Electric Logs (E-logs)
The E-log suite as it is usually referred to normally consists of a Spontaneous Potential (SP), 16″ Normal (short normal), 64″ Normal (Long normal), and Single Point Resistance (SPR). These electric logs can only be collected in a fluid filled borehole (mud or freshwater). In addition, you may come across an 8″ Normal, 80″ Normal and a 72″ or greater lateral log. Southwest Exploration uses 3 different tools manufactured by Gearhart Industries, Mount Sopris Instrument Co and Geovista.
The SP compnonent of electronic logs responds to changes in the fluids chemistry in the borehole which changes the self potential of the formation. These changes can be caused by differing electrochemical components of the borehole fluids and the formational fluids, physical effects such as fluid flowing through the well or drilling mud effects on the fluid. Redox reactions can also effect variations in the measurements as sulfides oxidize. The SP is a qualitative measurement.
Resistivity measurements in electronic logs are quantitative. The normal Resistivity measurement is a method of measuring the physical properties of a formation. The longer the separation, 8″ to 80″ the further out into the formation you will read. The 64″ Normal reads deeper into the formation where as the 16″ Normal shallower, and the 8″ Normal even shallower. Changes in the Resistivity measurements can be attributed to or controlled by the following; borehole or formation fluids, mineralogy, rock fabric, grain size or shape and porosity of the formation. Some basic associations can be made for resistivity. High resistivity is anticipated in low porosity non-mineralized or altered volcanics and sandstones. Lower resistivity may be indicative of more conductive fluids or increased porosity, increased conductive mineralization or higher clay content.
The larger 80″ Normal and the 6Æ or greater Lateral logs are used when the client or operator want to get a good handle on the formational resistivity and get out an away from the borehole effects on the tools. This can be very important and aid in larger bed boundries/structures.
The Single Point Resistance (SPR) measurement in electronic logs is usually a 3″ or 4″ electrode which is a qualitative measurement and usually additive within the borehole. It responds to changes in the resistance of the formation and usually gives a higher vertical resolution than the normals.