Fracture controlled hydrothermal dolomitisation - From outcrop to full 3-D reservoir facies model

Spectacular outcrops from the Zagros fold belt in SW Iran allow detailed mapping and sampling of reservoir quality dolostone bodies developed within Aptian-Turonian aged carbonates. Dolomite bodies are 100m to several km in width, have plume-like geometry, with both fracture (fault/joint) and diagenetic contacts with undolomitised country rock. "Fingers" of dolostone extend away from dolomite bodies along steeply dipping fault/fracture zones, and as strata-bound bodies following permeable depositional/diagenetic facies. Vertical barriers to dolomitisation are low permeability mudstones, below which dolomitising fluids moved laterally. Where these barriers are cut by fault/fracture zones, dolomitisation can be observed to have advanced upwards, indicating that faults/fractures were fluid migration conduits.
The resultant dolomite reservoir geometry is complex in 3D, but well connected. Dolomitisation caused porosity redistribution, enlargement, and formation of new porosity. An understanding of which fault/joint systems were active during dolomitisation, coupled with a facies control on stratabound dolostones, allows semi quantitative prediction and conceptual modelling of dolomite geobodies in a 3-D reservoir facies model. These geobodies are stochastically distributed in the grid and are heavily dependent on the fracture and stratigraphic framework developed at outcrop. A sensitivity study on the reservoir simulation model indicates significant production profile differences depending on the size and number of dolomite objects modelled. This study highlights the importance of producing P10 and P90 reservoir models during the early phase of field development to quantify uncertainty in STOIIP calculations and production profiles.