ABSTRACT

Much of the remaining prospectivity in the Ula trend (Norwegian Central Graben) is deep (>3.5 km). A major risk to successful petroleum exploration in the trend is reservoir effectiveness. A few oil discoveries are not yet commercial because they occur in low-permeability sandstone. No simple porosity-depth relationship exists for the whole of the Ula trend. As such, mapping of economic basement is difficult. There are, however, simple porosity vs. depth relationships within the two main producing fields: Ula and Gyda.

The porosity-depth relationships in the fields are due to downflank cementation by quartz. Quartz cementation was synchronous with oil emplacement, and evidence from petroleum-filled fluid inclusions has led to the conclusion that cementing fluids and petroleum competed in a "race for space." The Ula trend displays evidence of all three outcomes of such a race: petroleum emplacement ahead of cementation, synchronous processes, and cementation ahead of petroleum emplacement.

Porosity prediction for undrilled prospects and prospect segments was made by risking the three possible outcomes of such a race for space. The reservoir in prospect 7/12-JU4 was predicted to be oil bearing and have a mean porosity of about 16.4%: a function of synchronous petroleum emplacement and cementation. The well, however, was dry. It had a mean porosity of 14%; this compares well with the predicted porosity (13.9%) at the well location for a system in which cementation was completed before oil emplacement (equivalent to a porosity estimate for a dry hole).

1Present address: Monument Oil and Gas plc, London, United Kingdom.

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