High Permeability Streaks Characterisations in Middle East Carbonate
Authors: Poli Emmanuelle , Maza Carine , Virgone Aurlien , Gisquet Franck , Fraisse Christian , Pabian-Goyheneche Cecile 
Published: May 2016

This reference is for an abstract only. A full paper was not submitted for this conference.

One of the main difficulties for modeling and producing carbonate rock oil reservoir in the Middle East is linked to the fact that these reservoirs often display numerous and variable heterogeneities, ranging from plug micro-scale heterogeneity (texture, pore types, cementation, …) to field mega-scale heterogeneity (geometries and facies, fracturation and faulting). Because these heterogeneities often result in significant variations in permeability and saturation, and hence in productivity it is absolutely necessary to capture all of these scales of heterogeneities and understand their origin. Various factors can be implied alone or combined between them: depositional environment, sequence stratigraphy, early or late diagenetic overprint and fracturation scheme.

The aim of this paper is to present an integrated approach used to characterize very heterogeneous reservoirs and in particular those presenting highly permeable levels. Some comparative studies using different approaches have been carried out simultaneously in order to provide key information on origin and geometry of these highly heterogeneous carbonate reservoirs.

The first approach is based on outcrop analogs. In order to provide 2D/3D characterization of the main depositional bodies some outcrop analog studies have been carried out. The main objectives of this approach were to understand the sedimentary architecture and distribution of the reservoir facies related with sedimentology, diagenesis and sequence stratigraphy.

The lateral continuity of sedimentary bodies and particularly of highly permeable levels is one of the most important parameter to take into account during the modelling process. Diagenesis effects and modeling must also be closely considered in order to be able to address its relationship with sequence stratigraphy, and its impact on the reservoir characteristics. In some cases, there is a strong relationship between vertical facies heterogeneity and sea level change. Reservoir drains may result from confined deposition or early dissolutions caused by meteoric or mixed waters linked with exposure surfaces. In these cases, heterogeneity is mainly due to early diagenesis. Corresponding facies and associated petrophysical characteristics have to be properly described on outcrop.

However early diagenesis has to be distinguish from late diagenesis. Some high permeability levels are often associated with late dolomite type such as saddle dolomite. These facies, resulting from hydrothermal water circulation are systematically associated with fault and fractures network. In this case, analog studies allowed a better understanding of the geometrical relationships between facies, of the diagenetic effects with respect to the distance from fault, and of the fracturation associated network. All these are key information to better assess the variographic parameters into the reservoir models.

So, outcrop analog approach provides information about: sedimentary architecture, size of sedimentary bodies and quantitative information about primary and secondary diagenesis effect such as diagenesis parameters distribution, facies envelop size and diagenesis timing. These three kinds of information are necessary to build and populate an accurate geological model. The second approach is based on subsurface studies. Cores have to be described in detail in order to determine sedimentological facies families having homogeneous lithology and representing specific depositional environment. Cement types and porous network, mainly linked to diagenesis have also to be described precisely from thin sections. However, none of such detailed studies is useful without confronting the so-defined sedimentological facies of a reservoir with their corresponding petrophysical and wireline log properties. For each sedimentological facies, the associated diagenetic overprint has to be analysed in order to define the relationships between facies and diagenesis, the impact of their stratigraphic position on their behaviour, and their relations with petrophysical and wireline log properties. The challenge will consist in integrating many sources of data from cores, thin sections, plug (CCAL & SCAL), wireline logs and reservoir engineering data (well tests, production logs…) in order to derive pertinent Petro Diagenetic Groups. A good correlation between the defined Petro Diagenetic Groups and wireline logs will permit to use Petro Diagenetic Groups as learning set in a statistic multi well analysis to define Electrofacies that will be extrapolated to the whole set of non-cored and/or undescribed wells. Of course, an accurate log data screening and if necessary processing (decompaction, normalisation, HC corrections, shoulder bed effects and thin beds lower than log resolution removal) is mandatory for good results. A blind test is necessary at this stage to control the quality of the generated Electrofacies. At least this approach based on subsurface studies will produce Electrofacies along all cored and uncored wells. The resulting Electrofacies are well defined in terms of sedimentology and depositional environment, diagenesis (pore and cement types) and petrophysics. Consequently, because they keep a geological consistency and because they are well defined in terms of petrophysics, they can be used as the building blocks of the geological and reservoir models. In conclusion, the confrontation of the conceptual sedimentological and diagenetic models coming from outcrop studies giving information such as size and geometries of sedimentary bodies and results coming from subsurface studies will help to define variographic parameters and trend maps to be used to constraint the 3D geological model. Electrofacies have to be propagated in the reservoir through depositional and diagenetic conceptual models, and being petrophysically meaningful (consistent in conventional cores and log domains). If possible, these results have to be confronted with 2D or 3D seismic to constrain the distribution of reservoir facies between wells, inter wells connectivities and faults dynamic impact. This integrated approach promotes discussion and exchange between geologists, geophysicists, petrophysicists and reservoir engineers, essential to optimise reservoir modelling of heterogeneous carbonate reservoirs.