Stylolite, porosity controls, and dense zones in lower cretaceous reservoirs of the Sahil Field, Abu Dhabi, U.A.E.

Abstract

Sylolitization (i.e. chemical compaction) is one of the significant diagenetic processes that could considerably affect the reservoir quality of carbonate successions. Stylolitization has been recorded in the Lower Cretaceous Kharaib Formation particularly reservoir zone B as well as the two dense zones (A&B;) bounding reservoir zone B unit. The Kharaib Formation is part of the Thamama carbonate Group, which was deposited in a shallow-water marine ramp setting. This study aims to: (1) better understand the origin and the distribution of stylolite in flank (i.e. water zone) versus crest (i.e. oil zone) of the Thamama limestone from Sahil Field, onshore Abu Dhabi, UAE; (2) reconstruct the paragenetic sequence in the Kharaib Formation; (3) constrain the origin of the two dense zones (A&B;); and (4) compare the impacts of diagenesis on reservoir quality evolution in the water versus oil zone. The study includes core description, detailed petrographic examination (conventional, cathodoluminescence, and fluorescence techniques, scanning electron microscopy and Energy-Dispersive X-Ray analyser), geochemical, organic geochemistry and fluid inclusion microthermometry. The study reveals that the Kharaib Formation is affected by: (1) dissolution, (2) calcite, dolomite, pyrite and kaolin cementation, (3) dolomitization, (4) micritization, (5) recrystallization, and (6) chemical compaction. Petrographic, geochemical and fluid inclusion analyses indicate that most of the micro-fracturing and cementation phases are closely linked to stylolite formation. Calcite, coarse-crystalline pyrite, and saddle dolomite cements, dolomitization, micrite recrystallization, were mediated by hot (125 ºC to 140ºC) basinal brine (salinity up to 24 wt% NaCl eq.). These brines might have been migrated along the stylolites and has affected mainly carbonate rocks in the water zone. The presence of kaolin suggests flux (possibly along stylolites) of organo-aluminum complexes related to hydrocarbon maturation in the source rocks. Moreover, pyrite indicates that the fluids were affected by thermo-chemical sulfate reduction (TSR) probably in the Khuff and/or Arab formations. Distribution of stylolite and its impacts on reservoir quality evolution is more extensive in water zone compared to the oil zone. Overall, stylolitization resulted in either reservoir quality enhancement or deterioration in both water and oil zone. This study demonstrates that understanding the formation and evolution of stylolites is crucial for: (1) constraining fluid flow in sedimentary basins and related overall diagenetic evolution of carbonate successions, and (2) unraveling the impact of diagenesis on reservoir quality evolution.

Date of Award	2013
Original language	American English
Supervisor	Sadoon Morad (Supervisor)