The Importance of Combining Geological and Petrophysical Knowledge for Carbonate Reservoir Characterization

Gregor P. Eberli¹ and Jose Luis Masaferro^2
1 Comparative Sedimentology Laboratory, University of Miami, 4600 Rickenbacker Causeway, Miami, FL 33149
² Shell International E&P, Technology Applications and Research, Carbonate Development Team, Volmerlaan 8, 2280 AB Rijswijk, The Netherlands

The petrophysical characteristics of carbonate reservoirs are the combined result of depositional fabric and the post-depositional diagenetic processes. Thus, an understanding of both processes and their influence on rock properties is paramount for building robust reservoir models. Each reservoir, however, is unique and requires extraction of the specific sedimentary, diagenetic and stratigraphic information from subsurface data by integrating of core, log, seismic data as well as results from focused laboratory experiments. 

The smallest building blocks for reservoirs are sedimentary cycles that are partitioned by sedimentary processes into transgressive and regressive hemicycles with potentially different facies, composition and diagenetic alteration. As these cycles stack with time, the changing interference of high- and low-frequency sea-level changes, subsidence and sediment supply, continuously changes the partitioning in each cycle and bundles them in cycle sets and sequences. These ever-changing characteristics can best be captured by high-resolution sequence stratigraphy that allows assessing the lateral and vertical facies and diagenetic variations. It also allows up-scaling of the strata in a consistent matter. The integration of laboratory measurements of petrophysical properties correlates the sedimentologic/stratigraphic information to log and seismic data. These laboratory data also help to assess the causes and amount of petrophysical variability in a specific reservoir. 
Geometries are paramount to delineate facies distribution and internal sequence architecture that, integrated with core, wire line log suites and 3-D acoustic impedance cubes, are used to build robust reservoir models. Recent advances in seismic acquisition, processing and volume visualization techniques provide the opportunity to image the carbonate reservoir architecture with unprecedented resolution. In particular, the increased acquisition of 3-D seismic data and the emergence of filtering techniques has contributed to these advances and resulted in imaging the sedimentary bodies in their entirety. In addition, the analysis of seismic attributes is a developing methodology to quantify the volumes and rock properties of these bodies. Consequently, subsurface strata that had been interpreted as having a horizontal layering based on core and log data need to be re-evaluated when high-quality seismic data becomes available. For example, calculated volume dip and azimuth from the seismic cubes to detect previously unnoticed subtle stratigraphic features such as low-angle progradation units and shoal-type mound seismic facies in the Permian Khuff and Upper Cretaceous Natih E reservoirs in Oman.