Abstract

Full citation: 
Abstract: 

Salt-cored domal structures in the subsurface represent common reservoir settings. The properties of the fracture network in the layers overlying salt diapirs are therefore crucial for the production of oil and gas and the storage of carbon dioxide in such reservoirs. Properties of the fracture network include fracture orientations, which are controlled by regional stress fields as well as the emplacement of the salt. The chronology of different fracture sets determines the connectivity of the fracture network, and thus influences fluid flow. Furthermore, the type of the fractures, which is determined by the failure mechanism, impacts on the capacity of fractures to seal. We carried out fracture analyses at six locations across Jebel Madar, Oman. Fracture orientation was measured and the chronology of different fracture sets determined using abutting relationships. In addition, we took measurements of the angle enclosed by the fracture and bedding plane across the salt dome in order to determine the fracture type. The brecciation textures of the fracture infillings were also analysed in order to understand the mechanism of brittle failure. Two dominant regional fracture sets were determined across Jebel Madar: A NE-SW trending fracture set which predates a fracture set that is oriented NW-SE. The major structure dissecting Jebel Madar is controlled by the NE-SW trending fracture set. We observe only extensional rather than compressional structures. Large-scale listric normal faults observed both in the field and in published seismic data indicate shear failure. We suggest this indicates a low neutral surface on or within the salt with the layers above this surface being subject to extension. The majority of fractures are of extensional nature with fracture-bounded blocks beginning to slip as the diapir continues to rise and the layers above extend. Thus, these fractures have elements of both extension and shear. We suggest that fracture orientation at Jebel Madar is strongly controlled by regional stress fields rather than the emplacement of the salt. Furthermore, the misinterpretation of the nature of fractures above a salt diapir based on seismic data, assuming they are shear rather than extensional fractures could lead to a misinterpretation of their capacity to impede fluid flow. This project is jointly funded by Qatar Petroleum, Shell, and the Qatar Science & Technology Park.

Publication date: 
Thursday, April 10, 2014