Bowland Basin fractured reservoirs near Clitheroe - Highland Geology Limited
15845
page-template-default,page,page-id-15845,bridge-core-2.8.7,qode-page-transition-enabled,ajax_fade,page_not_loaded,,qode-child-theme-ver-1.0,qode-theme-ver-27.1,qode-theme-bridge,disabled_footer_top,disabled_footer_bottom,wpb-js-composer js-comp-ver-6.6.0,vc_responsive

Bowland Basin fractured reservoirs near Clitheroe

Two sequences are interesting for geothermal wells. One is Chatburn Limestone Formation, about 500 metres thick. Below it the rest of the CL Group is a Worston-type succession of mudstone and several brittle carbonates with natural fracture systems.
Second, below the Chatburn Limestone Group base are Tournaisian dolomites and dolomitic limestones, the dolomite attributed to hypersaline lagoonal sabkha development. These are probably Ballagan-equivalents, seen in all the northern Carboniferous basins.

The geological map around Clitheroe-Gisburn displays the arcuate array of anticlines obliquely dissected by north-vergent thrusts. Quarry and stream outcrops are locally very good but glacial deposits cover much of the complex geology and there are certainly more thrust surfaces than the mapping recognises. Pale blue and yellow areas are collectively the Lower Carboniferous Bowland High Group and Craven Group, the pale blue being limestones and yellow represents mudstone, siltstone and sandstone formations.

The Gisburn to Clitheroe high is actually three arcuate folds, each fold has a thrust on its north flank, the dashed reds are the fold axial zones as mapped at around 500 milliseconds twt (about 1000 metres depth); and the purple dashed line is a closing contour, to show the dome shape.

The Clitheroe quarry complex is the orange heavy dashed line. The blue arrow is a possible drilling location there for a Clitheroe geothermal, subject to confirmation from re-mapping.

This 1988 Enterprise Oil intra-Chatburn marker map comes from UKOGL’s archives. We have sketched a modified fault pattern, to give an interlinked network defining a pile of northwest-moving thrust wedges: which is what the Gisburn-Clitheroe dome is. Each thrust slice (“duplex”) will be heavily fractured, below the imaging resolution of the seismic lines. Seismic resolution is around 40 metres, so we only see the larger faults and fractures.

It’s a very large fold, the grey area is 4-way dip closure over 10 km in length, with three compartments separated by NW to WNW-trending fractures. The fold axes drawn in pale blue are offset by these NW faults, which are oblique sidewalls to the numerous arcuate thrusts which dip south-eastwards. None of the three west-east rollovers has been drilled, to date (Swindon being a stratigraphic borehole).
Red lines are the seismic network, heavily drawn ones are interpreted here by HGL. Faults trending to northwest drawn in black are sidewall, strike-slip faults, notably the Clitheroe and Barnoldswick Faults, these allowed the fold slices to move by variable amounts. The crestal area corresponds to greatest net displacement north-westwards, in the fold core. This structure warrants 3D seismic cover for best definition.

Dashed yellow is the Clitheroe quarry group’s outline, which lies on the SW margin of the culmination closed area. Line -08 across the quarry is shown in the next slide. Blue arrow is a possible trial well location.

Clitheroe Anticline north limb and underlying duplex

NW-SE 23 km line E87C-08 across the Clitheroe quarry complex shows a highly-fractured structure which is an excellent candidate location for a producer well. Its close by the Heidelberg Materials cement kilns and so it’s favourably located for borehole planning and environment approval. Yellow tracks are potential well trajectories. Purple level duplexes are possible producer targets, with combination of structural dip and wrap-over by several shears in mainly-shale sequence, to provide high-potential caprock. With significant fracturing destroying event continuity the red fault is a possible very large fluid migration zone; it could have enormous capacity to deliver hot water. 3D seismic shooting would be justified for detail here.