microlog, Oil & Gas Glossary

What Is a Microlog?

A microlog is a shallow-investigation resistivity logging tool that uses two closely spaced microelectrodes pressed against the borehole wall to measure near-borehole resistivity at two different depths of investigation, allowing detection of mudcake presence as evidence of permeable formation invasion, identification of non-permeable tight zones, and qualitative assessment of formation permeability through the separation between the two resistivity curves.

Key Takeaways

The microlog measures resistivity at two depths: micro-normal (~5 cm) and micro-inverse (~2.5 cm) investigation.
Positive separation (micro-normal > micro-inverse) indicates mudcake presence and therefore formation permeability.
Negative separation or no separation indicates a tight, impermeable zone where invasion has not occurred.
The microlog pad must be pressed firmly against the borehole wall; rugose or washed-out holes degrade data quality significantly.
The microlog is used to identify permeable beds for core cutting, perforating intervals, and Rxo estimation.

How the Microlog Measures Near-Borehole Resistivity

The microlog tool uses a rubber pad that is hydraulically pressed against the borehole wall during logging. Mounted on the pad face are two pairs of button electrodes spaced approximately 2.5 cm and 5 cm apart. Current injected from these electrodes flows through the formation at very shallow depths, sampling primarily the zone immediately behind the mudcake and the flushed zone of invasion. The two electrode spacings give different depths of investigation: the closer-spaced micro-inverse electrode reads primarily the mudcake and immediately adjacent flushed zone, while the wider micro-normal electrode penetrates slightly deeper into the flushed zone.

When a permeable formation has been invaded by drilling fluid filtrate, a mudcake forms on the borehole wall. The mudcake is compacted, low-porosity, and typically has high resistivity because the filter cake solids (clay and weighting material) are not conductive. The flushed zone behind the mudcake contains mud filtrate at near-100% water saturation, typically with lower resistivity than the uncontaminated formation if oil is present. This configuration creates a resistivity profile in which the micro-inverse (shallower) reads a higher value (mudcake resistivity) than the micro-normal (deeper, which reads flushed zone). This positive separation — where micro-normal resistivity exceeds micro-inverse resistivity — is the classic indicator of mudcake and therefore permeability. When no invasion has occurred (impermeable zone), no mudcake forms and both electrodes read the same borehole wall resistivity, giving zero separation.

Microlog Applications Across International Jurisdictions

In Canada, microlog data is used in WCSB well petrophysical evaluations to identify permeable sands within the Cardium, Viking, Mannville, and Montney formations, distinguishing porous permeable intervals from tight calcareous or cemented streaks within otherwise productive zones. AER well data submissions may include microlog data as part of the composite log suite for pool establishment applications where bed-by-bed permeability indication is needed to define pay intervals. In Montney horizontal wells where the microlog cannot be run due to high hole angles, formation microimager tools serve as partial substitutes for near-borehole resistivity characterisation of permeable intervals.

In the United States, the microlog has historically been a standard component of Gulf Coast wireline log suites for sand-shale sequences, where it provides bed boundary definition and permeability flagging on a centimetre scale that complements the coarser resolution of deep resistivity tools. BSEE formation evaluation requirements for OCS wells include resistivity data; micrologs satisfy this requirement for shallow-investigation resistivity data. In Norway, Sodir well data requirements include shallow resistivity measurements; microlog or microresistivity data from pad tools satisfies this requirement for exploration wells. In the Middle East, Saudi Aramco's Arab Formation evaluation programmes use microlog data as part of the standard suite to identify permeable carbonate and anhydrite intervals before formation testing, supporting the selection of DST intervals in exploration wells.

Fast Facts

The microlog pad applies a contact force of approximately 50-150 pounds against the borehole wall, maintained by a spring-loaded arm that extends from the tool body. In boreholes larger than 16 inches diameter, or in rugose and washed-out sections, the pad may not maintain firm contact, causing the measured resistivities to reflect borehole fluid rather than formation. A simple quality check compares the microlog mudcake thickness reading against the caliper log: when the borehole diameter is close to bit size and the microlog shows positive separation, the data is reliable. When the borehole is significantly enlarged, microlog readings are unreliable and should be flagged on the composite log presentation.

