Modern rotary drilling usually uses a heavy mud as a lubricant and as a means of producing a confining pressure against the formation face in the borehole, preventing blowouts.
But controlling blowouts has drawbacks—mud filtrate soaks into the formation around the borehole and a mud cake plasters the sides of the hole.
Further complicating the problem is the widespread occurrence of small amounts of petroleum in the rocks of many sedimentary provinces.
Today, a wellsite geologist or mudlogger uses a low powered stereoscopic microscope to determine the lithology of the formation being drilled and to estimate porosity and possible oil staining.
At such a time there is a flood of foreign material knocked from the borehole walls (cavings), making the mudloggers task all the more difficult.
Often the plug breaks while drilling, usually in shales or fractures and the core barrel jams, slowly grinding the rocks in front of it to powder.
The mooring cables ideally pull the hollow bullets and the enclosed plug of formation loose and the gun carries them to the surface.
It might seem that cuttings and cores are very direct samples but the problem is whether the formation at depth will produce oil or gas.
The total gas, chromatograph record, lithological sample, pore pressure, shale density, D-exponent, etc.
The oil and gas industry uses wireline logging to obtain a continuous record of a formation's rock properties.
Wireline logging can be defined as being "The acquisition and analysis of geophysical data performed as a function of well bore depth, together with the provision of related services."
The measurements are made referenced to "TAH" - True Along Hole depth: these and the associated analysis can then be used to infer further properties, such as hydrocarbon saturation and formation pressure, and to make further drilling and production decisions.
Logging tools developed over the years measure the natural gamma ray, electrical, acoustic, stimulated radioactive responses, electromagnetic, nuclear magnetic resonance, pressure and other properties of the rocks and their contained fluids.
The two data sets are then merged using the common time base to create an instrument response versus depth log.
Electric logs have been improved to a high degree of precision and sophistication since that time, but the basic principle has not changed.
Multiple transmitting and receiving coils are used to focus formation current loops both radially (depth of investigation) and axially (vertical resolution).
Until the late 80's, the workhorse of induction logging has been the 6FF40 sonde which is made up of six coils with a nominal spacing of 40 inches (1,000 mm).
The amount of energy loss by Compton scattering is related to the number electrons per unit volume of formation.
Since hydrogen is mostly present in pore fluids (water, hydrocarbons) the count rate can be converted into apparent porosity.
The combination of neutron and density logs takes advantage of the fact that lithology has opposite effects on these two porosity measurements.
Sonic logs use a pinger and microphone arrangement to measure the velocity of sound in the formation from one end of the sonde to the other.
Modern sonic configurations with pingers and microphones at both ends of the log, combined with computer analysis, minimize the averaging somewhat.
A caliper arm on the sonde measures the profile of the borehole and a correction is calculated and incorporated in the porosity reading.
A salinity difference between the drilling mud and the formation water acts as a natural battery and will cause several voltage effects.
The copper surface stake provides a reference point against which the SP voltage is measured for each part of the borehole.
Others have a thin layer of water molecules wetting the surface of the rock, with gas or oil filling the rest of the pore.
As the SP logging tool is drawn up the hole it measures the voltage difference between the reference stake and the mud opposite shale and sandstone or limestone sections.
With experience in an area, a good SP curve can even allow a skilled interpreter to infer sedimentary environments such as deltas, point bars or offshore tidal deposits.
Thus the presence or absence of gamma rays in a borehole is an indication of the amount of shale or clay in the surrounding formation.
The gamma ray log is useful in holes drilled with air or with oil based muds, as these wells have no SP voltage.