EVOLUTION AND KARSTIFICATION OF THE EOCENE-MIOCENE CARBONATES OVERLYING THE TANGGUH GAS FIELDS IN WESTERN PAPUA - OBSERVATIONS FROM 3D SEISMIC AND IMPACT ON DRILLING OPERATIONS

摘要

The Tangguh gas fields in the Birdshead region of West Papua comprise the giant Vorwata field (currently producing gas to supply two 3.8 MTPA LNG trains) plus three undeveloped smaller structures. The traps have a structural and stratigraphic component, with shallow-marine Jurassic and deep-marine Paleocene clastic reservoirs sourced from deeply buried Jurassic and older shales. Overlying the fields is a complex overburden which presents significant drilling challenges, including the presence of shallow gas, overpressured sands, faults and karst features which lead to losses and hole-instability. Many of these drilling NPT (Non Productive Time) issues are related to drilling through a thick (3000 ft) carbonate sequence of Eocene to Miocene age, locally called the Faumai and Kais sequences.rnThe bulk of the Tangguh area is covered by 3D seismic of a variety of vintages and acquisition techniques, including towed-streamer, land, transition zone and OBC. Modern wide-azimuth OBC seismic is designed for reservoir imaging, while the more laterally-extensive narrow-azimuth towed-streamer dataset (acquired in 1997, and reprocessed several times) gives high-quality images of the overburden down to the carbonate intervals. Coherency and spectral decomposition techniques reveal startling karst features with a variety of geometries and dimensions at a number of key sequence boundaries. Often these seem to be structurally controlled, i.e. following the fabric of the strike-slip influenced faulting that occurred during the protracted and complicated collision between the Australian and the Pacific Plates in the early Tertiary.rnThe evolution and deformation of the Faumai and Kais has been investigated, with evidence for multiple episodes of exposure and karstification punctuating periods of carbonate production. Structural history and original carbonate facies strongly controls the karst development, and can be linked to past drilling problems.