Wireline quality
Previous logging showed issues affecting depth confidence, response consistency and the reliable identification of thin coal boundaries.
Anonymised prior project experience · Bengkulu Province, Indonesia
When thin seams make every depth correction matter.
Following a change of ownership, the incoming project team inherited wireline records and a geological model containing material inconsistencies. The work focused on restoring a defensible relationship between borehole evidence, topography, coal intersections and the resource model.
Shallow coalSequence encountered from approximately 20 m depth0.5–2 mTypical thickness of economic coal intervalsData assuranceWireline, drilling, survey and geology reconciled
Why it mattered
Small errors could move or distort an economic interval.
Where the target intervals were commonly only 0.5–2 m thick, an incorrect wireline depth reference, poor log response or mistaken seam pick could materially affect correlation between boreholes. In shallow terrain, inconsistencies between collar elevation, topographic surfaces and downhole data could then propagate into seam geometry, thickness estimates and the geological model.
The issue was therefore larger than the appearance of an individual log. The reliability of subsequent resource interpretation depended on identifying which observations could be trusted, corrected or excluded.
Inherited issues
The review separated symptoms from root causes.
Data reconciliation
Wireline picks did not consistently align with drilling records, geological observations and the spatial relationships expected between nearby holes.
Topographic control
Collar and surface information required review because elevation errors can distort a shallow seam surface even when downhole depth appears plausible.
Model inheritance
The incoming team needed to understand which assumptions from the previous model remained supportable before relying on it for reporting or planning.
Technical workflow
A traceable route from raw record to model input.
- Assemble and preserveCatalogue original wireline files, drilling logs, collar surveys, topographic information, seam picks and model inputs without overwriting the source record.
- Check log qualityReview depth references, tool response, repeat sections, acquisition conditions, calibration information and evidence of offsets or unsuitable data.
- Reconcile the boreholeCompare wireline signatures with core or cuttings descriptions, driller depths, recovery, lithology and observed coal contacts.
- Test spatial continuityExamine corrected intersections against neighbouring holes, structural expectations, topography and plausible seam thickness and elevation trends.
- Assign confidenceRecord whether each interval is accepted, corrected, qualified or excluded, together with the reason and supporting evidence.
- Update with governanceProvide an auditable dataset and issue trail for geological modelling and review by the responsible geologist and JORC Competent Person.
Result
A model based on reconciled evidence.
The revised workflow identified material discrepancies in inherited wireline interpretation and geological modelling, allowed affected intervals to be corrected or appropriately qualified, and improved the basis for correlating thin economic coal seams across the project.
The resulting evidence package supported geological model review and JORC-aligned resource reporting. Responsibility for any public Mineral Resource statement and its classification remained with the formally appointed Competent Person.
How this informs Axis
Acquisition, validation and interpretation cannot be separated.
Design for the decision
Set logging resolution, depth-control and QA/QC requirements around the thickness and consequence of the target.
Preserve provenance
Keep raw data, corrections, exclusions and approvals visible so later users can reconstruct the decision.
Integrate early
Compare wireline, drilling, geology, survey and topography before discrepancies become embedded in a model.
Escalate uncertainty
Make limitations explicit and place resource classification and public reporting decisions with the responsible Competent Person.
The operator and project are intentionally not identified. Metrics are presented at a level suitable for demonstrating the technical problem. This case study describes prior project-team experience that informs Axis Subsurface’s methods and does not claim that Axis was the project owner, reporting entity or JORC Competent Person.