The Upper Jurassic Arab and Hith Formations, which consist predominantly of evaporites, form one of the most important and extensive seals in the Middle Eastern region (Powers, 1962; Alsharhan & Kendall, 1994). Their significance for the hydrocarbon systems is widely known as they form the seal of the world’s largest oil fields, including the prolific, super-giant Ghawar oil field. Additionally, their value as a regional seal for CO₂ storage is now also being evaluated (Ye et al., 2022). They are of crucial economic interest to Saudi Arabia and the region as a whole. In the East of Saudi Arabia, where the Ghawar oil field is located, the Hith Formation lies buried beneath approx. 2 km of overburden. However, towards the West, near the capital Riyadh, is an escarpment where these Jurassic rocks outcrop. While the under- and overlaying carbonate strata are well preserved, the evaporites (anhydrites) of the Hith and Arab Formations are almost all dissolved (with the exception of the Dahl Hith cave, where the Hith Formation outcrops) and represented by collapse-breccias.

The Hith Formation consists mostly of anhydrite (almost 80%) and shows cm- to dm-scale bedding patterns. At the Dahl Hith cave, close to the drilling site of the well that provides the raw data for this study, the Hith Formation consists of nodular anhydrite facies with interbedded chicken-wire aggregates (associated with a sabkha setting) and laminated varied facies (associated with a playa setting) (Alsharan & Kendall, 1994). Below the Hith Formation is the Arab Formation, which consists of the Arab sub-members, and these are characterized by cyclical anhydrite and carbonate packages (Azer & Peebles, 1998).

This study focuses on the facies heterogeneity and sealing capacity of the Hith and Arab anhydrites, which depends on their mineralogical composition and rock strength. Even though both formations are known as important seals, they have shown seal integrity failure, leading to upward migration of hydrocarbons (e.g. Al Shaheen Field, Qatar; van Buchem et al., 2014). Understanding the effect of exposure to dissolved and/or supercritical CO₂ is crucial for de-risking the Hith and Arab member anhydrites as important seals for long-term, geological CO₂ storage.