Abstract

Full citation: 
Abstract: 

Reservoir properties of limestones can be significantly altered by burial dolomitization, which warrants careful investigation of this not fully understood diagenetic process in reservoirs. Therefore late-diagenetic dolomite bodies were studied in Permian outcrops in the central Oman Mountains as a potential surface analogue for the subsurface lower Khuff Formation where fault-related dolomite bodies are characterized by extreme variations in porosity and permeability. The current study aims at a better understanding of the occurrence, distribution, size, and shape of late-diagenetic dolomite occurring mainly at the contact between Permian limestone and overlying early-diagenetic dolomite. Intensive fieldwork followed by petrographic and cathodoluminescence microscopic analysis of thin sections, geochemical elemental analysis and stable carbon, oxygen and strontium isotope analysis are used to gain knowledge on the processes that control the occurrence of late dolomite. The results of fieldwork integrated into a new GIS model show that the distribution of the late-diagenetic red (weathering color) dolomite bodies in the Jebel Akhdar tectonic window is dominated by two principal localities of dolomitization, one in Wadi Sahtan and another one in Wadi Mistal. In between these two localities, no red dolomite is present in Permian host rocks. Field observations demonstrate that the red dolomite is concentrated above the Precambrian-Permian unconformity, either within the limestone that sits below or at the base of the contact with the early-diagenetic brown dolomite. Red dolomite bodies occur both as large roughly bedding parallel tabular bodies and as discordant subvertical bodies. Tabular bodies are continuous over hundreds of meters, but patches of limestone of centimeter to meter scale seem to be preserved within them. Discordant subvertical dolomite bodies occur in the oldest early Permian units and connect upsection with the tabular bodies. Bedding perpendicular dolomitic structures are either related to fault/ fracture planes or massive breccias. Breccias are composed of gray dolomite clasts and a red brown dolomitic matrix and both these breccia structures and faults may have acted as preferential, permeable pathway for dolomitizing fluids. We propose that the geometry of the late-diagenetic dolomite bodies hosted in Permian limestones is impacted by the distribution of (early) diagenetic products, the location of faults/fractures, disconformity surfaces and the Precambrian-Permian unconformity that most likely acted as migration pathways for Mg-rich fluids. This study is funded jointly by Qatar Petroleum, Shell and the Qatar Science & Technology Park.

Publication date: 
Thursday, April 10, 2014