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

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 non-stratabound 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. Introduction Building 3D carbonate reservoir models for flow simulation is a major challenge for the petroleum industry, primarily due to the occurrence within carbonates of a wide range of heterogeneities occurring at a wide range of scales. Sources of heterogeneity in carbonate reservoirs can be divided into 3 main categories:Primary (depositional) heterogeneitySecondary (diagenetic) heterogeneityStructural (fracture) heterogeneity When a carbonate reservoir is heavily fractured, the reservoir typically has dual porosity/permeability behaviour, and it becomes important to understand and capture the flow dynamics of both the matrix (which acts as the hydrocarbon storage unit) and the fractures (which act as the main permeability pathways draining the reservoir) when building the reservoir model.

Fracture Analysis in the Outcrops of the Qara Chauq Anticlines, Zagros Fold and Thrust Belt of Kurdistan Region in Northern Iraq

“…Furthermore, the occurrence of highly permeable fracture zones in wells are of concern because they may cause early water breakthrough, as demonstrated by numerical analysis (Jonoud et al, 2013). Therefore, a careful use of scaled outcrop data can, to some extent, improve reservoir modelling and constrain uncertainties (Sharp et al, 2006;Wennberg et al, 2006). In terms of geometry, the dominant NE-SW open and part-open fracture strike orientation is perpendicular to the fold axes, which indicates anisotropic permeability with the maximum fracture permeability aligning in the NE-SW direction.…”

“…Antonellini & Mollema, 2000;Aydin, 2000;Nelson, 2001;Stephenson et al, 2007;Lacombe et al, 2011). However, the scaled outcrop data should be used with care in order to improve reservoir modelling and constrain uncertainties (Sharp et al, 2006;Wennberg et al, 2006;Barr et al, 2007;Lapponi et al, 2011). The fracture attributes of potential carbonate reservoirs are investigated within Kurdistan Region of Iraq, as represented by the Kirkuk Group formations (Oligocene) in Figure (1).…”

A geocellular modelling workflow for partially dolomitized remobilized carbonates: An example from the Hammam Faraun Fault block, Gulf of Suez, Egypt

“…Two methods were employed to reflect the natural variability in the location and volume of dolomitized debris flows observed in the field. The first method, following Sharp et al (2006) used a percentage to populate a certain volume of the R-1 facies. The percent dolomitized bodies were calculated from logs (23%); however, the results were found to underestimate the number of dolomitized R-1 facies, based on comparison with field photographs.…”

“…In carbonate reservoirs, there is significantly less data available to describe sedimentary body geometry. Carbonate reservoirs are heterogeneous and complex, owing to the changes in depositional systems through determined by what components of the carbonate factory are present or absent (Pomar and Hallock, 2008) and diagenetic overprinting, which influence the physical rock properties of carbonate faciesand fracture abundance (Sharp et al, 2006;Pöppelreiter et al, 2008;Pyrcz and Deutsch, 2014). In deepwater carbonate systems, further complexity is introduced by the sparse database of mass carbonate transport facies may be due to lack of study, and almost certainly to the difficulty associated with prediction, imaging, and modelling complex carbonate facies in outcrop and in seismici.e.…”

The impact of heterogeneous mixed siliciclastic–carbonate systems on CO 2 geological storage

“…A detailed understanding of facies can be achieved by using outcrop analogue to construct realistic 3D reservoir models (Howell et al 2014;Barnaby and Ward 2007). Analogue reservoir models were previously used to simulate and investigate the effects of reservoir architecture on fluid movement (e.g., Zuluaga et al 2016;Wilson et al 2011;Rotevatn et al 2009;Sharp et al 2006;Brandsaeter et al 2005). The application of analogues in the study of subsurface CO2 storage has also been studied (Newell et al 2019;Newell and Shariatipour 2016).…”