Elastic Dislocation Modelling of Fractures in the Carboniferous Limestone: East Midlands, UK*

The Strelley-1 well encountered a minimum 27 m of hydrocarbon column in fractured Dinantian Limestone in the East Midlands, UK. Fault population statistics and a first-pass elastic dislocation forward model of the limestone reservoir in the Strelley up-dip structure has been created. These indicate that the optimum trajectory of a planned appraisal / development well to intersect tensional fracture should penetrate the reservoir near the top of the structure and then track sub-horizontally down the flank of the structure in a southeasterly direction towards the discovery well. Seismic interpretation of 2D lines along sometimes very winding English country roads was aided by using a sophisticated seismic interpretation package that displays seismic panels in true perspective from any view point. The seismic image is often poor, but the use of fault statistics and mapping of wall-rock shear and longitudinal strains helped constrain the 3D structural model. The top reservoir horizon in this 3D model was then forward modelled for displacement on fault surfaces within the up-dip Strelley structure. Using elastic dislocation theory, prediction of fracture type and orientation were then modelled by assuming that fracturing within the limestone was predominantly caused by Variscan rifting/extension. This model predicts that high palaeo-shear stresses and resulting shear plane failure are limited to within 400 m of the footwalls of the main bounding faults of the structure. Minor faults nearby to the well have very little influence over fracture type and orientations. Low paleo-shear stresses and associated tensile fractures occur in an orthogonal orientation to a projected sub-horizontal trajectory of a well, planned over a 2 km interval, between the crest of th