Electrical Log
An electrical log is a wireline log of formation resistivity produced by a simple, unfocused arrangement of current-emitting and potential-measuring electrodes that measures the electrical resistance of rock and pore fluids in the formations surrounding the borehole — the earliest and most historically significant logging tool in the history of formation evaluation, with the first electrical log recorded by Henri-Georges Doll on September 5, 1927, in the Pechelbronn field in Alsace, France, under the direction of Conrad and Marcel Schlumberger; conventional electrical logging devices use four electrodes (current-emitting electrode A, current return electrode B, measure electrode M, and measure reference electrode N) in two principal configurations: the normal device (M and N widely spaced, measuring approximately hemispherical current flow with a depth of investigation proportional to the AM spacing) and the lateral device (A between M and N, measuring at a fixed point in a configuration sensitive to invasion and bed thickness); the electrical log was the dominant formation evaluation tool through the 1940s and 1950s and remains important for historical log interpretation because it is present in the archives of most wells drilled during that era, but it has been replaced in new drilling by focused resistivity tools (laterolog, induction log) that correct for the borehole and invasion effects that make the unfocused electrical log difficult to interpret quantitatively.
Key Takeaways
- Normal device configuration and depth of investigation are defined by the AM electrode spacing — the most common configurations were the short normal (16-inch, AM = 16 inches = 40 cm) and long normal (64-inch, AM = 64 inches = 162 cm); in a normal device, the current electrode A and measure electrode M are separated by the nominal spacing while the current return B and reference potential N are placed far above A on the cable; the measured apparent resistivity Ra = K × delta_V / I, where K is a geometric factor proportional to AM spacing (K = 4π × AM for the normal device), delta_V is the potential difference between M and the remote reference N, and I is the current; the short normal's shallow investigation (approximately equal to the AM spacing, about 16 inches into the formation) makes it sensitive to invaded zone resistivity Ri and mudcake effects, while the long normal's deeper investigation (approximately 64 inches, about 1.6 meters) provides a closer approximation to true formation resistivity Rt in moderately invaded formations; beds thinner than the AM spacing are not resolved by the normal device, and the log shows a characteristic pattern of peak anomalies at bed boundaries rather than flat plateaus within thin beds.
- Lateral device configuration measures resistivity at a fixed point determined by the AO distance (from the current electrode A to the midpoint O of the M-N electrode pair, typically 18 feet 8 inches or 5.7 meters in the standard configuration) — unlike the normal device, the lateral device has strong directional sensitivity; when the tool moves upward through the borehole during logging, the current electrode A is above the measure point O, and the resistivity measurement represents the formation approximately 18 feet above the current electrode rather than immediately adjacent to it; this geometric offset creates the asymmetric "blind zone" behavior of the lateral log, where the tool responds to a resistive bed from below (showing suppressed readings as the current electrode approaches the bed) before showing the full resistive signal when A is within the bed and then returning to background as A passes above; the lateral device is particularly sensitive to bed boundaries and formation fluid contacts in thick beds but is nearly uninterpretable in beds thinner than the AO spacing; experienced log analysts recognize the lateral log's characteristic sawtooth pattern at resistive bed tops and bottoms as a diagnostic signature of the unfocused lateral geometry.
- Historical log interpretation of electrical logs in legacy well archives requires understanding the specific electrode spacing and configuration used, because the same formation will produce different apparent resistivity readings with the short normal, long normal, and lateral configurations — without knowing which configuration was run (documented in the log header, if legible), applying a quantitative Archie saturation calculation to a legacy electrical log can introduce factor-of-2 or greater errors in water saturation; the legacy electrical log era (approximately 1930 to 1960) also used low-frequency alternating current (less than 500 Hz) that is more susceptible to electrochemical electrode polarization effects than modern tools, further complicating quantitative interpretation; for legacy formation evaluation in mature basins (WCSB Cardium, US Anadarko Basin, North Sea Rotliegend), experienced log analysts interpret electrical logs qualitatively (identifying hydrocarbon-bearing intervals from elevated resistivity relative to water-bearing zones in the same formation) rather than applying strict Archie equations, and use the long normal as the best available proxy for Rt when deeper focused resistivity is absent.
- Conversion of electrical log apparent resistivity to true formation resistivity Rt requires correction for borehole effects, invasion, and bed thickness using charts specific to the normal or lateral configuration — borehole correction factors depend on borehole diameter and mud resistivity Rm; invasion correction requires knowledge of the invasion diameter (estimated from caliper and knowledge of the mud system overbalance); bed thickness correction accounts for the fact that the unfocused device integrates resistivity over a zone larger than the target bed; the correction charts developed by Schlumberger in the 1950s and 1960s (and reproduced in legacy chart books available in major oil company archives) allow approximate corrections to be applied, but the large number of corrections and their interdependencies mean that the corrected Rt from a legacy electrical log has an uncertainty of typically ±20 to 50 percent for beds thinner than 10 feet in moderate to high resistivity formations; for thick, clean, high-resistivity formations (greater than 50 ohm-m) in low-permeability environments with minimal invasion, the long normal apparent resistivity closely approximates Rt and can be used directly in Archie calculations with acceptable accuracy.
