Hydrocarbons in an Overmature Basin: II, Is There a Thermal Maturity Limit to Methane Production in Arkoma Basin, Oklahoma and Arkansas?

David W. Houseknecht, Lori A. Hathon

Since the pioneering work of David White in the Appalachians, it has been "known" that there are thermal maturity limits on the occurrence of oil and gas. Most recent literature indicates that no commercial gas accumulations exist above a vitrinite reflectance (R_o) of about 3%, not because methane is thermally degraded, but because reservoir quality is believed to be destroyed by high temperatures. However, in the Arkoma basin, methane is produced from rocks that range from less than 1 to about 5% R_o, with no apparent relationship between thermal maturity and gas production patterns.

Petrography of several Atoka reservoir sandstones indicates that diagenetic processes that occurred during shallow burial preserved porosity in some sandstones and destroyed porosity in others. During deeper burial, accumulation of hydrocarbons (including oil in some reservoirs) in porous sandstones located in favorable structural positions effectively terminated inorganic diagenesis and prevented further deterioration of reservoir quality. However, inorganic diagenesis proceeded below hydrocarbon-water contacts, resulting in nearly total destruction of porosity. This post-accumulation diagenesis occurred during or following organic metagenesis (overmaturation), as evidenced by quartz cement that fills bubbles and cracks in pyrobitumen.

Good reservoir quality was thus preserved in Atokan sandstones from which water had been displaced by hydrocarbon accumulation, whereas reservoir quality was totally destroyed by high temperature diagenesis in "wet" sandstones. This indicates that methane exploration is viable in strata characterized by R_o values of up to 5%, if it can be demonstrated that trap formation and hydrocarbon accumulation predated thermal overmaturation.

AAPG Search and Discovery Article #91039©1987 AAPG Mid-Continent Section Meeting, Tulsa, Oklahoma, September 27-29, 1987.