The Jansz-Io Gas Field, Northwest Shelf Australia: A Giant Stratigraphic Trap

Abstract

The Jansz-Io gas field is located in production licenses WA-36-L, WA-39-L, and WA-40-L within the Carnarvon Basin, northwest shelf, Australia. It is 70 km (43 mi) northwest of the Gorgon gas field, 140 km (87 mi) northwest of Barrow Island, and 250 km (155 mi) from Dampier on the northwest coast of Western Australia. Water depths vary from 1200 to 1400 m (3937 to 4593 ft) across the field. The Jansz-Io gas field was discovered in 2000 by the Jansz-1 exploration well. A three-dimensional (3-D) seismic survey was acquired in 2004, and a further five wells were drilled between 2000 and 2009 to further delineate the field extent and size and characterize the resource to facilitate progress toward development. The Jansz-Io hydrocarbon trap extends over 2000 km2 (772 mi2) with both structural (faulted anticline) and stratigraphic (reservoir pinch-out) components. The stratigraphic component of the trap is defined by the reservoir extent, which is limited by depositional downlap to the northwest, and erosional truncation by Upper Jurassic and Lower Cretaceous unconformities to the southeast. The reservoir comprises muddy, bioturbated, predominantly very fine- to fine-grained sandstones deposited in a shallow-marine environment and is divided into two units. The upper wedge reservoir has 25 to 35% total porosity with 10 to 1000 md permeability, and the lower wedge reservoir has 15 to 25% porosity with 0.01 to 10 md permeability. Both reservoir units are expected to contribute gas during production. The original gas in place (OGIP) for the Jansz-Io Oxfordian reservoir has a probabilistic range from 320 to 946 Gm3 (11 to 33 tcf), with a P50 value of 632 Gm3 (22 tcf). The ultimate recovered gas for the field will depend on both the development plan and the reservoir performance over field life. For the current 15-well development plan, the resource estimates range from 201 to 442 Gm3 (7 to 16 tcf). The Jansz-Io gas field is a key part of the greater Gorgon liquified natural gas (LNG) project and will supply gas to the LNG plant that is being constructed on Barrow Island. The development concept includes subsea completions from three drill centers placed on the seafloor connected to a subsea production pipeline to carry gas to the LNG processing plant. For the first stage of field development, 10 development wells were successfully drilled and completed during 2012 and 2014. The second drilling campaign is planned to commence after field start-up with the timing dependant on field performance.