Abstract: Recognition of Faults, Unconformities, and Sequence Boundaries Using Cumulative Dip Plots

Neil F. Hurley

Oil and gas wells commonly encounter faults, unconformities, or both. Unless there are good wireline-log correlations between wells, it may be difficult to identify faults and unconformities. This is especially true for faults with small offset (10 m or less).

Cumulative dip plots are cross plots of bedding-plane orientation vs. depth. Although results from oriented cores or dipmeters could be used, borehole-imaging logs are the preferred data source. The technique involves cross plotting cumulative bedding-plane dip vs. either depth or an arbitrary bedding-plane number (a function of depth). In the latter approach, bedding planes are numbered consecutively from top to bottom of the logged interval, and dip directions are color coded by compass quadrant (NE, SE, SW, NW). Because bedding-plane dips commonly differ above and below faults and unconformities, such discontinuities appear as inflection points in the otherwise straight line of the cumulative dip plot. An advantage of the technique is that it can be used in single-well studies wher no correlative logs are available.

Cumulative dip plots are useful in reservoirs where faults and unconformities define isolated compartments. Such compartments are logical targets for infill drilling. Cumulative dip plots are also valuable in structurally complex areas where they can be used to define dip domains, or groups of dips, that may be structural blocks at seismic scale. Finally, cumulative dip plots offer a practical approach to the detection of sequence boundaries. Such boundaries can be detected even when dip discordances are very small (10 or less).

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994