title: Clay minerals and clay-coat in modern estuarine cores as analogues for ancient and deeply buried sandstones
Creators: name: Simon, Naboth
Publisher: name: University of Liverpool Repository
Year: 2024

description: Tide-dominated estuaries are highly dynamic environments due to the interaction of a large number of terrestrial and marine processes. Estuarine sediments are therefore heterogeneous in terms of both composition and spatial distribution. Their diversity is important to an improved understanding of reservoir quality properties, i.e., the quantity and distribution of clay mineral and clay coats, thus reservoir quality, which are still poorly understood. The main cause of porosity- and permeability-loss in sandstone reservoirs, during prolonged burial and heating, is authigenic quartz. Clay minerals attached to grain surfaces as coats have been reported to inhibit quartz cementation during prolonged burial and heating, thus preserving reservoir quality in deeply buried sandstone. Being able to detail the degree of clay-coat coverage, as well as the distribution of clay minerals, as a function of depositional environment is crucial in facilitating predictive models for the distribution of clay-coat-induced anomalously-good reservoir quality. This study adopted a high resolution modern analogue approach which can be used to help predict reservoir quality in deeply buried and ancient sandstone. The study focused on surface (&lt; 2 cm), near-surface (&lt; 1 m) and Holocene (&lt; 15 m) sediments from the Ravenglass Estuary, UK. Four hundred and eighty-two surface sediment samples (&lt; 2 cm) were collected from the estuary and nearby coast and detailed ground surveys were made of the variety of marginal-marine depositional environments. Samples from near-surface (&lt; 1 m) and Holocene (&lt; 15 m) cores were used to determine clay minerals and detrital clay-coat coverage. Samples used for grain size analysis were collected at regular intervals to ensure coverage of each sub-depositional environment in near-surface (&lt; 1 m) and at every 5 cm in Holocene (&lt; 15 m) cores. Grain size was determined using a Beckman Coulter Laser Particles Size Analyser (LPSA) and GRADISTAT© soft-ware. Sub-samples for clay minerals and clay coats were analysed using a suite of X-ray diffraction approaches, SEM, Petrog software and statistical techniques. The spatial distribution of grain size, sorting, kurtosis and skewness in surface sediment of the Ravenglass Estuary, UK, were determined at an unprecedented high-resolution using ArcGIS. A novel automatic textural classification scheme was then developed to predict palaeo sub-depositional environments from core through a combination of visual discrimination of gravel and vegetated surfaces, principal component analysis and recursive partitioning routine (RPART) in R statistical software. The novel classification approach was applied to Holocene cores from Ravenglass resulting in a much-improved way of determining ancient (Holocene) depositional environments that supersedes descriptive sedimentology and lithostratigraphic correlation. The distribution of clay minerals and detrital clay coats relate to specific sub-depositional environments. Illite is relatively most abundant in the salt-marsh, mud flat and mixed sub-depositional environments. Chlorite relative abundance increases from the fluvial to the marine sub-environments and is most abundant in tidal inlet/north foreshore sediment. Kaolinite is relatively homogeneous throughout the estuary. Clay coat distribution tends to be high in low energy sub-depositional environments e.g. saltmarsh, mud flat and mixed flat, and low in high energy sub-depositional environments e.g. sand flat, tidal bars and tidal inlet/north foreshore. Based upon findings of this research and typical burial diagenetic pathways, mud-flats and mixed-flats sub-depositional environments are likely to express low porosity and low permeability in deeply buried sandstones due to pore-filling clays that block pore-throats. Despite chlorite being most abundant in the marine sub-depositional environments, tidal inlet/north foreshore sediment is likely to be extensively quartz cemented due to insufficient volumes of clay-grade material to create porosity preserving continuous clay coats, and thus have low porosity and low permeability. In contrast, sand flat and tidal-bar sub-depositional environments may contain enough clay grade material to form continuous clay coats that are porosity-preserving. In addition, this study revealed that surface clay mineral and clay coat distribution patterns have not been over-printed by post-depositional processes such as bioturbation or mechanical infiltration. 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.