Petroleum geochemistry of the Frøy field and Rind discovery, Norwegian Continental Shelf. Implications for reservoir characterization, compartmentalization and basin scale hydrocarbon migration patterns in the region

Abstract

Petroleum geochemistry provides an excellent tool for understanding reservoir characteristics with respect to oil flow and tracing out elements of basin scale migrational patterns and reservoir compartmentalization. Organic geochemical characterization has been carried out on 580 core extracts and 9 oils from 11 wells of the Frøy field and Rind discovery in the South Viking Graben. Two different drainage areas and source rock systems have been identified for the hydrocarbons in the Frøy field and the Rind discovery on the basis of differences in maturity and gas to oil ratio in addition to facies differences like % C 28-αα-sterane content and the ratio of bisnorhopane to bisnorhopane plus norhopane. Results show that the Rind discovery contains oil of higher maturity compared to the oil from Frøy field. The petroleum in the Rind discovery has a more terrestrial character and is interpreted to have been derived from source rocks other than the Draupne Formation in its typical anoxic distal development. A likely candidate is the Heather Formation. In addition, a small contribution of immature oil has been identified in a separate small sub-compartment in the Frøy field. The petroleum in this sub-compartment shows a high pristane to phytane ratio and a very prominent odd-to-even predominance. Well 25-14, drilled northeast of the Frøy field, penetrates a water-bearing, hydrostatic structure. The detailed geochemical characterization including petroleum inclusion studies have shown that this structure never received oil or gas, nor did it have any communication with the Frøy field. Hence it is inferred that this structure never communicated with the hydrocarbon producing basin. One of the two compartments in the Rind discovery is hydrostatic and contains petroleum of a comparatively large maturity span whilst residual core bitumen in a high-pressured neighboring compartment has a very small maturity span and contains presently only water as producible phase. The ratios of the 20S to 20S + 20R isomers of C 29-αα-steranes, the ratio of C 20 to C 20 + C 28 triaromatic steroids and the C 28 triaromatic steroid to C 28 triaromatic steroid + C 29 monoaromatic steroid ratio in the latter compartment have almost uniform values indicating a single oil pulse, i.e. a short filling history, whereas in the other compartment, these ratios vary over a large range indicating a longer filling history. As both compartments contain petroleum with the same source rock facies signatures, it is suggested that these at some time were part of the same migration system. Communication between these compartments is inferred to have been disrupted before seal failure occurred in the structure which was penetrated by well 25-15R2. The analyses carried out indicate a high degree of chemical homogeneity in the petroleum of the Frøy field reservoir which, in comparison with the Rind discovery, is more similar to extracts from typical anoxic developed distal shales of the Upper Jurassic source rock in the Viking Graben, i.e. the Kimmeridge equivalent Draupne Formation. However, the upper reservoir units in the northern well 25/5-A7 contain extracts with lower maturity and more terrestrial characteristics compared with the extracts in the deeper reservoir sections. In addition, well 25/5-1, which penetrates the highest point of the Frøy structure, contains petroleum of slightly higher maturity than found elsewhere in the Frøy field. The results further suggest that the faults in the Frøy field including the main E-W trending fault separating the northern and the southern parts of the of the Frøy field did not adversely affect lateral communication during filling. To conclude, there is no petroleum geochemical evidence to suggest that intra-reservoir faults have caused significant compartmentalization of the Frøy field except for the N-S trending fault which separates a small block to the west from the Frøy field main reservoir. Thus with respect to petroleum production, it is concluded that primary sedimentological features are more important in terms of exerting a first order control for determining the vertical and lateral flow of hydrocarbons in this reservoir than faults.