Four-way Dip Closure Research Articles

Palaeomagnetic analysis of fold closure growth and volumetrics

Abstract Fold closures (four-way dip closures) occur in several tectonic environments. These closures must attain lengths and widths of several kilometres before they can contain economically significant amounts of hydrocarbons. The growth kinematics of such large structures differ in some respects from those of geometrically similar, but smaller structures. Key aspects of the kinematics of large-scale fold structures are described, illustrated by new palaeomagnetic data from an example in the Salt Range of Pakistan. A numerical model describing the geometry of four-way dip closure is developed, relating the bulk volume to tangible geological parameters (limb dip, fold amplitude). An understanding of growth kinematics enables the application of a ‘fold test’, which provides a restoration of the palaeomagnetic data and host beds back to their attitudes at the time of magnetization. Comparison of the restored data with magnetic directions of known ages reveals the time of magnetization, so the geometry of the structure, including bulk volume at a known time, can be estimated. These data enable a curve describing the evolution of bulk volume through time to be calculated, which can be compared with the burial history of local source rocks — hence the trap volume available at the time of hydrocarbon generation can be estimated.

Pickens Field-A New Look at an Old Field

ABSTRACT Pickens Field in Madison and Yazoo Counties, Mississippi was discovered by the Kingwood Oil Company No. 1 Wilburn in March, 1940. This well was drilled in eastern Yazoo County six months after the discovery of Tinsley Field in western Yazoo County. Pickens was the fourth oil and gas field discovered in Mississippi, and has produced 22,270,565 BBL of 38-40 gravity oil and 2,527,603 MCFG through the end of 1991 from seven Upper and Lower Cretaceous reservoirs. The Upper Cretaceous Eutaw Wilburn Sand has produced 20,387,303 BBLS (91.6%) of the oil and 2,157,485 MCFG (85.4%) of the gas. Second most significant reservoir is fractured Selma Chalk, which was discovered in March, 1963 and has produced 1,731,391 BBLS (7.8%) of oil and 329,860 MCFG (13.1%) from twenty-seven (27) vertical completions. These two reservoirs continued to be productive through the end of 1991. Eutaw Wilburn Sand and fractured Selma Chalk oil production is only partially coincident. Eleven boreholes extending over a four mile distance west of the Eutaw producing area have produced seventy-seven per cent (77%) of the Selma oil. Four reservoirs in the Lower Cretaceous sequence have not produced significant volumes of reserves. An Upper Tuscaloosa reservoir discovered in 1989 remains to be more fully exploited. A migrated CDP seismic line shot across Pickens Field and the Pickens graben fault system in 1982 displays a structural complexity not evident from subsurface mapping. Remaining productive potential may remain in Lower Cretaceous angular unconformity traps, fault blocks with four-way dip closure, selected untested upthrown fault closures in the Upper Cretaceous and fractured Selma Chalk in fault blocks adjacent to certain faults that may be associated with vertical migration.

The Cyrus Field, Block 16/28, UK North Sea

Abstract The Cyrus Oilfield is located in Block 16/28 of the UK sector of the North Sea approximately 250 km (155 miles) NE of Aberdeen and 55 km (34 miles) NE of the Forties Field. The trap consists of a broad, very low relief four-way dip closure developed over a deeper tilted fault block. The reservoir consists of submarine-fan sandstones of late Palaeocene age, belonging to the Andrew Formation. Provenance was to the NW resulting from the early Tertiary sea-level fall which exposed the East Shetland Platform. The reservoir has been sub-divided into two zones, an upper zone of interbedded sandstones and mudstones with net to gross ratios of 0.4 to 0.6 and sandstone porositites of 12% to 18%, and a lower zone of massive fine-grained sandstones plus subordinate thin shales and limestones, with net to gross ratios in excess of 0.9 and porosities averaging 20%. The reservoir is filled with undersaturated oil of 35° API and is normally pressured. The estimate of initial oil-in-place is 75 MMBBL. Development of the field is centred on the use of BP's SWOPS (Single Well Offshore Production System) vessel using two horizontal field development wells which feed into a single seabed template for offtake. Ultimate recovery from the field is estimated to be approximately 12 MMBBL.

The geological and geophysical characteristics of structural?stratigraphic plays in the Offshore Gippsland Basin

Recent developments in the Offshore Gippsland Basin have suggested that not all large oil accumulations are necessarily restricted to four-way dip, closed structures at Top Latrobe level; the discovery of the Fortescue oil field in September 1978 indicated that further substantial oil accumulations may be trapped by a mechanism of combined structural-stratigraphic closure. Integrated well log correlations, micropalaeontologicaI dating, stratigraphic interpretation, geochemistry and reservoir pressure analyses have all been cited to indicate that the Fortescue oil accumulation consists of a series of discrete sandstone reservoirs lying on the western flank of, but separated from, the giant Halibut oil field. Seismic data have been incorporated with detailed geological data to develop a model by which such structural-stratigraphic plays can be recognised. Truncated seismic events, associated with the stratigraphic truncation of Upper Palaeocene to Lower Eocene delta-top coal swamp deposits, can be mapped across the area of the Fortescue field such that the existence of permeability barriers (basal and lateral seals) can be recognised and predicted. By combining the geometry of such permeability barriers with partial structural reversal at the level of the Top Latrobe unconformity, the geometry of the Fortescue oil field can be predicted. Palaeogeographical studies suggested that similar Palaeocene-Eocene coal swamp deposits might provide adequate permeability barriers to oil accumulations elsewhere in the Gippsland Basin. The structural-stratigraphic model was therefore extrapolated to other parts of the basin in an attempt to recognise such potential plays. It is suggested that the West Kingfish and Yellowtail oil fields, perhaps not fully defined at Top Latrobe level by complete four-way dip closure, could be further examples of the structural-stratigraphic concept. The Pisces No. 1 well, recently drilled by Union Texas Australia Incorporated and its Joint Venture Partners in Permit Vic P/12 on the southern margin of the Gippsland Basin, was a further test of the structural-stratigraphic play model. While the well proved to be dry, probably due to the lack of adequate vertical seal, the structural-stratigraphic model was essentially vindicated.