The Jurassic shales of the Wessex area: geology and shale oil and shale gas resource estimation

This report on the Jurassic shale oil and gas potential of the Wessex area follows previous assessments of the potential distribution and in-place resource for shale oil and gas onshore UK, including the Carboniferous shales of the Midland Valley of Scotland (Monaghan, 2014), the Carboniferous shales of the Bowland-Hodder (Andrews, 2013), and the Jurassic shales of the Weald (Andrews, 2014); it is intended as an addendum to the Weald Basin report. Following the methodology used in the assessment of the Weald Basin (Andrews, 2014), a preliminary in-place oil resource calculation has been performed for the main Jurassic shale intervals in the Wessex area. As in the Weald Basin, no significant shale gas resource is recognised in the Jurassic of the Wessex area. The resource assessment is of the hydrocarbons present in shale strata and does not include volumes which have migrated into potential tight conventional or hybrid plays. Organic-rich shales (with total organic carbon (TOC) > 2%) occur regionally in the Kimmeridge Clay, in the lower section of the Oxford Clay and in the Lower Lias. The most significant shales in terms of TOC and S2 are the Kimmeridge Clay and all five shale intervals that comprise the Lower Lias. A distinct difference to the Weald Basin is the organic richness of the Lower Lias; the previous study identified only limited potential in this interval, yet in the Wessex area there are rich source intervals throughout the strata. Shales with an oil saturation index (Jarvie, 2012) of greater than 100 are identified in all of the intervals of the Lower Lias, and therefore can be considered to have excellent source potential. Interpreting the presence of producible oil in the organic-rich shales allows for an in-place resource volume to be calculated with a broad range of probabilities. The determination of oil-in-place was undertaken using the same methodology applied in the Weald study (Andrews, 2014) and is described in detail in Andrews et al. (2014). The total volume of potentially productive shale in the Wessex area was estimated using a 3D geological model built using seismic mapping integrated with well data. This gross volume was then reduced to a net mature organic-rich shale volume using a maximum, pre-uplift burial depth corresponding to a vitrinite reflectance of 0.6% (modelled at 7000 ft/2130 m and 8000 ft/2440 m). A further upwards truncation was then applied at two alternative levels – firstly at a depth of c. 3950 ft (1200 m) and secondly at a depth of c. 5000 ft (1500 m) (as proposed by Charpentier & Cook, 2011) below surface. The volumes of potentially productive shale and average oil yields were used as the input parameters for a statistical calculation (using a Monte Carlo simulation, in which all the parameters were varied within their set distributions over 50,000 iterations) of the in-place oil resource (see Andrews, 2014). This study offers a range of total in-place oil resource estimates for the various Jurassic shales of the Wessex area of 0.2-1.1-2.8 billion bbl (32-149-378 million tonnes) (P90-P50-P10). It should be emphasised that these ‘oil-in-place’ figures refer to an estimate for the entire volume of oil contained in the rock formation, not how much can be recovered. A more refined methodology, like the USGS’s Technically Recoverable Resource “top-down” estimates, requires production data from wells, as yet unavailable for the study area. Given the paucity of data, there is a high degree of uncertainty in these figures. There is likely to be little or no ‘free oil’ for the Oxford Clay, Upper Lias and Middle Lias based on the oil saturation index and although the Kimmeridge Clay does show excellent source potential, it is likely to be immature regionally. The Lower Lias appears to be the only interval with shale oil potential onshore, albeit with relatively small volumes in a localised area largely south of the Purbeck-Isle of Wight Disturbance.