Request for technical advice: 300×150 m drill spacing to estimate resources in heterogeneous limesto
Hello everyone,
I would like to seek your technical advice regarding a limestone exploration permit located in a sedimentary environment composed of limestone, marl, and clay layers. Prior to my involvement, an initial geophysical survey had been conducted, and five core drillholes were installed across the area.
The results of these early drillings indicated that limestone occurs at greater depths than expected, around 7 meters below the overburden, contradicting the geophysical predictions. Three additional drillholes, spaced at least 70 meters apart, revealed significant lateral heterogeneity with no clear continuity: alternating limestone–calcareous sandstone, more massive limestone beds at another location, and alternating limestone–marl layers at a third point.
These observations clearly demonstrate the lack of geological continuity of the limestone at the local scale, raising important questions regarding the drilling strategy for the next phase.
We are planning a new geophysical survey covering two blocks of approximately 1 km² each, prior to a more extensive drilling campaign over a 6 km² area of interest. At this stage, I am considering a 300 m × 150 m drill spacing for the next phase.
My question is as follows: Given the observed lack of continuity in the initial drillholes, do you consider a 300 × 150 m spacing technically appropriate to support an estimate of Inferred Resources, and potentially Indicated Resources, in a heterogeneous limestone deposit intended for quarrying?
Specifically, I would greatly appreciate your technical insights regarding:
- the suitability of this spacing in a context where limestone shows significant variations from point to point;
- the potential conditions under which Indicated Resources could still be considered despite this heterogeneity;
- the geological or statistical criteria (variograms, sedimentary structures, geophysical signatures, etc.) that should be monitored to adjust the drilling pattern.
My aim is to design a drilling pattern that is credible and technically defensible, in accordance with best practices under JORC, NI 43-101, and CRIRSCO standards for bulk industrial minerals.
Your experience and advice will be invaluable in refining the planning of our next drilling campaign and ensuring that our resource estimates are robust and reliable.
Thank you in advance for your thoughtful feedback.
Comments
-
Under JORC, NI 43-101 and other CRIRSCO-aligned codes, drill spacing by itself doesn’t determine whether something can be reported as Inferred or Indicated. What really matters is the confidence in the geological interpretation, continuity of the units, and how well the data supports your model. That final call always sits with the Competent Person.
In practice, this usually means putting effort into strengthening the geological model first, especially in heterogeneous sedimentary settings like yours. Variography, stratigraphic mapping, geophysical inversion results, and good QAQC from the first round of drilling can help highlight where extra holes will genuinely reduce uncertainty rather than just add metres.
If you’re looking for background reading, the CIM Industrial Minerals Leading Practice Guidelines (2023) are a great high-level resource for bulk/industrial mineral deposits. There was also an excellent paper at the MREC 2023 conference “Drill hole spacing analysis for classification and cost optimisation – a critical review of techniques” which gives a solid overview of different drill-spacing approaches and how they fit into classification decisions.
From our side, Seequent can’t give site-specific drilling or classification recommendations, but we’re always happy to help you make the most of the tools and workflows in our software that support good interpretation and build solid workflows. If you’re after more tailored technical advice, we can also point you toward consultants who specialise in industrial minerals.
I invite others in the community, especially those with experience in limestone or similarly variable sedimentary units, to provide further advice.
1 -
Thank you very much, KathrynGall, for your detailed and thoughtful response, as well as for the very relevant resources you shared.
Your clarification that drill spacing alone does not determine whether a resource can be classified as Inferred or Indicated under JORC, NI 43-101, and other CRIRSCO-aligned standards is extremely valuable. Your emphasis on the need to build sufficient confidence in the geological interpretation rather than relying purely on geometric spacing provides an important framework for guiding our next steps. Given the significant lateral variability observed in our initial drillholes, your reminder about the importance of strengthening the geological model before defining a definitive drilling grid is particularly relevant.
I will take the time to explore the references you mentioned, including the CIM Industrial Minerals Leading Practice Guidelines (2023) and the MREC 2023 paper on drill hole spacing optimisation. These documents will help us better structure our thinking regarding classification thresholds and the level of geological support needed for each category of resources.
As you kindly mentioned in your Regarding software, we are in the process of acquiring Res2D Inv and Res3DInv software.
I would be very pleased and truly honoured to be put in contact with consultants or experienced professionals who have worked on industrial mineral deposits, particularly in heterogeneous limestone settings similar to ours. Any recommendation you could provide would be greatly appreciated and would contribute significantly to strengthening the credibility and technical robustness of our approach.
I also warmly welcome contributions from other community members with experience in comparable sedimentary environments. Insights on geological variability, continuity interpretation techniques, practical approaches to drill spacing, or effective workflows for combining drilling, stratigraphy, and geophysical inversion results would be extremely valuable for optimising the design of our next drilling campaign.
Thank you again, KathrynGall, for your availability, the clarity of your explanations, and the quality of your guidance. Your input is highly appreciated and will play an important role in refining our decision-making process.
0
