Th/K and PE logs misfits and red/orange clays

As I am working in a shallow basin and drillmuds don't require heavy barite to maintain formation pressure we have been inculding PEF logs in our standard log suite for quite some time. In the past Interpretation (of the mainly fluvial sands) used Th/K spikes from the SGR (Spectral Gammaray Log) as main correlative measure. It also allowed us to predict and model Clay type (ranging mainly from illite to kaolinite) as the high Th/K values (> 13) related to kaolinitic horizons, while lower values represented Illite.

After re-evaluation of this clay typing by using the PEF logs (using PEF < 1.5 for Kaolinite and > 3.5 for Illite) I noticed a good correlation with our Th/K logs. However on some occasions a huge mismatch occurs in areas with large Th/K spikes and high PEF values. I could not figure this out and have now started pulling SideWall cores from our corelab and noticed these intervals correspond to orange to red clays which seem heavily oxidized and mottled. Awaiting microscopic analysis I would love to hear your oppinion on these intervals. What could this be? has anybody encountered such clays whith these log characteristcs (Oh... the GR is rather low (<70 GAPI)?

Any ideas are welcome and please ask for extra info if needed.

tanks,

Jose

To me this sounds much like mottled clays. Iron oxide and other oxidization products produce low GR repsonses and often Potassium (accounting to the low GR reading and high Th/K spike as you mention) is leached in these soils. Also the PEF will have higher readings as these heavy minerals remain in the soils and lighter minerals are leached out. Also the coloration points towards oxidization.

We have a core sample of one of such intervals in our database. I included an image of it.

More info can be found here: http://www.spgindia.org/conference/5thc ... /32937.PDF

Hope this helped you..

Attachments

This core shows a paleosol comprising of strong redoxomorphic features, or mottling. Mottles are composed of khaki goethite (FeOOH), lurid red hematite, and pale green to white kaolinite. These features indicate alternate wetting and drying.