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Changes In Mudstone Properties and ‘Shale’ Reservoir Potential through Geological Time — The Impact of Biological and Oceanic Evolution


‘Shale’ reservoir pore-system components have changed significantly due to biological and oceanographic evolution, influencing such key elements as hydrocarbon yields, kinetics, kerogen/pyrobitumen porosity development, and occurrence, composition, and diagenetic behavior of brittle components for fracability. Mudstone reservoirs contain a variety of biogenic and mineral pore types: inter-granular, intra-granular, intra-kerogen, and intra-‘bitumen’. Our work indicates that ‘shale’ reservoir intervals with multiple porosity components tend to have higher resource densities and better well performance-- and that multi-component systems are more common in post-Paleozoic strata. Evolutionary events that affected the development of mudstone reservoirs include changes in dominant planktonic and benthic organisms, land plant type and distribution, and ocean circulation and chemistry. These changes resulted in an overall evolution of the dominant biogenic hard parts of mudstone reservoirs from minimal through the early Paleozoic (due to dominance of chlorophytae and cyanobacteria) to siliceous radiolaria (mid to late Paleozoic) to calcareous coccolithophores (Mesozoic) to siliceous diatoms (Cenozoic). Benthic taxa evolution influenced paleobathymetry and lateral reservoir distribution. Effective and prospective mudstone reservoir plays span more than 1 Gy age range-- we have identified 7 key intervals in that time span, using our integrated multi-variate approach of depositional sequence stratigraphy analysis of hydrocarbon systems within a genetic basin framework, building on our worldwide knowledge of mudstones, hydrocarbon source rocks, and benthic carbonate depositional systems. The key intervals (starting times listed below) are demarked by the evolutionary rise to dominance of key taxa (mostly photoautotrophs) and significant changes in ocean state and nutrient budget: 1) Archaeozoic-- cyanobacteria; 2) Proterozoic-- chlorophytae; 3) late Cambrian-- radiolaria; 4) mid Devonian-- tracheophytes; mid Triassic -- coccolithophores; 6) Paleocene-- diatoms; 7) Oligocene-- grasses. This approach enhances characterization and prediction of reservoir potential, identifies critical factors, and adds an essential dimension for selection of appropriate analogs and high-grading basins and plays.