Dating reservoir filling — a case history from the North Sea

Abstract

Secondary petroleum inclusions in reservoir sandstones in the Frøy Field and the Rind Discovery are used in combination with burial history modelling, reservoir oils and core extracts to shed light on how and when these structures received their hydrocarbon charges. Analysis of normal alkane and biomarker distributions in these three data sets: fluid inclusions, core extracts and drill stem tests (DSTs) provide information on the changes in organic facies and maturity of petroleum in the various reservoir strata over time. The geochemistry of core extracts and DST oils in Rind and Frøy reveal that the maturity of the Rind petroleum system is higher and also of slightly less anoxic facies compared to the oil present in the Frøy system. Biodegraded oil is found today in a small sub-compartment of the Frøy Field. Using the burial history of the Frøy and the assumption that biodegradation effectively comes to a halt at 70–80°C, we tentatively conclude that the filling of this sub-compartment and the biodegradation of the oil must have occurred earlier than 30–40 m.y.b.p. and before oil from the Upper Jurassic Draupne shales more recently homogeneously filled the main Frøy structure. At 30–40 m.y.b.p., the Frøy structure was at a depth of 1.5–2 km, compared to the present depth of 3.5–4 km, and it is more than likely that neither the Heather nor the Draupne Formations were mature in the paleo-drainage area of the field at this time. Still, the stratigraphically deeper Dunlin Formation could have been mature and the geochemical signatures of the now biodegraded oil correlates with known signatures from the Dunlin Formation in this region. In the Rind Discovery, no fluorescent petroleum inclusions are observed in K-feldspar overgrowths. However, the number of inclusions in quartz and plagioclase is larger than that observed in the shallower Frøy Field. From the mean homogenization temperatures of the petroleum inclusions, which are close to the present reservoir temperature, filling of the Frøy Field and main Rind structure is suggested to have been a recent event. A presently dry compartment penetrated by well 25/2-15R2 in the Rind Discovery contains high residual oil saturations, indicating loss of oil from this structure either by direct cap rock leakage or displacement of oil by gas and subsequent cap rock leakage. Numerous petroleum and coexisting water inclusions were found in the presently dry overpressured structure and it is clear from measurements of homogenization temperatures that the water and the petroleum inclusions are not both saturated with respect to gas. Due to the lower solubility of hydrocarbon gas in water compared to in oil, it is more likely that water inclusions are closer to saturation than are the petroleum inclusions. Using the difference in homogenization temperatures in these two systems and the burial history, we tentatively suggest that the cap rock leakage occurred later than 10 m.y.b.p. Such information on time for reservoir filling and seal failure, when used to calibrate basin modelling, should facilitate better models for understanding movement of hydrocarbon fluids in basins.