Origin and Early Diagenesis of Evaporites in a Coastal Sab-kha

  • (Didi) Sher Ooi

Student thesis: Doctoral ThesisDoctor of Philosophy (PhD)

Abstract

In most carbonate reservoirs in the Middle East and Cretaceous reservoirs, anhydrite is one of the major pore-occluding mineral phases that affects hydrocarbon reservoir quality. Whilst an-hydrite is a commonly observed CaSO4 mineral in the subsurface, the most common CaSO4 mineral deposited in modern setting today is gypsum. Sabkha system is commonly used to under-stand the formation of this diagenetic evaporites with a focus on dolomitization, but little studies have focused on the distributions and drives for the formation of diagenetic gypsum. The classic sabkha model is currently based on Abu Dhabi, which is a windward carbonate sabkha setting.
This integrated study focuses on Mesaieed sabkha, a 75 km2 leeward, prograding and a mixed siliciclastic-carbonate Holocene sabkha overlying an Eocene Dammam Formation. The sedimen-tology of four spatial sabkha zones within the sabkha was studied in detail, and this zonation is evident by geochemical and hydrodynamic observations. The sabkha geomorphology offers evi-dence of spit formation, marine flooding, meteoric input in the landward zone and hints of aeolian deposition during shamal events.
Gypsum precipitation is ubiquitous at the vadose zone and the phreatic zone in the upper and middle sabkha zone, and in the Dammam Formation underlying the middle and lower sabkha zones. Hydrogeology studies suggest that the vertical flow is about an order of magnitude larger than lateral flow, with the greatest evaporitic drawdown in the middle sabkha. Gypsum dissolution in the outcropping Dammam Formation updip is evident from the excess of calcium and sulphate in the continental waters, which provides the solutes downdip for the proximal and upper sabkha. Evaporation is a main driver of the hydrodynamic and consequently the geochemistry, which drives precipitation of poikilotopic and displacive gypsum within the sabkha, resulting in the en-richment of conservative ions and linear depletion in both aqueous calcium and sulphate ions. 1:1 magnesium depletion and calcium enrichment providing evidence of secondary dolomitization. Parts of the sabkha are not enriched by calcium but are depleted in magnesium, potassium, and the trace element silica and aluminium, suggest precipitation of K-clays.
This holistic sedimentology-hydrogeochemical workflow can be applied to improve accuracy of facies modeling, reservoir quality estimation and the assessment of the uncertainty for pre-drill value of information studies.
Date of Award1 Oct 2019
Original languageEnglish
Awarding Institution
  • The University of Bristol
SupervisorFiona F Whitaker (Supervisor), Jeremy Jameson (Supervisor) & Christian Strohmenger (Supervisor)

Cite this

Origin and Early Diagenesis of Evaporites in a Coastal Sab-kha
Ooi, (. S. (Author). 1 Oct 2019

Student thesis: Doctoral ThesisDoctor of Philosophy (PhD)