FLOW UNIT CHARACTERISATION IN THE  PERMIAN-TRIASSIC CARBONATE RESERVOIR  SUCCESSION AT SOUTH PARS   GASFIELD, OFFSHORE IRAN

A.H. Enayati-Bidgoli*, H. Rahimpour-Bonab, and H. Mehrabi

The South Pars gas field (offshore southern Iran) has been investigated in detail in recent studies in terms of depositional, diagenetic and reservoir properties of the Permian-Triassic carbonate succession. In the present paper, a variety of flow unit approaches were applied to identify reservoir (flow) and non-reservoir (baffle or barrier) units within the Permian-Triassic carbonates.

The zonation scheme was based on three approaches; (i) flow units were identified using the stratigraphic modified Lorenz plot (SMLP) method; (ii) hydraulic flow units were identified using a parameter known as the flow zone indicator (FZI); and (iii) petrophysical flow units (PFUs) were determined using the pore throat radius (R35) and water saturation (Sw) parameters.

Studies of flow units at both macro- and micro-scales showed that flow properties were controlled by both depositional and diagenetic features. In order to construct a reservoir flow model, the flow units and PFUs were correlated between the four wells studied within a sequence stratigraphic framework. SMLP-derived flow units appeared to be distributed homogenously within the reservoir succession resulting in a layer-cake architecture.

By contrast, the FZI-derived hydraulic flow units drew attention to the presence of small-scale heterogeneities within the reservoir. A comparison between these methods showed that the flow model derived from PFUs included greater vertical and horizontal heterogeneities, especially in the Upper Dalan Member (upper K4 reservoir unit). This was due to depositional/diagenetic heterogeneities in both lateral and vertical directions, and the parameters applied in the PFU method. The PFU-derived flow model showed a closer relationship to the actual reservoir performance than the flow units derived by
the other methods and can, therefore, be used as the basis for future dynamic flow simulation.
South Pars gas field and its extension within Qatari territorial waters (North field) comprises the world’s largest offshore gas accumulation (Halbouty, 2003; Insalaco et al., 2006; Ehrenberg et al., 2007) (Fig. 1).
Gas is produced from reservoir units in the Permian-Triassic (Khuff-equivalent) Dalan and Kangan Formations (Kashfi, 1992; Aali et al., 2006; Alsharhan, 2006). These formations have been studied in detail at South Pars and nearby fields.
studies have focused on depositional and diagenetic characteristics, palaeo-environmental conditions and geochemical properties and on factors governing reservoir quality (e.g. Insalaco et al., 2006; Ehrenberg, 2006; Rahimpour-Bonab, 2007; Ehrenberg et al., 2008; Rahimpour-Bonab et al., 2009, 2010;Esrafili-Dizaji and Rahimpour-Bonab, 2009, 2013;Tavakoli et al., 2011; Tavakoli and Rahimpour-Bonab,2012; Aleali et al., 2013).
In a recent study (Rahimpour-Bonab et al., 2014), barrier units within the Upper Dalan and Lower Kangan succession (intrareservoir barriers) were evaluated using petrographic, petrophysical (SMLP) and geochemical approaches.

In the present paper, after the detection of reservoir compartmentalization (Rahimpour-Bonab, 2007; Rahimpour-Bonab et al., 2014), different methods are applied to characterize the distribution of flow units and their relationship with geological properties in the wells studied. The paper thus attempts to differentiate flow, baffle and barrier units in the reservoir succession at South Pars using flow-unit characterisation, and will lead to a better understanding of the lateral and vertical distribution of reservoir and non-reservoir units.

A flow unit is a part of a reservoir which has lateral and vertical continuity and homogeneous flow and bedding characteristics (Hearn et al., 1984). Flow units can be used to divide a reservoir into geobodies appropriate for flow simulation (Bhattacharya et al.,2008) and to investigate production behavior. Flow unit modeling can be used in field development planning, water flooding programmes and the dynamic simulation of hydrocarbon production (Borgomano et al., 2008), as well as to predict reservoir performance. Flow units are based on both geological and petrophysical data representing reservoir heterogeneity at different scales (Ebanks et al., 1992) from the well-bore to the field scale (Slatt and Galloway, 1992).