Abstract In Chapter 2, an historical overview of the development of S-waves and VSP in the 20th century is presented. Early efforts focused on building strong S-wave sources; but after advancements in P-wave AVO and recognition of S-wave splitting, PS-waves began to dominate industry research in the 1990s. Although converted waves were well understood, VSP technology focused primarily on P-wave applications. As experience increased, the scope of applications broadened to include S-wave splitting and imaging through gas in addition to lithology discrimination.

Abstract The Varg Field is located in the Norwegian North Sea, by the UK border c . 240 km west of Stavanger. The Varg Field (Licence PL 038) is unique on the Norwegian shelf because it is the first successful tail-end production by a new and small operator, Pertra AS. Since the acquisition of the licence, Pertra has achieved higher production levels and better reservoir management in the Varg Field due to careful re-evaluation of all available data and close collaboration between different disciplines to improve the understanding of the subsurface. This paper outlines how an improved understanding of the subsurface has been achieved through a range of specialized geological and geophysical techniques, such as: ichnofabric analysis, dipmeter and wellbore image analyses, re-evaluation of the biostratigraphical data, trace inversion, forward seismic modelling and fluid substitution modelling. As part of the Pertra work programme, a new multi azimuth seismic dataset was acquired. After pre-stack depth migrating, this became a new and valuable dataset for reservoir description. The enhanced oil recovery analysis, utilizing all the above re-evaluated data, has yielded positive results in terms of new reserves and increased production levels. From an economics point of view the cost of more geo-manpower is negligible when compared to the cost of the development. This has been the most important lesson learnt.

Abstract The Clair Field is a giant oilfield containing in the region of 6–7 Bbbl of stock tank oil initially in place, located approximately 75 km west of the Shetland Islands. As such, it represents the single biggest hydrocarbon accumulation on the UK Continental Shelf. Clair was discovered in 1977, but first production did not occur from Phase 1 until 2005, after a lengthy appraisal period. The major appraisal milestone occurred in 1991 after well 206/8-8 proved up fractured clastic red beds of the Devonian Lower Clair Group. This was followed up with an extended well test on 206/8-10Z, which demonstrated the longer-term performance of the reservoir. Further appraisal on Clair Ridge led to the sanction of the Clair Ridge, which came on stream in November 2018. Following the Greater Clair appraisal programme in 2013–15, development options are currently being worked for Clair South, which will develop the Lower Clair Group reservoirs together with overlying shallow-marine reservoirs of the Cretaceous and Jurassic.