Fracture-Controlled Burial Dolomitization: Outcrop Studies in Northern Oman

Vandeginste, Veerle; John, Cedric; Cosgrove, John

Dolomitization, a significant diagenetic process affecting carbonate rocks, can occur as replacement of limestone (potentially creating porosity linked to mineral volume decrease) or as dolomite cementation (reducing porosity). A common form of burial dolomite is fracture or fault-related dolomite, which is often hydrothermal. Fracture-related dolomite has generally porous textures with breccias and zebra fabrics that include saddle dolomite, and it is found to host hydrocarbons in different tectonic settings. Although depositional fabric generally influences the extent and type of dolomitization, porosity-permeability within Middle Eastern reservoirs can be extremely variable and hard to predict based on sedimentary textures. This heterogeneity turns reservoir prediction into a challenging task. In addition, major dolomitized subsurface fractures that are undetectable on seismic lines represent significant geohazards for drilling operations.

The current study presents research on fracture-related dolomite bodies in outcrops of northern Oman. The aim is to evaluate geochemical and textural changes within dolomite bodies as potential predictive tool to identify major fractures in the subsurface. Three systems of fracture-related dolomite were studied that are hosted in limestones of the Precambrian Khufai Formation, Permian Saiq Formation (Khuff equivalent) and Jurassic Sahtan Group. The fracture-related dolomite bodies are generally less than 15 m wide, but can be up to a few hundreds of meters long following the fracture/fault plane. The burial dolomite in the Saiq Formation is controlled by the diagenetic contact of limestone to overlying fine crystalline dolomite; it is a few tens of meters thick and has been identified in separate wadi's about 40 km apart. All three systems comprise zebra dolomite, but the dolomites are geochemically distinct. The dolomite in the different systems formed at different times, implying a complex fluid flow history involving several pulses of dolomitizing fluid that affected the central Oman Mountains. Significant geochemical evidence (especially strontium isotope) shows that, although the obducted Semail ophiolite is a potential source for Mg, dolomitizing fluids were derived from interaction with deeper siliciclastic units, most likely the crystalline basement.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013