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Magpie

The Magpie field is situated 50 kilometres to the northeast of Seria. Waterdepths over the field vary between 35 m on the crest and 52 m on the eastern flank (Figures 5.58 , 5.59 and 5.60). The field was discovered in 1975 by well Magpie-1 (MG1), which found 14 metres of net-gas sand and 35 metres of net-oil sand in the F and G sands, on the northeastern flank of the Magpie structure.

A further eight exploration / appraisal wells quickly followed before the decision to develop the field was taken. In 1978, the first platform (MGDP-1) was installed, close to the discovery well. Appraisal and development drilling were initiated from the drilling platform and first production commenced at the end of that year. A second platform, MGDP-2, was installed in 1980, approximately 1.3 kilometres to the northwest of MGDP-1, to develop the northern part of the field and to appraise the west flank of the structure. In 1981, a third platform, MGDP-3, was installed approximately 1.6 kilometres to the southeast of MGDP-1, to allow production and further appraisal in the southern part of the field.

To date (1/1/96), 33 wells have been drilled in Magpie: 10 exploration/appraisal and 23 development wells. The 1995 average production rate was approximately 1,500 m3/d of liquids from 12 wells. The expected ultimate recovery for the field is 21.1 million m3 (44 % recovery) of oil and 3.1 109 m3 of gas (58% recovery).

The main hydrocarbon accumulations are trapped by fault-dip closures against the westerly dipping Magpie main fault. This fault separates the eastern blocks, which contain the main hydrocarbon accumulation, from the 'relatively unappraised' and strongly faulted western blocks.

Hydrocarbons are found in shallow (850-1,150 metres subsea) clean, virtually unconsolidated sands of Late Miocene to Pliocene age. For the main reservoirs, sand deposition may be partly structurally controlled. An overall thinning towards the crest of the structure from the eastern flank in the main F-reservoirs, suggests deposition against a palaeo-high. Thickening against faults suggests synsedimentary structural growth.

The main reservoir sequence (F sands) consists of well sorted and rounded shallow marine to coastal sands. Reservoir qualities deteriorate to the northwest. Porosity ranges from approximately 10 to 35% with associated permeabilities of 100 to 2,000 mD (0.1-2 m2).

A wide range of oil gravities (23-43° API or 915-810 kg/m3) is found even within individual fault blocks and sands. This variation is attributed to bacterial transformation, as indicated by geochemical analyses, rather than natural gravity fractionation or different times of expulsion for the various crudes.

The fault blocks are connected to regionally extensive aquifers, and material balance calculations indicate a significant waterdrive. It is expected that a 44% ultimate recovery of STOIIP (or even higher) will, in general, be achieved with the available strong water drive energy.

The Magpie field consists of 53 oil and gas reservoirs. Their STOIIP/GIIPs are reasonably well known, partly as a result of seismic studies by which fluid contacts could in some reservoirs be mapped directly.

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