Reservoir characterization is one of the most important step in exploration and development phases of any prospect. It combines the results of different analyses to reduce the risk and uncertainties and to enhance understanding of reservoirs. In this study an integrated approach; petrophysical analysis, compaction study and rock physics diagnostics is applied to characterize the reservoirs of the Balder field using log data from nine exploration and sixteen appraisal wells.
The Balder field is located in the Norwegian sector of the North Sea.The field was discovered in 1967. It compromises reservoir sands in three stratigraphic horizons of the Balder, Hermod and Heimdal Formations of Paleocene to Eocene age. The reservoir intervals are of turbidite systems which pinch out against the Utsira High. These reservoir rocks are unconsolidated to poorly cemented intervals of sands and sandstones (a post-rift petroleum play) interbedded with overpressure shales that acting as seal/cap rocks. These reservoir sands contain commercial quantities of hydrocarbons (mostly oil) which are being produced since 1999. The exploration and production gap of more than thirty years is due to the complex stratigraphy and structure of the reservoirs.
Parameters like porosity, shale volume and water saturation are calculated under petrophysical analyses using Interactive Petrophysics software. In addition, facies analysis as well as net-to-gross ratio estimation are performed by the Interactive Petrophysics. However, the compaction study helps to identify mechanical and checmical compaction regimes as well as transition zone between mechanical and chemical compactions. It also helps to understand the compaction behavior of the source, reservoir and cap rocks comparing the field data to model results. Finally the rock physics diagnostics are carried out to characterize the reservoir sands using a combination of different softwares (Excel, Petrel and Interactive Petrophysics). In this part cementation and hydrocarbon effect on the rock properties are carefully investigated.
An average, all the reservoir sand intervals possess a high porosity values (34%). These are moderately to well sorted sands contain a minor amount of clay, with a high net-to-gross ratio (90%). The Balder Formation is mostly mechanically compacted, the high temperature regime, where quartz cementation initiated put it in the transition zone of mechanical and chemical compactions. The Hermod and Heimdal formations are also in transition zone but still possess high porosity values. Rock physics diagnostics reveal that some reservoir intervals may contain a small amount of cement in the pores which, however may not be enough to derive the sediments more stiffen. Rock physics templates illustrate that rock properties in the reservoir intervals are influnced primarily by the depositional processes rather than the depth-related diagenetic trends.
In conclusion, it can be stated that the integration of petrophysical analysis, compaction study and rock physics diagnostics helps to understand and to characterize the reservoirs much better compared to any other single technique. This approach is useful especially when core data are not available. This integrated study give quick results and can help geophysicists, geologists and reservoir engineers to identify the risks and to enhance the opportunity. It can dramatically increase the geological models, reduce risks and improve process efficiency during the development, production and management phases.