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Reservoir characterisation (Formation Evaluation)

Representation of petrophysical rock parameters in a bore hole (well log)

After a well has been drilled,  the data collected needs to be interpreted in order to identify any hydrocarbons that may have been encountered. This evaluation is done using the following methods:

Well logging

Petrophysical rock parameters (resistivity, self-potential, natural radioactivity, density, sonic-velocity, bedding dips, temperature, formation pressure, etc.) are recorded and interpreted to gain information on depth, lithology, thickness, orientation and porosity of the reservoir. They may also characterise any fluid types (Gas, Oil, Brine) and the pore space saturation.

Drilling core with core plugs taken to determine petrophysical properties. Sandstone reservoir: Oil or gas can be stored within tiny pore spaces (coloured in blue) of the sandstone

Core analysis

Drilling cores provide the most valuable geological information from a well.  They allow the direct observation of the subsurface rocks. Macroscopic rock properties such as sedimentary structures, grain size and types of bedding surfaces describe the genesis and facies properties of the sediments. Rock composition can be determined and quantified using  thin sections. The mineral composition of grain particles in the (sub)microscopic scale can be obtained by x-ray diffraction methods. Thus the shape, type and generation of the rock´s pore spaces and diagenesis history can be inferred. Petrophysical properties (porosity, permeability, oil and water saturation) can also be determined from core material.

Representation of micro fossils found in drill cores or cuttings, visualized by a scanning electron microscope


Biostratigraphy (fossils) can determine the age and depositional environment of sediments. If present, fossil information from cuttings or drill cores that can be used to determine rock formation ages, depositional environments, burial history and to support well correlation.

Open Hole Drillstem Test in the uncased bore hole

Well testing

If the petrophysical interpretation indicates hydrocarbon bearing intervals, they need to be confirmed by well tests. A successful test will also indicate size and production capacity of a specific reservoir. An Open Hole Drillstem Test is performed when reservoir fluids are produced into the uncased bore hole and through the drill pipe. When the well is flow tested through perforations in the casing, it is called Cased Hole Drillstem Test.