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Reservoir characteristics of the Upper Jurassic sequence in the 35/8-2 discovery, Porcupine Basin

Abstract The 35/8-2 well, drilled by Phillips Petroleum in 1981, tested oil and gas from a Late Jurassic reservoir within a plunging, tilted fault block. The reservoir is overpressured by 5000 psi and, despite having good porosity and high net to gross ratio, exhibits very low permeability. Three hydrocarbon-bearing sandstone intervals were encountered in the well, and more than 120 m of core was taken from these reservoir sections. The sedimentological structures identified in core are clearly indicative of a deep-water (below storm wave base) submarine fan system, comprising a wide spectrum of turbidite and debris-flow, channel and lobe deposits. Detailed petrographic analysis of the reservoir was undertaken to identify the key factors affecting reservoir quality. The sandstones are chemically immature lithic arenites, containing abundant quartz, feldspar and rock fragments. Reservoir quality is generally poor, largely as a result of the chemical instability of the sandstones. Diagenetic overprinting is the main agent of downgrading the sandstones, with kaolinites and carbonate cements blocking pore throats and reducing permeability. Facies type, sand to mud ratio and ultimately the clastic provenance of the sandstones are also critical factors affecting the reservoir quality in these sandstones. Hydrocarbon flow was predominantly from a single sandstone facies (B1) and was confined to the ‘A’ sand interval in well 35/8-2, but it is inferred that this facies may occur away from the wellbore in all three sand intervals. Uncertainties in the results of the drillstem test and pressure data demonstrate that it is not possible to confirm whether the hydrocarbons present in the reservoir are gas condensate or volatile oil. Uncertainty in the extent of the productive reservoir facies, hydrocarbon columns and hydrocarbon type are challenges that must be overcome before the reserves contained within the 35/8-2 structure can be developed.