Positive Structural Inversion: Reflection Seismic Examples and Physical Modelling

Many extensional basins worldwide are modified by subsequent compressional episodes which lead to positive structural inversion. The inverted structures formed by the reactivation of pre-existing faults provide effective subsurface traps for hydrocarbons. Whereas there is a long history of research focusing on the kinematics of inverted structures since the 1980s, the importance of the variation in the ratio between the thickness of the syn- versus post-rift successions in inverted structures was just recently recognized. As a result, positive inversion tectonics was subdivided into two modes. Mode I inversion structures are characterized with a thicker syn-rift succession than that of the post-rift sequence, and Mode II inversion occurs when the syn-rift strata is thinner than the post-rift one in a structure. Physical modelling efforts to date addressed only Mode I structures even though there are numerous examples well constrained by hydrocarbon exploration and production efforts where inversion occurred in Mode II. Therefore, this work specifically addressed the modelling of Mode II inversion structures. Our physical experiments building on earlier ones were designed to address positively inverted structures in the higher than 2:1 post-rift to syn-rift ratio range. The structural patterns observed both on seismic reflection sections and in sandbox modelling show that the propagation of the major bounding thrust faults up-section within the post-rift cover is progressively inhibited by a thicker post-rift sequence. Instead of breaking through the post-rift sequence in a concave-upward manner producing the hallmark harpoon geometry, the bounding reverse faults tend to tip out. This leads typically to a tri-shear geometry above fault tip. Therefore, Mode II inversion structures manifest themselves as slightly asymmetric, unfaulted large open folds at higher post-rift stratigraphic levels. This 3D geometric pattern of simple four-way structural closures makes Mode II inverted structures especially attractive for prospecting.