The Hith Formation: a potential regional seal for CO₂ sequestration

The Late Jurassic Hith Formation is an anhydrite deposit with an average thickness of > 90 m. It forms an important regional seal for major hydrocarbon accumulations over large parts of the Arabian Peninsula. In Saudi Arabia, the only location where the Hith Formation outcrops is the Dahal Hit cave south of the capital of Riyadh. The Hith is being studied as an important seal for CO2 sequestration. To achieve this, it is vital to know its geomechanical properties and chemical reactivity, especially when exposed to CO2 under reservoir temperature and pressure conditions. From a literature review of anhydrites, we obtained global value ranges for strength, stiffness and acoustic properties. All of which indicate that anhydrites are generally stiff and strong rocks. This enables us to benchmark properties from the Hith anhydrite measured on hand specimens taken from the Hit cave. Resulting compressive strength of our samples averages 145 MPa, which is within the range reported in the literature. Compressional velocities average at 5300 m/s, not as high as literature values. Shear velocities on the other hand match the literature reported values of around 3250 m/s. We further reviewed and investigated the effect of brine composition and CO2 on the chemo-mechanical behavior of the Hith anhydrite through micro-scratch testing. Experiments confirm the capability of the micro-scratch test to quantify chemo-mechanical alteration.