Prediction of clay minerals and grain-coatings in sandstone reservoirs utilising ancient examples and modern analogue studies

• Main Author: Dowey, Patrick
• Other Authors: Worden, Richard; Hodgson, David
• Published: University of Liverpool 2012
• Subjects: 550
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.579333

Understanding how clay minerals affect reservoir quality in sandstone reservoirs is important for improving prediction of recoverable volumes of petroleum. Grain-coating clay minerals may preserve porosity and permeability in sandstones by limiting the development of authigenic minerals such as quartz. Authigenic clay minerals may also reduce porosity and permeability, through mineral growth into pore volumes and ductile deformation during compaction. This thesis draws upon published literature of chlorite-coats in reservoir sandstones and modern analogue studies of estuary sediments, utilising quantitative mineral analysis, microscopy and grain-size analysis. It seeks to address issues concerning clay mineral formation, development and distributions to better enable the prediction of these important minerals. Authigenic chlorite-coats formed from precursor clay minerals during diagenesis have been reported in varying depositional environments and settings in published literature. Depositional age, paleolatitude of deposition, and chemistry of chlorite are all important factors in the occurrence and impact of chlorite-coats in sandstone reservoirs. Quantitative data compiled from published literature indicate that delta-related environments are the most common host for chlorite-coats, with rivers the second most common. Chlorite-coats are also more common in younger examples although this may be due to a drilling bias. Chlorite-coated examples are not reported in polar regions. Chlorite-coats tend to preserve porosity and are typically an iron-rich variety. The most likely host for chlorite-coated sands are delta settings in proximity to river systems in a warm and wet climate, with sediment supplied from a mixed lithology hinterland. The surface distribution of clay minerals was studied in two modern estuarine analogues in Iceland and Spain, in order to quantitatively assess their formation, development and distribution. Findings show that the majority of clay minerals were derived directly from hinterland bedrock or associated sediments. Quantitative mineral data indicate that geochemical alteration of some clay mineral species occurs, in Iceland as a result of bioturbation of estuarine sediments and in Spain as a result of reducing conditions within centimetres of the sediment surface. The distribution of clay minerals and siliciclastic minerals appears to be influenced by marine processes; concentrations of these minerals are lower where carbonate minerals are in higher concentrations in the Spanish estuary. Both estuaries show varying concentrations of clay minerals within the estuary surface sediments; this may be related to estuarine transport and depositional processes that could be partly controlled by the physical characteristics of the clay minerals. In order to understand early grain-coat formation in a modern estuary analogue, grain-coat textures, mineralogy and average percentage coverage on sand grains were investigated in shallow sediment cores. Results show that grain-coat coverage varies, but is typically low (<25%). Grain-coats consist of mixtures of detrital siliciclastics, bioclastic debris and clay minerals. The average percentage coat coverage is controlled by the concentration of fine fraction within the sediments; less sorted sediments have greater average coat coverage. These sediments occur on the margins of the estuary. Sediments proximal to high-energy marine influence generally have lower average coat coverage. The cause of grain-coating may be varied; co-deposition of fine-grained sediment, bioturbation, and mechanical infiltration/clay illuviation are all potential grain-coating mechanisms. The differences in textures between modern and ancient grain-coats indicate that diagenetic overprinting has an important influence in authigenic coat development. This thesis draws together diverse approaches to understanding clay minerals and their impact upon reservoir sandstones. It encompasses a variety of geographical, physical and temporal scales in order to better understand this broad research area. The data and interpretations presented are potentially useful in petroleum exploration, and could be implemented in future academic and industry research, at a range of scales, to aid the prediction of clay minerals and grain-coats in sediments.