Abstract

Location of an oil-water contact is crucial for estimating reserves and for designing production strategies. Identifying hydrodynamic barriers or baffles is also important for optimizing both recovery and flow rates. In some oil fields, identifying fluid contacts and barriers is not always possible using conventional wireline and formation testing methods.

We have used a novel technique, fluid inclusion stratigraphy (FIS), to tackle these problems in the Upper Jurassic Magnus oil field in the North Sea. This technique involves systematic cleaning and crushing of small quantities of a large number of drill-cuttings or core samples followed by mass spectrometry without prior GC (gas chromatography) separation. The quantity and specific types of petroleum fluid trapped in pores in mineral grains are determined and the stratigraphic and spatial pattern of organic compounds constructed. For this case study, these data were integrated with quantitative mineralogical data derived from gamma-ray, density, sonic, and neutron porosity logs, together with electrical resistivity.

The conventional resistivity data resulted in ambiguity concerning the oil-water transition zone in both wells; interpreting the resistivity logs was locally hampered by vertical changes in reservoir lithology. The FIS data suggested more complex transition zones (locally variable water saturations in the transition zone controlled by shales and dolomites) and petroleum extending deeper than suggested by the resistivity log (possibly due to lack of sensitivity of the latter to low petroleum saturations in water-wet reservoirs). The FIS data also suggested the presence of potential stratigraphic (shale) and diagenetic (dolomite) barriers or baffles to fluid flow within the reservoir, illustrated by changes in abundance and composition of petroleum-bearing inclusions.

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