USING ACCOMMODATION PLOTS FOR OUTCROP-TO-SUBSURFACE CORRELATION

The Devonian and Mississippian of the Appalachian Basin is penetrated by 1000s of wells and has numerous exceptional outcrop exposures. The problem has always been how to correlate the two types of data sets together easily. In the past researchers have used cores, well-cuttings, and outcrop gamma ray to tie the outcrop data into the subsurface, but all these methods have their limitations. This study examines the usefulness of Accommodation Plots (Fisher Plots) as a means of outcrop-to-subsurface correlation. Gamma ray logs that penetrate the Middle Devonian from three locations in Preston County West Virginia, one location in Clinton County, Pennsylvania (PA), and one location in Lycoming County, PA are used to test the usefulness of accommodation plots as correlation tools. Several wells with gamma ray logs that penetrate the Upper Mississippian (Chesterian) Greenbrier Group of West Virginia are also used to test the usefulness of accommodation plots as correlation tools. Accommodation cycles are defined as a rise and fall of gamma ray intensity on the gamma ray logs, based on the assumption sea level controls the majority of clay content. Accommodation cycle thicknesses are measured from a printout of a gamma ray log, printed at one inch per ten feet. The accommodation cycle thickness data is entered into Antun Husinec’s FISHERPLOTS program to produce accommodation plots. Next well-documented outcrop based sequence stratigraphy is used to help interpret the results of the accommodation plots. The Middle Devonian (Eifelian-Givetian) has nine 3rd order sequences. The Upper Mississippian (Chesterian) Greenbrier Group has twelve 4th order sequences. Accommodation plots of the Middle Devonian of West Virginia and Central Pennsylvania both show the presence of all nine well documented 3rd order sequences, thus allowing the correlation of the two locations using only gamma ray logs and marker beds. The twelve 4th order sequences of the Upper Mississippian Greenbrier Group are also easily correlated from outcrop-to-subsurface using only the accommodation plots. This study demonstrates that accommodation plots can produce useful outcrop-to-subsurface correlations for the Middle Devonian and Upper Mississippian of the Appalachian Basin.