STRUCTURAL CONTROL ON SUBSURFACE FLOW OF DOLOMITIZING FLUIDS: EXAMPLES FROM OMAN

The interplay between rock matrix, tectonic stress, fracturing, fluid flow and physico-chemical reactions is important in controlling structural and diagenetic processes in the subsurface. The complex interaction between these components makes predictions of petrophysical properties and their vertical and lateral variability difficult. Although ‘geobodies’ with defined porosities and permeabilities are often related to sedimentologic fabrics, they can also be controlled by diagenesis and structures, particularly fractures. Defining the dimension and location of these geobodies is important as input for fluid flow reservoir modeling.

Dolomitization of limestone typically changes the original petrophysical properties of the rocks, but can either enhance or reduce porosity and permeability. The current study concentrates on structurally-controlled dolomite geobodies in the Central Oman Mountains. The tectonic framework involves the closure of the Neotethys Ocean as a result of the convergence of Laurasia and Gondwana during Cretaceous to Neogene times. Intra-oceanic subduction along the eastern margin of the Arabian Plate started in the Cenomanian and continued into the Middle Turonian to Early Campanian and resulted in the southwest-directed obduction of the Semail ophiolite. This obduction led to deep burial of the studied rocks resulting in low-permeability recrystallized limestones.

Structurally-controlled dolomite bodies have been observed in outcrop at several stratigraphic horizons, i.e. Precambrian, Permian, Jurassic and Cretaceous layers in north Oman. Both a fracture analysis and a diagenetic study were carried out at the different outcrops. Diagenetic characterization of both the dolomite bodies and the host rock limestone includes petrography, stable oxygen, carbon and strontium isotopes, elemental analysis and (on coarse dolomite samples only) fluid inclusion microthermometry. Our results show that the dolomite bodies in the different stratigraphic horizons have a distinct chemical signature and that they formed at different times and different structural framework. This implies that we have a complex fluid flow history affecting the central Oman Mountains, involving several pulses of flow of dolomitizing fluids that occur relatively locally.