Deposition and Diagenesis of Basin-Center Evaporites, Green River Formation

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1 Deposition and Diagenesis of Basin-Center Evaporites, Green River Formation Tim Lowenstein, State University of New York at Binghamton Acknowledgments: Deidre LaClair & Robert Demicco, Binghamton University John Dyni, U.S. Geological Survey Joseph Smoot, U.S. Geological Survey

2 How can study of evaporite deposition and diagenesis in the Green River Formation help us further understand the accumulation, preservation, and development of oil shale?

3 1. Paleoenvironments in which evaporites and oil shales were deposited. 2. Fluid inclusions: temperatures of paleolakes and diagenesis. 3. Hydrocarbon droplets in fluid inclusions. 4. Sodium carbonates and Eocene pco 2 5. Mineralogical puzzles?

4 Lacustrine Deposits of the Eocene Green River Formation GRB: Green River Basin WB: Washakie Basin PCB: Piceance Creek Basin UB: Uinta Basin Minerals: Nahcolite = NaHCO 3 Trona = NaHCO 3 Na 2 CO 3 2H 2 O Largest Reserves in World From Dyni, 1998

5 Stratigraphic section, Green River Formation, Piceance Creek basin. Oil Shale resources: predominantly saline lake deposits. from Dyni, 2006.

6 Green River Formation, Piceance Creek Basin, Colorado Fine laminae with nahcolite, halite, dolomite, dawsonite [NaAl(OH) 2 CO 3 ], and organic matter: detrital sediment and chemical precipitates Vertical, bottom growth of halite with no dissolution of saline minerals Density stratified saline lake-preservation of organic matter

7 Saline Valley, California, March Halophilic Archaea and Bacteria and algae (Dunaliella salina); salinity 26-30%. Red carotenoid pigments in halophilic Archaeal membranes and Dunaliella (β-carotene) cells/ml

8 2. Fluid inclusion Microthermometry: Primary fluid inclusions trapped in halite and other saline minerals: temperatures of paleolakes temperatures of diagenetic mineral formation

9 Fluid Inclusion Microthermometry: Single-phase brine trapped during crystal growth (left). Cool in freezer so vapor bubbles nucleate (right). Heat sample slowly to temperature at which vapor bubble disappears- HOMOGENIZATION TEMPERATURE (temperature of brine in which halite crystal grew).

10 Death Valley Paleolake Temperatures Lowenstein et al. 1998

11 3. Hydrocarbon droplets in fluid inclusions. Some primary fluid inclusions in halite from the Piceance Creek basin that crystallized at the brine bottom contain liquid hydrocarbons.

12 Linear arrays of negative cubic fluid inclusions Bottom-growth halite and chemicaldetrital mud layers 1 cm Cubic fluid inclusion with brine + liquid hydrocarbon droplets on needle of nahcolite (?)

13 Primary fluid inclusions in halite contain liquid hydrocarbons and nahcolite, both trapped during crystal growth at brine bottom. What does this mean? Hydrocarbon seeps on lake floor?

14 4. Sodium carbonates and Eocene pco 2 Sodium carbonate mineral equilibria and partial pressure of CO2 in the system NaCl- NaHCO3-Na2CO3-H2O Total pressure 1 atm, Solution + minerals in equilibrium Minerals: Nahcolite = NaHCO 3 Trona = NaHCO 3 Na 2 CO 3 2H 2 O Natron = Na 2 CO 3 10H 2 O Lowenstein and Demicco, 2006; modified from Eugster, 1966

15 Modern Lake Magadi, Kenya: Trona Dean and Fouch, 1983

16 Trona + Halite + Mud Owens Lake, California: Trona, less than 100 years old Trona

17 Green River Formation: Precipitation of Nahcolite and Halite cumulates at the airwater interface, in equilibrium with atmospheric CO 2

18 Conclusions: Lowenstein and Demicco, 2006 Crystallization of nahcolite and halite from waters in contact with the atmosphere establishes that early Eocene pco 2 > 1125 parts per million by volume.

19 5. Mineralogical Problem: trona and nahcolite occur in deposits of the same age! (1) During deposition, GRB trona may have formed at higher temperatures than the PCB nahcolite.

20 (2) Green River trona may have formed from precursor phase (nahcolite?) during diagenesis at elevated temperatures.

21 (3) Different brine chemistries, may have favored precipitation of trona in the Green River basin. 2NaHCO 3 (nahc) + 2H 2 O + Na + NaHCO 3 Na 2 CO 3 2H 2 O (tr) + H + indicates that trona forms in preference to nahcolite at high activity of Na + (a Na+ ) and high ph..

22 (4) High pco 2 needed to form nahcolite may have been produced in lake waters and bottom sediments by organic processes operating in the Piceance Creek saline lake system.

23 1. Saline lake paleoenvironments in which evaporites and oil shales accumulated. 2. Fluid inclusions: temperatures of paleolakes and diagenesis 3. Hydrocarbon droplets in fluid inclusions 4. Sodium carbonates and Eocene CO 2 5. Mineralogical puzzles? How is this useful for development of oil shale?