Published by the DTI Oil & Gas Directorate for the reservoir engineering and IOR community in the UK.
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A Post-Well Analysis of Recent Years Exploration Drilling in the Atlantic Margin

Nick Loizou (Nick.Loizou@dti.gsi.gov.uk), of the DTI's Oil and Gas Licensing, Exploration and Development Branch (OGLED) presents a personal analysis of results relating to the drilling of the more recent exploration wells west of Britain.  Nick has been with the DTI for over four years working as a Senior Geoscientist in exploration. Prior to this, he has gained extensive experience and expertise with a number of major oil companies. He has a comprehensive geologic and seismic interpretation background mainly in exploration covering many of the provinces in the UK Continental Shelf, Atlantic Margin, Offshore West Africa, Offshore Germany, Offshore Peru, Saudi Arabia, Kuwait and Algeria.

Analysis of both successful wells and failures can contribute to the understanding of petroleum systems. The identification of key geologic reasons for success or failure (e.g. trap, reservoir or charge) provides insight into which factors are essential for trapping ‘commercial’ quantities of hydrocarbons. The description ‘success’ is defined here as a hydrocarbon accumulation that if tested would flow to surface. It does not necessarily indicate the commercial potential of the discovery.

Prior to these wells, the exploration wells drilled in this area had approximately a 1 in 7 success rate. Of the subsequent wells reviewed here, a 1 in 5 success rate was achieved. None of the discoveries were tested but companies have indicated that these wells would probably have flowed at significant rates (greater than 1000 BOPD or 15 mmcfpd). This analysis shows that the success rate can be significantly improved by targeting ‘reliable, robust, structural closures’.

The Foinaven discovery in 1992 fundamentally shifted the focus of exploration toward the Palaeocene. However, apart from the nearby discoveries to Foinaven, for example Schiehallion and Suilven, there has been a period of generally low success, which has not yielded the expected results. Firstly a general lack of success in extending the Palaeocene play away from the BP/Shell discoveries in the Foinaven Sub-Basin. Secondly in extending the Laggan/Torridon gas trend, and thirdly for developing new plays in other areas.
The evaluation of the more recent wells has revealed some intriguing results that generally reflect a need for better understanding of play concepts, trapping mechanisms, charge history and source distribution. Even today this lack of knowledge is not helped by the inability to quickly trade well and seismic data, which contributes to a reduced understanding of the area, and ultimately adds to a much lower success rate.

The main exploration focus for many of the wells under review has been the Palaeocene due to the proximity of Foinaven, Schiehallion and the Laggan/Torridon gas area. Many of the Palaeocene targeted exploration wells in this more recent period failed to find commercial hydrocarbons. Some of these wells tested deeper Mesozoic prospectivity but unfortunately were not successful. 
Of the recent exploration wells drilled west of Britain, most were located west of Shetlands and a few in the greater Rockall Trough area. Unlike the North Sea in much of the licensed acreage particularly in the Flett Sub-Basin, the Tertiary is generally formed of monoclinal sequences that dip to the west with little structuring. As a result four-way dip closures and combination structural/stratigraphic are limited and therefore, more risky stratigraphic traps form the main objectives.

In an area like the West of Shetlands and Rockall, the identification and successful mapping of potential Tertiary plays requires high quality, high-resolution 3D seismic data with true amplitude preservation correctly tied to nearby well control. Many of the prospects/leads were originally mapped on a loose grid of 2D seismic data from various vintages. Superior interpretation using 3D seismic data and additional well control, improved understanding of the geology, which in some areas condemned early play ideas and prospects. 

Figure 1: The three main play types west of Britain and the overall percentage success and failure for the recent wells

Figure 2 shows how effective the trap was post well analysis. Approximately 71% of the wells lacked a sound trap configuration with a further 6% in which the trap is considered marginal. Overall, only 23% of the wells were located on a good effective trap. A good trap can be defined as a robust structural closure or a combination structural/stratigraphic feature that can be mapped with high confidence utilising good quality seismic and other key data.

A small number of the recent exploration wells were drilled on ‘prominent ridges’, testing Mesozoic prospectivity. Similar ‘crestal wells’ have been drilled located on ridge type features, which have generally not been successful in finding commercial hydrocarbons. Other examples include several wells along the Rona Ridge and wells drilled on the Westray Ridge.

Figure 2: Trap effectiveness for the recent exploration wells

Figure 3 shows the post well analysis for success and failure for wells drilled on AVO features. Most of the wells positioned on Tertiary stratigraphic ‘prospects’ at the pre-drill stage had a favourable AVO response (hydrocarbons present) but results were generally disappointing. The only wells where AVO successfully worked are associated with robust structural closures. These represent about 19% of all the wells drilled and intriguingly had a 100% AVO success rate.

On the whole there appears to be considerable difficulty in separating amplitudes relating to lithologies with amplitudes directly associated with hydrocarbons. In areas like offshore Angola, AVO in general works effectively in predicting hydrocarbons, for instance in the Oligo-Miocene turbidite sands. For the west of Britain, in terms of equivalent burial depth, the closest comparable sequence would be the Eocene and to a lesser extent the shallower Palaeocene sands.

Figure 3: The application of AVO on recent exploration drilled on amplitude anomalies west of Shetlands and Rockall Trough

Figure 4 shows the post well analysis of reservoir quality for all recent wells studied. Another outcome relating to the recent wells drilled was that for 45% of the wells the main objective encountered poor reservoir quality or the reservoir was absent or extremely thin. A further 10% found what could be termed moderately poor quality reservoir.

