Back to Basics - Building an Exploration Geological Model from Drill Cuttings

A drill cuttings based review of three deep (> 5000m below rotary table) exploration wells located in the Middle East was undertaken to determine if there is a geological reason/s for twist-off.

The described intervals, when combined and interpreted, indicate an overall continental section comprising initially four composite halokinetic sequences (CHS), a post CHS affected by pedogenesis and five further cycles interpreted as indicating an alluvial/fluvial gross depositional environment (GDE). The deeper CHS's were interpreted as indicating a predominantly lacustrine GDE with features related to diaper-induced topography. The observations align with a ‘typical’ cap-rock sequence associated with salt movement and subsequent alteration including possible mineralisation via brines rich in base metals that are probably associated with hydrocarbon migration.

Accordingly, and based on this model, salt is predicted below the CHS's and is considered the (unobserved) cause of twist-off. In addition to the depositional model, reservoir quality was also described. It is best developed in the anhydrite (‘crystalline sands’) of CHS2 and in the fluvial quartz arenites of the post CHS. Integration and comparison with the greater data-set (in particular XRF and XRD data) after the independent cuttings analysis resulted in no change to the cuttings-only model.