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Low-temperature gas from marine shales.

Mango FD, Jarvie DM - Geochem. Trans. (2009)

Bottom Line: In sequential isothermal heating cycles (approximately 1 hour), nearly five times more gas was generated at 50 degrees C (57.4 microg C1-C5/g rock) than at 350 degrees C by thermal cracking (12 microg C1-C5/g rock).Our results indicate two paths to gas, a high-temperature thermal path, and a low-temperature catalytic path proceeding 300 degrees below the thermal path.It redefines the time-temperature dimensions of gas habitats and opens the possibility of gas generation at subsurface temperatures previously thought impossible.

View Article: PubMed Central - HTML - PubMed

Affiliation: Petroleum Habitats, 806 Soboda Ct, Houston, Texas 77079, USA. dinacat.mango@gmail.com

ABSTRACT
Thermal cracking of kerogens and bitumens is widely accepted as the major source of natural gas (thermal gas). Decomposition is believed to occur at high temperatures, between 100 and 200 degrees C in the subsurface and generally above 300 degrees C in the laboratory. Although there are examples of gas deposits possibly generated at lower temperatures, and reports of gas generation over long periods of time at 100 degrees C, robust gas generation below 100 degrees C under ordinary laboratory conditions is unprecedented. Here we report gas generation under anoxic helium flow at temperatures 300 degrees below thermal cracking temperatures. Gas is generated discontinuously, in distinct aperiodic episodes of near equal intensity. In one three-hour episode at 50 degrees C, six percent of the hydrocarbons (kerogen & bitumen) in a Mississippian marine shale decomposed to gas (C1-C5). The same shale generated 72% less gas with helium flow containing 10 ppm O2 and the two gases were compositionally distinct. In sequential isothermal heating cycles (approximately 1 hour), nearly five times more gas was generated at 50 degrees C (57.4 microg C1-C5/g rock) than at 350 degrees C by thermal cracking (12 microg C1-C5/g rock). The position that natural gas forms only at high temperatures over geologic time is based largely on pyrolysis experiments under oxic conditions and temperatures where low-temperature gas generation could be suppressed. Our results indicate two paths to gas, a high-temperature thermal path, and a low-temperature catalytic path proceeding 300 degrees below the thermal path. It redefines the time-temperature dimensions of gas habitats and opens the possibility of gas generation at subsurface temperatures previously thought impossible.

No MeSH data available.


Related in: MedlinePlus

The C1–C4 hydrocarbons produced from Floyd shale under helium flow at 50°C. The procedure (Anoxic Conditions) in Fig. 1 was repeated with another sample of Floyd shale at 1552 m. Products were periodically withdrawn from the reactor effluent gas stream and analyzed by GC. Gas compositions are concentrations (ppm vol) in the effluent gas stream over time. Under Oxic Conditions, an aliquot of the same shale was ground to 60 mesh in air, the reactor was not pressure flushed with pure helium, and gas flow at 50°C employed helium with 10 ± 1 ppm O2. Rock-Eval (before anoxic reaction) TOC = 5.78; Tmax = 449; S1 = 2.09; S2 = 10.8; S3 = 0.46. Rock-Eval (after anoxic reaction) TOC = 3.93; Tmax = 451; S1 = 1.84; S2 = 10.37; S3 = 0.45. Yields (integration): 0.83 mg C1–C5/g (Anoxic); 0.23 mg C1–C5/g (Oxic). Ground samples were injected directly into a 300°C chamber under helium flow in Rock-Eval analysis. Thus, any C1–C5 hydrocarbons desorbed under helium flow at 50°C in our experiments would have been integrated into the Rock-Eval S1 peak.
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Figure 2: The C1–C4 hydrocarbons produced from Floyd shale under helium flow at 50°C. The procedure (Anoxic Conditions) in Fig. 1 was repeated with another sample of Floyd shale at 1552 m. Products were periodically withdrawn from the reactor effluent gas stream and analyzed by GC. Gas compositions are concentrations (ppm vol) in the effluent gas stream over time. Under Oxic Conditions, an aliquot of the same shale was ground to 60 mesh in air, the reactor was not pressure flushed with pure helium, and gas flow at 50°C employed helium with 10 ± 1 ppm O2. Rock-Eval (before anoxic reaction) TOC = 5.78; Tmax = 449; S1 = 2.09; S2 = 10.8; S3 = 0.46. Rock-Eval (after anoxic reaction) TOC = 3.93; Tmax = 451; S1 = 1.84; S2 = 10.37; S3 = 0.45. Yields (integration): 0.83 mg C1–C5/g (Anoxic); 0.23 mg C1–C5/g (Oxic). Ground samples were injected directly into a 300°C chamber under helium flow in Rock-Eval analysis. Thus, any C1–C5 hydrocarbons desorbed under helium flow at 50°C in our experiments would have been integrated into the Rock-Eval S1 peak.

Mentions: 2) The Floyd shale in Fig. 2 desorbed only 0.25 mg free hydrocarbons over the course of reaction (S1 Rock-Eval peak before and after the run), but it released 0.83 mg C1–C5/g in the experiment (3 hours, 50°C). Since our Rock-Eval analysis would include any C1–C5 hydrocarbons in the S1 peak, desorption of pre-existing light hydrocarbons can only account for a small fraction of the gas released in this experiment.