In general, the Palaeocene reservoirs show an overall reduction in porosity with burial depth. However, some wells have shown porosity preservation at depth, and this porosity preservation or enhancement is best developed in sandstones from the Upper Vaila Formation. Although showing similar composition to the older sandstones, the Vaila sands are much better sorted.

Reservoir properties of the Palaeocene sandstones in the Flett Sub-basin vary widely, with porosities ranging from >25% to below 10% and permeabilities from 2 D to 0.1 mD. Although there is a severe reduction of reservoir quality with depth of burial, certain sandstone units have retained high porosities (>20%) and permeabilities (10 - 100 mD) at below 3 kilometres burial depth.

Figure 4: Quality of the reservoir in recent wells

The preservation of reservoir quality is due in part to depositional facies distribution, but mineralogical/diagenetic variability has also allowed the preservation of porosity and permeability. In particular, the presence of grain rimming chlorite in these sandstones appears to be fundamental in preserving porosity.

Regionally, there is a series of transition (transfer) zones, one of which is located between the Laggan and adjacent Torridon area. The difference in reservoir quality and distribution could possibly be linked to the presence of transfer zones. In the Torridon area several wells have poor quality sequence Vaila sands (Figure 7), which are devoid of chlorite.

Figure 5 shows the results of the seal analysis for the recent wells. Of these wells 55% had what can be described as an effective seal while in the remaining 45% the seal was either poorly developed, absent, or relatively thin. Many of the wells relied heavily on a stratigraphic trapping component; with seal integrity a major risk. Inadequate regional analysis and mapping of the seal contributed to the failure of several of the wells.

Figure 5: Seal analysis for the recent wells

The presence of a regional T35-T36 seal (Figure 7) and a favourable sandstone-shale ratio are significant positive ingredients for a successful Palaeocene play. However, trap definition is difficult, as most of the remaining structures rely on a stratigraphic trapping component and top reservoir definition on seismic, which is not straightforward. Higher quality and better-imaged 3D seismic data would be an essential tool in the hunt for these subtle traps.

Figure 6 shows the results of the charge element. Approximately 54% of the wells drilled had not encountered a trace of hydrocarbons. Part of this percentage relates to the trap being poorly defined but in many cases the source rock distribution and pre-Tertiary burial history was either poorly understood or very speculative. For many of the stratigraphic features that were drilled there was confidence that the AVO anomaly identified represented hydrocarbons. Very few AVO related stratigraphic features could be tied realistically with confidence to a mapped nearby source origin. Instead, in some cases sourcing and migration was simply based on a rather hypothetical nearby source kitchen.

One of the key issues to exploration success is the charting of vertical and lateral migration of hydrocarbons through time. This can only be achieved by detailed integrated geological and geophysical analysis.

Figure 7: Palaeocene Stratigraphy west of Shetlands (click image for larger view)

What can be learned from the drilling of recent exploration wells?

• Generally a success rate of 1 in 5 would be considered relatively poor. Of the failures or dry holes the most significant aspect to failure was the lack of understanding of what constitutes a robust and valid trap. Most of the failed wells analysed were essentially due to the overall trap being ineffective.
• The Palaeocene stratigraphic plays drilled west of Britain have failed largely because the trap has been invalid. The hydrocarbon discoveries to date appear to be complex combination traps, which, in most cases, have related deeper structuring. This is the case for Foinaven, Schiehallion, Suilven, Laggan and Torridon.
• Many of the high amplitude stratigraphic features that have been drilled within the Palaeocene are chiefly related to changes in lithology and not hydrocarbons. There is scope to acquire more ‘state-of-the-art’ 3D seismic with the aim of maximising the frequency content, improving the signal-to-noise ratio, suppressing multiples and hence improving the structural and stratigraphic imaging.
  In certain circumstances there has been poor integration of existing key well information. The inability to quickly trade well data has contributed to a number of dry holes.
• The mapping and identification of a good seal tied to solid well data is critical. If the seal becomes too thin then the trapping of significant hydrocarbons will be ineffective.
• Mesozoic ridges are not by any means true Jurassic/Triassic tilted fault blocks, which generally form better trapping configurations, contain better reservoir qualities and are usually effectively sealed by Lower Cretaceous or Jurassic shales. Potential ‘flank plays’, within the Jurassic, may exist down-dip, away from the crest of Corona and Westray Ridge.
• The quality of the Palaeocene reservoirs is unpredictable particularly the T31-T35 sands of the Flett Basin. Good porosity and permeability can be preserved at depths greater than 3000 metres below seabed, but the main challenge will be to find a method for prediction.

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 rather unique geologic position in terms of hydrocarbon generation, migration and trapping.

Many companies that have carried out exploration west of Britain perhaps have visualised that most of all the wells historically drilled were located on ‘valid prospects’, which is reflected in the overall poor success rate of 1 in 7. Interestingly, this analysis shows that from the recent wells drilled, less than 50% were located on what could be termed ‘reliable, robust structures’. However, by taking the wells that were located on the more reliable, robust structures a higher success rate of more than 1 in 2 is calculated.

West of Britain 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 west of Britain by carefully evaluating and integrating all the relevant data (including wells that have failed) to unravel the true geology. Moreover, there is scope in many areas, including areas with existing 3D seismic data, to improve the structural and stratigraphic imaging. In certain areas of the North Sea very modern ‘state-of-the-art’ acquisition technology has been shown to give a superior 3D seismic dataset to better define prospectivity. Furthermore, there is a need for improved methods to correctly analyse seismic amplitudes.

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