F field is located in the Middle East and was discovered in 1980's. The bulk of the proven liquid hydrocarbons are contained in the B formation (carbonate), which averages some 200m in thickness in the study area. The hydrocarbons are contained in a low relief NW-SE trending anticlinal structure. Deposition of the reservoir sequence occurred during the late Cemnomanian to Early Turonian in the Mesopotamian Basin.
Oil production started few years ago and currently there are more than 40 producing wells in the centre of the structure. The uneven well distribution limits the understanding of 3D reservoir characterisation, explicitly in the flank areas. Only one conventional core with good recovery was available for reservoir B, which makes it somewhat difficult to delineate the internal architecture of the carbonate ramp.
A total of seven lithofacies were identified which are Rudistic Floatstone Wackestone, Bioclastic Wackestone, Bioturbated Bioclastic Packstone (BBP), Rudistic Rudstone Wackestone, Bioclastic Floatstone Wackestone, Heterolithic Bioturbated Bioclastic Wackestone and Bioclastic Mudstone. A conceptual depositional environment with lithofacies association was generated by using core descriptions, regional studies and analogues.
A fit for purpose integrated reservoir characterisation study was carried out in 2014 with main inputs from Nuclear Magnetic Resonance (NMR) and image logs in conjunction with digital rock analysis (DRA), conventional open hole well logs, core laboratory analysis, mud logs, pressure and well test data. Several rock typing approaches were developed; including Flow Zone Indicator (FZI) or Rock Quality Index (RQI), Lucia Rock Class, Clerke Pore Size technique from Mercury Injection Capillary Pressure (MICP), multi-variate cluster analysis (biased and normalized), cluster analysis (non-biased and non-normalized) and self-organizing maps (biased but non-normalized). From these FZI, an optimum of seven Hydraulic Units (HU) were selected where FZI 1 being the best flow unit, and FZI 7 is the poorest.
A relationship between FZI with lithofacies was then established by comparing both on depth plot as well as porosity-permeability cross plot. Different FZI can be seen for one lithofacies, for instance BBP consists of FZI 1 to 4 which indicates the changes in reservoir quality within the same lithofacies. The same relationship was extended towards depofacies, and resulted with lateral segregation of depositional environment according to its reservoir quality.
This exercise confirms the heterogeneity within B formation, and it captures the changes in reservoir quality laterally and vertically. This detailed understanding of the carbonate architecture was translated into a 3D geological model in order to minimize the uncertainties for dynamic simulation and future field development plans.