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Title: Facies, diagenesis and pore characterisation of the Lower Carboniferous Hodder Mudstone Formation, Bowland Basin, UK
Author: Ohiara, Timothy
ISNI:       0000 0004 8501 2309
Awarding Body: University of Manchester
Current Institution: University of Manchester
Date of Award: 2019
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An understanding of the various controls in sediment deposition, burial processes and deformation of rock strata is required in the adequate estimation and conversion of hydrocarbon resources to reserves. In recent years, petroleum technology has evolved enabling oil and gas production from organic-rich mudstones. Understanding the distribution of organic and inorganic materials and how they relate to porosity development in fine-grained rocks is critical in predicting the rock's physical properties and successful hydrocarbon production. This thesis presents sedimentological, diagenetic and porosity characterisation of a potential UK unconventional shale gas reservoir; the Hodder Mudstone Formation of the Bowland Basin. This unconventional gas-bearing section is a ca. 900 m thick unit of organic-rich Viséan strata, primarily comprising hemipelagic mudstones and thinly laminated calcareous turbidites deposited on a carbonate ramp setting. A total of 1,679 m of continuous cores from 11 boreholes have been logged and sampled for this study. For sedimentological facies characterisation, 132 samples were selected for laboratory analyses after producing graphic core logs and lithologic description. 50, oriented 30 µm thick, polished, thin sections were further prepared from samples for optical and scanning electron microscopy and electron probe microanalysis. Whole-rock XRD mineral analysis of 76 samples was carried out and trace and major elemental analysis acquired from 67 samples to aid provenance and diagenetic study. To understand the organic matter properties and maturity, the total organic carbon content of 30 representative organic-rich core samples were determined. Bulk pyrolysis was also performed on the same samples and maturity data estimated from the pyrolysis data. These datasets were further combined with digital image analysis of pore structure and quantitative porosity measurements from nitrogen gas adsorption to characterise pores and evaluate the relationship between mineral distribution and the physical properties of associated pores. Results from these studies show that the succession comprises gravity flow, calciclastic sediments. Recognised facies were grouped into calciturbidites, densite mudstones and calcidebrites. Calciturbidites comprise mostly of high- to low-density, wavy-laminated bioclast-rich facies. Low-density densite mudstones are characterised by planar laminated and unlaminated mud-dominated facies. Calcidebrites are comprised of muddy or hyper-concentrated debris-flow deposits occurring as poorly-sorted, chaotic, mud-supported floatstones. These facies were deposited in a tectonically-controlled submarine fan setting. Primary sedimentary comprised intrabasinal skeletal debris, microscopic biogenic detritus and extrabasinal silt- and clay-sized siliciclastic (quartz and muscovite) detritus. Constituent diagenetic minerals include calcite, siderite, dolomite, ankerite, quartz, kaolinite, pyrite and marcasite with minor phosphate and chlorite. Samples show organic richness of 1.5% present-day TOC, and maturation analysis reveals an oil to gas widow mature source rock. The textural fabric of analysed samples shows significant diagenetic overprinting with a high abundance of authigenic carbonate and silicate minerals. Mineral authigenesis and precipitation were localised and controlled by primary constituents and the mobility of minerals. These changes affected the evolution and preservation of inter- and intra-particle pores within the studied samples. Inter-particle pores dominate argillaceous (>50% tectosilicates and phyllosilicates content) samples while intra-particle pores control porosity in calcareous (>50% carbonate content) samples. The calculated average porosity of calcareous samples is between 3.6 - 4.4 % while in more argillaceous samples is between 5.6 - 6.8 % porosity. The results from this research have allowed for the first time, the evaluation of submarine density flow deposits of the Viséan Bowland Basin succession. It has added a layer of knowledge on the mineral fabric, organic matter and diagenesis within a range of Hodder Mudstone facies. This will significantly enhance the understanding of reservoir quality in this potential shale play. The control on pore distribution and quartz diagenesis in the Hodder mudstones highlighted in this thesis has implications in the mechanical properties of the Hodder Mudstone as a target for hydraulic fracturing.
Supervisor: Taylor, Kevin ; Hollis, Catherine Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Carboniferous ; Pore systems ; Diagenesis ; Facies ; Bowland Basin ; Unconventional Reservoir