Basement Architecture and Mechanical Stratigraphy: Hidden Controls on the Formation of Complex Forelimb Structures
The Agogo and Kutubu structures are producing oil and gas fields within the Papua New Guinea Fold Belt. The fields are contained within two en-echelon NW-SE trending anticlines that are truncated by near-planar thrusts separating the gently folded hangingwall fields from the complexly deformed, near-vertical forelimb fields in the underlying footwalls. The two structures are located ~15 km apart along the strike of the fold belt, and although their overall structural form is similar, they contain significant lateral variations in subsurface geometry that indicate the presence of local controls on structural style. In the absence of high quality seismic capable of imaging the core of steeply-dipping or complexly deformed structures, sandbox models provide important analogues for understanding subsurface structure and evolution, including the relative influences of basement architecture and the mechanical stratigraphy of the overlying sediments. In this study, 14 sandbox experiments were undertaken inside an X-ray tomography machine, allowing observations of the models to be made in three dimensions through time. Using materials that simulate brittle and ductile behaviour within rocks, initial parameters were varied between models as well as laterally within the same model. Observations can be made on the impact of varying the mechanical behaviour of the system, both at the scale of changing the rheology of individual layers as well as the brittle-to-ductile ratio of the whole stratigraphic column. Other variables explored included the rate of deformation, the geometry and inversion of basement features, and the influence of pre- and syn-tectonic sedimentation and erosion. The resultant scanned models provide digital realisations at time steps through the modelled deformation, analogous to 4D seismic. This enables the evolution of structural geometries to be interrogated, including assessing strike lines and oblique slices through the model – orientations that are inherently difficult to interpret in traditional 2D seismic. The models have significantly improved our understanding of the spatio-temporal evolution of the Agogo and Kutubu forelimb structures. The structural styles and mechanical coupling of the lithological layers in the models were found to be most sensitive to changes in the rheology of the overburden shales and the deeper syn-rift sediments that bound the relatively-stronger Late Jurassic to Early Cretaceous sandstone reservoir interval. Formation and resultant geometries of individual structures within the models were most heavily influenced by the underlying basement architecture, with the model best replicating the Agogo and Kutubu structures requiring a combination of multiple basement graben steps, a relatively strong stratigraphic sequence, and an increase in rate of compression over time, coupled with early inversion on the basement structures. In addition to an improved understanding of the Agogo and Kutubu structures, the methodical approach employed in this study allowed for inferences to be made regarding other structural features observed in the PNG Fold Belt, including NE verging backthrusts, small duplexes in transfer zones, and partially de-coupled deformation of the Darai Limestone with the Toro reservoir structures at depth. Sandbox modelling represents a powerful tool for understanding the relative role of basement architecture, mechanical stratigraphy and rates of deformation on structural style, with applicability to both near-field discoveries and wildcat targets, rendering the models a valuable addition to our standard geometric-based structural workflows.
AAPG Datapages/Search and Discovery Article #90371 © 2020 AAPG Asia Pacific Region, The 1st AAPG/EAGE PNG Geosciences Conference, PNG’s Oil and Gas Industry: Maturing Through Exploration and Production, Port Moresby, Papua New Guinea, February 25-27, 2020