Economic Geology is a Predictive Science: Some Assorted Observations

Economic Geology is a Predictive Science: Some Assorted Observations

Economic geologists strive to understand processes that lead to distribution of resources. If we know how rocks are deposited and what happens to them after they are deposited (such as compaction, fluid migration, cementation, diagenesis, faulting, folding, extension, etc.) then we can predict which rocks can host economic resources. We try to determine the distribution of certain kinds of host rocks through techniques like basin modeling, facies mapping, porosity mapping, reconstruction of structural history, pressure gradient mapping, TOC mapping, fracture mapping, and thermal maturity mapping. Mapping by contouring surfaces, values, chemical components, and thicknesses is the main technique in the economic geology of sedimentary rocks. We also utilize depositional shapes based on sediment source areas and rates of sedimentation. Ore geology utilizes chemical distribution variations within volcanogenic massive sulfide deposits, pegmatites, and other ore deposits to find precious metals and minerals.

  

In order to map spatial distributions we need data points based on real and reliable techniques like rock sample analysis and geophysical well logs. More is almost always better. In absence of sufficient data points there are other reconnaissance techniques like seismic, gravity, aeromagnetic, and remote sensing surveys. Many of these techniques, while quite valuable, are subject to high margins of error due to lack of resolution on small scales, effects of interference, and processing assumptions. They are mainly supplemental to more precise data like well logs, drill cuttings, cores, and borehole tests. They are very valuable in determining regional geology and in broadly imaging geological structures such as large faults and folds that can influence the success of resource and mineral extraction. 

Our data is gathered with investigative methods including visual description, component identification, and geophysical exploration. We gather, organize, study, and interpret our data so that we can make useful predictions. That is the goal. We try to predict the most resource and economize it.

There are different levels of predictability in different processes of economic geology. Success in mapping, say mapping a rock formation boundary, seems to be dependent on how closely the data match the scale. Generally speaking, the more data the more accurate the picture. Hydrocarbon geologists use the term “well control” to convey the distribution of the well data to be mapped. Less data points or less in certain areas of the whole map range can lead to skewed interpretations by both mapping programs and by mappers. Abundant and even coverage is usually best. There are macroscales and microscales. Making cross-sections with a network of seismic data and some well logs is one way to study and model large areas (macroscale) at basin-scale. Then one can perhaps zoom into facies scale, dividing up the basin in terms of primary sedimentary processes (clastic influx from source area, deposition in depo-center, marine deposition, etc.). Further zooming in leads to individual features, then perhaps to individual lobes of those features.

One of my former colleagues, a land manager, used to quip that geology is not an exact science. Well I would say that it is only as exact as the available data allows it to be. We map and characterize the subsurface so data acquisition is a big issue and that is usually limited compared to other sciences. Once data is gathered we can be fairly creative in how it is interpreted. Most geologists I know have been wrong many many times. It is the nature of the beast. Educated guesses must be made. In economic geology or any predictive science many decisions must be made: what acreage to recommend, where to drill or mine, whether to continue after initial failures, etc. Analysis of failures is very important in exploration. When you are looking for something, knowing where it isn’t is valuable information. One needs to understand the limitations of computerized contouring, know how and in what orientations to skew the contours to better reflect the realistic likely shape of structures and bodies, and notice unrealistic shapes in the computer-generated results. 

Geology and many sciences rely on our evolved human abilities of pattern recognition and detection of anomaly through contrast. Statistics and data manipulation can enhance identification of anomalies. Of course, they can also generate false anomalies, so educated analysis is important. Sample data like grain size, degree of sorting, orientation of grains, and types and degree of cementation can yield clues of the origin, facies, and most importantly, the likelihood of those rocks to host economic materials. Familiarity with large and varied data sets and other detailed studies are both important for developing skill in geological exploration. Model analogues from other areas are often utilized.

In geology we are often reconstructing the past through a minimal amount of clues. There is much educated guesswork and only real data from outcrops, wellbores and geophysical techniques can confirm the guesswork that typically derives from modelling. As we work with data we come to understand that we can work with data and manipulate it in various ways to squeeze out previously hidden information from the data. There is much deductive and detective work with science. Sometimes it is a matter of statistical likelihood and other times it is a matter of noticing things in the data that others have missed.

In order to optimize one’s predictive capacity one also needs to keep abreast of new techniques and new discoveries about economic deposits. New technologies can shift the rules of the quest emphasizing new parameters over old ones. Better understanding of host rocks and local subsurface fluid mechanics can only help the explorationist. The mindset of the explorationist is toward discovery of value. In that sense economic geology is like a quest, and in many ways like a scientific treasure hunt. Happy hunting!