depth matched, Oil & Gas Glossary

What Is Depth Matching in Well Logs?

Depth matching is the process of aligning two or more wireline or LWD log runs from the same wellbore to a common depth scale by applying depth shifts to one or more runs based on correlation of common features, typically gamma ray or density signatures, ensuring that measurements from different logging passes sample the same formation depth before they are compared in petrophysical interpretation.

Key Takeaways

Depth mismatch between log runs arises from cable stretch, wellbore rugosity, different tool weights, and logging speed variations.
The gamma ray log is the standard depth-correlation reference because it is present on nearly all logging runs and responds consistently to formation.
Depth mismatches of 0.5-3 metres are common between logging runs and can cause significant errors if uncorrected in petrophysical interpretation.
LWD logs are depth-referenced to drill bit position; wireline logs are depth-referenced to cable length and require correlation to the LWD depth scale.
Well-to-well depth correlation requires additional adjustment for different datum elevations and drill floor heights between wells.

Why Depth Matching Is Necessary

Every wireline logging run acquires depth independently. The depth reference for a wireline tool is the length of cable paid out from the surface sheave wheel, corrected for cable elastic stretch. Cable stretch depends on the tool weight, wellbore angle, mud weight, and the speed at which the cable is moving. Different logging runs use different tool strings with different weights and lengths; even identical tools run at different times on the same well will have slightly different effective cable stretch. LWD logs are depth-referenced to the drill bit position calculated from the measured depth of the drill string, which has its own accuracy limitations from measurement while drilling. The cumulative result is that two logging runs in the same wellbore may have depth scales that differ by 0.3-3 metres or more throughout the logged interval.

This mismatch has direct consequences for petrophysical interpretation. If a density log from one run is depth-shifted by 1 metre relative to a resistivity log from a different run, the computed water saturation at any given depth will combine density porosity from one formation sample with resistivity from an adjacent formation sample. In a thinly bedded sequence with alternating sand and shale, a 1-metre mismatch is sufficient to combine the porosity of a sand with the resistivity of the adjacent shale, completely invalidating the saturation calculation for that interval. Depth matching before petrophysical integration is therefore not optional — it is a prerequisite for valid quantitative interpretation.

Depth Matching Applications Across International Jurisdictions

In Canada, depth matching is a documented requirement in WCSB well log data quality control guidelines for AER pool establishment submissions. AER Directive 065 petrophysical report requirements include documentation that multi-run logs have been depth matched before integration; unmatched logs used in reserve submissions are flagged by independent auditors as a data quality concern. Canadian petrophysical service companies routinely provide depth-matched log composites as a standard deliverable when integrating LWD logs from horizontal drilling with wireline evaluation logs run in the same well.

In the United States, BSEE reserve submissions require that log data used for proved reserve determinations has undergone quality control including depth matching; independent reserve engineering firms (Ryder Scott, Netherland Sewell) verify this as part of their audit procedures. In Norway, Sodir requires that submitted well log data meet specified data quality standards; Equinor's internal petrophysical workflows include depth matching as a mandatory preprocessing step before any multi-run composite log dataset is used in formation evaluation. In the Middle East, Saudi Aramco's petrophysical quality assurance programme verifies depth matching between LWD and wireline logs in horizontal Arab Formation wells where the lateral heterogeneity is centimetre-scale and a 1-metre depth mismatch would corrupt the facies classification used for completion optimisation.

Fast Facts

The cable stretch that causes depth mismatch in wireline logging amounts to approximately 1 metre of additional effective cable length per 1,000 metres of depth for a standard 4 cm wireline cable under typical tool-string weights. For a 4,000-metre well, uncorrected cable stretch could cause an error of approximately 4 metres in the absolute depth of any logged formation top. Modern logging systems apply real-time stretch corrections based on measured cable tension and cable stiffness, reducing absolute depth errors to typically less than 0.5 metres. The residual relative depth mismatch between two runs after cable stretch correction is usually 0.3-1 metre and is removed by cross-correlation-based depth matching.

