UNDERSTANDING THE DYNAMIC SEDIMENTOLOGY OF INTERWELL-SCALE CARBONATE-EVAPORITE CYCLES

Quantifying geometrical and geochemical properties of carbonate-evaporite cycles in a subsurface outcrop is critical in improving our understanding of interwell-scale sedimentological heterogeneity in evaporite caprock. Evaporite seals may provide important barriers to CO₂ migration, however, variability in facies and their lateral continuity could alter caprock quality, reducing CO₂ storage efficiency. We aim to understand the process of deposition and lateral variation in thickness of carbonate-evaporite cycles in the Jurassic Purbeck Formation, UK, as an analogue for the Hith Formation of the Middle East. We incorporate fieldwork in a subsurface mine with lab work (thin sections, stable isotopes, XRD) and three-dimensional bed thickness models. The stratigraphic interval intersected by the mine contains five facies comprising a basal unit of gypsum and three well-defined rhythmic carbonate-anhydrite couplets. Based on textures and morphological features, the environment of deposition is interpreted as shallow supratidal and intertidal. Accumulation of algal limestone, shale, and mudstone occurred in a tidal flat and episodic flooding of a sabkha is recorded by deposition of marine carbonates. We propose that these observations indicate a similar mode of deposition to the modern-day sabkha of Qatar and Abu Dhabi. Rhythmic sequences are characterized by a similar succession of facies, in stratigraphic order: (1) brown to dark brown-grey algal limestone, (2) black, fissile, shale with satin spars; and (3) dark brown laminar mudstone. Carbonate sequences capped by nodular anhydrite exhibit small-scale variations in cycle packaging, lithology, and bed thickness distributions. Spatial patterns reveal thicker carbonates towards the west. The cycles are interpreted as meter-scale shallowing-upward parasequences. Thickness variation is likely controlled by paleotopography. This work provides new insight into process-based sedimentology to predict the distribution of intra-seal facies at an inter-well scale, which can shed light on how an evaporite seal will respond when subjected to CCS operations.

We gratefully acknowledge funding from the Qatar Carbonates and Carbon Storage Research Centre (QCCSRC), provided jointly by Qatar Petroleum, Shell, and Qatar Science & Technology Park.