BOREHOLE EVIDENCE OF DEEP FRACTURE NETWORKS – ALLEGHENY PLATEAU

In 2022 Cornell University completed the Cornell University Borehole Observatory (CUBO), a 3-km deep exploratory well, to assess the feasibility and guide the development of deep direct-use geothermal energy for district heating of the Ithaca campus. Downhole logging was conducted, allowing fracture characterization between 2,379 and 2,984 m deep within Ordovician to Precambrian strata of the north central Allegheny Plateau. Fractures identified in CUBO well logs are not influenced by weathering and surficial mechanical processes and provide information about the deformation history of the Allegheny Plateau.

Downhole fracture data analysis includes near-field fracture picks from resistivity borehole image (BHI) surveys and far-field fracture picks based on Stoneley wave analysis of borehole sonic scanner data. Fracture analysis of BHI surveys reveals multiple depth intervals containing fractures within Ordovician/Cambrian sedimentary strata and the Precambrian basement complex. The largest set of BHI survey fractures strike ENE and dip moderately SE. Analysis of the lower resolution far-field fractures identified through sonic logs shows a principal NW-SE striking set exhibiting dips exceeding 80° at distances up to 48 m beyond the borehole. Orientations of interpreted far-field fractures are nearly orthogonal to most of the near-borehole fractures identified in BHI surveys and to regional ENE anticlinal fold axes that terminate downward at the Salina décollement. This difference likely reflects the lower resolution of the far-field sonic method and the overall characteristics of the large-scale fracture network with primarily ENE fractures intersected by a secondary fracture set trending NNW. Similar observations and intepretations have been noted on published maps in the CUBO area. Together the CUBO analysis coupled with regional fold orientations and near-surface fracture observations imply the existence of deep penetrative fracture networks of complex histories.