SUBSURFACE EXPLORATION METHODS

The wildcat well is defined as the first well to be drilled in a geographic region. The
drilling of the wildcat well is the beginning of the final stages of exploration. This is the
first opportunity to actually bring back to the surface for analysis samples of the
subsurface rocks and fluids. It is important to obtain as much information as possible
relative to subsurface conditions including rock properties, fluid properties and any other
significant data, which might be obtained.
There are many potential sources of important information from a wildcat well. Some of
these information sources may provide data not otherwise available from other sources, or
confirm data obtained from one or more of the potential sources of information. These
include rock cuttings, reservoir fluid samples, mud logs, cores, well logs and Drill Stem
Tests.
Rock Cuttings
During the drilling operation, rock removed from the subsurface formations by the drill
bit, are being returned to the surface on a continuous basis. These samples are analysed
in order to describe the subsurface geology and for indications of hydrocarbon presence
within the cuttings. A cuttings analysis with well depth is used to complete a stratigraphic
column as a summary of subsurface geology.
Reservoir Fluid Samples
Reservoir fluid samples are collected from any reservoir rocks that are of potential
interest. Various collection techniques are available, such as the pressure bomb. These
fluid samples are sent to the laboratory for a P-V-T analysis. This provides important
reservoir fluid data such as chemical composition, fluid formation volume factors, bubble
point pressure, solution gas oil ratio, viscosity and density.
Mud Logs
The drilling fluid, pumped through the inside of the drill string and exiting the drill bit
while drilling, carries rock samples back to the surface in the drilling mud. When drilling
into a rock formation containing hydrocarbons, traces of reservoir fluids encountered will
be returned to the surface in the drilling mud. Surface samples of the mud are collected
and analysed for hydrocarbon presence. This is known as a mud log. The mid log
contains description of the rock type based on inspection under a microscope, plot of
penetration rate, gas composition based on gas chromatography, oil cut based on washing
the cuttings in toluene and ultraviolet fluorescence to determine presence of oil.
Cores
When a formation of interest is encountered while drilling, one of the most important
sources of downhole information is the core of the reservoir rock. A typical core is a rock
cylinder, normally 4” to 6” in diameter, of the reservoir rock retrieved from the wellbore
to the surface in a core barrel.
The core is sent to the laboratory for analysis. Potential information obtained includes
rock type, rock characteristics, source of the sediments, depositional environments,
porosity, permeability, radioactive properties and estimates of fluid saturations in the
rock. Sidewall cores, which are less than 1” in diameter and less than 3” in length can be
taken instead of the full hole cores which can be 30 – 60 feet in length.