From outcrop to 3D modelling: a case study of a dolomitized carbonate reservoir, Zagros Mountains, Iran

Outcrop data derived from fieldwork, remote sensing satellite and LiDAR-derived 3D models were used to build an integrated dual porosity-permeability static reservoir model which captured stratigraphic, diagenetic and structural heterogeneities. The study focuses upon the Mishrif-Mauddud/Sarvak interval, one of the most prolific reservoir units in the Middle East. The study area is exceptionally well exposed in deep gorges which cut transversally across anticlines of the Simply Folded Belt of the Zagros Mountains. The outcrops reveal volumetrically significant dolomitization of the latest Albian to Turonian carbonates of the Lower and Upper Sarvak formations. Three different dolomite bodies, which are spatially connected and genetically linked to the same fluid flow event, were recognized and mapped: (1) a thick dolomite body replacing the Lower Sarvak and forming a massive dolomite core; (2) horizontally extensive stratabound dolomite bodies (sheets), emanating laterally from the massive dolomite; and (3) vertically elongated dolomite pipes, rooted in the massive dolomite and typically replacing slope facies of the Upper Sarvak Formation.

The widespread development of tight, non-planar dolomite textures (a typical feature of high temperature dolomitization) drastically reduces the reservoir potential of the dolomitized geobody in the study area. In particular, this is present in the massive dolomitized body. Vuggy porosity seems to increase porosity only locally and to a limited extent, developing a non-connected pore network. The dominant porous dolomite textures are more abundant in the peripheral part of the geobody (dolomite sheets), where they are strongly controlled by precursor facies and diagenesis. Three main dolomite pore types were identified (intercrystalline, interparticle and mouldic), linked to the depositional environment of the precursor limestone. These pore types were used for petrophysical modelling. The approach adopted in this study allowed the distribution of rock properties in the dolomitized geobody to mimic the main depositional facies architecture.

The study area is characterized by a simple fracture network. Two main fracture sets and two major sets of conjugate normal faults were recognized in the field and mapped on 3D virtual outcrop data. Non-stratabound fracture density varied according to stratigraphic unit and/or dolomite body type (pipes/massive/sheets), showing a general increase from precursor limestone to dolomite. Fracture density also varied according to distance from faults (fault damage zone). This was particularly true in the limestone. The data also showed a prominent increase in fracture height from limestone to dolomite bodies, indicating that the dolomitized geobodies are likely sites for high production and early water breakthrough.