The Jansz gas field is located in permit WA-268-P, 70 km northwest of the Gorgon gas field in the Carnarvon Basin. The Jansz–1 discovery well was drilled in April 2000 and intersected 29 m of net gas pay in an Oxfordian age shallow marine sandstone reservoir. The well drilled a stratigraphic trap on the western limb of the Kangaroo Syncline.The Io–1 well was drilled in January 2001 in the adjacent permit WA-267-P (18 km from Jansz–1) and intersected the same Oxfordian sandstone reservoir penetrated by Jansz–1, with a total of 44 m of net gas pay. The Tithonian and the Upper Triassic Brigadier Sandstone gas reservoirs at Geryon–1 (1999) and Callirhoe–1 (2001) in WA-267-P are in pressure communication with the Oxfordian gas reservoir at Jansz–1 and Io–1. Consequently, the three different age reservoirs comprise a single gas pool, with a common gas/water contact.The Jansz gas field has been delineated by four wells and 2D seismic. The gas sandstones have a prominent amplitude versus offset response, which defines the field limits. The Jansz gas field is confirmed by drilling to be an areally extensive (2,000 km2) gas accumulation with a gross column height of 400 m and an estimated 20 TCF (566 G.m3) recoverable sales gas, which represents 40% of the discovered gas resources in the deepwater Carnarvon Basin. The size of the Jansz gas field and its remoteness from existing pipeline gas markets suggests that an export LNG project will be the basis for its development.
The Geryon, Orthrus, Maenad and Urania Gas Fields are located in permit WA-267-P in approximately 1,200 m of water, and between 35 km northwest and 70 km north of the Gorgon Gas Field in the offshore Carnarvon Basin of Western Australia. Five wells were drilled in these fields between August 1999 and February 2001 as part of a six-well, three-year obligatory drilling program. The primary objectives were late Triassic sandstones of the upper Mungaroo Formation. The Geryon and Urania Fields are three-way footwall structures, while the Orthrus and Maenad Fields comprise four-way horst structures where progressively older units subcrop against the Callovian Unconformity. All objective reservoirs were amplitude associated and had strong AVO signatures, which was instrumental in the high exploration success rate and excellent exploration prediction of OGIP from seismic data.This paper will briefly discuss the description of late Triassic and early Jurassic reservoirs and the transition of the AA sand of the Mungaroo Formation from fluvial to marginal marine facies in the Greater Gorgon Area, the recent drilling results of the Triassic Prospects in WA-267-P, and the geophysical attributes of the AA sand Mungaroo Formation reservoirs.The WA-267-P Triassic Gas Fields are estimated to contain approximately 210 billion m3 (7.4 TCF) recoverable sales gas. The close proximity of these Triassic gas fields to each other, the clean gas composition and size of resource base suggests these fields are excellent candidates for a future gas development in Western Australia.
The Wheatstone gas discovery is located 110 km northnorthwest of Barrow Island in the Dampier sub-basin, northwest Australia. The field comprises two non-conformable and interconnected reservoir units consisting of shallow dipping, Triassic fluvial sandstones, and an overlying Tithonian transgressive sand. These units are partially separated by Late Triassic to Early Jurassic sediments.The Tithonian sand reservoir is instrumental in hydrocarbon accumulation in the Greater Gorgon Area due to its unconformable connectivity to underlying Triassic reservoirs, and its ability to act as a thief zone. It is therefore a significant component in hydrocarbon entrapment. This paper discusses the transgressive Tithonian sand palaeogeography, and environment of deposition as a predictive tool of reservoir risk assessment and play fairway geography.This study compares and contrasts the rock properties of the latest Triassic sequences and proposes the palaeogeographic make up within the basin. A review of the reservoir properties has identified differences between units which may impact the formation evaluation approach used. The thinly bedded character of the latest Triassic sequences necessitated the use of a non-traditional formation evaluation model that has improved the accuracy of the wireline predicted reservoir properties. Acquisition of probe permeability data through conventionally cored intervals identified the limitations associated with standard core plug sampling procedures, and ensured better net reservoir definition through the sequence.
The Perdido fold belt, located in the northwest part of the Gulf of Mexico basin, is defined by a series of large-scale fold structures that extend southwest into Mexican waters and northeast beneath the Sigsbee salt nappe. Within the Alaminos Canyon OCS lease area, the fold belt consists of northeast-southwest trending, sub-parallel, concentric, box folds cut on one or both of their flanks by high-angle reverse faults. The folds are slightly asymmetric and verge both landward and basinward, a geometry typical of contractional fold belts formed above a weak detachment layer. The folds uplift the regional middle Cretaceous sequence boundary (MCSB) by up to 3 km, with a basinward decrease in height and amplitude of the folds. Detailed structural mapping has led to a model for the structural evolution of the Perdido fold belt that is consistent with sequence stratigraphic analysis of the seismic data. Minor salt movement occurred during the Late Jurassic and Early Cretaceous, as indicated by onlapping and thickness variations within the relatively thin overlying section at that time. Salt mobilization before the main phase of shortening led to early growth of some fold structures during the Eocene-early Oligocene. The main phase of compressional deformation occurred during the late Oligocene-early Miocene by gravity sliding on a detachment within the Jurassic Louann Salt. The basinward limit of autochthonous salt deposition defined the southeastern margin of the foldbelt. Detailed analyses of onlapping middle to upper Miocene strata indicate that separate folds had different evolutionary histories and developed variable along-strike geometries. Subsequent Pliocene to present-day reactivation of the highest-relief structures further modified the fold geometries. Topographic relief over the highest folds has in turn influenced the evolution of the allochthonous Sigsbee salt nappe. The advancing salt nappe has been deflected around the highest fold structures, resulting in a complex allochthonous salt sheet geometry. Previous studies of the Perdido fold belt have produced conflicting interpretations for the evolution of the fold geometries. These include salt or shale-cored interpretations and the development of the fold geometries by imbrication and fault-bend folding. Our interpretation favours an origin as salt-cored detachment folds, with late modification by re-mobilization of salt in the cores of the folds.
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