ABSTRACT

Successful prior-to-drilling prediction of anomalously good reservoir quality in prospects at deep burial requires an understanding of diagenetic processes and quantitative models on how porosity is related to sandstone composition and to burial history. Quartz cementation and compaction are, in many cases, the most important porosity-reducing processes in quartz-and feldspar-rich arenites, capable of destroying all useful porosity during burial toward 4000 m. Hence, the recognition of factors that may hinder porosity loss by these processes, and thereby preserve good reservoir quality to depths beneath those usually considered as economic basement, is crucial during prospect evaluation of deep structures.

In two deep (>4000 m) oil discoveries in Upper Jurassic sandstones in the Norwegian Central Graben, high porosity (>20%) appears to be preserved due to the presence of a ubiquitous microquartz coating on framework grains, and not due to any burial history-dependent factor such as high pore pressure, low thermal maturity, or early oil emplacement. In these sandstones, the microquartz coating has hindered quartz precipitation and late diagenetic chemical compaction. In interbedded sandstones without

1 Present address: Norsk Hydro Exploration, Stabekk, Norway.

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microquartz coating, the porosity is low (<10%) due to extensive quartz cementation. The microquartz coating appears within specific isochronous layers, and its presence is probably caused by input of amorphous silica (volcanic glass and sponge spicules) during deposition. The recognition of the inhibiting effect of this coating on quartz cementation, combined with quantitative models on the relationship between sandstone composition and diagenetic processes such as compaction and quartz cementation, allows confident porosity predictions. Hence, future porosity prediction in deeply buried Upper Jurassic sandstone in this area should focus on establishing sedimentological models addressing prediction of sandstone facies within intervals deposited during periods with high amorphous silica production and deposition.