A stratigraphic pick in a well is a point defined in three dimensions (X, Y and Z).The accuracy of the pick, either in measured depth from the kelly bushing (KB), or with respect to sea level, is difficult to quantify. Sources of error include:
- incorrectly recorded well surface or bottom hole location, which results in an error in either X, Y, or both.
- incorrectly recorded ground or KB elevation, which results in an error in the Z dimension.
- incorrect stratigraphic top pick elevation values, or an incorrectly identified formation result in error in the Z dimension.
- errors in data entry or data transfer, resulting in errors in X, Y and/or Z values.
Any errors in the initial data points were reduced in the final data set by geostatistical evaluation to identify and remove outliers.
Step 1: (Input) 4175 picks of the sub-Cretaceous unconformity within the study area boundary were used to identify the extent of the subcropping stratigraphic units and delineate the subcropping polygons.
Step 2: (Creating subcrop boundaries) Surfaces for the stratigraphic units underlying the sub-Cretaceous unconformity were independently modelled in ArcMap using picks that were made in unison with unconformity picks. These picks were made to determine the form and geometry of the stratigraphic units below the sub-Cretaceous unconformity. These geostatistically modelled surfaces were integrated into a 3D geocellular model using Petrel 2013 and were then projected through the modelled sub-Cretaceous unconformity surface, creating lines of intersection representing the subcrop boundaries. To ensure that the modelled subcrop lines were consistent with the eroded unit picks, the results were validated with well control data, and the subcrop lines were adjusted where required.