Diagenetic Geobodies: Fracture-Controlled Burial Dolomite in Outcrops From Northern Oman

Abstract

SummaryDiagenetic heterogeneities are difficult to predict in the subsurface. Nevertheless, such heterogeneities can be crucial in hydrocarbon exploration. Diagenetic processes can significantly alter petrophysical properties of reservoir rocks, especially in carbonate rocks because of the reactive nature of the carbonate minerals. Dolomitization [i.e., the transformation of calcite (limestone: Lmst) into dolomite] is a common diagenetic process in carbonate rocks. Resulting dolomite bodies have a different pore network than the original Lmst and respond also differently to tectonic stress causing different fracture networks than in the original Lmst. The paper presents an overview of the learning outcomes gained by studying fracture-related dolomite in outcrops of Oman and subsequent laboratory analysis conducted over the last 4 years. A combined structural, petrographic, and geochemical approach was taken to study three dolomite systems occurring in different stratigraphic host-rock (HR) intervals. Structurally controlled dolomitization (i.e., dolomitization along faults and fractures) typically occurs in burial conditions, and the resulting strong permeability anisotropies caused by the dolomite textures can cause major challenges for hydrocarbon exploration.Dolomite bodies in the Precambrian Khufai formation are related to N/S to NNE/SSW fractures, whereas dolomite bodies that mainly occur in the Jurassic HRs occur along reactivated WNW/ESE normal faults. These fracture-related dolomite bodies are generally less than 15 m wide, but can be up to a few hundred meters long. Late-diagenetic dolomite bodies were also recognized in Permian HRs, where they occur at or close to the contact between Permian Lmst and early-diagenetic dolomite. This late-diagenetic dolomite system can be traced laterally for at least hundreds of meters and occurs in wadis that are approximately 40 km apart. Our data indicate that there were several dolomitization events in the geological history of the succession, generating dolomite bodies with different characteristics. This paper highlights the need to understand timing and structural setting of dolomite bodies in the subsurface to improve reservoir management.