Microlog Curve Separation and Permeability Interpretation

The conventional interpretation of the microlog uses the separation between the micro-normal and micro-inverse curves as a permeability indicator. Positive separation of 2 ohm-m or greater is considered a definitive permeability indicator when the borehole is in good condition. Very large separation (greater than 10 ohm-m) typically indicates fresh mud filtrate invasion with significant oil saturation contrast between the flushed zone and formation; the micro-normal reads the lower-resistivity flushed zone while the micro-inverse reads the higher-resistivity mudcake. In gas-bearing formations, both curves may be elevated because gas in the flushed zone increases resistivity; positive separation persists but the absolute values are higher than in oil or water-bearing permeable zones.

In carbonate formations, the microlog requires careful interpretation because vugular porosity and fractures can create localised high permeability while the matrix adjacent to the pad may be tight. A fractured carbonate may show intermittent separation on the microlog where fractures intersect the borehole, while the unfractured matrix shows no separation. This pattern is used to identify fracture-dominated permeability zones in carbonate reservoirs, though borehole image logs provide more reliable fracture identification due to their full 360-degree borehole coverage compared to the microlog's single pad orientation.

Tip: When interpreting microlog data in a well with oil-based or synthetic-base mud, remember that OBM filtrate is non-conductive. The mudcake in an OBM well has different resistivity characteristics than in a water-based mud well, and the conventional positive-separation permeability indicator may not apply in the same way. In OBM wells, the micro-resistivity separation pattern depends on the contrast between the OBM-invaded flushed zone and the formation beyond. If you are using the microlog from an OBM well to identify permeability, validate your interpretation against the caliper log and any available formation pressure data rather than relying solely on the curve separation pattern calibrated for WBM environments.

Microlog is also referenced as:

ML — the standard abbreviation used on composite log headers and in petrophysical software; tracks labeled "ML1" and "ML2" or "MINV" and "MNOR" represent the two electrode spacings
Micro-resistivity log — the descriptive name used in formation evaluation textbooks to encompass the microlog and its successors (microlatlog, microfocused log); used when the broader category of shallow-pad resistivity tools is being discussed
Contact resistivity log — an older term for the microlog that emphasises the physical contact between the pad and the borehole wall; used in pre-1970 literature

Related terms: mudcake, flushed zone, resistivity, invasion, permeability

Frequently Asked Questions

What is the difference between a microlog and a microlaterolog?

The microlog and microlaterolog (MLL) both use pad-mounted electrodes for shallow resistivity measurement, but they have different electrode configurations and depth of investigation. The microlog uses simple current and potential electrodes that provide unfocused shallow measurements of 2.5 cm and 5 cm depth, sensitive primarily to mudcake and the immediately adjacent flushed zone. The microlaterolog uses a focused guard electrode configuration (similar to a laterolog but miniaturised) that directs current deeper into the flushed zone — approximately 10-15 cm. The MLL is better suited for measuring the flushed zone resistivity (Rxo) quantitatively, which is used to calculate residual oil saturation and water saturation in the invaded zone. The microlog is better for mudcake detection and permeability indication but is less suitable for quantitative Rxo derivation than the MLL.

Can the microlog be run in horizontal wells?

The microlog in its conventional wireline form is rarely used in high-angle or horizontal wells because the tool relies on gravity-controlled pad deployment that works best in near-vertical boreholes. In horizontal wells, one side of the borehole consistently faces downward (the low side) and the single pad on a conventional microlog will contact only one azimuthal sector of the borehole wall. For high-angle and horizontal wells, azimuthal microresistivity tools such as the formation microimager (FMI) or resistivity-at-bit (RAB) LWD tools provide equivalent or superior near-borehole resistivity information without the orientation limitations of a single-pad tool. The formation microimager, in particular, provides 80-100% borehole wall coverage with multiple pads and flaps, giving a complete picture of shallow-investigation resistivity around the entire borehole circumference that the single-pad microlog cannot achieve.

Why the Microlog Matters in Oil and Gas

The microlog addresses one of the most fundamental questions in formation evaluation: which intervals are permeable and therefore potentially productive? In a typical well with multiple sand or carbonate beds, not all formations that show porosity on the neutron-density crossplot will be permeable — tight cemented streaks, diagenetically altered beds, and facies transitions can create high-porosity but low-permeability intervals that will not produce hydrocarbons at economic rates. The microlog distinguishes permeable from non-permeable intervals by detecting the presence or absence of mudcake, providing a simple, unambiguous permeability flag that guides perforation interval selection, core cutting programme design, and formation testing decisions. In mature basin development drilling where well costs are high and completion decisions must be accurate, the microlog's centimetre-scale resolution of permeable bed boundaries provides a cost-effective petrophysical quality check that reduces completion failures from perforating tight intervals that cannot produce.

Microlog: Definition, Mudcake Detection, and Permeability Indication in Well Logging