Recognition of faults, unconformities, and sequence boundaries using cumulative dip plots

In exploration and production settings, wells commonly encounter faults, unconformities, or both. Unless the geology is very "layercake" and logs from nearby wells correlate peak-for-peak with the subject well, identifying a fault or unconformity with any degree of certainty may be difficult. This is especially true for faults with offsets of 10 m (30 ft) or less. Cumulative dip plots are crossplots of bedding-plane orientation vs. depth. Although results from oriented cores or dipmeters could be used as input for bedding-plane dips, borehole-imaging logs are the preferred data source. The technique involves crossplotting cumulative bedding-plane dip (direction and magnitude) vs. either depth or an arbitrary bedding-plane number that is 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 (northeast, southeast, southwest, northwest). 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 where no correlative logs exist. Cumulative dip plots could have significant economic impact in areas like the Gulf of Mexico, where faults and unconformities define isolated reservoir compartments. Such compartments are logical targets for infill drilling. Cumulative dip plots also are 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. Once dip domains are defined, structural dips can be accurately removed in studies of paleocurrent orientations, for example. Finally, with the current high level of interest in sequence stratigraphy, cumulative dip plots offer a practical approach to the detection of some sequence boundaries. Such boundaries can be detected even when dip discordances are small, i.e., on the order of 1 degrees or less.