Reasons for exploration failures of anticlinal prospects in Yamal and Gydan regions of West-Siberia

Aleksandr A. Kurkin NOVATEK STC Tyumen, Russian Federation
The statistic shows that in the north of West Siberia basin about 60% of all prospects turned out to be water-bearing. Most of them are structural prospects, that were not confirmed by more recent seismic data. To avoid mistakes in the future, and to increase the exploration drilling success rate, one needs to establish the reasons for these failures. For this purpose, the so called dry hole analysis or post well analysis was carried out for all dry wells, drilled in the region in 1970-90s. During the analysis the values of seismic depth prediction errors in comparison with the estimated structural uncertainty were determined. Based on collected data, typical morphological features and seismic amplitude characteristics of water-bearing, false anticlines were revealed.
Materials and methods
The basic data for analysis was: archival seismicsurvey reports, prospects passports, well historical data, R&D, exploration and other reports, on the basis of which well locations were determined. During the analysis the history of well placement and drilling, the anticline morphology and estimated structural uncertainty before drilling were recovered. Structural maps before and after drilling were compared. Seismic velocity derived from modern surveys was analyzed, especially considering near surface effect. The total number of unsuccessful (dry) wells from the analyzed wells was 68, including 61 exploration wells and 37 failed prospects.
As a result of post drilling analysis, the main reasons for exploration failures of anticlinal prospects were determined. Two of the five causes are of methodological nature: drilling the anticlines that were not closed and drilling based on deep horizons structural maps.
The other three matters are the most common ones. They are related to the problems of processing and interpretation of seismic data: an underestimation of near-surface and deep velocity anomalies and errors in the horizon picking or isochrone mapping. These problems determine relatively high structural uncertainty, in comparison with the anticline size. This uncertainty was systematically underestimated before drilling. In archival seismic interpretation reports the estimated accuracy was often too high, and prognostic standard error was too low. Moreover, the geometrical size of prospects was often overestimated, that was the reason for evaluated hydrocarbon resources beingtoo optimistic. This underlines the importance of estimation of all possible componentsof uncertainty, and prediction of the lateral distribution of structural uncertainty.
Typical morphological features and seismic amplitude characteristics of water-bearing, false anticlines were revealed:
absence of seismic anomalies in the interval of Cenomanian and Albian productive formations;
unconformable nature of anticline structural features;
irregular, broken shape of the anticline closure contour;
no change of thickness in the anticline crestalong whole section;
no anticlinal closure within seismic survey area, even when interpreters have strong reasons to believe that the structure is closed in the vicinity of the survey;
almost all of the failed anticlines have the area of less than 80 km2; the majority of anticlines with the height less than 10 m turn out to be false.
The most common methodological errors, pitfalls and problems in seismic data interpretation and processing, which led to exploration drilling failures, were established. The solution lies in the careful consideration of near-surface and deep velocity anomalies during seismic imaging and consistent structural uncertainty assessment.
The above stated, empirically identified features of water-bearing, false anticlines are suggested to use as an additional tool during near-surface effects correction.
These features also are suggested to be considered in geological risks assessment. If several of the stated features apply to the prospect, then a low success probability can be assigned, since most of the dry wells of the region were drilled on such false structures. However, the prospect should be written off only if it is clearly not confirmed by modern seismic data, processed and interpreted considering all velocity anomalies and uncertainties.
Appropriate consideration of identified seismic imaging problems and pitfalls, methodological errors and comprehensive geological risks assessment during exploration well placement will contribute the improvement of drilling success rate of the region.
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dry hole analysis retrospective analysis seismic interpretation time-depth conversion velocity anomaly Yamal Gy