Applications of Graphic Correlation in Operational Activities

Richard A. Denne

When log and seismic correlations are ambiguous or too coarse, graphic correlation is effective in increasing the accuracy of correlations during operational activities. The initial cross-plot of horizons (biostratigraphic and log) from the drill well and a nearby well can be used to estimate depths of horizons not initially recognized or misidentified. The final plot can be used to locate and determine the amount of missing section caused by faults or unconformities. In one example, a plot of 27 biostratigraphic and log picks indicates that two unconformities and a fault in the drill well removed sands found in the control well.

If a horizon's depth needs to be accurately predicted (abnormal pressure, casing or coring point), graphic correlation is useful. An example is an attempt to core an interval during its initial penetration, which a control well indicates is overlain by the Calcidiscus macintyrei extinction. At a drilling stoppage the plot is extrapolated, indicating that the C. macintyrei extinction should be found 800 ft below TD. Drilling can then continue at a normal rate before decreasing near the target's top, saving drill time.

The plot can also be used to illustrate whether an objective has been penetrated. In an example, initial log correlations indicate a well has reached its target, just above the Discoaster quinqueramus extinction. However, no D. quinqueramus specimens are found. Graphic correlation indicates a linear relative sedimentation rate between the drill well and control well, and projects the D. quinqueramus extinction below the TD, persuading management to continue drilling.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California