This article was first published on Oiledge.
As exploration around the globe gets more difficult, more “unconventional oil” is being brought forward. Of these, the “least unconventional” are a) light crudes that are waxy and b) certain heavy oils.
The ‘line’ these crudes need to cross is that separating a categorisation in a Competent Person’s Report (CPR) as “contingent resources” from the inestimably more valuable status of “reserves”. Despite some CEOs’ apparent wish that this line be crossed simply because they say so, in fact professional reserves auditors – who write CPRs and audit reserves annually for companies – set quite significant hurdles for this line to happen.
Anyone familiar with theis evolution for conventional resources where, for example, 3D seismic can play a much more significant role, will recognise that significantly more dynamic evidence is required for unconventionals – see an excellent review by Chernik.
The first steps for defining the geology are similar whatever the oil quality: understand the regional geology and depositional environment; identify the areal extent of the formation/play; shoot a 3D seismic survey; drill vertical appraisal wells to take core, cutting samples and logs (drill enough wells to have ‘triangulation’ and to identify reservoir transitions within the acreage); take pressure measurements; complete core and cuttings studies to identify geology, mineralogy, geochemical, petrophysical and geomechanical properties, and rock-fluid compatibility; correlate logs to core, and then use all available data to generate geological mapping and to verify the regional geological and sedimentological models.
The resulting ‘static’ geological model integrates - and resolves conflicts between - seismic, a sedimentological model, core-correlated well log information and rock properties. It is then vital to add an assessment of the ‘dynamic’ behaviour of the reservoir. In circumstances where reservoir performance is already well known – for example in a ‘mature’ area such as the North Sea – or can be reliably predicted – for example dealing with light oils in deep water clastic reservoirs, this can be achieved by collecting an adequate number of drill stem tests (DSTs) in which wells are flowed for a relatively short time (24 hours or a day or two). However, DST data is too short term for unconventional oils where questions arise as to the longer term flow of oil - out of the reservoir, and then through the well bore, the sub-sea installations and any…
