Evolution of Porosity and Pore Systems in Organic-Rich Posidonia and Wealden Shales

Free gas in shales occurs mainly in larger mesopores (width >3 nm) and macropores (width >50 nm) and is likely to be the first or even main contributor to gas production. To determine the maximum free gas porosities shale samples from the Lower Jurassic Posidonia and Lower Cretaceous Wealden formations covering a wide range of maturities have been subjected to a set of porosity determination techniques. A decrease of the original total porosity by a factor of two from the immature state (10%) towards the peak oil window (4%) have been documented and ascribed to a combination of factors: compaction, bitumen saturation and swelling. The subsequent regaining of the original porosities in the gas window samples has been proved as a mutual effect of bitumen conversion and gas generation. In contrast to their immature equivalents, in the gas window shales this porosity is associated with the organic phase with over 60% of organic particles visibly porous. Utilizing the integrated approach the connectivity not only of the whole pore system, but also of its organic component has been suggested. An organic matter related gas transport is expected to omit flow barrier problems encountered in water saturated mineral matrix.