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Title: Mineralogical and textural variation in modern estuarine sands : implications for sandstone reservoir quality
Author: Griffiths, J.
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2018
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The spatial and temporal variability of primary depositional mineralogy and sediment texture (e.g. detrital clay coat coverage and matrix abundance) in sandstones are poorly-understood, and therefore empirical models often fail to accurately predict reservoir quality in ancient and deeply-buried petroleum-bearing sandstones. To address this challenge, surface sediment (< 2 cm) samples and one-metre cores were collected from the Ravenglass Estuary, NW England, and detailed ground surveys were made of the variety of marginal-marine depositional environments. Samples were analysed using a suite of X-ray diffraction approaches, SEM-EDS, laser particle size analysis and statistical techniques. The spatial distribution of quartz, feldspar, carbonates and clay minerals (chlorite, illite and kaolinite) in surface sediment of the Ravenglass Estuary, UK, have here been mapped at an unprecedented high-resolution, at a scale similar to many oil and gas reservoirs. Furthermore, clay mineral, clay coat and Fe-sulphide distribution patterns in near-surface sediment (< 1 m below the sediment surface) have been analysed to establish whether postdepositional processes e.g. bioturbation and mechanical infiltration, may over-print surface distribution patterns. Results show that estuarine composition is largely controlled by provenance; both the character of bedrock and drift-sediment in the drainage basin. Quartz, feldspar, clay mineral, carbonate and clay coat distribution patterns, are primarily controlled by the grain size of specific minerals (e.g. rigid versus brittle grains), estuarine hydrodynamics, and processes active in the top few centimetres of the primary deposition environment. Surface mineral distribution patterns are not over-printed by post-depositional processes such as bioturbation or mechanical infiltration. The distribution of smectite and pyrite is primarily controlled by geochemical conditions in the primary depositional environment, which are strongly influenced by topographic relief, bioturbation type and intensity, and extent of groundwater-flushing. Results of this study show that the abundance of quartz, feldspar, carbonates and clay minerals, and the extent of detrital clay coat coverage on sand grains, are predictable as a function of depositional environment (lithofacies) and mean grain size. However, the relative abundance of specific clay minerals (chlorite, illite and kaolinite) is much more sensitive to local specific hydrodynamic conditions e.g. wavedirection. This integrated study may be used, by analogy, to better predict sandstone reservoir quality during oil and gas exploration, field appraisal and in planning well locations in ancient and deeply-buried marginal-marine sandstones. Based upon findings of this research and typical burial diagenetic pathways, reservoir quality in analogous ancient and deeply buried sandstone reservoirs is likely to vary accordingly. Outer estuarine sediment is likely to be extensively quartz cemented (and so have low porosity and low permeability), due to insufficient volumes of clay-grade material to create porosity preserving continuous clay coats. Mud-flats and mixed-flats at the margin of the inner estuary and central basin are likely to express low porosity and low permeability due to pore-filling clays that block porethroats. In contrast, low amplitude dunes and tidal-bars in the central basin and inner estuary may contain sufficient quantities of clay grade material to form continuous clay coats that are porosity-preserving. Furthermore, low amplitude dunes and tidal-bars in the central basin and inner estuary are relatively chlorite-enriched and are often intensely bioturbated. Increased bioturbation is likely to inhibit Fe-sulphide development (reduce Fe-sequestration) permitting the chloritization of none Fe-bearing precursor minerals such as kaolinite. Reservoir quality is therefore likely to be greatest in low amplitude dunes and tidal-bars in the central basin and inner zones in marginal-marine sandstones.
Supervisor: Worden, Richard Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral