Geological evaluation of posidonia and wealden organic-rich shales : geochemical and diagenetic controls on pore system evolution

Abstract

Free gas in shales occurs mainly in larger mesopores (width >6 nm) and macropores (width >50 nm) and is likely to be the first or even main contributor to gas production. Because evaluation of the storage capacity and final recovery of gas depends on distribution and connectivity of these pores, their correct quantification has become a focus point of advanced research. A major step for understanding pore systems in organic rich shales was made by recognition that under increasing thermal stress, decomposition of kerogen should progressively lead to development of organic porosity. Despite this, many questions concerning fate of organic porosity in organic rich rocks still remain unresolved. To date, several important attempts to link evolution of organic pores with maturation and organic matter content gave inconclusive and contradictory results. In this study, pore systems of the Lower Jurassic Posidonia and Lower Cretaceous Wealden shale, representing different mudrock types and covering a range of maturities, have been characterised. By integrating geochemical and petrophysical measurements, and with a detailed analysis of microscopic images we offered a unique approach for measuring porosity and pore characteristics on micrometre and centimetre scales with thorough understanding for a micrometer lithological variation. Key aims were to quantify the evolution of porosity associated with both organic matter and inorganic rock matrix as a function of maturity, and address the influence of mudrock heterogeneity on porosity change. Our experiments revealed a non-linear trend of porosity change with maturity in pores of all sizes, with an initial drop in the oil window as a result of mechanical compaction, chemical diagenesis, as well as pore-filling oil and bitumen. At comparable maturities, porosity and distribution of pores depend on the content of clays, organic matter, microfossils, silt grains and pore filling cement. In both Posidonia and Wealden, macropores (> 50 nm) account for merely up to 20% of total porosity physically measured, with the lowest percentage in the least mature samples. It was also demonstrated that gas sorption micropores are controlled by the amount of organic matter and clay minerals, and thus their microporous nature was confirmed. In terms of organic porosity development, we provided evidence that organic matter content and the path of its thermal decomposition control total porosities of the gas window shale. Importantly, neoformed intraorganic porosity is highly heterogeneous with 35% of organic particles containing visible pores (> 6 nm in diameter), and porosities of individual particles ranging from 0–50%. As a key result, we confirmed that porous zones in the gas window are associated with sites of bitumen retention and degradation. That indicates iii that the location of potential reservoirs of free gas should be linked to rigid zones, such as fossiliferous faecal pellets, or compaction shadows of mineral grains. Combined mercury injection and SEM data also showed that visible but potentially isolated macropores are connected, but only through throats below 20 nm. With the evolution of the porous network of bitumen saturating the shale matrix in the gas window, connectivity of the system changes from inorganic to organic dominated. The size of the pore throats, and the connectivity of the organic system in shales are likely key controls on the delivery of gas from pore to fracture and then to wellbore.