Modelling Complex Structures in Papua New Guinea – Challenges and Insights

Abstract

Oil and gas fields in the Papua New Guinea (PNG) highlands fold and thrust belt have complex structural geometries that are difficult to accurately represent in 3D geologic models. Some of the most challenging features to model are low-angle thrust faults, steeply dipping to overturned beds, tight fold axes, repeat or missing sections, faulted faults and folded faults. Common issues encountered when modelling these features include, but are not limited to: (1) challenging pillar gridding due to low-angle faults, (2) inconsistent projection of isopachs, and (3) difficulty in building multi-Z overturned forelimbs. Due to the inherent structural uncertainty of the PNG highlands, most models in PNG rely heavily on iteration with production data, where it exists, to constrain and refine the structural models. Therefore, building 3D geologic models that are relatively simple and easy to update is important so that production data can be incorporated early in the modelling process. To keep the model efficient and simple, not all interpreted faults are modelled; only key faults integral to the structural framework and faults that define the original fluid contacts are built into the 3D structural framework, and simulation faults are iterated into the model to obtain a history match. This approach has been used in the Hides gas field where the simple model was used to test multiple scenarios in the dynamic model without requiring continuous updates of the static model. Some compromises and simplification of the framework are necessary to build a 3D geologic model that can be simulated within a reasonable timeframe. One key simplification which significantly improves pillar gridding and thickness output is to steepen low-angle faults in the model to be more orthogonal to the surface bed dip. Low-angle faults in combination with steep bed dip, often lead to artificial thinning or thickening of true stratigraphic thickness (TST) in the model. Given the uncertainty associated with mapping the true location of these low-angle faults, this compromise is deemed acceptable. However, merely steepening of the faults is not always sufficient, and it is sometimes necessary to retain the low-angle faults in the 3D geologic model. Issues that often still persist include: (1) grid distortion especially around steeply dipping area, (2) inconsistent thickness across faults causing incorrect fault juxtaposition, and (3) artificial thinning along tight fold axis. Multiple methods are available to overcome these issues. However, complex situations usually involve significant trial-and-error and the application of one or more framework modelling tools. One of the most effective approaches is to add pseudo faults into the framework to control pillar gridding and reduce grid distortion. In general, pseudo faults are added to rotate the pillars to be perpendicular to the surfaces. Pseudo faults are also essential in building overturned geometries; pseudo faults added along the fold axis, act as a segment boundary so that different surface inputs can be used in each segment. Horizon-fault-lines are also used extensively to control thickness across faults; ensuring accurate fault juxtaposition and honoring important fluid contacts spills and breakovers. To ensure consistent thickness throughout the model, a base model depth surface created from a TST map is used as a model horizon input to control the total thickness output of the model.

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