Thermal-maturity limit for primary thermogenic-gas generation from humic coals as determined by hydrous pyrolysis

AAPG Bulletin
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Abstract

Hydrous-pyrolysis experiments at 360°C (680°F) for 72 h were conducted on 53 humic coals representing ranks from lignite through anthracite to determine the upper maturity limit for hydrocarbon-gas generation from their kerogen and associated bitumen (i.e., primary gas generation). These experimental conditions are below those needed for oil cracking to ensure that generated gas was not derived from the decomposition of expelled oil generated from some of the coals (i.e., secondary gas generation). Experimental results showed that generation of hydrocarbon gas ends before a vitrinite reflectance BLTN13204eq1 of 2.0%. This reflectance is equivalent to Rock-Eval maximum-yield temperature BLTN13204eq2 and hydrogen indices (HIs) of 555°C (1031°F) and 35 mg/g total organic carbon (TOC), respectively. At these maturity levels, essentially no soluble bitumen is present in the coals before or after hydrous pyrolysis. The equivalent kerogen atomic H/C ratio is 0.50 at the primary gas-generation limit and indicates that no alkyl moieties are remaining to source hydrocarbon gases. The convergence of atomic H/C ratios of type-II and -I kerogen to this same value at a reflectance of BLTN13204eq3 indicates that the primary gas-generation limits for humic coal and type-III kerogen also apply to oil-prone kerogen. Although gas generation from source rocks does not exceed vitrinite reflectance values greater than BLTN13204eq4, trapped hydrocarbon gases can remain stable at higher reflectance values. Distinguishing trapped gas from generated gas in hydrous-pyrolysis experiments is readily determined by BLTN13204eq5 of the hydrocarbon gases when a BLTN13204eq6-depleted water is used in the experiments. Water serves as a source of hydrogen in hydrous pyrolysis and, as a result, the use of BLTN13204eq7-depleted water is reflected in the generated gases but not pre-existing trapped gases.

Additional publication details

Publication type Article
Publication Subtype Journal Article
Title Thermal-maturity limit for primary thermogenic-gas generation from humic coals as determined by hydrous pyrolysis
Series title AAPG Bulletin
DOI 10.1306/06021413204
Volume 98
Issue 12
Year Published 2014
Language English
Publisher AAPG
Contributing office(s) Central Energy Resources Science Center
Description 30 p.
First page 2581
Last page 2610