Testing the Gas Plume Hypothesis Using Fracture Distribution above Marcellus
Joints in the Middle and Upper Devonian formations of the Appalachian Basin, the Catskill Delta Complex, are common, particularly in the silt- and sandstone layers. The planar surface and cyclic plumose morphology indicate these joints are gas driven. Generally two sets of joints can be seen: J1 and J2 (Engelder et al. 2009). The strike of J1 joints range from 060o to 085o, while those of J2 joints range from 320o to 010o. J1 is present in organic rich shales and rapidly disappears above these source rocks, while J2 appears in coarser clastic rocks of the delta complex as much as 2000 m above the Marcellus. This pattern was discovered through mapping 281 outcrops in Northeastern Pennsylvania and Central New York. The fraction of outcrops with joints present decreases up section above either Marcellus Shale or the Geneseo/Burket when present. Gas-driven joints are present in 100% of the Brallier Formation outcrops, 76% of the Lock Haven Formation, 58.2% of the Catskill Formation, 20% of the Huntley Mountain Formation and 0% of the Monongahela Formation outcrops. Joint density also decreases with vertical height above the source rocks when compared on an outcrop by outcrop basis.
To explain the distribution of gas-driven J2 joints, we propose a gas plume model. Gas plumes reflect pervasive leakage above source rock due to buoyancy of overpressured methane gas. A gas column is more likely to overcome the least horizontal rock stress at the top of the column. The core of the gas plume is taken to be the volume of rock just above the source rock where joint density is highest. Further up in the section where methane pressure dissipates, the density of joints decreases in a logarithmic trend away from the gas plume core. Some areas above source rocks are more densely jointed than expected and these are interpreted as gas chimneys.
Most joints in the Catskill Delta Complex are not mineralized, further evidence for pervasive methane. The joints of the gas plume may well have served as a conduit for methane migration from the deep black shales to charge shallow sand layers during late Palaeozoic maturation of the Devonian source rocks.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California