Principle of shallow
stratigraphic drilling
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. |
