Disturbed Zones and Their Relationship to Hydrocarbon Traps in Central Appalachian Valley and Ridge Province

Howard A. Pohn

Disturbed zones are thrust-faulted and folded rocks that occur either in the hanging walls or footwalls adjacent to thrust faults or between closely spaced pairs of thrust faults. They are common in the central Appalachian Valley and Ridge province and are found predominantly in ductile lithologic units. Proprietary seismic reflection profiles indicate that these zones are the surface manifestations of imbricate thrust faults that are rooted to detachments at depth. Because field evidence shows that the disturbed zones formed before or contemporaneously with associated folds, these fault zones control, to a great degree, the wavelengths of the associated folds. Disturbed zones are generally tens to hundreds of meters thick and can be followed along strike for a few to ten of kilometers. Although they generally form in alternating siltstone and shale sequences, some disturbed zones occur in more competent rocks. The intense folding and faulting associated with disturbed zones in competent rocks such as in the Oriskany Sandstone, appear to localize domains of high fracture porosity in the subsurface and thus create potential hydrocarbon traps. Many of the gas fields in the central Appalachian Valley and Ridge province are parallel with and slightly offset from concentrations of disturbed zones. Other areas of concentrations of disturbed zones. Other areas of concentrations of disturbed zones present intriguing possibilities for possible future hydrocarbon exploration.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.