AAPG Middle East Region Geoscience Technology Workshop:
3rd Edition Carbonate Reservoirs of the Middle East

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Geological Characterization of a Late Jurassic Carbonate Mudstone, Saudi Arabia: Sedimentology, Geochemistry, And Geomechanics


Five cores from a late Jurassic (Kimmeridgian) carbonate mudstone succession from Saudi Arabia have been evaluated for sedimentologic, organic geochemical, and geomechanical properties. This interval was deposited in a basinal setting and is dominated by calcite mineralogy, with variable admixtures of lesser amounts of quartz and clays, and minor amounts of authigenic dolomite and pyrite. Twelve lithofacies have been identified through centimeter-scale core descriptions and standard petrography. Lithofacies are grouped into three principal facies associations, with differences related to processes such as intermixed traction-suspension, traction-bioturbation, and bioturbation-suspension deposition/modification. Organic-rich facies of calcareous and mixed-calcareous mudstones with low bioturbation indices, high-redox sensitive elements, and medium to high paleo-productivity indices, corresponded to anoxic to dysoxic oxygen levels. In contrast, calcareous- and silica-rich mudstones with high bioturbation indices and low redox-sensitive elements were likely deposited in more oxygenated water. The twelve lithofacies have also been characterized by their organic richness and hydrocarbon potential using open-system pyrolysis. Organic richness in the more anoxic facies ranges up to 5.6% TOC. Geochemical attributes indicate oil- to gas-prone kerogen. Further geochemical analysis suggests that the organic-rich lithofacies fall in the category of good to very good source rock and above average hydrocarbon generation potential. Analyses of slabbed core samples were used to develop a high-resolution mechanical stratigraphy using micro-rebound testing, impulse hammer measurements, scratch testing, and ultrasonic velocity measurement methods. Calculated Leeb Hardness values range from 300 to 800 HLD, with low values associated with increased TOC and clay content. Techniques were used to compare/calibrate measured elastic properties with wireline logs. Comparisons show very similar ranges, variations, and trends, however, HLD profiles provide higher-resolution variability than matched wireline logs.