Companies unite for Carnarvon Exploration Cube
Multiple exploration companies united to prefund Fugro’s largest non-exclusive 3D seismic survey in the Carnarvon Basin. The Cretaceous Barrow Group highs and Triassic Mungaroo tilted fault blocks identified in the Fast Track Cube have been the highlight of initial investigations into the dataset.
The Eendracht 3D project has been motivated by the need for large volumes of high quality exploration 3D seismic data in a relatively underexplored section of the Carnarvon Basin. The project is located in a very exposed part of the North West Shelf, subject to extreme and unpredictable currents, which makes the area challenging for 3D seismic acquisition. Infill acquisition is always required to ensure adequate subsurface seismic reflection coverage, as areas may be affected by adverse currents and sub-optimal streamer feather angle matching, adding significantly to project costs.
With this in mind, the Eendracht 3D multi client survey has been the flagship project for the Fresnel Zone Binning (FZB) acquisition criteria and processing technique. By combining steerable streamer technology with the Fresnel zone binning analysis, streamers can be positioned in a manner to assist in optimal subsurface seismic coverage. After calculating the coverage tolerances for all offsets using the Fresnel zone analysis, the maximum lateral distance between adjacent streamers can be calculated along each streamer, and the lateral steering then assists in positioning the streamers to maximise coverage.
Large discoveries in Carnarvon Basin
In a maturing province such as the North West Shelf it is essential to have large quantities of good quality 3D seismic data in areas which are relatively underexplored within proven oil and gas fairways. The Carnarvon Basin is home to some of the biggest gas discoveries in recent years and the new acquisition of Eendracht has been strategically designed to meet industry needs for large high quality 3D seismic cubes.
The Exmouth Sub-basin, along with the Barrow, Dampier and Beagle Sub-basins, is part of the large Mesozoic intra-cratonic rift system of the Northern Carnarvon Basin, which developed in the Early Jurassic Sinemurian to Pliensbachian (196 – 183 MA) with the onset of the breakup of Gondwana. These en-echelon sub-basins now form a complex, elongate series of structural depressions trending sub-parallel to the present day coastline. Their development was accompanied by significant wrench and strike-slip faulting.
The Io Jansz gas field, about 200km offshore, is Australia’s largest gas field, estimated to hold over 20 Tcf of gas reserves. 3D seismic data over the field provided significant uplift over the existing 2D coverage in the area.
The eastern and western boundaries of the Exmouth Sub-basin are defined by the elongate Alpha Arch and broad Exmouth Plateau respectively. The Exmouth Plateau and northern extension of the Alpha Arch merge further to the north forming a loosely defined northern margin to the Exmouth Sub-basin. Although they have thin to absent Jurassic syn-rift sediments, these Triassic-cored structural highs exerted a major influence on the tectono-stratigraphic evolution of the Exmouth Sub-basin, as they enabled the deposition of very thick, restricted marine sediments, the major oil source for the area.
Many oil and gas plays
The Barrow Sub-basin hosts many new discoveries in the proven Lower Cretaceous Flag Sandstone play fairway. There have been more recent discoveries within the Upper Triassic to Lower Jurassic fluvio-deltaic/littoral sandstones in structural traps along the Gipsy-Rose-Lee trend. Exploration in deeper Jurassic and Triassic strata has also been successful with potential for deeper hydrocarbon plays.
The John Brookes 1/ST 1 gas discovery shed light on a purely structural play up–dip from Tryal Rocks 1, suggesting that the John Brookes 1/ST 1 gas reservoir was a direct continuation of the sandstones at Tryal Rocks 1. A review of the 3D seismic data over the field and seismic modelling supported this palaeo-depositional model.
The southern Exmouth Sub-basin has oil discoveries that define the Early Cretaceous Pyrenees Member play fairway, and high quality 3D seismic played a strong part in the recognition of hydrocarbon related seismic attributes in this. Ravensworth 1 intersected a 37m gross oil column below a 7m gas cap and is a complex structural/stratigraphic trap located on a north-east to south-west trending fault terrace. The 3D seismic data quality was integral to its discovery.
Marine transgressions inundated the Exmouth Plateau in the latest Triassic. In the Vinck 1 and Eendracht 1 wells there was approximately 50m of Rhaetian to Early Jurassic marl, the equivalent of the Brigadier Formation, which caps the Mungaroo Formation. The marl was deposited in a marine shelf environment. In Vinck 1 it is overlain by a thin Jurassic section of calcareous sandstones and marl deposited in a shallow marine environment.
The Dampier Sub-basin has received more than its fair share of attention with the success of Gorgon and Wheatstone, as extension discoveries to both are announced seemingly with no end. The Wheatstone gas discovery comprises two non-conformable and interconnected reservoir units consisting of shallow dipping, Triassic fluvial sandstones. It lies on the south-western flank of the Rankin Platform/Alpha Arch at the confluence of the continental shelf and shelf break. In the same area there is also a tilted north to north-east trending horst block situated at the northern end of an en-echelon horst block complex that includes the West Tryal Rocks and Sultan horsts.
The field represents a test of two possible interconnecting reservoirs; a northerly dipping Late Triassic fluvial sand unit which sub-crops against the regional intra-Jurassic/Callovian break-up unconformity, and a thin Oxfordian to Tithonian age marine reworked sand-draped unconformity.
High quality dataset
The Eendracht non-exclusive 3D seismic survey has proven to be a high quality dataset with excellent results achieved with the Fast Track Cube, which shows very good fault resolution with coherent imaging of deep reflectors. The 3D survey is well situated to extend the successes of the Carnarvon Basin to the west. Good prospectivity has been identified in the Exmouth Sub-basin, while the Barrow Sub-basin has seen the continuation of many play types, as well as the discovery of new plays, like the Pyrenees Member play. High quality 3D seismic cubes will be key to future oil and gas discoveries.
