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High TOC Source Rocks, Multiple Oils and Low Permeability Reservoir Rocks: Mississippian Heath Formation Hybrid Oil Petroleum System, Central Montana, USA


The late Mississippian Heath Formation of central Montana contains interstratified organic-rich carbonate mudstone source rocks and low permeability oil-saturated (“tight oil”) carbonate reservoir rocks. A study of source rocks, extracted oils and bitumens, and produced oils indicates the presence of at least three hydrocarbon sources within the Heath, including in descending order the Heath Limestone, Cox Ranch Oil Shale, and Van Dusen Zone. Oil API gravity and maturity increase with increasing source rock thermal maturity indicating little or no lateral migration within Heath reservoirs. Aryl isoprenoids in the oils indicate photic zone euxinia (PZE) existed during source rock deposition. The presence of gammacerane in the oils is indicative of a stratified water column and evaporitic or hypersaline environments. The Heath Limestone includes tan dolomite, gray marine limestone, calcareous mudstone, and anhydrite and has been the primary target of most of the horizontal wells drilled in the Heath to date. Source beds contain TOC up to 11 wt% and HI up to 793 mgHC/gTOC; reservoirs are best developed in intertidal to supratidal dolomites with porosity up to 18.7% and permeability up to 1.1md. The Cox Ranch Oil Shale contains interbedded dark brown to black calcareous mudstone, gray marine limestone, and minor dark gray fissile shales. Source beds contain TOC up to 26 wt% and HI up to 757 mgHC/gTOC; limestone reservoirs have 2–6% porosity and mudstones have up to 21% porosity. The Van Dusen zone contains dolomitized limestones, gray marine limestones, dark brown to black calcareous mudstones, coals, and greenish-gray claystones. Source beds, including coals, contain TOC up to 73 wt% and HI up to 646 mgHC/gTOC; oils generated by these sources contain abundant terrestrial biomarkers. Van Dusen reservoirs include fractured limestones with 1–5% porosity and mudstones with up to 26% porosity. Interpretation of biomarkers suggests that each of the Heath Limestone, Cox Ranch Oil Shale, and Van Dusen Zone generate distinct oils. Produced oils examined from horizontal wells with laterals drilled in the Heath Limestone contain only Heath Limestone biomarkers, suggesting that drilling and completion techniques applied to date have not contacted the entire pay thickness in the Heath.