Shale Facies and Seal Variability in Deep Marine Depositional Systems
William C. Dawson, William R. Almon, 2004. "Shale Facies and Seal Variability in Deep Marine Depositional Systems", Depositional Processes and Reservoir Characteristics of Siltstones, Mudstones and Shales, Erik D. Scott, Arnold H. Bouma
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Sealing characteristics of marine shales are among the least understood aspects of petroleum systems. Petrophysical measurements indicate that the largest interconnected pore throats ultimately control seal behavior. Pore throat diameter, determined from mercury-injection capillary pressure (MICP) analysis, is influenced by numerous factors, including: composition (total clay content, and organic enrichment), fabric and texture (fissility, silt content and bioturbation), and diagenesis. Data from deepwater wells in the Gulf of Mexico and offshore Angola document the variability of shale microfacies and sealing character in marine depositional settings. This variability can be quantified and predicted where considered within the context of sequence stratigraphy and shale sedimentology.
The analyzed Tertiary-aged marine shales record deposition in middle to lower slope paleoenvironments and are interstratified with sandstones representing lowstand fan lithofacies. Six shale microfacies can be defined based on differences in fabric and petrophysical properties: 1) well-laminated, slightly silty, organically-enriched shales; 2) moderately silty, partially laminated shales; 3) moderately silty mottled shales; 4) very silty mottled shales; 5) very silty shales interlaminated with siltstones and very fine sandstones; and 6) calcareous shales and claystones. Shale types 1, 2 and 6 consistently exhibit better than average seal capacities. Shale types 3 and 4 are moderate to good seals, and Type 5 shales are typically poor to very poor seals. Top seal capacity increases as clay content increases and decreases as the content of detrital silt increases. Depositional fabric appears to exert primary control on seal character, and early marine cementation can significantly enhance seal capacity. The texture and composition of marine shales vary systematically within depositional sequences and correlate with variations in sealing capacity.
Silt-rich shales in highstand and lowstand systems tracts have 10% non-wetting (MICP) saturations that are consistently low relative to those of transgressive shales. The highest 10% non-wetting mercury-injection capillary pressure (MICP) saturations values correspond to transgressive and condensed shales containing significant percentages of authigenic carbonates. Shales occurring within the upper part of third-order transgressive systems tracts are typically excellent to exceptional top seals. These finely laminated, silt-poor, transgressive shales commonly have elevated percentages of organic matter and authigenic iron minerals. Seal capacity generally increases, basinward, from near shore to distal offshore marine settings. Where a transgressive shale is the controlling top seal for a lowstand reservoir, a thick waste zone commonly separates the seal and the subjacent reservoir.