Improved Reservoir Description of the Heidrun Field, Norway, using a Combination of Sedimentological, Geochemical and Pressure Data

Ger W. Van Graas, Wenche Odden, and Knud Egil Svela

The Tilje Formation on the Heidrun Field was deposited in a shallow marine to paralic setting dominated by tidal processes. The resulting, often heterolithic, sedimentary sequences can potentially act as barriers to fluid flow due to lack of lateral continuity of sand bodies, and restricted vertical permeability due to clay drapes and lenticular bedding. Based on the sedimentological model and RFT pressure data, a number of potential barriers to fluid flow were identified. In order to substantiate the existence of these barriers a suite of core samples was collected for geochemical analysis. Using core extracts instead of test oils allows for a denser sampling of the reservoir, providing a more detailed description of any variations in reservoir fluid composition.

Gas chromatograms of the core extracts show the presence of two oil types in Tilje reservoirs: (A) a strongly biodegraded oil, (B) a mixture of degraded and non-degraded oil. Intervals containing different oil types are usually sharply delineated, with no suggestion of a gradual change from one type to the other. Both oil types are also found at different relative positions, i.e. type A is found both above and below type B within one well. These observations can be explained by a dual charge model: (i) a first charge of biodegraded oil filling the entire Tilje reservoir; (ii) a second (or continuous) charge of non-degraded oil, arriving after the reservoir has passed the temperature threshold for biodegradation (approx. 70°C), which was unable to penetrate the entire reservoir. The resulting distribution pattern clearly outlines the presence of barriers to fluid flow.

The reservoir units thus identified agree in many cases with the model based on the sedimentological and pressure data. in a few cases a different depth is indicated for a barrier, while also some additional barriers are indicated by the geochemical data.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California