Petrophysical Properties of North Sea Shales

This study presents a data set comprising estimated permeabilities of 22 natural mudstones which also have been characterised in terms of their pore size distribution, porosity, grain density and Total Organic Carbon (TOC) content. Pore size distribution and grain density were determined using mercury porosimetry and pycnometer method. TOC was determ ined by direct combustion of the organic carbon in LECO RLS-100 Carbon Sulphur analyser. Permeabilities were estimated based on the measured pore throat size distribution plus assumed pore alignment and pore shape using the model of Yang and Aplin. TOC and grain density ranges from 3.44- 9.98% and 2.61-2.70 g/cm3, respectively. Porosity decreases from ~24-3% between the depth range of 1515-4781 m. The pore size distribution data show that the m ean pore radius decreases from ~36-2 nm within the same depth range, the shift to smaller mean pore radii is driven primarily by the collapse of pores larger than 10nm with increase in consolidation. These values represent some of the smallest pore sizes known for rocks and in part provide a qualitative explanation for the low shale permeabilities. Estimated vertical permeabilities range from 9.5×10-23 to 8.6×10-21 m2. In general, permeability decreases logarithmically with porosity.