Stratigraphic drilling combines continuous bedrock coring with petrophysical logging and high resolution seismic data (2D) to provide "hard" geological data at a very early stage of exploration.
As the coring is concentrated in areas where dipping bedrock strata a can be reached within a few hundred metres below seabed, the concept has been referred to as "offset drilling".
High resolution multichannel seismics is used to spot the optimal drilling sites, to exactly correlate the cored section to conventional seismic data for further extrapolation into the prospect areas.
The main coring method applies well-proven onshore mineral exploration techniques which are adapted to offshore conditions. The slim-hole diamond coring machine rides in a piggy-back position on top of a heave-compensated outer drill string which acts as a riser in seawater and as a casing in unconsolidated sediment. Coring is carried out inside this drill string, and the cores are retrieved by a wire-line system. Typical production rates are 40-70 m of coring per day.
The boreholes are routinely logged using gamma, sonic, neutron porosity, density, dipmeter and caliper tools. Sonic velocity and spectral gamma ray measurements are carried out on the cores immediately after retrieval to provide information for a first, rough correlation with seismic and wireline log data.
Once on shore, the cores are fully analysed using biostratigraphic, sedimentological, mineralogical/petrographic, petrophysical and petroleum geochemical methods to obtain a comprehensive characterisation of the drilled rock intervals with respect to age, lithology, depositional environment, source- and reservoir rock properties etc. A refined correlation between cores, shallow seismic and deep seismic, together with an understanding of the overall geological setting allows extrapolation into areas only covered by conventional seismic.