With the aim of building environments of deposition (EOD) maps supporting the prediction of the reservoir quality in the Mishrif Formation investigated in the subsurface of the Zubair field, sedimentary facies distribution and reservoir internal architecture were defined through integration of 3D seismic interpretation with well data (core facies, biostratigraphy, petrophysical parameters and log-derived facies obtained extending the log character of the different core facies to the non-cored wells).The workflow comprised the sedimentological core study, the seismo-stratigraphic interpretation tied to the well stacking pattern and the environment of deposition mapping including the interpretation of attribute volumes derived from seismic inversion (porosity and frame flexibility facies volume based on the internal Eni approach CaSRC -Seismic Reservoir Characterization for Carbonates-which put into relationship the rock physic parameters to the pore structure).Mishrif Formation consists of shallow water, bioclastic carbonates deposited in a well-developed carbonate ramp system. Geometrical evidences observed on seismic allowed to depict the evolution of the ramp passing through a main phase of aggradation, a subsequent step of progradation and, finally, a stage of aggradation-progradation. For each identified system tract, thickness maps were computed and depositional settings distributed as inner, middle and outer ramp domains. Each domain is represented by a group of facies characterized by its own configuration of porosity and pore types. Porosity is excellent in rudist bearing shoal facies deposited in the middle ramp setting and poor in mudstone and wackestones of the outer ramp. The inner ramp domain is characterized by packstone with poor to moderate porosity, improved at the very top of the Formation by diagenetic alteration associated with subaerial exposure.Once established the relationships between facies types, environments of deposition and porosity, the EOD maps were utilized to estimate the porosity distribution within each system tract. In this view, the interpretation of the volumes derived from the seismic inversion were included in the mapping process to refine the outlining of the boundaries between depositional domains characterized by different porosity configurations.The approach proved to be very useful in predicting the distribution of facies and related reservoir properties in areas of the field poorly controlled by well-data.

The objective of the work was to characterize the pore system of the Mishrif carbonates of the Zubair Field (onshore Iraq), using the Simultaneous Seismic Inversion results, geostatistical information (extracted from wells and seismic data) and rock physics modeling, which allowed to robustly transform impedance to porosity and additionally, to implement the internal Eni approach CaSRC (Carbonate Seismic Reservoir Characterization), including the Frame Flexibility Factor (FFF) to discriminate reservoir quality facies in terms of hydrocarbons productivity. A seismic inversion was performed to produce elastic properties from seismic data. These property volumes were used to describe the lateral distribution of petrophysical parameters, while well data provided information about the vertical distribution of porosity and its relationships with the elastic properties. These data were then quantitatively integrated by applying a geostatistical approach to generate a porosity volume that honors all the available data and to compute a Seismic Facies volume, as additional property related to FFF, according to the estimated relationships between elastic and gross rock frame. Finally, an optimal average velocity volume was built by integrating structural information and inverted velocities, to properly locate the delivered seismic properties in a calibrated depth domain, suitable for reservoir modeling activities. The analysis of seismically derived properties within the Mishrif Formation was carried out by vertically dividing the carbonate sequence in three units and estimating at each level the porosity distribution through an appropriate property mapping. The results highlighted a different lateral distribution for each layer. The upper one shows better porosities in the northwestern field sector, while the middle interval confirms an improvement of reservoir quality moving to the central-southwestern part. The lower interval shows globally poor porosity values along the field. As well as porosity, the seismic facies volume (driven by FFF) was analyzed separately layer by layer and mode maps were extracted. These maps shown that the lateral distribution of the best quality rocks is different among the intervals and change laterally inside of them. Furthermore, they shown that the main facies distribution trends, even though correlated to the predicted porosity, can provide additional information in terms of gross rock frame, where porosity type and mineral distribution represent key elements in assessing reservoir quality and, ultimately, suitable new well locations. The interpretation of property maps using geological keys allowed to improve the robustness of recognized depositional trends, successfully used with the delivered volumes as driver for facies and property population within the generalized reservoir modeling workflow.

This paper provides an overview on the geophysical experience gained in Southern Iraq from seismic acquisition to seismic reservoir characterization to support the development of Mishrif reservoir.