Oil & Gas - Maximising Recovery Programme (formerly SHARP) IOR Views

Exploration

Since 1972, 147 exploration wells have been drilled along the UK Atlantic Margin with an overall success rate for the whole of the UK Atlantic Margin of about 1 in 6. Only 33% of the exploration wells were located on what could be termed a “reliable, robust structure”. Moreover, for wells located on more reliable, robust structures a higher success rate of about 1 in 2 had been achieved.

27 different operators have tried to unlock the exploration potential and despite the hoard of chancers and dreamers, keen to add to their company’s fortune, exploration success has been generally disappointing.

Key Reasons for Failure

Approximately 63% of the exploration wells were positioned on 2D seismic data, which was often very poor quality. A large number of wells were generally based on weak and somewhat flawed geologic concepts. A number of the dry holes can be attributed to simultaneous or back-to-back exploration drilling, testing similar play concepts over a short time period. The inability to quickly trade well data has also contributed to a large number of dry holes. Not surprisingly, the key explanation for approximately 67% of failed exploration wells relates to the fundamental lack of a genuine trap.

Figure 1: Time Line of Exploration and Appraisal Wells Along the UK Atlantic Margin

Exploration on the UK Atlantic Margin began in 1972 (Figure 1) and took twenty years and over 100 exploration and appraisal wells before the first commercially viable discovery was made – the BP operated Foinaven field. A year later, the Schiehallion field was discovered, subsequently other nearby accumulations were made including the Loyal and Suilven fields. Apart from the Foinaven area the Clair Field, discovered back in 1977 is the only other current commercially viable development west of the Shetlands.

Summary of all Failed Wells Along the UK Atlantic Margin

Mainly as a result of large number of mergers and relinquishments, there are currently just ten remaining companies operating along the Atlantic Margin. Many companies who have been involved along the UK Atlantic Margin have developed a more pessimistic view of future prospectivity as a consequence of the number of dry holes. However, very few if any have been in the position to evaluate all the exploration wells and therefore unfold a story that demonstrates that reality shows that a large number of exploration wells have not tested valid traps.

There is a view that limited source rock distribution west of Shetlands is the main cause for the lack of exploration success, but much of this is not backed by solid evidence. Much of the published material is over simplified and quite misleading particularly to geoscientists and new companies who have never worked along the UK Atlantic Margin but may be thinking of future involvement in the area.

The UK Atlantic Margin is part of a passive continental margin that formed as a result of multiphase extension and comprises a large area of rifted basins associated with North Atlantic rifting during the Mesozoic and Palaeocene. However, nobody has managed to convincingly estimate the hydrocarbons generated and expelled in the area prior to the rifting phase.

Figure 2: Approximate Percentage Cause of Failure for all the Unsuccessful Exploration Wells Along The UK Atlantic Margin

What are the Key Elements That Have Attributed to the Number of Dry Holes

Analysis was carried out on trap, reservoir, seal and source migration to determine the reasons for failure. Furthermore, all wells that were also based on an AVO anomaly were also analysed. Many of the wells failed on a combination of geologic reasons but for this particular analysis if the trap constituted more than 50% towards the well failing to find hydrocarbons then traps have been assigned as the key element for failure.

Figure 2 shows an approximation of overall percentage cause of failure for the 115 unsuccessful exploration wells along the UK Atlantic Margin. The largest percentage (67%) failed as a result of a poorly defined trap. A further 21% of the wells failed as a result of thin or absent reservoir. Approximately 7% failed due to the seal being either thin or absent. Not surprisingly, only around 7% specifically failed as a result of source rock. The majority of failed wells related to source were located in areas where the source kitchen is quite shallow and therefore generation of hydrocarbons is unlikely.

The Problem of AVO

Since the acquisition of 3D seismic data approximately 39 wells have been positioned with respect to an AVO anomaly (see Figure 3) of which 9 of the wells encountered hydrocarbons. The majority of the failures have been positioned on poorly defined Palaeocene stratigraphic wedges many of which had associated amplitude features, strange AVO artefacts and spurious DHIs (including multiples). Not surprisingly all the wells that encountered hydrocarbons were associated with structural closures (including combination traps).

Figure 3: The Application of AVO on Exploration Drilled on Amplitude Anomalies Since 1993

Although source rocks have only been encountered in a few of the wells west of Shetland, identifying and extensively mapping the source rocks remains problematic. However, based on analysis of 115 failed exploration wells, source is not the key “geologic” problem, particularly for west of Shetlands. The key failure for approximately 67% of the exploration wells has been the lack of a viable trap, therefore, to increase future success, it is important to identify and explore for valid and robust traps.

Conclusions

Following the discovery of the Foinaven and Schiehallion fields in the early 1990s, exploration success outside Quadrant 204 acreage of the Faroe-Shetland Basin has been rather limited. It has become clear that the Foinaven area is in a unique geologic position in terms of hydrocarbon generation, migration and trapping.

Two-thirds of all failed exploration wells failed because of a lack of a “reliable, robust trap”. Where wells were located on robust structures a more than 1 in 2 success rate has been achieved.

Compared to other parts of the world the overall success rate on the UK Atlantic Margin has been quite poor, particularly in finding commercial hydrocarbons and even in recent years exploration has been generally disappointing. The UK Atlantic Margin can still be a major hydrocarbon province but there are lessons to be learnt from the activity so far. The challenge ahead lies in enhancing the possibilities of finding further hydrocarbons in commercial quantities. Undoubtedly there is capacity to improve the exploration success rate on the UK Atlantic Margin by carefully evaluating and integrating all the relevant data to unravel the true geology.