Full citation: 

Sequences of laterally extensive evaporite-carbonate cycles in the margin of the Weald Basin form the basis of an interwell-scale sedimentological heterogeneity study of anhydrite caprock. Specifically, we examine an analogue to the subsurface Hith Formation, a major evaporitic seal for oil and gas reservoirs in the Middle East. We aim to understand process-based sedimentology and diagenesis to predict the distribution of intra-seal facies at the inter-well scale. Scant surface outcrops have preserved anhydrite because the mineral is generally removed by meteoric dissolution; however, a mine subcrop provides an accessible anhydrite field locality. We selected a study area consisting of a ~120x200 m horizontally leveled grid system located at ~300 m below surface at Brightling Mine, UK. We undertook vertical and lateral stratigraphic measurements of the seal architecture coupled with a full lithologic description and sampling. Geologic logs were taken at 40-meter intervals and preliminary 3-D correlations reveal relatively small-scale heterogeneities and changes in bed thicknesses. Four sabkha cycles exhibit evaporite-carbonate lithologic oscillations. The basal evaporite bed consists of anhydrite, which becomes increasingly gypsiferous towards the base. Incipient vertically oriented phenocrysts surround the periphery of horizontally elongated nodules of anhydrite and gypsum. Horizons of horizontally elongated chert nodules are observed. Carbonate stringers consist of stromatolitic-bearing algal limestone, mud, and shale. All gypsum in this seam has been diagenetically altered to anhydrite, with the exception of the basal unit, which subsequently underwent rehydration back to gypsum. Abundant secondary (alabastrine) gypsum crystallized along horizontal fractures, with fibers oriented vertically (i.e., the long axes of the fibers are normal to the edge of the vein). These satin spars occur within shale beds and/or are bracketed by a thin (1-2 mm) layer of shale; spars show evidence of displacement and thus likely formed subsequent to host rock consolidation. Enterolothic veins in anhydrite are observed in the uppermost bed. We aim to enhance this investigation utilizing X-ray diffraction, δ13C, δ18O, and clumped isotope (Δ47) analyses on carbonates. The ultimate outcome of this research will provide a fundamental stratigraphic framework that will inform subsurface models of caprock integrity, and provide the basis for further CO2-brine-caprock interaction and fracture propogation studies. These will aid in providing a comprehensive understanding of caprock systems.

Publication date: 
Thursday, April 10, 2014