Origins, characteristics, controls, and economic viabilities of deep-basin gas resources

Dry-gas deposits (methane greater than or equal to 95% of the hydrocarbon (HC) gases) are thought to originate from in-reservoir thermal cracking of oil and C-2+ HC gases to methane. However, because methanes from Anadarko Basin dry-gas deposits do not carry the isotopic signature characteristic of C-15+ HC destruction, an origin of these methanes from this process is considered improbable. Instead, the isotopic signature of these methanes suggests that they were cogenerated with C-15+ HC's. Other gas-composition data from both nature and laboratory experiments also suggest that most methane in dry-gas deposits originates from a post-expulsion fractionation of wet gases generated with C-15+ HC's into methane-rich deposits. Only a limited resource of deep-basin gas deposits may be expected by the accepted model for the origin of dry-gas deposits because of a limited number of deep-basin oil deposits originally available to be thermally converted to dry gas. However, by the models of this paper (inefficient source-rock oil and gas expulsion, closed fluid systems in petroleum-basin depocenters, and most dry-gas methane cogenerated with C-15+ HC's), very large, previously unrecognized, unconventional, deep-basin gas resources are expected. Nonetheless, economic recovery of these gas resources will depend on drilling, completion, stimulation, production, and maintenance procedures applicable to the unique characteristics of each unconventional gas-resource base. Isotopic values of methanes from nature and laboratory experiments suggest that most 'thermogenic' methanes with delta(13)C values of -50 to -45 may have a significant component of biogenic methane. Also, laboratory experiments demonstrate that organic-matter facies variations have as strong a control on HC gas chemical and isotopic values as maturity. Thus, estimation of gas maturities by gas isotopic or chemical characteristics can be misleading.