EAS 430: Petroleum Geology Lab 2: Interpreting Geophysical Well Logs

Historical Aspect Schlumberger brothers, Conrad and Marcel, are credited with inventing electrical well-logs. On September 5, 1927, the first “well-log” was created in a small village named Pechelbroon in France. In 1931, the first SP (spontaneous potential) log was recorded. Discovered when the galvanometer began “wiggling” even though no current was being applied. The SP effect was produced naturally by the borehole mud at the boundaries of permeable beds. By simultaneously recording SP and resistivity, loggers could distinguish between permeable oil-bearing beds and impermeable nonproducing beds.

Types of Logs a) Gamma Ray b) SP (spontaneous potential) c) Resistivity (Induction) d) Sonic e) Density/Neutron f) Caliper

a) Gamma Ray The gamma ray measures the natural radioactivity of the rocks, and does not measure any hydrocarbon or water present within the rocks. Shales: radioactive potassium is a common component, and because of their cation exchange capacity, uranium and thorium are often absorbed as well. Therefore, very often shales will display high gamma ray responses, while sandstones and limestone will typically show lower responses.

The scale for GR is in API (American Petroleum Institute) and runs from 0-125 units. There are often 10 divisions in a GR log, so each division represents 12.5 units. Typical distinction between between a sandstone/limestone and shale occurs between 50-60 units. Often, very clean sandstones or carbonates will display values within the 20 units range.

b) SP (Spontaneous Potential) The SP log records the electric potential between an electrode pulled up a hole and a reference electrode at the surface. This potenital exists because of the electrochemical differences between the waters within the formation and the drilling mud. The potenital is measured in millivolts on a relative scale only since the absolute value depends on the properties of the drilling mud.

In shaly sections, the maximum SP response to the right can be used to define a “shale line”. Deflections of the SP log from this line indicates zones of permeable lithologies with interstitial fluids containing salinities differing from the drilling fluid. SP logs are good indicators of lithology where sandstones are permeable and water saturated. However, if the lithologies are filled with fresh water, the SP can become suppressed or even reversed. Also, they are poor in areas where the permeabilities are very low, sandstones are tighly cemented or the interval is completely bitumen saturated (ie- oil sands).

c) Resistivity (Induction) Resistivity logs record the resistance of interstitial fluids to the flow of an electric current, either transmitted directly to the rock through an electrode, or magnetically induced deeper into the formation from the hole. Therefore, the measure the ability of rocks to conduct electrical currents and are scaled in units of ohm-meters. On most modern logs, there will be three curves, each measuring the resistance of section to the flow of electricity.

Porous formations filled with salt water (which is very common) have very low resistivities (often only ranging from 1-10 ohms-meter). Formations that contain oil/gas generally have much higher resisitivities (often ranging from 10-500 ohms-meter). With regards to the three lines, the one we are most interested in is the one marked “deep”. This is because this curve looks into the formation at a depth of six meters (or greater), thereby representing the portion of the formation most unlikely undisturbed by the drilling process. One must be careful of “extremely” high values, as they will often represent zones of either anhydrite or other non-porous intervals.

d) Sonic Sonic logs (or acoustic) measure the porosity of the rock. Hence, they measure the travel time of an elastic wave through a formation (measured in ∆T- microseconds per meter). Intervals containing greater pore space will result in greater travel time and vice versa for non-porous sections. Must be used in combination with other logs, particularly gamma rays and resistivity, thereby allowing one to better understand the reservoir petrophysics.

e) Density/Neutron Density logs measure the bulk electron density of the formation, and is measured in kilograms per cubic meter (gm/cm3 or kg/m3). Thus, the density tool emits gamma radiation which is scattered back to a detector in amounts proportional to the electron density of the formation. The higher the gamma ray reflected, the greater the porosity of the rock. Electron density is directly related to the density of the formation (except in evaporates) and amount of density of interstitial fluids. Helpful in distinguishing lithologies, especially between dolomite (2.85 kg/m3) and limestone (2.71 kg/m3).

Neutron Logs measure the amounts of hydrogen present in the water atoms of a rock, and can be used to measure porosity. This is done by bombarding the the formation with neutrons, and determing how many become “captured” by the hydrogen nuclei. Because shales have high amounts of water, the neutron log will read quite high porosities- thus it must be used in conjunction with GR logs. However, porosities recorded in shale-free sections are a reasonable estimate of the pore spaces that could produce water.

It is very common to see both neutron and density logs recorded on the same section, and are often shown as an overlay on a common scale (calibrated for either sandstones or limestone’s). This overlay allows for better opportunity of distinguishing lithologies and making better estimates of the true porosity. * When natural gas is present, there becomes a big spread (or crossing) of the two logs, known as the “gas effect”.

Example of dolomite overlying limestone, as distinguished by the neutron/density.

f) Caliper Caliper Logs record the diameter of the hole. It is very useful in relaying information about the quality of the hole and hence reliability of the other logs. An example includes a large hole where dissolution, caving or falling of the rock wall occurred, leading to errors in other log responses. Most caliper logs are run with GR logs and typically will remain constant throughout.