Recommendations for the Proper Handling of Cores and Sedimentological Analysis of Core Sequences

Abstract

The detailed sedimentological analysis of whole-diameter cores is greatly enhanced by the core being in good condition; ideally, cores should be complete (100% recovery) and well labeled with all pieces in correct order. Considerable information loss occurs when cores are mishandled; therefore, specific procedures should be followed. At the well site care should be exercised to preserve correct orientation of individual core segments and the core should be properly marked. Hügel orientation grooves cut in the core will facilitate core reconstruction. Commercial core analysis laboratories usually generate the greatest amount of information loss; therefore, special care should be used during subsampling for fluid-saturation analyses and porosity-permeability measurements. A core gamma-ray scan should be obtained whenever possible to aid in later core-to-log correlation. Finally, prior to detailed examination, it is recommended that the core be slabbed, lapped and photographed. X-ray radiography may be needed to enhance subtle sedimentary structures in massive-appearing sandstones and mudrocks. For proper sedimentological core analysis we recommend a process-sedimentology approach which emphasizes detailed lithologic description and the recognition of genetic units within the vertical sequence. A continuous, detailed sketch should be made and a description made using a check list of important lithologic features. An understanding of Walther’s Law is required for maximum use of the vertical sequence. At least four types of genetic units can be delineated to interprete the physical, biological and chemical processes responsible for generating the sedimentary rock product of the core. Those types are: sedimentation unit, ichnogenetic unit, soft-sediment-deformation (s-s-d) unit, and diagenetic unit. Finally, the sedimentological core analysis should be used to calibrate the wire-line logs and associated sursurface data. The main steps in such calibration are to first determine the core-to-log depth correction and then to determine the level at which genetic units and lithofacies can be recognized on the logs. Such calibration leads to better correlation of sedimentological information to nearby non-cored wells and allows for lateral extension of predictive sedimentological models throughout the subsurface study area.

Seismites from a well core of palustrine deposits as a tool for reconstructing the palaeoseismic history of a fault

“…The core has been described following the recommendations suggested by Selley (1978), Siemers and Tillman (1981) and Ethridge (1993). Furthermore we have taken into account that well cores do not allow the observation of lateral variations of sedimentary levels, neither of large-scale sedimentary and deformation structures (see discussion section).…”

Lithofacies and Depositional Environment from Geophysical Logs of EMK Field, Deepwater Niger Delta, Nigeria

“…The description and interpretation of the core and wireline log of clastic reservoir is the foundation of lithofacies analysis (Siemers and Tillman, 1981). Most times, core samples are not available for the analysis, thereby leading to the sole dependency on wireline logs.…”