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Thermally immature mineral-free kerogens were pyrolyzed in the laboratory under air-free conditions at temperatures from 250 to 450°C for up to several weeks. Significant quantities of bitumen, gaseous hydrocarbons, water, and carbon dioxide were formed. Elemental composition of each kerogen type was measured before and after pyrolysis to monitor increased carbonization, decreased hydrogen/carbon ratios, and other elemental changes.
Immature kerogens with atomic H/C ratios greater than 1.00 generate liquid hydrocarbons on pyrolysis, whereas those with lower atomic H/C ratios generate predominantly gaseous hydrocarbons. Principal generation of liquid hydrocarbons generally occurs when the weight percent of carbon in the kerogen is between 77 and 85%, and the atomic H/C ratio is above 0.80. Principal gas generation occurs between 85 and 89% carbon and between atomic H/C ratios of 0.40 and 0.80. Thus, principal gas generation occurs after principal oil generation. Actually, most kerogens have completed hydrocarbon generation when the carbon content reaches approximately 90% by weight and the atomic H/C ratio declines to 0.40. Kerogens with unusually high initial hydrogen contents undergo principal oil generation up o 87% carbon and principal gas generation up to 92% carbon, but within the same atomic H/C ratio limits as normal oil-generating kerogens. Pennsylvanian-age coal, which is composed of gas-generating kerogen with a very low hydrogen content, has principal gas generation from 88 to 93% carbon.
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