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The Southwest Ampa field (Figures 5.37 , 5.38 ,5.39 ,5.40 and 5.41) is situated some 25 kilometres west-northwest of Seria in water depths of 10 to 40 metres. The field was discovered in 1963 by well Southwest Ampa-1 (SWA-1).
Further exploration and appraisal drilling in the mid-1960's showed that the field is a northeast-southwest trending anticlinal structure, dissected by northeasterly trending growth faults. The field is divided into three major hydrocarbon-bearing areas: 1) Main field, 2) Southern area and 3) the 21-area (named after the discovery well Southwest Ampa-21, Figure 5.37).
Oil production from the Main field began in 1965 and reached 9,500 m3/d in the period between 1968 and 1970. In 1973, peak production rates from both the Main field and 21-Area totalled some 20,000 m3/d, and gas began to be supplied to the Liquefied Natural Gas (LNG) plant at Lumut.
To date, 279 wells have been drilled in the field and 161 are currently (1/1/96) producing. Most wells can produce from several intervals and many have dual string completions. The average 1995 production rates were some 8,400 m3/d liquids (6,950 m3/d of oil and 1,460 m3/d of condensate) and 21 million m3/d gas. The ultimate recovery for the field is estimated at 128 million m3 oil (36% recovery), 35 million m3 of condensate (42% recovery) and 345 109 m3 gas (86% recovery).
Oil reserves are evenly distributed between the Main field and 21-Area. The Southern Area contains only gas. Gas reserves consist of non-associated gas and gas contained in gas caps to oil reservoirs.
The reservoirs are heterogeneous, stratified, fine to very fine grained deltaic sands with varying clay content, separated vertically by shales of great lateral extent. All sands silt-out northeastwards across the field. Typical permeabilities are 100-300 mDarcy (0.1-0.3 m2) at 25% porosity. There are some 625 reservoir units with STOIIP's ranging from 0.002 to 26.7 million m3, between 1,000 and 3,500 metres subsea. The Southwest Ampa oil is light (40° API, 825 kg/m3) with an initial low viscosity (0.35 cp), hence the mobility ratio is favourable. Initial solution gas/oil ratio (GOR) ranges from 70-2,300 m3/m3. In the deeper accumulations, most oil reservoirs have extensive gas caps with initially about one-half the hydrocarbon pore volume being gas filled.
Depletion and gas cap drives are predominant for most Southwest Ampa reservoirs with limited aquifer influx. The total water influx is estimated to be less than 30% of the underground withdrawal.
In the deep reservoirs on the open unfaulted, westerly flank of the 21 Area, a transient aquifer behaviour, resulting in increasing water influx after a highly advanced stage of depletion, has been noted. Gas/Oil Ratio (GOR) control is exercised to maintain offtake rates and thereby maximise oil recovery through use of the gas cap energy. This offtake policy also serves to stabilise the oil rim against the tendency of it being pushed up dip by the advancing aquifer.
The performance of the shallow reservoirs, especially in the 21 Area, is governed by depletion drive and gravity drainage or water drive. Moderate pressure declines are observed and these reservoirs are still producing at low GOR's and almost zero water cuts. Through optimally placed drainage points and by control of the GOR quite high recovery factors are anticipated.
Oil recovery factors are likely to vary between 25 and 40% of the total oil (STOIIP) with the higher recoveries for the larger reservoirs, mainly in the 21-Area. An estimated 70% (range 50-80%) of gas initially in place (GIIP) is likely to be recovered.
Most of the oil reserves in Southwest Ampa have been produced, but it will remain a major gas source for many years. The field is the principal contributor for the export of liquefied natural gas via the Liquefied Natural Gas plant at Lumut.
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