Bend Arch–Fort Worth Basin - Thermal Maturity

Thermal Maturity

Low maturation levels in the Barnett Shale at vitrinite reflectance (Ro), estimated at 0.6-0.7%, yield oils of 38° API gravity in Brown County. Oils found in Shackelford, Throckmorton, and Callahan Counties as well, as in Montague County, are derived from Barnett Shale at the middle of the zone of oil generation (oil window) thermal maturities levels (≈0.9% Ro). Although condensate is associated with gas production in Wise County, Barnett source rock maturity is generally 1.1% Ro or greater. The zone of wet gas generation is in the 1.1-1.4% Ro range, whereas the primary zone of dry gas generation (main gas window) begins at a Ro of 1.4%.

Thermal maturity of Barnett Shale can also be derived from TOC and Rock-Eval (Tmax) measurements. Although Tmax is not very reliable for high maturity kerogens due to poor pyrolysis peak yields and peak shape, the extent of kerogen transformation can be utilized. For example, Barnett Shale having a 4.5% TOC and a hydrogen index of less than 100 is in the wet or dry gas windows with equivalent Ro values greater than 1.1% TOC. In contrast, low maturity Barnett Shale from Lampasas County outcrops have initial TOC values averaging about 12% with hydrocarbon potentials averaging 9.85% by volume. A good average value for Barnett Shale is derived from the Mitcham #1 well in Brown County where TOC is 4.2% and hydrocarbon potential is 3.37% by volume. Using these data we can determine TOC values will decrease 36% during maturation from the immature stage to the gas-generation window. Samples from the T. P. Simms well in the Newark East gas-producing area have average TOC values of 4.5%, but greater than 90% of the organic matter is converted to hydrocarbons. Thus, its original TOC was about 7.0% with an initial estimated potential of 5.64% by volume. Any oil generated would be expelled into shallow (or deeper) horizons as in the west and north, or cracked to gas where measured vitrinite reflectance is above 1.1% Ro.