Fracture related dolomitization in the Central Oman mountains

Abstract

The characteristics of late diagenetic dolomite bodies (DT2) were studied focussing on distribution patterns, compositional trends, diagenetic history as well as diagenetic and sedimentary fabrics. DT2 dolomite bodies are exposed in Lower Permian platform carbonates and represent a stratigraphic analogue for subsurface Khuff deposits. Two types of dolomitisation are interpreted to have affected the platform carbonate succession referred to as early diagenetic (ED) dolomite (shallow burial origin) and late diagenetic (DT2) dolomite (medium to deep burial). DT2 bodies were found to follow predominantly the strike direction of fracture planes, bed-tobed contacts, the Precambrian-Permian unconformity and the bottom rim of early diagenetic dolomite. DT2 dolomite bodies are furthermore associated with brecciated fabrics of up to tens of metres in diameter interpreted as collapsed karst cavities formed by either meteoric or hypogenic fluids. The timing of the brecciation is interpreted to postdate shallow burial early diagenetic dolomitisation. The dolomitised matrix indicates a migration of late dolomitising fluids subsequent to or postdating the collapse of the karstic cavities. Two groups of diagenetic fabrics were observed in DT2 dolomite bodies in association to fractures and bedding planes. First, vug related fabrics comprise vuggy pores and zebra dolomite. Second, breccia related fabrics (centimetre scale) consist of either small-scale crackle/ mosaic breccia fabrics and centimetre sized vugs followed by bedding-parallel vugs. Christmas tree geometries, saddle dolomite cements and zebra textures indicate a DT2 dolomite formation by ascending fluids of at least 60 degrees. The regional ascent of dolomitising fluids is interpreted to be triggered by large scale faults in Precambrian rocks underlying Permian platform carbonates in the area ofWadi Mistal and Sahtan. The lateral distribution of fluids between both Wadis is most likely controlled by cherts, fine grained turbidites and volcanoclastics of the Fara Formation. The edges of DT2 dolomite bodies were subsequently exploited by hot deep basinal brines which potentially caused recrystallisation. Hyperspectral imaging enabled the detection of inaccessible DT2 dolomite bodies. However, the study revealed strong spectral variations in the spectra of DT2 dolomite across the Central Oman Mountains which is linked to weathering and dedolomitisation.