Based in Mahone Bay, nova scotia, agile libre is an independent publisher of technical books about the subsurface.  

Scaling the outcrop

Feeling gravity taking over and pulling one down while struggling up the debris aprons formed by pieces of heterolithic Neill Klinter Group (exposed along the cliffs of Jameson Land, East Greenland) is not an encouraging feeling. The cliffs have to be reached though, because the succession is packed with relevant information for analogous hydrocarbon-bearing formations, such as those on the mid-Norwegian continental shelf. Three steps up and two down is the rule; the loose scree material lying at the angle of repose forces you to be patient and persistent. With all safety precautions taken, and most accessibility issues overcome, a tremendously rewarding day lies ahead.

At the end of the day, yet another section has been completed, and numerous samples have been taken and stored. The results — such as qualitative and quantitative data, panoramic photographs, insights, a proper feeling of geological scale — will be highly relevant. They will allow for further reconstruction of an ancient, continuously changing depositional system and its complex geomorphological and stratigraphic architecture. The laboratory of a sedimentologist (or, in fact, any geologist) is an outcrop, all outcrops. Only outcrops can provide the quality and quantity of information and insight with which to better understand subsurface analogs. Just as a biologist or a physicist needs a laboratory to carry out his or her experiments, so a sedimentologist needs outcrops. And yes, the adage is true: the more outcrops you have seen and studied, the more value you add to your work.

Why is it that hydrocarbon-bearing successions in the subsurface at several kilometres depth can never be sufficiently understood without the aid of quantitative and qualitative outcrop data? This data is used to build conceptual models, build three-dimensional, geostatistically based computer models, populate software with realistic information and numbers, test assumptions, and so much more. Although the instruments used to collect information in the subsurface are very advanced, they represent indirect information only. They provide very high-density but spatially discrete datasets in one, two, or three dimensions but they have a large range in resolution and can simply not compete with continuous cliff sections that may stretch for up to tens of kilometres in length and cover hundreds of metres of stratigraphy. All hydrocarbon com- panies supplement their subsurface evaluation with outcrop analog studies because they realize that outcrop information is an essential ingredient for better reservoir description and for finding the most appropriate drainage strategies.

But maybe more than anything else, outcrop studies allow for the assessment of natural variability and uncertainty related to any type of data. Getting it right in one place does not mean you will get it right in another. Underestimating natural variability leads to errors in your work. As long as you are aware of this you will want to see more in order to learn and improve. Therefore, it is essential that you study not just one example of a particular depositional system but several more as well. Although classifications can be made and general rules extracted, natural variability is always larger and we need to understand it well.

The necessary skills for outcrop analysis and data acquisition have to be consistently taught (and re-taught) at our universities so that outcrop analog studies can continue to provide crucial information to the hydrocarbon industry. The industry is still using outcrop analog data widely and exhaustively but cannot provide all the training to acquire the basic acquisition skills. The geoscience community needs to preserve the right balance between much-needed modern computer workstation skills and the equally valuable but more traditional field geology skills. Remember the Neill Klinter slopes — relying solely on workstations may lead us to take two steps up but three steps down.

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