Trash to Treasure (unearthed)

From Trash to Treasure (unearthed)

Sifting through old data with new tools and techniques to unlock the next generation of high value discoveries in the UKCS

Ronnie Parr, BP Aberdeen

Fifty years of exploration in the UK sector of North Sea Basin has delivered over 44bbnoe of recoverable resource from hundreds of oil and gas fields. It has also resulted in thousands of wells that are still providing clues to identify additional potential and future discoveries in the form of overlooked and underappreciated hydrocarbon shows and occurrences. Most of these wells were drilled in the 1970’s and 1980s before the advent of modern 3D seismic. This provides the experienced explorer a wealth of opportunities to re-assess wells through utilization of modern geoscience analysis integrated with surgical mapping of high quality 3D seismic.

BP has successfully utilised this approach to discover a number of relatively small but highly valuable fields located in subtle trapping configurations around the margins of well-established reservoir fairways across the UK sector of the North Sea. In many of these areas, the initial phase of was focused on deep targets in the Jurassic and Triassic. Numerous shows and thin pay zones were encountered in Paleocene and Cretaceous levels but were largely written off following limited assessment. Acquisition of modern vintage, high quality 3D seismic prompted a re-assessment across a number of previously explored areas with emphasis on understanding the shallow hydrocarbon indicators identified on composite logs and end of well reports. The methodology employed was to carefully apply high resolution stratigraphical, geochemical and geophysical interpretation to create a high quality, integrated description of the remaining prospectivity.

The Kinnoull, Vorlich and Capercaillie Fields are all examples of successful discoveries that were made utilizing this approach. All were originally ‘encountered’ in older wells but were either overlooked or underappreciated for a variety of reasons which include poor quality data and reporting in ‘overburden’ sections, thin or poor-quality reservoirs at the well location, and lack of integration between subsurface datasets.

Vorlich and Capercaillie are subtle oil and gas discoveries that rely on stratigraphic trap edges located along the distal margins of the Sele deep water sand fairway. Regionally, the Sele reservoir is characterized by a complex stratigraphy with significant variability in sand distribution and thickness. The thin pay zone that corresponds to what was later identified as the Vorlich prospect was not recognized at the time of drilling the 30/1C-3 well in 1985. It remained in obscurity for over 20 years until a scan of the composite log revealed a thin sand with an intriguing resistivity response. The oil pay was substantiated through careful re-assessment of the original field prints of the openhole logs and geochemical extract analysis of the bagged cuttings. Subsequent interpretation of newly acquired 3D seismic revealed a corresponding amplitude anomaly that was drilled in 2014 and resulted in the discovery of the Vorlich Field. In contrast, Capercaillie was originally defined as separate oil and gas discoveries encountered in wells drilled to appraise a deeper HPHT gas field. Both wells were tested and pressure data suggested they were likely in communication but the small ‘discoveries’ remained undeveloped until recent re-interpretation of the well data and modern 3D seismic. This work provided support for the connection of the two discoveries as one accumulation consisting of a gas cap and oil rim. This was confirmed with the successful appraisal of the Capercaillie Sele oil rim in 2017.

Kinnoull is north of Vorlich and Capercaillie and is a subtle low relief 4-way trap along the distal margins of the Lista deepwater sand fairway. The 16/23-1 well, drilled in 1972, was the first exploration well in the area. The well encountered thin sands with gas shows and minor oil pay, but was abandoned as a failure. A further 3 wells were drilled for deeper targets around the Kinnoull structure between 1977 and 1990 before careful interrogation of the offset wells and interpretation of modern vintage 3D led to the discovery of the Kinnoull oil field which has been on production since 2014.

The approach is not full proof and there have been failures along the way. The South Farragon prospect, located to the south of Kinnoull, is one such example. The trap and reservoir framework are very similar to Kinnoull and the presence of hydrocarbons was supported by an older exploration well drilled along the southern edge of the prospect area. The prospect was drilled in 2016 and the well failed to find mobile hydrocarbons. It has not been possible to assign a definitive failure mode, though there is significant indirect evidence that the likely cause is top seal breach associate with a sub-seismic sand injectite.

New approaches to old data in a mature hydrocarbon province has provided BP with a number of recent discoveries that have generated significant value and helped extend vital infrastructure in the basin. This requires attention to detail, deep integration of subsurface datasets and a healthy dose of curiosity but with countless unidentified and underappreciated hydrocarbon shows remaining, this approach will continue to deliver high value discoveries for decades to come.