Proximity Log
The proximity log is a shallow-reading focused resistivity tool that uses a concentric guard electrode arrangement to minimize mudcake and borehole influence and measure the resistivity of the flushed zone (Rxo) immediately adjacent to the borehole wall, providing a shallow investigation measurement used alongside deep and medium induction or laterolog resistivity to calculate the Rxo/Rt ratio for moveable hydrocarbon identification and to quantify invasion diameter.
Key Takeaways
- The proximity log belongs to the microresistivity tool family, which includes the microlog, microlaterolog (MLL), proximity log, and microspherically focused log (MSFL), each with a different depth of investigation from about 1 inch to 10 inches into the formation.
- The proximity log has a depth of investigation of approximately 4 to 8 inches, deeper than the microlog (1 to 2 inches) but shallower than a standard laterolog, making it suitable for reading Rxo in deeply invaded formations where the microlaterolog would still be influenced by the mudcake.
- Electrode focusing in the proximity log uses guard electrodes above and below the central measurement electrode to force current horizontally into the formation and prevent current from following the low-resistance mudcake or borehole fluid, isolating the formation signal.
- The Rxo/Rt ratio (shallow flushed zone resistivity divided by deep true formation resistivity) is a moveable hydrocarbon index: if Rxo/Rt is much greater than 1, hydrocarbons have been flushed from the near-wellbore zone, indicating moveable oil or gas; if Rxo/Rt is near 1, the invasion has not altered saturation significantly.
- Rugose boreholes degrade proximity log quality because poor pad contact introduces air gaps between the electrode pad and the borehole wall, causing the measurement to read higher than true Rxo; the caliper log is used to quality-control proximity log data in irregular holes.
Fast Facts
The proximity log was developed by Schlumberger in the 1960s as an improvement over the microlaterolog for deeply invaded formations. The tool is pad-mounted on a backup arm assembly that presses the electrode array against the borehole wall. Current electrode spacing is approximately 6 inches, providing the 4 to 8 inch depth of investigation. The tool reads Rxo directly in ohm-meters and is usually presented on the same track as the MSFL and microlog for comparison. Resistivity values in the flushed zone typically range from 0.1 to 100 ohm-m depending on the formation water salinity and mud filtrate salinity.
Tip: When interpreting proximity log data in a freshwater mud system, remember that the mud filtrate resistivity (Rmf) is much higher than the formation water resistivity (Rw) in a saline formation. This means the flushed zone will always show elevated Rxo even in water-saturated formations, which can be misinterpreted as moveable hydrocarbons. Always compute the flushed zone water saturation (Sxo) using the correct Rmf before applying the Rxo/Rt moveable hydrocarbon criterion.
What Is a Proximity Log
The proximity log is a wireline resistivity tool designed to measure the electrical resistivity of the flushed zone, the annular region immediately surrounding the borehole from which native formation fluids have been displaced by drilling fluid filtrate during the drilling process. The flushed zone extends approximately 1 to 6 inches from the borehole wall in most formations and is the nearest measurable formation volume from the wellbore. Because it is so close to the borehole, measuring it accurately requires a tool that can reject the influence of the conductive mudcake plastered on the borehole wall and the borehole fluid itself, both of which would otherwise dominate the resistivity measurement.
The proximity log achieves this rejection through electrode focusing: the central measurement electrode is surrounded by concentric guard electrodes that emit current to constrain the measurement current into horizontal sheets perpendicular to the tool axis, minimizing vertical leakage through the borehole fluid and mudcake. This focusing gives the tool its name: the measurement is focused to investigate a zone proximate to the borehole wall at a slightly greater depth than the microlaterolog, which is optimized for very shallow investigation but is more susceptible to mudcake effects in deeply invaded formations.
How the Proximity Log Works
The proximity log tool is mounted on a pad that is pressed hydraulically against the borehole wall by a backup arm. The pad contains the central measurement electrode (A0) flanked by two monitor electrodes (M1, M2) and two outer guard or bucking electrodes (A1, A2) arranged concentrically. A constant current is emitted from A0 and the bucking electrodes are adjusted so that no current flows between M1 and M2, a condition that ensures the A0 current flows radially outward into the formation rather than along the borehole surface. The voltage required to maintain this null condition is proportional to the formation resistivity at the depth of investigation.
The measurement is recorded continuously as the tool is pulled up the wellbore at logging speed (typically 1,800 feet per hour for most wireline tools). Data is presented as a resistivity curve in ohm-meters, alongside the microlog and MSFL curves for comparison. The separation between the proximity log curve and the deeper-reading laterolog or induction curves indicates the degree of invasion: large separation suggests significant invasion with high contrast between Rxo and Rt, while overlapping curves suggest little invasion or similar fluid resistivities in the flushed and uninvaded zones.
