Facies Control on Fluvial Reservoir Quality

Abstract

Fluvial sandstones form important hydrocarbon reservoirs and aquifers in many regions of the world and more recently been identified as potential sites for carbon dioxide and hydrogen subsurface storage. The characterization of fluvial reservoirs is however challenging due to the complex heterogeneities (internal and external) associated with the variable lithologies and sedimentary architecture. Understanding the main controls on the heterogeneities is essential for building accurate reservoir models. One of the main controls is depositional facies, which in turn has a major influence on early and late burial diagenesis. Although depositional facies is widely known to exert a primary control on fluvial lithological variability and heterogeneity, its role in clay-coat distribution and authigenesis remains poorly constrained. In this study, a multidisciplinary approach involving outcrop analogues, core analysis, petrography, electron microscopy, burial history/quartz cement modelling, clay coat and stable isotope analysis has been employed to understand the controls on fluvial reservoir quality and overall heterogeneity. A total of 293 samples comprising of core and outcrop samples from the Triassic Skagerrak Formation (UK Central North Sea), St Bees Sandstone Formation (West Cumbria, UK) and Buntsandstein facies (Central Iberian Basin, Spain) were investigated. This research clearly identifies that reservoir quality/heterogeneity is controlled by facies, grain size and clay/ductile grains content. Channel facies offer the best reservoir quality while floodplain facies offer poor quality. In the channel sandstones, porosity and permeability range from 0-24% and 0.01-1150 mD, respectively, while in the floodplain facies, they range from 0-7.3% and 0.004-0.51 mD, respectively. Grain size is a first order control on reservoir quality. Coarser-grained channel sandstones have a higher reservoir quality than finer-grained channel sandstones due to their lower clay and ductile grains content. This study reveals that within channel bodies, there is a significant variation in reservoir quality, with the channel centres having the best reservoir quality. Furthermore, this research reveals that the extent of coverage of clay coats governs its ability to effectively inhibit quartz cementation, and significantly correlates with grain size, clay content, and depositional energy. The most extensive clay coat coverage (70-98%) is associated with the finer-grained, low energy channel sandstones and possibly crevasse channel intervals containing between 5 and 10% clay coat by volume. The results of this study have significant implications for reservoir quality prediction and development of fluvial reservoir models especially in high pressure high temperature (HPHT) environments. Finer-grained, dirty sandstones that are often overlooked during exploration offer potentially better reservoir quality at depth and could provide better underground storage sites.