Interwell-scale Sedimentological Heterogeneity and Sequence Stratigraphic Framework in Anhydrite Caprock

S. Abbott¹, C. John¹, and A. Fraser¹

Abstract

Sequences of laterally extensive Jurassic evaporite-carbonate sabkha 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 a subsurface evaporite formation, which acts as 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 preserve anhydrite because the mineral is removed by meteoric dissolution and/or replaced by gypsum; however, mine subcrops provide accessible anhydrite field localities. The ~120x200 m study area is a horizontally leveled grid system ~300 m below surface at Brightling Mine, UK. We undertook vertical and lateral stratigraphic measurements of the seal architecture coupled with geologic logs. Preliminary 3-D correlations reveal small-scale heterogeneities and changes in bed thicknesses. All gypsum has been diagenetically altered to anhydrite, with the exception of the basal unit, which subsequently rehydrated back to gypsum. 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. 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). 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. Future work will consist of studying the evaporite caprock of the Upper Permian Zechstein Group in the Southern Permian Basin. 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.

AAPG Search and Discovery Article #90188 ©GEO-2014, 11th Middle East Geosciences Conference and Exhibition, 10-12 March 2014, Manama, Bahrain