--> Abstract: Burial and Thermal History of the Haynesville Shale: Implications for Gas Generation, Overpressure, and Natural Hydrofracture, by Jeffrey Nunn; #90124 (2011)

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Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA

Burial and Thermal History of the Haynesville Shale: Implications for Gas Generation, Overpressure, and Natural Hydrofracture

Jeffrey Nunn1

(1) Geology & Geophysics, Louisiana State University, Baton Rouge, LA.

The Haynesville Shale is a thin organic rich sedimentary rock found in Northwest Louisiana, Eastern Texas, and Southwest Arkansas. It was deposited during the Late Jurassic in a shallow marine environment. The Haynesville Shale is typically found at depths of 3 km (10,000 ft) or more and is characterized by ultra low permeability. Nunn et al. (1984) showed that subsidence of the Northern Gulf Coast is consistent with crustal extension by a factor of 1.5 to 2 during Late Triassic to Early Jurassic. Results from a thermal-mechanical model suggest that Jurassic temperature gradients were more than twice the current regional value of 25 to 35 °C/km. Thus, Jurassic age sediments have been close to their current temperatures for the last 100 m.y. Using subsurface data, a simple model of heat transport and fluid flow was used to estimate temperature versus time for the Haynesville Shale. Three erosion cases associated with the Mid-Cretaceous unconformity were considered: no erosion; erosion prior to deposition of the Tuscaloosa-Eagle Ford; and erosion both prior and after deposition of the Tuscaloosa-Eagle Ford. In all cases, total burial at time present is the same and heat flow is assumed to decay with elapsed time since rifting. Two cases for present day basal heat flow are considered: Low Heat Flow (60 mW/m2) and High Heat Flow (75 mW/m2). Additional deposition followed by erosion generates temperatures as much as 10 °C higher in the Cretaceous than the no erosion case. As expected, the high basal heat flow case generates substantially higher temperatures at all times. Computed vitrinite reflectance for the low heat flow case indicates that the Haynesville Shale should not reach gas generation. The high heat flow simulation has gas generation for all three erosion cases. However, gas generation occurs 10-25 m.y. early for the erosion cases. For ultralow permeability (~ nanoDarcy), disequilibrium compaction can generate significant overpressure within the Haynesville Shale. This overpressure cannot be maintained over geologic time because the unit is too thin. However, if overpressure generation due to formation of oil and gas is synchronous with rapid sediment loading, then it is possible that fluid pressures would be sufficient to naturally hydrofracture the Haynesville Shale at approximately 105 Ma.