Relationship between karstification and burial dolomitization in Permian platform carbonates (Lower Khuff - Oman)

Large breccia fabrics associated with karst constitute an important structure in massive limestone successions. The dimensions and shapes of breccia structures are controlled by the initial fracture pattern of the limestone and preferential pathways of the karstifying fluids, but subsequently breccia fabrics can also govern the migration of later fluids. Therefore, breccias are highly relevant features to capture for reservoir characterisation. Outcrop analogues for Lower Khuff units in the Middle East present in the Central Oman Mountains reveal brecciated fabrics up to I Os of metres in diameter. These brecciated units are closely associated with dolomite bodies of late diagenetic origin. Based on an integrated set of data, the breccias are interpreted as collapsed karst cavities either formed by meteoric or hypogenic fluids. The exact origin of the fluids could not be constrained due to an overprint by later dolomitizing fluids. Based on the composition of the clasts and matrix in the breccias, two dolomitization events are interpreted to have affected the succession, one prior to (early diagenetic [ED] dolomite) and one after brecdation (late diagenetic [DT2] dolomite). Dolomite of shallow burial origin (ED dolomite) was only observed as clasts within breccia and is much more frequent than late diagenetic (medium to deep burial) dolomite clasts. Thus, the timing of the brecciation and collapse is assumed to postdate shallow burial early diagenetic dolomitization. late diagenetic replacive dolomite (DT2 dolomite) forms 90% of the matrix in the breccia fabrics with the exception of a small area that was not affected by dolomitization, but is rarely present as clasts. Stable isotope measurements [delta O-18: -2.5 parts per thousand to -6 parts per thousand VPDB and delta C-13: 2.9 parts per thousand to 4.8 parts per thousand VPDB] suggest a burial origin for the late diagenetic dolomite potentially with the participation of hydrothermal fluids. The dolomitized matrix indicates a migration of late dolomitizing fluids subsequent to or postdating the collapse of the karstic cavities. Thus, early karstification processes seem to have played a big role in controlling subsequent loci of late dolomitization in the Oman Mountains, and potentially in other similar settings elsewhere. (C) 2016 Elsevier B.V. All rights reserved.