Interpreting the Wireline Logs

In most exploration holes, the total depth drilled is to reach the primary target formation for oil indicated by exploration methods. In Small County, all exploration wells are drilled to "basement", that is the Precambrian crystalline rocks that underlie the Phanerozoic sedimentary section. The tool-pusher, drilling crew, and well-site geologist all recognize when the drill-bit reaches basement when the drilling rate slows markedly and chips of granite are brought up in the circulating mud column. The arrival at total depth is already anticipated from the prognosis prediction and the revised formation tops picked by the geologist.

A logging service company would have been contacted to be ready to log the hole at this time, using logging tools and measurements decided by the oil company as useful data to evaluate the production potential of the borehole. A logging truck arrives at the well site and the logging crew works to assemble and calibrate the logging tools. These can range from 40 to 100 feet in length. When the drill crew has tripped out the drill string from the borehole, the logging crew lowers a logging tool to the bottom of the hole suspended on a cable or "wireline". The records of the properties measured by this tool, plotted versus depth, are often referred to as "wireline logs", although other terms, such as simply "well logs", are also used.

The logging process starts at the bottom of the hole as the tool is pulled up the hole. A variety of electrical and nuclear measurements are made and the data used to calculate porosity volumes and hydrocarbon saturations in potential reservoir formations. The logs are also used by the geologist to revise the picks of the tops of formations, if necessary.

In Small County wells, the logging program selected is a combination of natural gamma-ray measurement, formation density, and formation resistivity.


The Gamma-Ray Log

The gamma-ray log records natural radiation from sources of potassium-40, uranium and thorium isotopes and their radioactive daughter isotopes. These radioactive sources are more abundant in shales than other rocks in sedimentary successions and so are a good "shale log" that differentiates shales from potential hydrocarbon reservoirs in sandstones, limestones and dolomites. Most of the shales in Small County have gamma-ray values ranging from 120 to 130 API units, easily distinguishable from the gamma-ray values of reservoir rocks, which range from about 5 to 20 API units. The Francisco and Possum Trot Shales are notably more radioactive, with gamma-ray values around 160 API units, making them distinctive "marker" horizons in the stratigraphic sequence.


The Density Log

The density log measures the bulk density of formations penetrated in units of grams per cubic centimeter (cc). In a prospective reservoir zone, the density log can be used to estimate the volume of pores in the rock, by rescaling the density from zero porosity, corresponding to the density of the reservoir rock mineral, to 100% porosity at a density of 1 gram/cc for a fresh-water drilling mud. Assuming a fluid density of of 1 g/cc, the bulk density (rhob) is given by:

rhob = porosity + (1-porosity)*rhom

where rhom represents the density of the rock matrix. If rhom is known, then porosity can be estimated from a density log reading (rhob) using:

porosity = (rhom - rhob) / (rhom - 1)

So, a decrease in the density log reading indicates an increase in porosity if the lithology remains the same. For reference, the expected matrix densities (rhom) for the porous lithologies encountered in Small County are:

and the expected bulk densities (rhob) for the (effectively) non-porous lithologies are:


The Resistivity Log

Even if the reservoir rock has a high and favorable porosity, it must still be determined if the pore space is filled with water or partially filled with hydrocarbons. This determination is made from the resistivity log, because pores filled with salty formation water will have much lower resistivities than those with hydrocarbons which have exceedingly high resistivities. The calculation of the amount of oil saturation is made using the Archie equation, which combines porosity and resistivity log readings to estimate water saturation. The water saturation is the fraction of the pore space filled with water which, when subtracted from unity, gives the fraction of nonconductive hydrocarbon. The results of the log analysis may reveal potential hydrocarbon zones that went unnoticed during drilling.

For the porous lithologies in Small County (limestone, dolomite, and calcite), the water saturation (Sw) can be estimated from readings on the resistivity log (Rt) using a slightly simplified form of the Archie equation:

Sw = sqrt( Rw / (por^m * Rt ) )

where Rw is the formation water resistivity at depth and por^m means porosity to the power m. m is the cementation exponent, which is 2.0 for the limestones and dolomites and 1.8 for the sandstones. Formation water resistivities need to be corrected for the difference between the temperature at which they are measured (at land surface, perhaps in a lab) and the temperature at formation depth. For the potentially productive intervals in Small County, the measured formation water resistivities (Rwm) are:

These are all measured at Tm = 75 degrees Fahrenheit. The water resistivity (Rw) at formation depth is estimated from:

Rw = Rwm*(Tm + 6.77) / (T + 6.77) = Rwm*81.77 / (T + 6.77)

where T is the temperature at formation depth, in degrees Fahrenheit. The average annual surface temperature in Small County is 55 degrees and the average geothermal gradient is 1.2 degrees per hundred feet, so T can be estimated as:

T = 55 + 1.2*depth/100

For the (effectively) non-porous lithologies, the expected bulk resistivities are:

These values will simply help in the recognition of these lithologies on the logs. You would generally not be interested in computing saturations in these units.