Online Journal for E&P Geoscientists

Strohmenger, Christian J.¹; Jameson, Jeremy¹
(1) ExxonMobil Research Qatar, Doha, Qatar.

Anhydrite is one of the most common porosity destroying minerals in ancient, arid climate carbonate reservoirs like the Arab and Khuff formations. Reservoir quality is inversely proportional to the amount of pore-filling and replacement anhydrite-after-gypsum. This study reports on the first detailed three dimensional mapping of the occurrence and the distribution of gypsum in a Holocene sabkha of Qatar.

At the surface, gypsum occurs as either laminar or pustular, buckle crusts, ranging from up to 60 cm in thickness. The gypsum crusts tend to be composed of distinctive, elongate crystals, lensoid twins, or ‘fishtail’ twins. Most crystals are mm-sized, with cm-sized crystals forming in areas where water ponds at or near the surface. Between the surface crusts and the water table, gypsum occurs as cm-sized bladed crystals or gypsum rosettes and cements; preferentially forming along cross-stratification and burrows. Most extensive gypsum precipitation/cementation occurs from the water table to the base of the Holocene (approximately 3 to 4 meters), where dm-sized gypsum rosettes and extensive poikilotopic gypsum cementation occurs. Radiocarbon age dating reveals that pervasive gypsum cementation occurred in less than 6000 years.

The entire Holocene is heavily gypsum cemented around the landward limit of the sabkha. More seaward, gypsum is present as crusts and water table cement. Towards the seaward limit, gypsum is only found as surface crusts. This pattern suggests gypsum forms through evaporitic concentration and retrograde thermal precipitation from groundwater discharge into the coastal areas.