Luncheon – May 15, 2009

Title: Shale Gas Geochemistry Applications: E-TX Jurassic Gas Mixing and Ft. Worth Basin Burial History /Thermal Maturation

Speaker: Mark H. Tobey

Date: May 15, 2009

Publication: The Outcrop, May 2009, p. 19

Vertical migration of deeper, Bossier-sourced gas in shallower Cotton Valley reservoirs has been inferred through gas composition and isotopic data from approximately 50 producing wells in the Freestone-Limestone-Leon-­Robertson county area of East Texas. This vertical migration is most prevalent along the paleo-shelf edge. The geochemical tools used to demonstrate gas migration/mixing are based on apparent gas thermal maturity differences.

Gas geochemistry also reveals several important aspects of the Ft. Worth Basin Barnett Shale: (1) the basin has experienced relatively uniform heat flow over the area studied, but (2) the basin has undergone differential burial and uplift, and it is this burial history that controls the maturity across the basin, (3) and gas composition, as measured by wetness / specific gravity / BTU, does not correlate as closely to thermal maturity as the methane isotopic signatures, perhaps because the absence of the underlying Viola “seal” in the Western portion of the basin, leading to methane “depletion” via water dissolution into the underlying Ellenberger aquifer.

Produced gas methane carbon isotopic signatures tell the same story of differential burial and differential uplift across the basin as do the rock maturation signatures. The resulting thermal maturation map for the Barnett shale differs from published maturation maps in both degree of maturation and maturation line contours.