Proximity Log Across International Jurisdictions
In Canada and the WCSB, the proximity log and its successor the MSFL are standard components of the triple combo log suite (resistivity, density, neutron) run on exploration and delineation wells. Alberta Energy Regulator (AER) well records include microresistivity data as part of the standard log submission requirement for all wells where a full wireline suite is acquired. In Devonian carbonate plays of the Rainbow Lake area and the Swan Hills, proximity log data combined with deep dual laterolog measurements provides invasion analysis essential for determining true formation resistivity in high-contrast invasion environments where OBM or high-resistivity WBM filtrate deeply invades the near-wellbore zone.
In the United States, BSEE regulations for Gulf of Mexico wells require comprehensive logging programs that include shallow resistivity measurements. In deepwater turbidite plays such as the Miocene Wilcox and the Paleocene Frio, proximity log (or MSFL) data is used alongside array induction tools to construct Rxo profiles that constrain invasion models and improve Sw calculations. Operators including Shell, Chevron, and bp run microresistivity pad tools on all exploration and appraisal wells where formation evaluation quality is critical to field development decisions.
In Norway, proximity log data is acquired as part of comprehensive wireline suites on exploration wells on the NCS. Statoil (Equinor) formation evaluation standards include microresistivity requirements for all wells where invasion analysis is needed to compute accurate true resistivity from deep induction or laterolog measurements. In the Brent Group sandstones of the northern North Sea and the Jurassic Ula Formation in the Central Graben, invasion profiles derived from proximity log and deep dual laterolog data help distinguish tight cemented zones from producible hydrocarbon-bearing intervals.
In the Middle East, proximity log data is critical in Arab Formation carbonate reservoirs where oil-base mud is used and invasion contrast between OBM filtrate and formation brine creates pronounced resistivity profiles. Saudi Aramco's petrophysical evaluation workflows include standard invasion analysis using shallow (MSFL or proximity), medium, and deep resistivity data to correct apparent resistivity to true formation resistivity, which is essential for computing accurate water saturation in the billion-barrel reservoirs of the Ghawar field. ADNOC uses equivalent microresistivity suites in Abu Dhabi carbonate formations.
Synonyms and Related Terminology
The proximity log is part of the microresistivity tool family, which includes the microlog, microlaterolog (MLL), and microspherically focused log (MSFL). The flushed zone resistivity it measures (Rxo) is a key input to the Archie equation applied to the flushed zone to compute flushed zone water saturation (Sxo). The invasion diameter and Rxo/Rt ratio derived from combining the proximity log with deep resistivity tools are used in invasion correction workflows. The tool is also related to the dual laterolog and array induction log, which provide the deep and medium resistivity measurements used in conjunction with the proximity log.
Frequently Asked Questions
Q: Why was the proximity log developed if the microlaterolog already measured Rxo?
The microlaterolog has a very shallow depth of investigation (approximately 1 to 2 inches) and is strongly affected by mudcake when the cake is thick (greater than 3/8 inch). In formations with deep invasion and thick mudcake, the microlaterolog reads mudcake resistivity rather than formation Rxo. The proximity log was designed with a slightly deeper investigation (4 to 8 inches) to read past the mudcake into the true flushed zone, at the cost of some sensitivity to formation heterogeneity at that scale. The MSFL (microspherically focused log) is the modern successor that combines pad contact like the MLL with slightly deeper investigation.
Q: How does the Rxo/Rt ratio indicate moveable hydrocarbons?
When drilling fluid filtrate flushes into the formation, it displaces some of the native formation fluids. In an oil- or gas-bearing zone, the filtrate displaces hydrocarbons and leaves a flushed zone saturated largely with filtrate. Because hydrocarbons have very high resistivity and filtrate conductivity depends on its salinity, Rxo will differ from Rt depending on what fluid was displaced. The ratio Sxo/Sw (derived from Rxo/Rt) indicates how much of the original hydrocarbon was moved by invasion. If Sxo is much higher than Sw, original oil saturation was high and has been partially displaced: moveable hydrocarbon is present. If Sxo equals Sw, either no invasion occurred or no hydrocarbon is moveable.
Why the Proximity Log Matters
The proximity log fills a critical gap in the resistivity measurement suite by providing a reliable Rxo measurement that is robust to mudcake effects in deeply invaded formations. Without an accurate Rxo measurement, invasion correction of deep resistivity tools is unreliable, leading to overestimated or underestimated true formation resistivity and consequently incorrect water saturation calculations. In exploration wells where a single well must deliver the reserves estimate that justifies a development program, the accuracy of Sw directly affects the probabilistic resource estimate. The moveable hydrocarbon indication from Rxo/Rt also provides a quick-look productivity assessment before any test or completion is performed, guiding interval selection decisions with direct economic consequence.