Ed's note: This is a 2017 updated version of the article originally published in our magazine in late 2016.
Basement reservoirs are recognized throughout the world as being oil-bearing and commercially productive, yet remain largely overlooked. Hurricane Energy’s Robert Trice believes these rocks represent a huge, underdeveloped play for the UK and that the imaginative use of the plentiful available data could unlock the country’s basement resource potential.
“I first became interested in the potential of basement as a reservoir when I was working for Enterprise Oil in the 1990s,” explains Dr. Robert Trice, CEO of Hurricane Energy, a company dedicated to identifying and exploiting such reservoirs. “We were drilling a well in Vietnam, investigating a typical clastic reservoir, when we decided to deepen the well to evaluate the basement – and it was oil-bearing. This sparked my interest and I began researching the subject of fractured basement reservoirs. The Vietnam story was fascinating, as in recent years the country’s reserves have doubled, all due to fractured basement fields. The original discoveries in Vietnam were made by accident, and it took several years before the play was taken seriously.
“Could the same be true for the UKCS, I wondered? I believe it could. There have been several serendipitous North Sea basement discoveries, including Cairngorm and Emerald and, of course, the West of Shetland giant field, Clair. However, the majority of companies stopped drilling as soon as basement was encountered. I did some research and decided there was considerable potential for exploration in fractured basement in the UKCS.”
With a belief in this untapped and promising resource, Robert and a small staff of experts set up Hurricane in 2005. “Our aim was to seek out the right geological conditions in proven petroleum basins that will allow basement reservoirs to work, looking particularly for areas where previous drilling results indicated the presence of hydrocarbons in the basement but which were never followed up,” he explains. “The relatively unexplored but prospective UKCS West of Shetland area fits this description perfectly.
“The West of Shetland Basin has proven its petroleum potential, with the Clair, Foinaven and Schiehallion oil fields already in production. It also contains an estimated 17% of the UK’s remaining hydrocarbon reserves. At Hurricane, we believe that the area remains underexplored for oil, and that new exploration concepts are required to unlock its remaining potential,” he continues. “The Lewisian Basement, which is between 2.3 and 3.2 billion years old and composed predominantly of fractured, crystalline gneiss, forms extensive ridges and localized highs. These are charged by the prolific Kimmeridge Clay – the same rock which sourced much of the oil in the North Sea.
“Over the past eleven years we have tested this hypothesis and have outlined a number of prospects, drilled seven wells, and, most importantly, validated our model for basement exploration. We have made two discoveries in the West of Shetland area and identified three further promising prospects for drilling, and as a result have 450 MMb of 2C contingent oil resources on our acreage, and we believe there is plenty more out there.”
How Does the Oil Get There?
By definition, basement rocks cannot overlie younger sedimentary source rocks, and so hydrocarbons will not have migrated directly upwards into them, as occurs in a classic petroleum system. However, in areas where basement has been uplifted so that it is adjacent to a deep source, a migration pathway can exist either through the basement itself or through a carrier bed that connects the source rock to the basement. An alternative migration route could be from a source rock that is draped over the basement high, allowing hydrocarbons to be expelled directly into the basement underneath.
“In the West of Shetland Basin, the basement plays we are chasing are all in granitic rocks. The fracture networks develop from joint systems that exist as a result of the initial cooling of the granite. Subsequent tectonic events, which in the case of Lancaster include vertical uplift by as much as a kilometer, enhance the fracture network through the formation of ridges and domes,” Robert explains. “This movement importantly occurred during a key phase of hydrocarbon migration, meaning that as the fractures were opening, hydrocarbons were being emplaced.
“One of the great attractions of fractured basement reservoirs is that they can be considered to be stratigraphic traps, in that oil can be found outside of structural closure, which can lead to extensive flank oil accumulations. This has been demonstrated in basement discoveries in Yemen, and the potential also exists for the same to occur on Hurricane’s Typhoon Prospect.
Exploring Basement Reservoirs
Although basement rocks are complex, the process and workflow involved in exploring for hydrocarbons in them is not greatly different from that applied to reservoirs with intergranular porosity, Robert believes. “It is important to gather and analyze a wide range of data, in particular to assess if an effective fracture system is present,” he says. “By integrating a variety of technical information using off the shelf technology and proprietary techniques, the exploration potential of the basement can be comprehensively evaluated, geological risk minimized, and prospective well locations identified.”
