The 1st AAPG/EAGE PNG Geosciences Conference, PNG’s Oil and Gas Industry:
Maturing Through Exploration and Production

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An Overview of Structural Style, Trap Configuration and Reservoir Connectivity in the Western and Central Fold Belt, Papua New Guinea


Understanding the structural and stratigraphic relationships that control the distribution of hydrocarbons in a field improves the accuracy of reservoir models and aids the identification of near-field exploration targets. To help achieve this, a comprehensive re-assessment of structural style, trap configuration and reservoir connectivity was undertaken for oil and gas fields in the Western and Central Fold belt, including Hides, Juha, Angore, Agogo, Hedinia-Iagifu, Moran, Usano, SE Hedinia and Gobe. This work aimed to integrate geological, geophysical and engineering datasets to develop concepts and models that were consistent across all the fields. By taking a regional perspective, this work identified key commonalities and differences across the fold belt, leading to a more effective characterization of individual fields and improving our understanding of the processes that control the distribution of fluids across the fold belt. For each field, the first step in the workflow was to construct a detailed 3D structural model using the available surface, well and seismic reflection data. This was done by creating a series of balanced cross sections through the field using appropriate kinematic forward modelling and palinspastic restoration techniques to test evolutionary concepts and refine our interpretations. Balanced cross sections and dip panels defined by well intersections were then used to create the horizon and fault surfaces of the 3D structural model. The resulting fault framework was tested by examining displacement profiles for each horizon, adjusting where necessary to remove geological inconsistencies. Reservoir intervals in the 3D structural model were evaluated in the context of fluid pressures and composition data to identify potential compartments and to characterize the nature of fluid entry and exit points, e.g. structural spill points, fault-controlled leakage, capillary top seal leak and fluid breakover mechanisms. Fault juxtapositions and the shape of reservoir horizons were then modified to achieve a satisfactory history match. This presentation will summarize the key results of this work: describing new concepts and ideas on the structural style of the fold belt; discussing key structural differences and their impact on trap configuration; and evaluating the role of faults in reservoir connectivity within fields. Based on this work, we recognized four types of hydrocarbon traps in the Western and Central Fold belt: gas-flushed four-way dip traps, filled to the structural spill point, e.g. Hides; normal-pressured, multi-fluid traps leaking gas though large fault zones and spilling oil at structural or fault-controlled spill points, e.g. Agogo; over-pressured, underfilled traps leaking gas and oil though mechanical failure and capillary leak along large bounding fault zones, e.g. Moran; and under-pressured, gas-flushed traps, filled to a fault-controlled spill point, e.g. SE Hedinia. Based on these observations, we suggest the large thrust fault zones (collectively accommodating ca. 1 km of displacement) that bound the Type 2, 3 and 4 traps control the fluid composition and pressures within the fields. Our results indicate these large thrust fault zones cannot act as effective barriers to fluid flow over geological timescales, despite having high shale gouge ratios (ca. 70%) that would often be expected to seal. Within fields, our analysis indicates many of the smaller-scale faults (<200 m of displacement) cannot be sealing when sand layers are juxtaposed, allowing pressure communication between different reservoir intervals. These results help us to characterize the structural and stratigraphic relationships that control the distribution of hydrocarbons in the Western and Central Fold belt, allowing us to more accurately predict fluid compositions and pressures in adjacent prospects and untested compartments in the future.