--> ABSTRACT: Gypsum Precipitation Patterns in Holocene Sabkhas of Qatar: An Analogue for Ancient Anhydrite Distribution, by Strohmenger, Christian J.; Jameson, Jeremy; #90142 (2012)

Datapages, Inc.Print this page

Gypsum Precipitation Patterns in Holocene Sabkhas of Qatar: An Analogue for Ancient Anhydrite Distribution

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

Anhydrite is one of the most significant minerals that affect porosity and permeability in ancient, arid climate carbonate reservoirs like the Arab and Khuff formations (Arabian Plate), the Zechstein Formation (Northern Europe) and the Permian of West Texas. Reservoir quality is inversely proportional to the amount of pore-filling and replacement anhydrite. Models for the origin and distribution of early evaporites are poorly understood. This study reports on the first detailed, three dimensional mapping of the occurrence and the distribution of gypsum in a Holocene sabkha of Qatar, proposed as an analogue for ancient anhydrite-after-gypsum cementation.

At the surface, gypsum occurs as either laminar or pustular, buckle crusts, reaching up to 60 cm in thickness. After burial, dehydration and compaction surface crusts would be 15-20 cm thick and resolvable on wire-line logs. The gypsum surface 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. Massive water table precipitation would likely preserve its thickness during burial. Radiocarbon age dating reveals that pervasive gypsum cementation occurred in less than 6000 years.

Over the study area, 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.

Understanding the origin and distribution of modern gypsum helps to predict the porosity and permeability reducing anhydrite-after-gypsum distribution in ancient, arid climate carbonate reservoirs.

 

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California