Architecture, Facies Distribution and Reservoir Properties of the Mishrif Formation (Zubair Field, Southern Iraq): An Example of Integration of Seismic Interpretation with Well Data

摘要

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.