Pore systems in oil-window mudrocks of lower Green River Formation (Castle Peak Shale through Lower Garden Gulch), Uinta Basin, Utah

Stratigraphic, petrographic, geochemical, and petrophysical data from a core in the Central Basin region of the Uinta Basin indicate abundant hydrocarbon-saturated reservoir porosity in lower Green River Formation mudrocks containing abundant organic matter (OM). Pore systems evolved with variable detrital and near-surface diagenetic components and fabrics overprinted by differential burial diagenesis into the oil window (mean Ro = 0.97%).

The ~1100 ft core is comprised of cyclically interbedded sandstones and sandy conglomerates, OM-lean siliciclastic mudrocks, OM-bearing siliciclastic mudrocks and argillaceous conglomerates, and OM-bearing carbonate mudrocks and intraclast rudstones–floatstones. Sandstones, sandy conglomerates, and OM-lean siliciclastic mudrocks contain substantial pore volumes dominated by clay-hosted and interparticle micro- to nanopores; however, these are largely water-saturated. OM-bearing siliciclastic mudrocks contain interparticle, clay-hosted, and OM-hosted nanopores, with mean pore volumes of ~10% and mean hydrocarbon saturations of ~43%. OM-bearing carbonate mudrocks and intraclast rudstones–floatstones host interparticle, intercrystalline, and OM-hosted nanopores, with slightly lower mean pore volumes of ~8% but higher hydrocarbon saturations of ~64%. Although OM-bearing siliciclastic mudrocks have higher average total porosity, carbonate mudrocks contain greater volumes of hydrocarbon-saturated porosity (mean BVHC = 5.2% vs. 4.4%).

Near-surface mineralization, compaction, and thermal maturation of organic matter had variable impacts on pore systems in organic-bearing mudrocks. Crystals of dolomite, Fe-carbonate, calcite, feldspars, and sulfides are common (especially in carbonate mudrocks), many of which are interpreted as pore-filling cements. Clear examples of secondary OM filling former pore space do occur; however amorphous OM of ambiguous origin is widespread. Compaction strongly impacted mudrocks, especially ductile components like detrital clay and kerogen which are commonly deformed. OM-hosted porosity is patchy and best preserved where sheltered between rigid particles, particularly dolomite crystals. Higher hydrocarbon saturations and abundance of surface-coating hydrocarbons on dolomite surfaces suggest an affinity of hydrocarbons for dolomite surfaces and more oil-wet surface chemistry in carbonate mudrocks.