Outcrop Characterization of Fractured Cretaceous Carbonate Reservoirs, Zagros Mountains, Iran

Abstract

Abstract Well-exposed outcrops of Middle Cretaceous carbonates in the Zagros fold belt, Iran have been used to provide input data for a dual porosity reservoir model incorporating both matrix and fracture heterogeneity. The modelled outcrop area is located on the steeper dipping limb of a large-scale (10km wide, 100km long) asymmetric Tertiary anticline. The outcrop is structurally and stratigraphically analogous to producing and prospective anticlines in the nearby subsurface. Matrix heterogeneity can be divided into primary (depositional) and secondary (diagenetic) heterogeneity. Sources of primary heterogeneity include: 1)large-scale depositional architecture (e.g. shelf margin clinoforms, depositional sequence architecture), 2) macrofacies architecture and distribution, e.g. length scale distribution and geometry of carbonate sand shoals and rudist biostromes, 3)microfacies distribution and associated variability in primary pore types. Sources of secondary heterogeneity include diagenetic alteration of primary pore systems, karstification and hydrothermal dolomitisation. We demonstrate a close link between primary heterogeneity and secondary porosity-permeability distribution in the matrix. To construct the outcrop model, vertical stratigraphic logging and sampling were undertaken to establish a synthetic well log from which reservoir architecture, zonation, facies, microfacies, pore-type, and porosity-permeability data could be derived. "Walking-out" of depositional and diagenetic elements captured horizontal and vertical correlation length scales of porosity-permeability distribution and architectural features. The main fractures present are normal faults and joints. Faulting was characterized using measurements of fault kinematics, orientation, displacement, length, fault zone thickness and fault zone petrology. The normal faults are nonstratabound and have power-law scaling. Faults were stochastically simulated in a discrete fracture network and then scaled up to a reservoir simulation grid. Resulting porosity-permeability distributions are highly heterogeneous. Joints are stratabound and have an almost constant density within the same stratigraphic horizon, regardless of outcrop location (e.g. fold limb vs fold crest). In contrast, joint density varies greatly vertically according to lithology/facies. Joints were modelled using a simple parallel plate model approximation. Permeability due to joints is highly heterogeneous vertically, but laterally homogeneous. Construction of the outcrop model has been key to reducing uncertainty in geological interpretation and modelling of subsurface data, in particular with regard to characterization of porosity-permeability distribution and in interpretation of test data. The modelling workflow presented in this paper is directly relevant to producing and prospective fractured Middle Cretaceous carbonate reservoirs elsewhere in the Middle East and in Mexico.