Mineralogical development and fluid-rock reactions within mudstones from the Widmerpool Gulf and the Bowland Basin

Abstract

This PhD project combines 2D, 3D and 4D imaging and analysis methods to investigate the microtextures and mineralogical development within selected portions of the Carboniferous Bowland Shale, northern England (The Morridge Formation in the Widmerpool Gulf, and the Bowland Shale in the Bowland Basin). Fluid-rock experiments were conducted to investigate the dissolution rate of minerals and possible changes in mudstone mineralogy and microstructure as a result of exposure to carbon dioxide during geological carbon storage, in which mudstones form the seal. Mineral cements were identified in the Morridge Formation mudstone samples. Kaolinite commonly occurs within the intraskeletal porosity and has been replaced by micro-sized quartz and anhedral pyrite in some cases. Pyritized radiolarians are present in samples, which suggests that the skeleton of radiolarians likely provided the silica source for kaolinite and micro-sized quartz precipitation. The release of magnesium during clay diagenesis was a possible source for dolomite cements. The diagenetic release of elements into basinal brine, which was expelled along with hydrocarbon during basin inversion, potentially led to the mineralization on adjacent platforms. Integrated 2D and 3D imaging of the Bowland Shale allowed for an enhanced understanding of mineral texture evolution. Zoned dolomite-ferroan dolomite-ankerite is common and 3D imaging shows the presence of dissolution features on the ferroan dolomite rather than dolomite, indicating that ankerite was formed after ferroan dolomite dissolution. 3D imaging allowed for the observation of pores around silica nanospheres, which may preserve pore space for the subsequent mineral precipitation, such as dolomite. Time-lapse 3D imaging technique was utilized to investigate changes occurring within a mudstone during acid-rock interactions over time. Modelling was performed to provide further theoretical understanding of the experimental data for the reactions. The major changes that have occurred in mudstones include the closure of original fractures, formation of new vugs, calcite dissolution and sample swelling. Both the experimental and numerical results show that calcite dissolution rate and dissolution front migration rate decrease with time. Carbonates were commonly dissolved at the rock-fluid interface. New fractures were generally formed along the bedding planes, indicating that unlaminated mudstones are more suitable as caprocks than laminated mudstones for preventing CO2 leakage.

Date of Award	31 Dec 2022
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Kevin Taylor (Supervisor) & Catherine Hollis (Supervisor)