The Bowland Shale in the Roosecote Borehole of the Lancaster Fells sub-Basin,Craven Basin,UK:a Potential UK Shale gas Play?

摘要

A re-evaluation of the Roosecote Borehole,located in the northern part of the Craven Basin(northern England),places the Bowland Shale in a modern geological context and allows an improved understanding of basin evolution and shale hydrocarbon prospectivity.In northern England during the Early Carboniferous,rapidly subsiding sub-basins developed between local highs in the Craven Basin.Subsequently,during the thermal relaxation phase,a period of regional subsidence allowed thick accumulations,locally in excess of 500 m,of hemi-pelagic mudstone to be deposited as a transgressive systems tract. The Roosecote Borehole is a stratotype section for the Bowland Shale Formation,located close to the northern basin-to-shelf margin of the Craven Basin.The upper part of the Mississippian(Lower Carboniferous)Bowland Shale Formation is the principal potential shale gas play in the British Isles.The Bowland Shale and other argillaceous units of the Craven Group are rich in organic matter,have a similar depositional style to coeval units in the USA including the world-class Barnett Shale of the Fort Worth Basin,Texas,USA.Throughout England,Wales and Ireland it is currently being investigated by several exploration companies,with at least 4 dedicated shale exploration wells now drilled. Rock evaluation indicates the TOC at between 1.73–3.72%.Many samples indicate a mixture of type III and IV kerogens; there was no evidence for type I or II kerogens found.Thermal maturity for the sequence is within the oil window,with results indicating liquid oil and wet and dry gas have been generated.However,an estimate of the organic matter transformation ratio of 0.36(36%kerogen conversion to hydrocarbons being possible)compares poorly to the Barnett Shale.Consequently we conclude that the Bowland Shale within the Roosecote Borehole has some,but possibly lower,potential as a shale oil play.