How to Log Core - by D.M.Kent and J.H.Lake

Abstract:

The authors presented this paper at the Energy, Resources Conservation Board Core Research Centre, Calgary, Alberta, May 13-14, 2010. This story is less about technique and more about interpreting sedimentary depositional environments. Geologists have an innate ability to envision the third dimension that Engineers seldom possess (left brain-right brain). We should take advantage of this gift when logging core and interpreting lateral facies changes and stratigraphic traps. The Era of Lithostratigraphy is over. This misguided approach involved crossing time lines by joining similar lithologies and identical log and seismic responses. We must model the third dimension in depositional sedimentology to see how hydrocarbons are trapped stratigraphically

Walther’s Law of Superposition is the key to interpreting facies changes. Lateral facies changes are preserved in a vertical succession in the core. In other words, the depositional environment changes laterally with the facies we see vertically in the core (an easy concept to state but a difficult one to visualise). One fundamental facies modelling concept is the interpretation of the strike and width of the reservoir facies (width of the shelf or platform). There is much evidence from horizontal wells that not all operators understand the limits of the reservoir and cross into non-reservoir. Only Geologists can define the facies/ reservoir width and these answers are in the core boxes. We have seen too many Geologists who return to the office to interpret their core descriptions in three dimensions. By then it’s too late - the thinking/modelling must happen during the core logging procedure. Computers are the ideal modelling tool, since we can hang the cross-sections of our core descriptions and build facies maps while developing the depositional model in three dimensions....