The role of poroelasticity during disequilibrium compaction and hydrocarbon generation on horizontal stress in the Devonian section of the Appalachian Basin

Open Access
Author:
Johnston, Thomas James
Graduate Program:
Geosciences
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
December 08, 2014
Committee Members:
  • Terry Engelder, Thesis Advisor
  • Rudy Slingerland, Thesis Advisor
  • Demian Saffer, Thesis Advisor
Keywords:
  • Marcellus
  • poroelasticity
  • modeling
  • gas shale
Abstract:
A borehole in McKean County, Pennsylvania, provides input information on detailed stress measurements; this is added to previous data from three boreholes near South Canisteo, New York (Evans et al., 1989a). The four boreholes penetrate bedded Devonian sandstones, limestones, and shales (including the Marcellus Formation) in the Appalachian Basin. A discontinuity at the base of the Rhinestreet Shale separates an upper package of rocks with a high stress gradient, from a lower package with a low stress gradient. To explain the stress discontinuity I explore the palaeo-overpressure drainage hypothesis proposed by Evans et al., 1989b, in the light of the new data and 1D basin modeling. Both modeling and in-situ stress measurements support a four-step stress evolution for the lower package: 1. Sedimentation rates in the late Devonian/early Mississippian must have been high enough to cause disequilibrium compaction; 2. Cementation and lithification must have occurred while the lower section was overpressured to maintain under-compaction below the base-Rhinestreet Shale; 3. At the time of maximum hydrocarbon generation, cracking of oil to natural gas must have generated a pore fluid volume increase (Tian et al., 2008). The fluid volume increase would have supplemented the initial compaction-related overpressure; and finally 4. The pore pressure must have bled off during erosional unroofing from 270 Ma to present. Pore pressure reduction then must cause a decrease of Shmin due to poroelastic relaxation. A plot of stress versus depth shows close agreement between observed data and model runs in the four boreholes. Where the data and model runs do not match perfectly, the separation of the upper, high stress package; and lower, low stress package is still corroborated. Therefore the palaeo-overpressure drainage hypothesis is further supported with this work. Extrapolating these local, well-scale models through reservoir-scale, to the basin-scale for the Appalachian Basin could allow better prediction of in-situ stress magnitudes. Furthermore, the Appalachian Basin four-step stress evolution may be a model for other gas shales in the world, and thereby lead to a better understanding of in-situ stress for gas shales.