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More than 30 years has passed since quantitative and probabilistic geological risk analysis was introduced to the oil and gas exploration industries, which is a method composed of two processes 1 to get Chance of Success CoS by multiplying grouped geological factors trap, reservoir, source rock and so on that are essential for hydrocarbon accumulation in the prospect and 2 to get probability density function PDF of resources by running Monte Carlo simulation.However, it is dif cult for some prospects to apply above steps simply due to their geological complexities, which include multi-reservoir, multi-segment and multi-scenario.Multi-reservoir prospects have more than one target reservoir. In this case, CoS and PDF for resources should be calculated for each reservoir at first, then combined CoS probability of at least one reservoir has hydrocarbon accumulation and combined PDF weighted summation of PDFs for all cases for single or multiple reservoir discoveries with relative probabilities . Dependencies of geological risk factors and resources calculation parameters between target reservoirs have to be considered.Multi-segment prospects are separated to more than one segment by faults or other geological obstructions, where combined CoS and PDF for resources can be calculated like as multi-reservoir prospects.In multi-scenario prospects, there are more than one aspect scenario for any of geological factors required for hydrocarbon accumulation. For example, some 4-way dipping anticlinal traps becomes fault-dependent 3-way dipping traps when hydrocarbon-water contacts are deeper than speci ed depth. In such prospects, probability that the fault seal works can give relative probabilities for both scenarios and separated PDFs for both scenarios can be combined by using those relative probabilities.Reasonable geological risk and uncertainty models can be constructed for most of prospects in oil and gas exploration businesses with careful combination of above three types of geological complexities.

Norway is one of the major oil and gas producing countries in Europe. Main production is done from the North Sea in lower latitude with the contribution from the Norwegian Sea and Barents Sea in higher latitude, and almost same undiscovered resources is expected in these areas. Petroleum potential in Norway is guaranteed by worldclass source rocks in Upper Jurassic, and therefore the nature and the habit of the petroleum system induced by these source rocks are discussed in this paper. In the Northern North Sea, several oil families were identi ed by biomarker composition, especially bisnorhopane and diasteranes. Each oil family is distributed in different areas related to specific hydrocarbon kitchens. Multidimensional basin modeling revealed that the contributions from 3 source rocks (Draupne, Heather and Brent coal) differs in each kitchen, which appears to result in above nature of oil families. The different contribution is caused by the relationship of 3 source rocks to regional carrier system in each kitchen. The established Halten and Dønna Terrace in the Norwegian Sea appear to have similar petroleum systems as the Northern North Sea. However, in the Vøring Basin, Jurassic source rocks deeply buried, and therefore the migration of oil and gas to the Cretaceous sandstones is more complex, which is the key for future exploration in the central part of the Norwegian Sea. In the Barents Sea, the Upper Jurassic source rock matured only in the Hammerfest Basin. Detailed biomarker analysis revealed the existence of extended tricyclic hopanes in some oils suggesting the supply from the Triassic source rock. Diamondoid analysis also suggested that marine carbonate petroleum system is working in the Barents Sea, which may be originated from the Permian source rock. The distribution of these source rocks is the key for future exploration in the Barents Sea.

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In the early 1970 s, oil and gas exploration was started in the west of Shetlands, United Kingdom, which is a part of the Atlantic Margin. One of the main exploration targets in the west of Shetlands is the Paleocene deepwater sandstones, some discoveries of which proceeded to development or production stages. The Laggan field discovered in 1986 is under development together with a neighboring discovery, Tormore field. The Foinaven field has been producing oil since 1997. These Paleocene fields are characterised by seismic attributes including amplitude and AVO anomalies, and understanding of these attributes enables reduced geological risks and estimation of reserves, not only in exploration but also in development and production stages. On the other hand, there are many failures of exploration wells in areas with seismic anomalies. Loizou 2005 examined all the exploration wells in the west of Shetlands drilled until 2005, and concluded that only 9 out of 39 wells, which have targeted Paleocene seismic anomalies, resulted in geological success. To improve the success-to-failure ratio in exploration associated with seismic anomalies, it is important to construct a plausible geological model including trapping mechanism, seal, reser voir, source rock and hydrocarbon migration that is consistent with hydrocarbon accumulations implied by seismic anomalies. The Tornado discovery was a prospect, which targeted the Paleocene deepmarine sandstone associated with a seismic amplitude anomaly. The rst exploration well con rmed a gas column with oil leg. Prior to the drilling, seismic data analysis was carried out together with a CSEM data acquisition. Those geophysical information was integrated with a geological model built on the information from nearby wells and other geological studies to reduce geological risks Pickering et al., 2011. This paper presents successful examples of integrated methods of exploration activities which do not rely only on seismic anomalies.