TWO OR THREE THIRD-ORDER STRATIGRAPHIC SEQUENCES? AN EXAMINATION OF THE MARCELLUS FORMATION IN THE NORTHERN SEGMENT OF THE ROME TROUGH, APPALACHIAN BASIN

The Middle Devonian Marcellus Formation is currently the most prolific unconventional shale reservoir within the Appalachian basin with more than 10,000 horizontal producing wells. The Rome trough is a northeast-southwest trending graben within the basin that initially formed during the Cambrian period and a portion of the southwestern “core” Marcellus production area is located within the trough. Previous studies on Marcellus stratigraphy interpret the formation to have two third-order stratigraphic sequences; however, these studies were not focused on the Rome trough. Initial well log analysis within the West Virginia portion of the trough suggests the possibility of three third-order sequences, which contrasts with the stratigraphic framework presented in previously published work. Currently, the extent of the three third-order stratigraphic framework is unknown.

This study focuses on well logs located in both West Virginia and Pennsylvania and the ESGP2 core from the northern segment of the Rome trough (Allegheny County, PA). Four core lithofacies were defined using the core description, XRD, and TOC data: 1) calcareous light grey-grey shale with low TOC, 2) dark grey shale (with clay, silica, or carbonate) with moderate TOC, 3) dark grey-black shale (having either elevated silica, carbonate, or clay) with low TOC, and 4) black shale with high TOC and elevated quartz content. XRD data indicates quartz percentages range from 17-70%, carbonate range from 0-37%, and clay minerals range from 13-79%. TOC content ranged from almost zero to 8.89%. Comparison of ESGP2 log and core data to surrounding well data suggests that the three third-order sequence interpretation from northern West Virginia may extend into southwestern Pennsylvania, as there are three high TOC, low clay, high quartz intervals in the core which may record three transgressive systems tracts. Future work includes detailed mapping of sequences and systems tracts throughout the trough, isopach mapping to assess depocenter changes, carbon isotope and hydrocarbon distribution analysis, and relating these data sets to trough structure and potential pre- and syn-depositional reactivation.