- Self-potential (SP) curve was almost universally recorded alongside the electrical log as the standard companion measurement — the SP curve (measured in millivolts) records the naturally occurring electrochemical potential difference between the borehole mud and the formation pore water, arising from membrane and liquid junction potentials at shale-sand interfaces and at the mudcake-formation contact; in permeable, clean sands saturated with formation water more saline than the mud filtrate, the SP deflects negatively (toward the left on the standard SP track) from the static SP baseline established in bounding shales; the SP deflection is proportional to the ratio of formation water resistivity Rw to mud filtrate resistivity Rmf (SP = -K × log(Rmf/Rw) in simplified form), allowing estimation of Rw from the SP response; the SP curve was the primary porosity-indicator and formation-water-resistivity-estimator before the introduction of neutron, density, and sonic porosity logs, and legacy SP data remains valuable for Rw estimation in basins where modern salinity data is sparse.
Fast Facts
The first electrical log, recorded on September 5, 1927, by H.G. Doll and the Schlumberger brothers Conrad and Marcel at Pechelbronn, Alsace, France, showed 500 meters of resistivity versus depth — a simple paper record that launched the entire wireline logging industry. The Pechelbronn log used a primitive normal device with a hand-cranked winch and required most of a day to acquire what modern tools record in an hour. Schlumberger patented the electrical logging method and began commercial operations in the United States by 1929, logging the first US commercial well in Ventura, California. Within 20 years, electrical logging was standard practice on virtually all exploratory wells worldwide. The Society of Petrophysicists and Well Log Analysts (SPWLA) recognizes September 5 as "Formation Evaluation Day" in honor of the 1927 Pechelbronn log. Today, the original paper log from the Pechelbronn field is preserved by Schlumberger and displayed at its heritage collection, representing the beginning of quantitative subsurface formation evaluation.
What Is an Electrical Log?
Before 1927, the only way to know what kind of rock lay behind the borehole wall was to pull a core — an expensive, time-consuming operation that sampled only a few feet of formation per run. On September 5, 1927, Conrad Schlumberger's team lowered a cable with electrodes attached into a borehole in the Pechelbronn oil field in France, passed an electrical current into the formation, and recorded how the formation's resistance to that current varied with depth. The resulting curve showed, for the first time, a continuous picture of formation resistivity from surface to total depth — an electrical log.
The principle was simple: oil and gas are poor conductors of electricity (high resistivity), while saltwater-saturated rock is a good conductor (low resistivity). A formation that shows elevated resistivity is likely hydrocarbon-bearing. A formation that shows low resistivity is likely wet. In the 70+ years before focused resistivity tools replaced it, the electrical log — with its paired short normal, long normal, and lateral curves plus the self-potential curve — was the primary formation evaluation tool used to identify pay zones in virtually every exploration well drilled in the world. Its legacy lives in the log archives of every major basin.
Electrical Log Technology and Successor Tools
The limitations of the unfocused electrical log drove the development of focused resistivity devices through the 1940s and 1950s. The primary limitation is that in conductive (saltwater) drilling mud, the electrical current from the unfocused electrodes tends to flow preferentially through the low-resistance mud column rather than into the formation, giving artificially low apparent resistivities (the borehole effect). Additionally, the simple unfocused geometry cannot distinguish invasion zones from virgin formation and cannot resolve thin beds. The laterolog (Schlumberger, 1950) solved the borehole problem by using a focusing guard electrode system that forces current into the formation perpendicular to the borehole. The induction log (Schlumberger, 1946) solved the saltwater mud problem entirely by using induced electromagnetic currents to measure formation resistivity without requiring current contact with the mud. By 1960, these focused tools had largely replaced the electrical log for new wells, but the archive of electrical logs from 1927 to 1960 — covering the exploration history of most of the world's producing basins — remains an irreplaceable record of formation resistivity that continues to be interpreted and reinterpreted as basin understanding evolves.
Electrical Log Interpretation Across International Petroleum Basins
Canada (AER / WCSB): AER's well record archives contain electrical logs from thousands of WCSB wells drilled between the 1940s and 1960s when the major Devonian reef fields and Cretaceous heavy oil zones of Alberta and Saskatchewan were discovered; the Leduc and Redwater Devonian reef oil pools discovered in 1947-1948, and the subsequent Pembina Cardium light oil discovery in 1953, were all identified and delineated using electrical logs with their characteristic high-resistivity signatures in porous carbonate reef facies and clean Cretaceous sands; AER's legacy log archive (now digitized through the Alberta Geological Survey) is used by modern explorationists to evaluate bypassed pay in mature fields, identify reservoir heterogeneity missed by early well spacing, and correlate formation tops across the basin using the distinctive SP and resistivity signatures of Mannville, Colorado, and Cardium formation markers that appear consistently on legacy electrical logs from hundreds of wells.