Low-temperature gas from marine shales.

Mango FD, Jarvie DM - Geochem. Trans. (2009)

The C1–C4 hydrocarbons produced from Floyd shale under helium flow at 50°C. The procedure (Anoxic Conditions) in Fig. 1 was repeated with another sample of Floyd shale at 1552 m. Products were periodically withdrawn from the reactor effluent gas stream and analyzed by GC. Gas compositions are concentrations (ppm vol) in the effluent gas stream over time. Under Oxic Conditions, an aliquot of the same shale was ground to 60 mesh in air, the reactor was not pressure flushed with pure helium, and gas flow at 50°C employed helium with 10 ± 1 ppm O2. Rock-Eval (before anoxic reaction) TOC = 5.78; Tmax = 449; S1 = 2.09; S2 = 10.8; S3 = 0.46. Rock-Eval (after anoxic reaction) TOC = 3.93; Tmax = 451; S1 = 1.84; S2 = 10.37; S3 = 0.45. Yields (integration): 0.83 mg C1–C5/g (Anoxic); 0.23 mg C1–C5/g (Oxic). Ground samples were injected directly into a 300°C chamber under helium flow in Rock-Eval analysis. Thus, any C1–C5 hydrocarbons desorbed under helium flow at 50°C in our experiments would have been integrated into the Rock-Eval S1 peak.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC2654466&req=5

Figure 2: The C1–C4 hydrocarbons produced from Floyd shale under helium flow at 50°C. The procedure (Anoxic Conditions) in Fig. 1 was repeated with another sample of Floyd shale at 1552 m. Products were periodically withdrawn from the reactor effluent gas stream and analyzed by GC. Gas compositions are concentrations (ppm vol) in the effluent gas stream over time. Under Oxic Conditions, an aliquot of the same shale was ground to 60 mesh in air, the reactor was not pressure flushed with pure helium, and gas flow at 50°C employed helium with 10 ± 1 ppm O2. Rock-Eval (before anoxic reaction) TOC = 5.78; Tmax = 449; S1 = 2.09; S2 = 10.8; S3 = 0.46. Rock-Eval (after anoxic reaction) TOC = 3.93; Tmax = 451; S1 = 1.84; S2 = 10.37; S3 = 0.45. Yields (integration): 0.83 mg C1–C5/g (Anoxic); 0.23 mg C1–C5/g (Oxic). Ground samples were injected directly into a 300°C chamber under helium flow in Rock-Eval analysis. Thus, any C1–C5 hydrocarbons desorbed under helium flow at 50°C in our experiments would have been integrated into the Rock-Eval S1 peak.
Mentions: 2) The Floyd shale in Fig. 2 desorbed only 0.25 mg free hydrocarbons over the course of reaction (S1 Rock-Eval peak before and after the run), but it released 0.83 mg C1–C5/g in the experiment (3 hours, 50°C). Since our Rock-Eval analysis would include any C1–C5 hydrocarbons in the S1 peak, desorption of pre-existing light hydrocarbons can only account for a small fraction of the gas released in this experiment.

Bottom Line: In sequential isothermal heating cycles (approximately 1 hour), nearly five times more gas was generated at 50 degrees C (57.4 microg C1-C5/g rock) than at 350 degrees C by thermal cracking (12 microg C1-C5/g rock).Our results indicate two paths to gas, a high-temperature thermal path, and a low-temperature catalytic path proceeding 300 degrees below the thermal path.It redefines the time-temperature dimensions of gas habitats and opens the possibility of gas generation at subsurface temperatures previously thought impossible.

View Article: PubMed Central - HTML - PubMed

Affiliation: Petroleum Habitats, 806 Soboda Ct, Houston, Texas 77079, USA. dinacat.mango@gmail.com

ABSTRACT
Thermal cracking of kerogens and bitumens is widely accepted as the major source of natural gas (thermal gas). Decomposition is believed to occur at high temperatures, between 100 and 200 degrees C in the subsurface and generally above 300 degrees C in the laboratory. Although there are examples of gas deposits possibly generated at lower temperatures, and reports of gas generation over long periods of time at 100 degrees C, robust gas generation below 100 degrees C under ordinary laboratory conditions is unprecedented. Here we report gas generation under anoxic helium flow at temperatures 300 degrees below thermal cracking temperatures. Gas is generated discontinuously, in distinct aperiodic episodes of near equal intensity. In one three-hour episode at 50 degrees C, six percent of the hydrocarbons (kerogen & bitumen) in a Mississippian marine shale decomposed to gas (C1-C5). The same shale generated 72% less gas with helium flow containing 10 ppm O2 and the two gases were compositionally distinct. In sequential isothermal heating cycles (approximately 1 hour), nearly five times more gas was generated at 50 degrees C (57.4 microg C1-C5/g rock) than at 350 degrees C by thermal cracking (12 microg C1-C5/g rock). The position that natural gas forms only at high temperatures over geologic time is based largely on pyrolysis experiments under oxic conditions and temperatures where low-temperature gas generation could be suppressed. Our results indicate two paths to gas, a high-temperature thermal path, and a low-temperature catalytic path proceeding 300 degrees below the thermal path. It redefines the time-temperature dimensions of gas habitats and opens the possibility of gas generation at subsurface temperatures previously thought impossible.

No MeSH data available.


Related in: MedlinePlus