Lengguru, Irian Jaya: prospect selection using field mapping, balanced cross-sections, and gravity modelling

Abstract

Abstract Exploration mapping involving acquisition of detailed structural, biostratigraphic and line gravity data has allowed effective prospect analysis across the 22 main leads in the frontier Lengguru Thrust-Fold Belt. Evaluation has centered around production of structurally viable retrodeformable balanced cross-sections constrained by gravity modelling ('gravity balanced'). Lengguru is an arcuate, NW-SE trending, Plio-Pleistocene thrust-fold belt comprising both inversion and non-inversion imbricate thrust structures. The external zone contains large-scale, detached, ramp anticlines with structures dominantly thin-skinned and no basement-involvement in the hangingwall. Basement is mostly through-going, but sub-thrust extensional systems which offset basement are suggested by regional gravity data at the boundary with the internal zone. Thick platform carbonates of the New Guinea Limestone Group (NGL) form a major competent unit. The internal zone comprises closely-spaced imbricates and smaller-scale ramp anticlines, many of which are breached to the Kembelangan Group. Close spacing of the thrusts reflects a lithological change from platform to distalfacies carbonates, and the boundary with the external zone represents the palaeo-shelf margin in limestone depositional times. Main exploration risks in Lengguru involve: (a) accurate identification and delineation of individual structural traps, and (b) recognition of whether or not a trap is within the productive fairway of the thrust-fold belt. Structural risk and ranking criteria center around two principal aspects: target reservoir volumes in hangingwall ramp anticlines, and investigation of possible inversion geometries. Prospects with the largest reservoir volumes in four-way dip closure with a probable inversion history were high ranked. Balanced cross-section construction and gravity modelling represent valuable low-cost exploration tools in the absence of seismic reflection data. Independently, neither technique could sufficiently constrain structural risk to allow selection of drilling locations in Lengguru. When used together, the resulting modelling significantly reduced exploration uncertainty.