of pore diameter, and TOC. As expected sample porosity, pore diameter and standard deviation of pore diameter are inversely related to sealing capacity. TOC is directly related to sealing capacity. Degree of bioturbation is more variable in samples with low sealing capacity and is generally lower in samples with high sealing capacity. There are no apparent correlations between MICP and permeability, grain density, average quartz grain size, standard deviation of quartz grain size, quartz grain roundness, standard deviation of quartz grain roundness, or CaCO3 content.
From a depositional setting and sequence stratigraphic viewpoint proximal slope deposits have the lowest average sealing capacity, highest porosity and permeability, highest pore aperture diameter and most poorly sorted pore diameters, highest overall grain size and degree of bioturbation, and lowest TOC. Basin floor deposits have the highest average sealing capacity, lowest porosity and permeability, smallest pore aperture diameter, and best sorted pore diameters, lowest overall grain size and degree of bioturbation, and highest TOC. Compositional variables, other than TOC, are less important than textural variables in determining the sealing capacity of these mudrocks.
Factors that influence the relative sealing capacity of these carbonate-rich mudstones are generally similar to those for non-calcareous shales from other foreland basins and include overall grain size, degree of disruption of depositional fabric by bioturbation, pore throat size and sorting, TOC, and depositional setting within the basin. The variables that most strongly favor high sealing capacity are most likely associated with deposits of deep water anoxic environments, hence the common association between good seals and upper transgressive systems tract deposits and condensed sections.
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Siltstones, mudstones and shales have been studied mainly with regard to general transportdeposition processes and clay mineralogy. A small group of investigators, with differing backgrounds, have worked on these fine-grained deposits. Recent studies on deepwater deposits from cores and outcrops indicate that the presence of finer-grained deposits greatly affect the fluid flow properties of deepwater reservoirs. Characteristics and rock properties of these deposits, which resulted from a variety of depositional processes, are just beginning to be understood.