Depth Matching Methods

Three methods are used to depth-match wireline and LWD logs. Visual matching involves displaying two gamma ray logs side by side on a computer workstation and manually shifting one relative to the other until corresponding features align visually. This is the most commonly used method for experienced interpreters because it is fast and allows geologically informed judgement about which features are reliable correlation markers versus noise. Cross-correlation is a quantitative method that calculates the statistical correlation coefficient between two log curves as a function of depth shift and selects the shift giving maximum correlation. Dynamic time warping is an advanced algorithm that allows non-uniform depth shifts to be applied along the log, accommodating differential stretching effects that vary with depth. The appropriate method depends on the quality and consistency of the logs being matched; visual matching with gamma ray remains the industry standard for most applications.

Tip: When depth-matching LWD logs to wireline logs in a horizontal well, be aware that the LWD log represents the formation as it was at the time of drilling while the wireline log represents the formation after mud invasion has altered the near-wellbore zone. Some tools (particularly neutron and density) will show systematic differences between LWD and wireline readings in addition to any depth shift, because the invasion profile changes the tool's measurement environment between LWD acquisition and wireline acquisition hours or days later. Depth match on the gamma ray (which is minimally affected by invasion) rather than on density or neutron to avoid confusing invasion-related response differences with depth mismatch errors.

Depth matched in log analysis is also referenced as:

Depth-shifted — used when a specific constant offset has been applied to a log to align it to a reference; less precise than depth-matched because it implies only a constant shift rather than the full matching process
Depth-correlated — the broader term encompassing both well-to-well formation top depth correlation and within-well log run alignment; context determines which application is meant
Tied to TD or tied to casing shoe — the informal operational description of depth matching wireline logs to a physical reference point (total depth or casing shoe) with a known depth from the drill string measurement

Frequently Asked Questions

How large a depth mismatch causes a significant petrophysical error?

The significance of a depth mismatch depends on the vertical resolution of the formation being evaluated. In a thick, uniform reservoir (10+ metres of homogeneous sand), a 1-metre depth mismatch has minimal effect because the log response does not change significantly over that interval. In a thinly bedded laminated sequence where individual sand layers are 0.5-2 metres thick, a 0.5-metre depth mismatch is large enough to pair porosity from a sand layer with resistivity from an adjacent shale, producing a calculated water saturation that has no physical meaning. As a practical rule, depth mismatch should be reduced to less than one-tenth of the minimum bed thickness of petrophysical interest — typically 0.1-0.3 metres for conventional formation evaluation in laminated reservoirs.

How is LWD log depth referenced differently from wireline log depth?

LWD logs are depth-referenced to the drill bit position as calculated from the measured depth of the drill string, which is the sum of the drill string component lengths measured before the drilling run. Drill string stretch and temperature effects cause small depth errors. Wireline logs are depth-referenced to the cable length paid out from the surface sheave wheel, corrected for cable stretch based on tension measurements. The two reference systems start at the same datum (the rotary table or drill floor) but accumulate different errors as depth increases. Typically, LWD logs are considered the primary depth reference in horizontal wells because the drill bit position is known without cable-stretch complications; wireline logs are depth-matched to the LWD reference using the gamma ray cross-correlation.

Why Depth Matching Matters in Oil and Gas

Petrophysical interpretation of oil and gas reservoirs combines multiple measurements from multiple logging runs into integrated formation models that determine net pay, water saturation, porosity, and permeability. Every calculation in this chain that combines data from different log runs is compromised if the logs are not depth-matched before integration. In tight formation plays where bed-by-bed facies classification determines perforation cluster placement in horizontal completions, and where individual beds are 0.5-2 metres thick, a 1-metre depth mismatch translates directly to misclassified beds, incorrectly placed perforation clusters, and suboptimal hydraulic fracturing that leaves producible reservoir unperforated. The few minutes required to depth-match log runs before petrophysical interpretation is one of the highest-return quality control investments in formation evaluation.