Reconstructing Basin Overpressure and Geomechanical History to Explain Observed Natural Fracture Patterns, Cleveland Basin, UK

Abstract

The Cleveland Basin exposes Lower to Middle Jurassic shallow marine shale, ironstone and sandstone along the North Yorkshire coastline. The basin is underlain by a Permo-Triassic rift and post-rift thermal subsidence allowed the slow deposition (<50m/Ma) of Jurassic and Cretaceous rocks exposed today. Maximum burial depth from VR data is approximately 2.6km, indicating deposition of approximately 1–1.25km of now-eroded Tertiary sediments. Inversion began during the Mid-Tertiary due to Alpine orogenic and Atlantic ridge-push events, resulting in the broad east-west trending Cleveland Anticline. Reconstruction of the overpressure history of the basin is possible using deposition rates and hiatus, as well as a knowledge of the thermal and diagenetic maturation stage of the shales. Due to a 10 Ma hiatus after deposition of Cretaceous chalk, which is assumed to have allowed all overpressure to have drained off, maximum overpressure caused by compaction disequilibrium alone is on the order of 3000 psi. However, field observations of horizontal, hydraulically-induced fractures indicate overpressure must have exceeded overburden pressure. Secondary overpressure generation must have been active to increase overpressure to near to overburden pressure and horizontal fractures indicate a vertical minimum principal stress. Natural fractures observed in the field vary in orientation. At Staithes, horizontal, bed-parallel fractures are observed within the Cleveland Ironstone Formation. Compressive tectonic stresses from Alpine and Atlantic events allow for an Andersonian σ3, as well as generation of additional overpressure. A later, secondary set of vertical fractures formed and observed arrest lines indicate they are also pressure-driven. A relaxation of compressive stresses is thought to be responsible for the geomechanical change, with vertical fracture propagation away from the horizontal fractures acting as fluid conduits. At Saltwick Nab, in the Whitby Mudstone Formation, two vertical fracture orientations trend N-S and NW-SE with complex abutment relationships. Here, maximum overpressure was limited due to the presence of the overlying Dogger aquifer; no horizontal fractures are observed and fractures must have formed under a normal or strike-slip regime. Our integrated overpressure, basin history, tectonic and geomechanical approach ties field observations to theoretical reconstructions made to complete the story of natural fracturing in the Cleveland Basin.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015