The most important recent technological development to assist in exploring basement reservoirs is probably the advent of 3D seismic data, which has allowed for effective imaging of fault networks. 3D seismic, combined with horizontal drilling and advances in both wireline and LWD imaging technology, has enabled fracture sweet spots to be targeted and effectively quantified by exploration and development wells.
“At the pre-drill stage, the detailed analysis of seismic data combined with 3D geological modeling is of particular importance in determining how fractured a basement reservoir target is likely to be,” Robert continues. “The use of data obtained by previous licence holders can be crucial, especially 3D seismic, well data and cores, and we are fortunate to have a lot of that vital information for our assets, with many of the old wells drilled on our acreage having found indications of oil in the basement. Cores are useful as they provide definitive evidence of hydrocarbon presence as well as an insight into the basement lithology.
“Once drilling for oil is underway, we use a range of specific data-gathering techniques at the wellsite to enable the reservoir to be fully evaluated and explored. This includes the analysis of oil and gas through advanced gas chromatography, as well as high resolution imaging acquired through the process of logging while drilling. Post drilling, we have utilized a range of wireline techniques including borehole image logs, sidewall cores and formation pressure and fluid sampling. A key factor in evaluating basement reservoirs is the determination of how well the hydrodynamic fracture network is connected, so we undertake well testing at the end of the drilling program. Obtaining production logging tool information and sufficient test data to carry out pressure transient analysis is key in detailing the effectiveness of the fracture network.”
Prospects West of Shetland
The 1977 Clair Field discovery wells were the first to identify significant quantities of oil in fractured basement in the West of Shetland area, but subsequent tests were disappointing. The field is on production, mainly from the overlying Devonian, although there may be connectivity to the basement beneath. Clair lies on the Rona Ridge about 50 to 75 km north-east of and on-trend with Hurricane’s assets. Basement oil at Clair together with oil shows elsewhere along the Rona Ridge provided plenty of evidence of oil presence and migration into the basement for Hurricane to investigate further
“The first prospect we drilled, in 2009, was Lancaster, where we had identified a four-way dip structural closure creating a conventional trapping mechanism,” says Robert. “We have now drilled five wells on the prospect and they have all identified oil within the structural closure. We also have oil well below the structural closure; our 2016 inclined well demonstrated that the Lancaster structure has an oil column height of some 670m. Our initial assessment of the 2016 well results suggest that the Lancaster Field reserves are likely to be significantly greater than 200 MMbo, our published 2C numbers. The second stage of our 2016 operations involved drilling a horizontal well, designed as a second future production well and also to provide additional information to help optimally plan field development. Early test results from this well have produced 14,500 bopd, which gives the company a great deal of encouragement.
“Our second basement discovery was Whirlwind, a large structure approximately 2,000m deeper than Lancaster, although charged from the same source. Initially drilled in 2010, the well flowed light oil/gas-condensate from an open hole test conducted over Lower Cretaceous limestones and underlying fractured basement, although it was not possible to determine precisely which intervals were flowing hydrocarbons. 2C contingent resources of 205 MMboe (oil case) or 179 MMboe (gas-condensate case) have been assigned to Whirlwind.”
Hurricane made its third basement discovery in 2016 with its first basement exploration well on Lincoln (well 205/26b-A). Gas chromatography and logging while drilling data indicate a very significant hydrocarbon column of at least 660 metres TVD, comparable to that found at Lancaster. The discovery also supports the presence of a sealing fault between Lancaster and Lincoln which provides an important insight into how oil is trapped within the Rona Ridge structure. From initial analysis of the well results Hurricane have indicated that the pre-well predicted Lincoln resource estimate of 250 million barrels of recoverable oil may be conservative.
Just Use Imagination
“We have another two promising prospects to drill on the Rona Ridge - Warwick and Halifax, both geologically similar to Lancaster - plus a number of other exciting prospects to follow up,” he continues. “Halifax was spudded on 15th January, so for the moment, working on those and getting Lancaster into production will keep us busy. However, there are interesting areas to investigate further south on the Rockall Ridge, which traverses both UKCS and Irish waters, and in the North Sea - and plenty more potentially prospective basement regions to consider beyond the UKCS.
“Basement plays offer massive potential throughout the world. They are considered by some to be more difficult and expensive to evaluate than clastic reservoirs, but I don’t agree. All that is restricting their exploration and exploitation is the procurement of good quality data - and imagination!” Robert concludes.