Methods, microstructure and mudrocks : towards an improved understanding of deep-water mudrocks

Abstract

Microstructure controls the petrophysical properties of mudrocks. Knowledge about mudrock microstructure in general is maturing and there is an on-going research field aimed at developing accurate, reliable and fast methods to characterise them. The focus of this research is on mudrocks from the deep-water setting, which are deposited by three principal processes operating in the deep-water: downslope turbidity currents (turbidites), along-slope bottom currents (contourites) and vertical fall-out from surface suspension (hemipelagites). Distinguishing between these respective deposits is challenging but very important in understanding the deep-water environment in terms of their petroleum systems and reservoir characteristics. Hence, the present research entails methodology development and documenting the microstructure of the three principal sedimentary facies in the deep-water. Scanning electron microscopy is a common technique for studying mudrock microstructure. An efficient and effective method for analysing grain size of mudrocks was developed, which gives closely comparable results to laser diffraction granulometry. A fast and reliable approach for characterising detailed mudrock microstructure using automated large-area, high-resolution scanning electron microscopy and image processing is also presented. The method is automated, free of human subjectivity and provides robust information on mudrock microstructure. Interestingly, the developed method gives comparable results with a synchrotron x-ray diffraction technique. Furthermore, the research presents exciting new insights on microstructure of deep-water fine grained sediments. A microfabric model is presented for the deep-water fine grained sediments. Turbidites have pronounced preferred bedding parallel fabric, produced by turbulence and high sedimentation rate, with little or no bioturbation effect. Contourites possess mixed fabric (random - semi random and parallel to the bedding). The mixed fabric is suggested to be developed by weak turbulence (bottom currents) and distortion of the fabric by bioturbation. Hemipelagites are characterised by random and oblique preferred microfabrics, which are produced by absence of current and pervasive bioturbation. The oblique preferred microfabric is suggested to be a product of extensive burrowing, in which grains are aligned along the length of the burrows. Additional important findings are that, depositional processes and sedimentation rate as well as burial depth are the most important controlling factors of microstructure development within the deep-water sediments. Mudrocks are prevalent is all sedimentary environments. The need for a cleaner source of energy for the 21st century has revolutionized interest in mudrocks, as they are recognised as potential unconventional reservoirs. They are also important in terms of carbon storage, as repositories for nuclear waste and as records of environmental change. The contributions presented on mudrock microstructure in this research are relevant to studying the generality of mudrocks without recourse to their environment and not just restricted to deep-water mudrocks.