Basement reservoirs potentially exist in many of the world's petroleum basins yet remain largely overlooked and under-explored.
Hurricane’s strategic niche is to look for the right geological conditions in proven petroleum basins for basement reservoirs to work, particularly in areas where previous drilling results have indicated the presence of hydrocarbons in the basement but not progressed the concept further. Our West of Shetland acreage is a classic example, with many of the old wells drilled on our acreage having found indications of oil in the basement.
Once basement opportunities have been identified, our exploration strategy is to capture the prospective acreage and its upside potential on a 100% basis and ideally on relatively low entry cost terms. In the UK, the offshore drilling Licensing Rounds held by DECC have provided us with the ideal route for securing such acreage.
By having control over the exploration phase, we can then implement our exploration work programme which is specifically geared towards the evaluation of basement reservoirs. Hurricane has built an experienced geoscience team who are able to evaluate basement reservoirs in a focussed and innovative way. By integrating a variety of technical information using proprietary techniques, the exploration potential of the basement can be comprehensively evaluated, geological risk minimised, and prospective well locations identified. At this pre-drill stage, the detailed analysis of seismic data combined with 3D geological modelling is of particular importance in determining how fractured a basement reservoir target is likely to be. The degree of fracturing is crucial as it determines how porous and permeable the basement could be and how much hydrocarbon could be stored.
Once drilling for oil is underway, we use a range of specific data-gathering techniques at the wellsite to enable the basement reservoir to be fully evaluated and explored. This includes the advanced analysis of oil and gas ‘shows’ in the drill cuttings being returned to the surface as well as the detailed downhole imaging of the fractures in the basement using state-of-the-art wireline logging tools. As a key factor in evaluating basement reservoirs is determining how well they will flow hydrocarbons to surface, we also prefer to undertake a well testing programme at the end of the drilling programme if hydrocarbons have been discovered.
Establishing which fractures contain oil and could contribute to a commercial discovery is a challenge. However the oil industry has invented specialised wireline tools (see panel on right) for fractured reservoirs and these are used by Hurricane’s geologists to discover where oil bearing fractures are, once the borehole has been drilled.
We have summarised this important process with the image below.
A Borehole with fracture
B Opening up the borehole to see how a fracture appears to a wireline image tool
C Example borehole image of a fracture from the Lancaster discovery
D Wireline tester isolates the fracture and measures the fracture pressure to determine if the fracture is permeable, and to acquire a fluid sample
E Production logging tool measures fluid flow from the fracture during well testing
This image illustrates a fracture, portrayed crossing a drilled borehole (A) The wireline tool perspective of the fracture is from the inside of the borehole and as a 360 ° view (B) An example of the output from the tool used on one fracture that produced oil in Hurricane’s Lancaster discovery is seen in (C) Once a fracture has been identified by our geologists, a wireline tester (D) is used to determine the fracture pressure from which it is possible to determine whether the fracture is permeable. To be absolutely sure a fracture can produce oil a production logging tool is used (E) this device is deployed whilst the well is being tested and is flowing oil to the surface. The production logging tool is capable of establishing which fractures are contributing to the oil flow as it is sensitive to small amounts of liquid flowing into the borehole and can distinguish between oil, gas and water.
All of these wireline measurements help Hurricane's geologists understand whether an exploration well is a discovery.
Of course not all wells result in a discovery, but after each well is drilled all of the relevant technical information is integrated in order to evaluate the potential of the discovery or to understand why the well didn’t work. In the event of success, the 3D geological model of the basement reservoir is updated and the volumes of both in-place and potentially recoverable hydrocarbons are calculated. The results are critical in deciding if a basement discovery is potentially commercial and whether it should be further evaluated through an later appraisal drilling programme.