Predicting Clay Mineralogy Distribution in Deeply Buried Sandstone Reservoirs Using a Modern Estuarine Analogue Approach

Abstract

One of the major causes for porosity- and permeability-loss in sandstones is the growth of quartz cements at depths >2.5 km (>80–90°C). Chlorite, an iron-magnesium rich clay, can form well-developed grain coats that preserve anomalously high porosity in deeply buried sandstone reservoirs, even at depths up to 5500 m, through the inhibition of authigenic quartz cements. In contrast, illite and kaolinite are typically considered to be detrimental for sandstone reservoir quality since they block pore throats and diminish permeability by orders of magnitude. The dominant control on the occurrence and type of clay (chlorite, illite or kaolinite) is the primary depositional mineralogy since the key ingredients in clay minerals (Fe-, Al- and Si-oxides) are so insoluble that clay diagenesis can be assumed to be isochemical during burial. As spatial resolution of oil field core is limited within tidally influenced petroleum bearing deposits, a modern high resolution estuarine analogue study of clay mineral distribution has been constructed to aid reservoir quality prediction of ancient and deeply buried systems. This research focuses on the origin, abundance and distribution of clay minerals within the Ravenglass estuary, UK. X-ray diffraction was performed on both fine clay (<2 μm) surface and shallow (< 1 m) core samples, to reveal the mineralogy and mineral proportions of the framework grains, bioclasts proportions and clay grade material. Clay mineral maps combined with LIDAR imagery, bioturbation intensity, grain size and salinity data aim to better understand the fundamental processes governing clay mineral distribution within the Ravenglass estuary. Hinterland geology and climate control clay mineral type and abundance within the Ravenglass estuary. Chlorite is relatively most abundant toward the foreshore, whereas illite is relatively most abundant within low energy tidal flat environments. Kaolinite displays a relatively ubiquitous distribution. It is suggested that grain size, local estuarine drainage patterns and bathymetry control clay mineral distribution patterns in this modern estuary. The remarkably high resolution dataset with transferable fundamental controls on clay mineral distribution will allow for the prediction of reservoir quality on a stratigraphic reservoir-scale basis.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016