--> Connectivity Analysis of Regional Aquifer Systems: Foreland, Fold & Thrust Belt, Papua New Guinea

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Connectivity Analysis of Regional Aquifer Systems: Foreland, Fold & Thrust Belt, Papua New Guinea

Abstract

Aquifers associated with oil and gas accumulations in PNG exhibit complex pressure & salinity distribution. This is investigated using approaches described in ‘Reservoir Connectivity Analysis’ (Vrolijk, 2005: SPE-93577). Pressure data from ~130 wells and regional structure maps are integrated to identify ~12 ‘aquifer pressure systems’ in Toro & older sandstones. Interpretation is limited by structural resolution; some systems may be connected, and others likely exist, but, in principle, multiple ‘local’ systems are interpreted. In contrast, previous interpretations invoke a regional Toro hydrodynamic system, with associated salinity changes, from Muller & Lavani outcrops through the fold and thrust belt fields to the foreland. The intent is to understand the entry, exit and distribution of fluids and pressures in each system. Excess aquifer pressure is likely caused by tectonic stress, hydrocarbon systems, surface recharge, and recent uplift. Pressure dissipation is continuous, by Darcy flow - or discrete, where perched waters in fields are controlled by breakovers (at Kutubu this has superseded interpretation of tilted contacts). Fluid & pressure ‘exit’ is interpreted: (1) at Toro sandstone outcrops on the Muller anticline; (2) at Darai limestone outcrops above frontal thrusts (where Ieru shale topseal is fully offset, hangingwall Toro juxtaposes footwall Darai, which is self-juxtaposed to Darai outcrops); (3) by topseal failure in isolated high pressure compartments at leak-off; (4) by topseal failure at more moderate pressures near volcanic stocks; (5) in the foreland, cross-fault connections occur at inverted normal faults, & locally, Darai overlies Toro unconformably due to late Cretaceous uplift & erosion (Darai is overlain by permeable clastics to surface). Previous regional hydrodynamic interpretations invoke Toro-to-Toro connections across major fault systems but do not consider: (1) the absence of biodegradation in light oil and wet gas fields at low temperatures is inconsistent with a regional through-going meteoric water system; (2) aquifer juxtapositions interpreted at frontal thrusts south of Mananda could connect surface and reservoir aquifers in static equilibrium without mixing or enabling surface organisms to access hydrocarbons that were ‘sanitized’ pre-uplift or migration; (3) observed, brackish aquifers could result from late Cretaceous or syn-depositional meteoric influx, or by freshwater generated by clay transformations.