Fig. 1

Examples

We present two (2) examples of dipping or more highly deformed beds, that lie above less deformed or more gently dipping beds. One example is from outcrop and the other example is from seismic. For purposes of demonstration, we present two (2) examples of Lateral Ramps, that laterally terminate fold and thrust belts in the strike direction. Often the lateral or strike ramps are better imaged on seismic data than are thrust faults on dip lines.

The seismic line (Figure 2), that trends east to west, is from the western termination of the North Monagoas Fold and Thrust Belt, Venezuela. The North Monagas fold-thrust bed produces hydrocarbons from the Furial and Carito and Tejero Fields that are thrust to the southeast. Major bed dip discontinuity can be observed at about 2.5 sec and several other smaller thrust faults seem to image at the I.8 sec level. Dipping reflections overlie reflections that dip at more gentle angles indicative of a major discontinuity.

Fig. 2

The outcrop example is in the Wyoming Thrust belt where folded hanging wall beds are seen to truncate along nearly horizontal footwall beds (Figure 3). The hanging wall is moving toward the observer. The truncation of hanging wall beds upon footwall beds is direct evidence of bed dip discontinuity and detachment.

Fig. 3

PROCEDURES

We recommend the following procedures for locating and mapping thrust faults on seismic sections. First, peruse the seismic data set in the strike and dip direction looking for bed dip discontinuities. Dip domain analysis helps in location changes in bed dips (Tearpock et al., 1994, p. 126). If you are working a 3-D data set, examine several arbitrary lines taken along the flanks of the structure in an attempt to locate lateral ramps. Lateral ramps are often better imaged in seismic data than are dip ramps. The following procedures should help generate a viable or admissible interpretation. After one or more dip discon­tinuities are located, fault mapping can proceed. The strike or arbitrary lines can then be loop tied to dip lines in order to map the fault. The fault surface is loop tied for the same reason that horizons are loop tied, that is, ensure that you are mapping the same event from line to line (fault or horizon).

Our examination of several hundreds, if not thousands, of fault surfaces in good data areas demonstrates that fault surfaces are smooth surfaces that do not contain kinks. Kinks in fault surfaces typically mean that two (2) or more faults are present. Also, remember that faults cannot cut across coherent reflections, and probably not across reflections that are semicoherent and likely to be continuous.

Another good rule to remember is that if an interpreted fault surface contains a ramp and if the beds moved up the ramp on the fault surface, then the reflections in the hanging wall must subparallel the dip of the ramp. An analogy would be a car moving up a ramp, onto a free­way. The pitch or dip of the car must conform to the pitch or dip of the ramp. The car cannot move up the ramp and remain the angle of the flat freeway. Thus, if beds move up a ramp of a thrust fault, the reflections of the beds should parallel or semiparallel the angle of the ramp. If the reflections are flat or subhorizontal, then there is no fault ramp.

Finally, remember that in the end the interpretation must be admissible, have three-dimensional struc­tural validity, must balance (Tearpock and Bischke, 1994.) and be retrodeformable (Tearpock et al., 1994. p. log). A simple fifteen (15) minute cut and paste retrodeformation of your interpretation to test its validity is certainly worth the cost of a multi­million-dollar dry hole.