The Control of fracturing and dolomitisation on 3D reservoir property distribution of the Asmari Formation (Oligocene-Lower Miocene),

Abstract

Abstract The Asmari Formation (Oligocene-early Miocene) is the most important reservoir in the oilfields in the Zagros foreland of the Dezful Embayment of SW Iran. The carbonate reservoirs of the Asmari Formation are characterized by low matrix permeability, and effective drainage is dependent on the occurrence of open fractures. The fractures have been formed during the Zagros Orogeny since late Miocene, which is also responsible for the geometry and formation of NW-SE trending anticlinal traps. Dolomitisation is another factor controlling reservoir qualities as dolostones in general have higher porosities than limestones. The fractures are typically stratabound and sub-perpendicular to bedding. The density and dimension of fractures are controlled by the mechanical stratigraphy, which is controlled by the depositional environment and cycles. In the platform top sequence, the Dunham's (1962) texture classification appears to be the most important factor controlling the Fracture Intensity (FI) rather than the mechanical layer thickness. Mud supported textures (mudstone and wackestone) have higher FI than grain supported (packstone and grainstone). The fracture density is high in the vicinity of faults. Two main types of dolomite fabric are recognised; finely crystalline pervasive dolomite (<20µ) replacing mud-rich facies; and combinations of finely crystalline replacive dolomite surrounded by coarser dolomite cement (up to 100µ) in grain-supported facies. Dolomites have an evaporitic signature, based on geochemical and petrographic data, with a general upward increase in the abundance of both anhydrite and dolomite. Dolomite replacing the mud-dominated facies occurs as thinner beds, less extensive aerially forming the top of stratigraphic cycles deposited during highstand, and has lower reservoir quality, particularly as microporosity. Contrary, dolomite replacing the grain-dominated facies exists in thicker and more extensive beds forming the bottom of stratigraphic cycles deposited during early transgression, and these have better reservoir quality, particularly if leached or coarsely crystallined. General Geology The Dezful Embayment of SW Iran is one of the rich oil provinces in Middle East, see Fig. 1. The structural and stratigraphic features of Dezful Embayment have been investigated continuously since mid-last century (e.g. Haynes and McQuillan, 1974; McQuillan, 1985; Blanc et al., 2003, Wennberg et al. 2006 & 2007, Ehrenberg et al 2007). Most oilfields in this region are asymmetric anticlines having NW-SE trend as in Fig. 1.