Paul C. Hackley1, Alana M. Bove1, Frank T. Dulong1, Michael D. Lewan2, Brett J. Valentine1 1U.S. Geological Survey, MS 956 National Center, Reston VA 201922U.S. Geological Survey, DFC Bldg. 20, MS 977, Lakewood CO 80225 } Problem vitrinite reflectance suppression } Hydrous Pyrolysis Approach } Sample Materials } Petrographic Results Reflectance Results } Conclusions Outline of this presentation Statement of Problem Petroleum formation is defined within a range of reflectance values Oil Window: Ro 0.6- 1.2%Gas Window: Ro >1.2%Gas Shales: Ro >1.2%Thermogenic CBM: Ro >0.9% If vitrinite reflectance is suppressed then it becomes more difficult to predict thermal history and petroleum formation. Important to characterize conditions which can cause vitrinite reflectance suppression and retardation. -overpressure, bitumen impregnation, a priori differences in composition and maturation kinetics, others ...(?) Vitrinite Reflectance Suppression vitrinite reflectance suppression, na reduction in vitrinite reflectance values below thermal maturity levels determined by geochemical or other petrographic parameters arising as a result of atypical hydrogen- rich vitrinite chemistry inherited from the precursor plant material or introduced into the vitrinite by the chemical microenviroments of deposition, diagenesis, and catagenesis ASTM, 2011 0.20.61.01.41.8 0.20.61.01.41.8 %Ro Original 300 310 320 330 340 350 360 HP Temperature (C) for 72h (Lewan, 1993) Blackhawk coal Frontier coal Wilcox lignite Phosphoria Shale Woodford Shale Alum Shale Skull Ck. Shale Mowry Shale 20C 360C 20C GeneratedGases ExpelledOil Source Rock 2.5- 5.0 g Water 12- 15 g SpentRock 72 h Preparation Pyrolysis Cool- down & Collection Heat- up Room Temp. Room Temp. Can be performed over a range of different temperatures and heating times to assess different levels of thermal maturation Hydrous Pyrolysis Experimental Approach GC ovens for hydrous pyrolysisSwageLok mini- reactorsSealing reactor 72 hr experiments at 300C, 320C, 340C, 350C, and 360C completed on seven source rocks (2 Wilcox coals, Green River, Woodford, Boquillas, Alum, and Huron shales) Reflectance microscopeFiltering productPellet mount for reflectance } Green River Shale (Eocene Mahogany Ledge) } Wilcox Coal (Paleocene- Eocene, Texas) } Wilcox Coal (Paleocene- Eocene, Louisiana) } Boquillas Shale (Cretaceous, Eagle Ford equiv.) } Woodford Shale (Devonian) } Huron Shale (Devonian) } Alum Shale (Cambrian- Ordovician) Samples Used in Experiments Samples are immature: Ro 0.30 to 0.50% XRD Mineralogy Green River, LOI 23.6 wt.% QTZFLDCARBILLITEKAOLCHLRPYOTHERBoquillas, LOI 5.7 wt.%QTZFLDCARBILLITEKAOLCHLRPYOTHERHuron, LOI 21.6 wt.% QTZFLDCARBILLITEKAOLCHLRPYOTHERWoodford, LOI 12.0 wt.% QTZFLDCARBILLITEKAOLCHLRPYOTHERAlum, LOI 10.0 wt.% QTZFLDCARBILLITEKAOLCHLRPYOTHERWilcox LA, LOI 79.3 wt.% QTZFLDCARBILLITEKAOLCHLRPYOTHEROrganic Geochemistry 0501001502002503000 10 20 30 40 50 60 70 80S2, mg HC/g rock) TOTAL ORGANIC CARBON (TOC, wt.%) TYPE I oil-prone usually lacustrine TYPE II oil-prone usually marine Mixed TYPE II-III oil-gas-prone TYPE III gas-prone TYPEIV inert l II III IV 010020030040050060070080090010000 20 40 60 80 100 120 140 160 180 200HI OI TYPE I TYPE II TYPE III TYPE IV l II III IV Green River Green River Coals Coals Boquillas Huron Boquillas Woodford Alum Huron Woodford Alum Petrographic Results Original sample; subbituminous coal with vitrinite, inertinite, fluorescent liptinite Wilcox Coal TX & LA Ro 0.46%Ro 0.89%300C; no organic fluorescence remaining, development of fluorescent radial carbonate(?)Ro 1.05%320C; pitting and cracking of vitrinite common; radial carbonate(?) 340C; pitting and cracking of vitrinite common; ragged vacuoles; radial carbonate(?) Ro 1.35%Wilcox Coal -TX & LA 350C; Pitted and cracked; ragged vacuoles; vitrinite same reflectance as relict funginite 360C; very similar to 350C sample; vitrinite is pitted and cracked, ragged vacuoles common Ro 1.44%Ro 1.57%Original sample; abundant AOM, Tasmanites; original solid bitumen texturally obvious Huron & Woodford Shales 300C; Tasmanites still brightly fluorescent but developing silky luster in white light and showing devolatilization features: shrinkage cracks; bitumen abundant 320C; Tasmanites fluorescence diminished, silky luster more prominent 340C; Tasmanites largely volatilized, replaced by carbonate and sulfide; bitumen abundant Ro 0.48%Ro 0.56%Ro 0.68%Ro 1.02%350C; Tasmanites completely replaced by carbonate/ sulfide, bitumen forms network groundmass, embayed against euhedral neo- carbonate 360C; Tasmanites completely replaced by carbonate/ sulfide, bitumen forms network groundmass Ro 1.21%Ro 1.28%Huron & Woodford Shales Original sample; abundant AOM, some original solid bitumen present 300C; neo- bitumen forms from AOM, original solid bitumen less common Green River Mahogany Ledge 320C; neo- bitumen forms a gummy tar which remobilizes during heat setting of pellets, original solid bitumen very scarce 340C; neo- bitumen very abundant, migrates to grain margins, entrains mineral matter, original solid bitumen no longer present Ro 0.30%Ro 0.09%Ro 0.15%Ro 0.49%Green River Mahogany Ledge 350C; neo- bitumen shows coked texture with mesophase, organic fluorescence no longer present, original bitumen has disappeared 360C; neo- bitumen mesophase domains have coalesced; formation of neo- sulfides and oxides; qualitatively less neo- bitumen is present than lower temperature experiments Ro 1.00%Ro 1.31%2803003203403603800.00 0.20 0.40 0.60 0.80T (C) Gas (g) Green River Gas Reflectance Results Reflectance Results Reflectance Results Reflectance Results Reflectance Results } Hydrous pyrolysis of coals and shale source rocks produces a host of physical changes, including: Tasmanites devolatilizes and is replaced by minerals Qualitatively greater quantities of bitumen forms at higher temperatures in the shales AOM in Green River becomes a neo- bitumen Vitrinite devolatilizes in coals leaving pits, cracks and ragged vacuoles } Kinetic barriers to bitumen maturation in response to thermal stress are higher than for vitrinite; the effect is more pronounced at lower heating levels } If petrographers mistake bitumen for vitrinite due to absence of void- filling textures ... this work explains reports of vitrinite reflectance suppression at low thermal maturity, particularly in the PaleozoicConclusions