Scaling Geologic Reservoir Description to Engineering Needs
Authors: Slatt Roger M. , Hopkins Graham L. 
Published: December 2007

Three types of reservoir description models have been developed for Balmoralfield, North Sea, for different applications. A depositional model provides ageological basis for constructing a layer model and for exploration for similardeposits. The layer model provides a framework for calculating reservoirvolumetrics. A flow-unit model integrates geological and petrophysicalproperties, thus providing the most properties, thus providing the mostcomprehensive description for simulation and reservoir management.

Introduction

As a result of the increased interaction between geologists and engineers inresolving reservoir problems, geologists have become more aware of the need tovary the scale and style of geologic description according to particularengineering applications. particular engineering applications. Conventionalgeological descriptions normally are too detailed for reservoir engineeringsimulations and often are not in an appropriate format for relating toreservoir performance. A geological reservoir description of Balmoral field, North Sea, is presented here as an example of different scales and styles ofreservoir description for different applications. This study had threeobjectives:to develop a depositional model that would provide the basisfor a layer model and for provide the basis for a layer model and for anexploration model for similar tertiary deposits in the North Sea,todevelop a geologically based layer model to provide the framework forcalculating reservoir volumetrics, andto develop a flow-unit model thatcombines geologic and petrophysical properties to describe the reservoirpetrophysical properties to describe the reservoir more comprehensively forreservoir management purposes.

Balmoral field, discovered by Sun Oil Co. in 1975, is located in the U.K. North Sea Block 16/21 a-b, 140 miles [225 km] off the northeast coast ofScotland (Fig. 1). Production from the field began in 1987 from a Productionfrom the field began in 1987 from a single floating production vessel. Thefield produces from Paleocene sands at an produces from Paleocene sands at anaverage subsea depth of 6,970 ft [2124 m] in water depths averaging 470 ft [143m]. The field is a 6 × 2-mile [9.7 × 3.2-km], northwest-trending, low-reliefanticline cut by a number of faults, trending predominantly northwest, with asecondary set trending northeast to east (Fig. 2). The approximate oil/watercontact (OWC) is at 7,050 ft [2149 m] subsea.

Geologic Setting

The Balmoral field is located on the southern tip of the Fladen Ground Spur, a southerly extension of the East Shetland Platform, North Sea. The FladenGround Platform, North Sea. The Fladen Ground Spur was a positive featurethroughout the basins of the Witch Ground graben in the west from the SouthViking graben in the east.

Thick Mesozoic sediments present in the flanking grabens are absent on theFladen Ground Spur. It was not until the Late Cretaceous Age that the FladenGround Spur began to subside with the surrounding areas, although it stillremained a relatively positive feature. From the end of the Paleocene positivefeature. From the end of the Paleocene Age to the present, the area of theBalmoral field subsided along with the rest of the North Sea and was filledwith clastics from the bordering massifs. During the Paleocene Age, theShetland Platform to the northwest was uplifted and tilted eastward, givingrise to an easterly directed drainage pattern. Consequently, Late Paleocene andyounger clastic sediments in the area are composed of deltaic, shelf, andsubmarine fan deposits that are thought to have been derived from the Scottishlandmass to the west and northwest.

Depositional Model

Lithologies and Vertical Stratification Sequences. Of the 25 wells in thefield, 19 were cored. These cores were examined for sedimentologiccharacteristics, described below.

Almost all the reservoir interval is composed of fine- to medium-grained, moderately to poorly sorted sand. Individual sand beds are typically massiveand unconsolidated except for occasional zones cemented with calcite. Dishstructures and vertical fluid-escape pipes are sometimes preserved within thecalcite-cemented zones. Many beds are probably amalgamated with sharp, erosivebases; however, these are usually not visible owing to the unconsolidatednature of the sand and, quite often, to heavy oil-staining.

The calcite-cemented -zones are up to a few feet thick in cores. These zonesappear on sonic logs as high-velocity zones or spikes; thus their presence canbe determined in uncored portions of wells, allowing correlation between wells(Fig. 3). Many of the zones can be correlated across several wells, while otherzones occur within a particular stratigraphic interval in only one particularstratigraphic interval in only one well.

JPT

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