AAPG Geoscience Technology Workshop

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Unconventional Offshore Petroleum-extracting oil from active source rocks of the Kimmeridge Clay Formation of the North Sea


Unconventional resources have become increasingly important in their contribution to the global energy base; exploration is spreading rapidly to onshore areas of North America, Europe, South America, Asia and Africa. Based on a review of public domain data and recent conferences, and publications, there appears to be no reported offshore unconventional exploration activity anywhere in the world. It is likely that unconventional offshore resources have largely been ignored even in the United States, due to abundant onshore production and offshore economics are controlled by drilling costs which are considerably higher than onshore. The Kimmeridge Clay Formation is the primary source rock for conventional oil and gas in the deep-water trough of the South Viking Graben area of the North Sea, and a potential unconventional shale oil reservoir. In the Middle Jurassic, rapid subsidence from structural extension produced an isolated deep water basin; this deep restricted basin accumulated organic-rich mudstone and fine-grained sand interbeds. This alternating lithofacies is interpreted to have been deposited by low-density turbidity currents on the outer submarine fan, inter-fan and in the basin plain environments. Burial history modelling calibrated against measured parameters such as vitrinite reflectance, Rock-Eval Tmax, Rock-Eval S1 and solvent extraction yields suggest a burial depth deeper than 3200 m below seabed for maturity and generation from the typical Type II oil-prone kerogen. North Sea Kimmeridge Clay Formation with interbedded mudstones and sandstones contains a significant amount of expelled oil in the sands, as well as retained oil in the muds. Migration of free oil (S1) from the mudstone into the interbedded sandstone involves a short lateral distance through the sandstone pores from the source to storage in the interbeds implying that the sand-rich samples have retained rather than drained oil. Distal and inter-fan areas allow development of a hybrid system (frack production from both mudstones and sandstones), and selection of optimum sand-shale ratio for unconventional oil exploitation. High silica content in the interbeds infers brittleness, a key factor in creating vertical fracture pattern that are large enough to connect the maximum amount of rock volume during hydraulic fracturing stimulation. Some of the advantages of exploring for unconventional offshore resources include: larger basin centres; the availability of existing under-utilised drilling platforms and pipelines, drilling deviated wells from onshore, existing seismic surveys and well data to define extent and thickness, database of geology, geochemistry and mineralogy from conventional drilling, limited environmental constraints compared to onshore, unlimited water supply to use in hydraulic fracturing and lack of significant political issues balance positively against the added costs of exploitation.