--> Integration of Lithofacies Description, Fracture Characteristics, Micropaleontology and Wireline Log Analysis to Define the Internal Architecture of Chalk Reservoirs in the Norwegian Sector of the North Sea, by C. D. Caldwell, W. T. Siemers, H. E. Farrell, C. R. Young, J. Yang-Logan, and M. B. Enderlin; #90986 (1994).

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Abstract: Integration of Lithofacies Description, Fracture Characteristics, Micropaleontology and Wireline Log Analysis to Define the Internal Architecture of Chalk Reservoirs in the Norwegian Sector of the North Sea

C. D. Caldwell, W. T. Siemers, H. E. Farrell, C. R. Young, J. Yang-Logan, M. B. Enderlin

Descriptions of lithofacies, fractures, and microfossils have been made on approximately 2,000 meters of core from the Late Cretaceous and Early Paleocene Chalk Group in the Greater Ekofisk area, Norwegian sector, North Sea. Composite wireline log-core plots were generated for a total of fifteen wells from Ekofisk, Eldfisk, Tor and Edda fields, to integrate data from all disciplines.

Ten chalk lithofacies were defined on the basis of bedding characteristics, sedimentary structures, carbonate grain types, and quartz and clay content. They are: Homogeneous Chalk, Burrowed Homogeneous Chalk, Pebble Floatstone, Deformed Chalk, Burrowed Argillaceous Chalk; less common Laminated Chalk, Burrowed Laminated Chalk; and rare Argillaceous Chalk, Calcarenite, and Hardground/Conglomeratic Chalk. These lithofacies record sediment gravity flow and less commonly pelagic sedimentation. Porosity in the chalks is mainly a function of lithofacies and diagenesis, in particular compaction and calcite cementation. The magnitude and character of log curves responding to rock volumes similar in scale to features described in the cores are being used to identify lithofacies on wireline logs These curves are: bulk density, density porosity, shallow resistivity and raw dipmeter.

Four types of natural fractures have been recognized in the chalks: tectonic, stylolite-associated, irregular and healed. The healed fractures are always gouge-filled and do not enhance reservoir permeability. All other fracture types are usually unmineralized within the reservoir and are the primary pathways for produced or injected fluids to and from the wellbore. Fracture intensities are generally highest in the lithofacies that form the best quality reservoir rocks.

Micropaleontological studies consist of identification and interpretation of assemblages of foraminifera, calcareous nannoplankton, ostracodes, radiolarians and miscellaneous biogenic particles. In addition to standard age zonations that have been used for interbasinal correlation, this work has provided high-resolution age zonations for intrabasinal correlations. Paleobathymetry interpretations from the benthic foram assemblages help determine the depositional settings and processes that control the areal distribution of the chalk lithofacies. In addition, it has been possible to use the biostratigraphic data to identify intervals of resedimented chalk. Variation in the nannoplankton assemblage has been shown to vary synchronously with mineralogical and petrophysical parameters such s quartz/silica content, porosity, water saturation and wetability.

Interpretation and integration of these diverse data types are proving critical in establishing the internal architecture or stratigraphy of the North Sea chalk reservoirs and are providing the basis for chalk exploration and exploitation models.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994