Quality control and risk assessment of seismic profiles using area-depth-strain analysis

Abstract

Area-depth-strain (ADS) analysis is a method for quantifying the structural style, balance, boundary displacement, detachment locations, and subseismic strain from a seismic profile, information that is particularly useful for quality control and risk analysis. The method is based on measurements of excess area, width, depth, and bed length of multiple horizons in a structure. A balanced structure is indicated by a well-defined line or lines on an area-depth graph. The structural uncertainty or risk of the interpretation is quantified using the fit of the data to the least-squares line(s), the match between the ADS detachment values and the interpreted geometry, and the magnitudes of the calculated strains. The method also clearly separates syntectonic (growth) units from nontectonic (pregrowth and no-growth) units. Different area-depth graph styles represent (1) classic detachment folds, fault-propagation folds, and ramp anticlines; (2) buckle-style detachment folds; and (3) fault-bend folds. The focus here is on the first two. Numerical models of the detachment fold styles show the similarities and differences between their geometries and ADS interpretations. Both styles are evaluated with seismic profiles across oilfield-scale structures. The Alpha/Bobo field, Nigeria, is a classic detachment fold, and an Angolan anticline is of the buckle style. Profiles across the Alpha/Bobo field before and after the first well was drilled demonstrate the improvement in the ADS interpretation of the lower detachment location and the reduction of layer-parallel strains in the revised profile. The Alpha/Bobo field and the Angola fold illustrate the distinction between growth and no-growth intervals and the interpretation of growth strata. Both fields show the use of predicted versus observed ADS results to suggest possible improvements to the interpretation.