Pores, porosity and pore size distribution of some Draupne Formation and Colorado Group shales and kerogens

Abstract

Organic rich shales are an important source of natural gas, where significant amounts of gas can be stored in the pore system of shales. The pore systems of organic rich shales are both highly variable and poorly understood, and within this context, the overall research aim is to characterise and investigate the pore system of organic rich shales and isolated kerogens, and to examine the relationship between shale pore structure and gas storage. The research focussed on two case studies: 1) the Draupne Formation (DF) of the North Sea, and 2) the Colorado Group (CG) of the Western Canada Sedimentary Basin of Canada. The DF shales have a range of thermal maturities, from immature to late oil window, allowing the effect of maturity on pore structure to be examined. The CG group shales are both immature and isomature, allowing the pore structure of pre-oil window kerogen and shale to be investigated. The geochemistry of the shales and isolated kerogen samples was characterised using TOC, Rock-Eval, Pyrolysis GC-MS, FT-IR, 13C-NMR, Elemental Analysis. The pore structure of the isolated kerogens and shales was investigated using 1) Electron microscopy, 2) Mercury intrusion porosimetry, and 3) low pressure gas adsorption methods, including nitrogen at - 78oC and CO2 at 0oC. The gas storage capacity of the shales was determined using high pressure methane at 30oC and up to 1 MPa. In the Draupne Formation sample suite, thermal maturity has a significant influence on the pore structure, with organic matter content and mineralogy having a secondary role. Mercury intrusion porosimetry indicates that the pore size distribution of the shales is dominated by pores < than 100 nm in size, and that the proportion of mesopores increases with maturity, at the expense of micropore content. The gas sorption pore volumes of both the isolated kerogens and shales exhibit a strong negative correlation to maturity, with the TOC normalised pore volumes strongly decreasing with increasing maturity. The negative correlation with maturity is also repeated with the Dubinin-Radushkevic (D-R) micropore volumes and the BET surface areas of the kerogens and shales. The absence of shale minerals in the kerogen concentrates indicates that the negative correlation between pore structure and maturity is controlled by the organic matter, and shale matrix mineralogy is a secondary influence on pore structure in this sample suite. In the Colorado Group case study, mineralogy is the primary influence on the pore structure of the shales, with organic matter content having a secondary role. The mercury intrusion porosimetry of the shale indicates that the pore size distribution is dominated by sub-100 nm pores, and the pore size distribution is constant, with no change across the sample suite. The gas sorption pore volumes of the isolated CG kerogens are almost identical to the isolated DF kerogens, suggesting that Type II algal kerogens have similar gas sorption pore volumes. For the CG shales, the sorption pore volume correlates strongly with illite content. The correlation is positive, with a correlation coefficient of R2 = 0.97. This indicates pore volume is primarily located in the shale mineral matrix, and organic matter content is secondary in the CG sample suite. This strong positive illite content is also observed in the D-R micropore volumes of the shales, indicating that illite is microporous. The N2 BET surface area also reflects this strong positive correlation to illite, with a R2 = 0.90.