Regional Jointing and Hydrocarbon Generation in Big Sandy Gas Field, Kentucky

Steven J. Loar

Producing wells in the Big Sandy gas field depend on well-developed subsurface joint systems to enhance migration into the borehole. To examine the joint systems, the surface joints were studied. Statistical analysis revealed eight regional joint sets: N30°W, N10°W, N20°E, N30°E, N40°E, N60°E, N80°E, and east-west. These sets have the same orientations as reservoir joints observed in oriented cores, indicating that they are part of the same systems. Field observations suggest that the regional joint sets formed in at least three phases, which can be classified as hydraulic, tectonic, and unloading. The hydraulic phase consists of the N10°W, N80°E, N20°E, and N30°E sets, which formed about 310 Ma. The tectonic phase co sists of the N30°W and N60°E sets, which formed about 280 Ma during Alleghenian deformation. Finally, the unloading phase consists of the N40°E set, which formed about 10 Ma and appears to be related to the contemporary stress field. The timing of hydrocarbons was calculated from a subsidence curve for Devonian and younger formations, which was constructed on the basis of published isopach, conodont alteration index, and well data. Plotting the maturation of the Ohio Shale with the regional jointing phases shows that the hydraulic joint sets formed before hydrocarbon generation began, and that the tectonic joint sets formed while the Ohio Shale was in the oil window. The oil and wet gas generated from the Ohio Shale have since migrated through the subsurface joint systems nto younger reservoirs.

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