Geological Prediction of Subseismic Deformation from Seismic-Reflection Profiles of Contractional Structures

Groshong, Richard H.; Withjack, Martha O.; Schlische, Roy W.

Subseismic (subresolution) deformation can significantly influence reservoir continuity, porosity, and permeability. The area-depth-strain (ADS) method is a rapid and inexpensive screening technique for recognizing potential locations and magnitudes of subseismic deformation. With this method, a graph of excess area vs. depth yields the boundary displacement. The displacement, together with measured bed lengths and widths of the structure at regional, allow for the calculation of subseismic strain for each marker. We present ADS results for three seismic profiles from fold-thrust belts: 1) a regional profile from the central Appalachian Valley and Ridge fold-thrust belt, 2) an oil-field-scale fault-bend fold from deep water offshore Nigeria, and 3) the fault-bend fold that produced the Rosario oil field in Venezuela. The results show that the ADS method permits the quantification of subseismic deformation at numerous stratigraphic horizons (including growth horizons) within the structures. For the Appalachian profile, the ADS method predicts a subresolution layer-parallel shortening of ~ 40% at the level of the basal Silurian strata. Outcrop and map data provide information about the character of this deformation and indicate at least two scales of substantial deformation not included on the regional profile: outcrop-scale folds and faults and small map-scale folds. The Nigerian example is a single thrust-ramp anticline with growth strata. ADS analysis indicates that layer-parallel strain varies considerably with substantial shortening (13-23%) at some stratigraphic levels and little at other levels. The differences are probably related to lithology, with stiffer, brittle units having less subseismic deformation. Horizons with high subseismic strains are likely to be thin-bedded or consist of an inherently more ductile lithology. The predicted subresolution strain is inversely proportional to the separation on the fault, suggesting a partitioning of displacement between layer-parallel shortening/thickening and fault slip. For the fault-bend fold forming the Rosario oil field, the ADS analysis indicates that layer-parallel shortening strains are small at all stratigraphic levels. Thus, the ADS analysis predicts that the Rosario structure has little subseismic deformation, in contrast to the Nigerian and Appalachian examples.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013