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Regional Sequence Stratigraphic Architecture and Paleogeographic Interpretation of the Tithonian to Berriasian Reservoir Systems , Papua New Guinea


The key clastic reservoir section in Papua New Guinea is Tithonian to Berriasian in age, spanning biostratigraphic sub-zones LJ5 to EK10. The use of litho-stratigraphic field-based reservoir nomenclature has resulted in significant miss-correlations when used on a regional scale. The gross reservoir system has previously been described as a general shoreline succession orientated northwest-southeast parallel to the main structural trend, but has not been subdivided regionally to address how individual sands relate to each other from field to field or how the system developed spatially through time. Individual fields typically have reservoir units termed Toro A, B, C, etc, but the use of this litho-stratigraphic nomenclature implies a reservoir correlation from field to field which is not supported by biostratigraphy. A detailed chronostratigraphic study was undertaken to resolve these discrepancies. Regional reservoir correlations were created using well log-based sequence stratigraphy guided by a standardized biostratigraphic subdivision and the SEPM eustatic curve, to provide an integrated description of how the reservoir system develops spatially through time. The relationship between second and third order eustatic controls on reservoir deposition was established. Paleogeographic maps were made for the main third order lowstand and transgressive intervals from the LJ5 through to EK10 subzones. The reservoir system represents a third order, lowstand dominated sequence set deposited during an overall second order eustatic fall on a low accommodation, shelfal ramp setting. Generally, lowstand and transgressive systems tracts are preserved, but highstands are typically eroded. Sand body development shows a spatial progression through time with some units having lateral shale equivalents or are not deposited. Integration of these results with concurrent regional structural studies have established a pre-existing structural grain which initially controls accommodation, captures up-dip stream inputs and influences the position of shorelines. Sediment enters the basin from the south and west across a faulted low relief structural platform that plunges to the southeast and steps into the basin deep to the northwest. This structural fabric results in shorelines that trend sub parallel to the present day fold belt through most of the Highlands, becoming more north-northeast orientated on the eastern margin. As accommodation fills in, eustatic changes exert a stronger influence. Early systems were focused in the southeast with depositional systems backstepping to the northwest through the EK14 interval. The EK13b through to EK12-1 sequences show widespread reservoir development through all the fields, although thickness and quality does vary depending on proximity to sediment input and accommodation. A further second order fall during the late Berriasian, brings a lowstand system to the southeast again after transgressive shale deposition during a brief second order rise. Higher order eustatic cycles combined with changes in estuary input position and prominent northwest along shore drift, are locally important during the development of the overall reservoir system, particularly in the later Toro cycles. The study has provided a consistent regional chronostratigraphic sequence architecture that explains how the reservoir systems develop through time and correlate from field to field. The maps provide paleogeographic control for predicting facies trends within individual third order systems tracts between fields.