The Moomba and Big Lake Gas Field area has been actively explored for 25 years. However, recent drilling and field studies have identified new reservoir objectives for appraisal of established fields and for exploration in wildcat areas. Cooper Basin reserves have been increased and further additions are likely. Integration of drilling, production and pressure data for the Moomba and Big Lake Fields has resulted in the discovery of a structural-stratigraphic trap on the south-west flank of the Moomba Dome. Moomba-65 flowed gas at 9.8 MMCFD (0.27 Mm3/d) from deltaic sandstone of the Epsilon Formation (Early Permian). Similar plays are likely to be found on the flanks of other Cooper Basin fields and will become increasingly important as opportunities for conventional crestal tests of anticlines diminish.Exploration to the south-west of the Moomba Field has established the first significant gas flows from rocks beneath the conventional reservoirs of the Cooper Basin. Lycosa-1 drilled a faulted anticline and achieved a maximum gas flow of 5.0 MMCFD (0.14 Mm3/d) from fractured metasiltstone of the Dullingari Group (Ordovician). Moo- lalla-1 drilled a low-side fault terrace and flowed gas at 9.6 MMCFD (0.27 Mm3/d) from 'protoquartzite' tentatively assigned to the Dullingari Group. Consequently, structures where 'basement' reservoirs are faulted against mature Patchawarra Formation source rocks are attractive exploration targets.Petrological studies have identified 'glauconitic illite' in the Cooper Basin sequence suggesting hitherto unrecognised marine conditions. A reassessment of the source and reservoir potential of the region will be necessary if the presence of marine environments is substantiated by further studies.

The Dulang Field is located offshore east coast of Peninsular Malaysia in water depths of approximately 75 m. The field, discovered in 1981, is about 24 km by 3.5 km. After drilling 14 exploration/appraisal wells by both Carigali and its partner Esso Production Malaysia Inc., the central part of the field was developed as a unitized area in November 1990. Three 32-slot platforms have been installed in the unitized area, and development drilling is ongoing. Production commenced in March 1991 and is currently maintained at approximately 50,000 BOPD. The estimated OIIP (oil-initially-in-place) for the unitized area is in the order of 700 million barrels. There are 19 reservoir sands in Groups D and E which are of Middle-Late Miocene age. During the exploration/appraisal phase, oil and gas were encountered in the Group E and only gas in the Lower D1 reservoirs. Wireline formation pressure test data taken in the Lower D 1 reservoir in these wells plotted along a common trend with a gradient of 0.06 psi/ft. The lowermost gas pressure point was only 6 m above the normal hydrostatic gradient. It was therefore concluded that an oil column, even if present, would be thin. At the time, it was understandable that the gas pressures plotted along the same trend because the hydrocarbon column of the Lower D1 reservoir was large and extended beyond the limits of the major faults, suggesting a common pool. However, during the development drilling phase, it was discovered that the Lower D1 sandstone was a major oil reservoir, with estimated oil-in-place of about 100 million barrels. Oil columns of 75 m and 40 m have been proven up in the northern and southern flanks of the field, respectively, in the Lower D1. In addition, development plans were flexible enough to be able to effectively exploit the discovery. The well and formation pressure test data suggest that the Lower D 1 has a common pressure system in the gas cap over the central part of the field but different systems in the oil columns. Faulting is suspected to have provided both conduit and seal at different times to accommodate this phenomenon. P. 385

The Dulang field, discovered in 1981, is a major oil field located offshore Malaysia in the Malay Basin. The Dulang Unit Area constitutes the central part of this exceedingly heterogeneous field. The Unit Area consists of 19 stacked shaly sandstone reservoirs which are divided into about 90 compartments with multiple fluid contacts owing to severe faulting. Current estimates put the Original-Oil-In-Place (OOIP) in the neighbourhood of 700 million stock tank barrels (MMSTB). Production commenced in March 1991 and the current production is more than 50,000 barrels of oil per day (BOPD). In addition to other more conventional means, reservoir simulation has been employed from the very start as a vital component of the overall strategy to develop and manage this challenging field. More than 10 modelling studies have been completed by Petronas Carigali Sdn. Bhd. (Carigali) at various times during the short life of this field thus far. To add to that, Esso Production Malaysia Inc. (EPMI) has simultaneously conducted a number of independent studies. These studies have dealt with undersaturated compartments as well as those with small and large gas caps. They have paved the way for improved reservoir characterization, optimum development planting and prudent production practices. This paper discusses the modelling approaches and highlights the crucial role these studies have played on an ongoing basis in the development and management of the complexly - faulted, multi-reservoir Dulang Unit Area. Introduction The Dulang field is one of the most significant offshore oil fields in the Malay Basin which is also home to several other major fields such as Seligi, Tapis, Guntong etc. Dulang field is located about 130 km off the east coast of Peninsular Malaysia in water depths of about 76 m. in the South China Sea (Figure 1). Following field discovery in 1981, an additional 13 exploration / appraisal wells were drilled confirming an oil and gas column of up to 130 m in 19 stacked sandstones (classified into Groups D and E) of Middle - Late Miocene age (Figure 2). The Dulang field is an anticline which is approximately 24 km long and 3.3 km wide. The western and eastern portions are operated separately by Carigali and EPMI, respectively. Carigali, in addition, operates the central area which was unitized in 1988 and is referred to as the Dulang Unit Area (Figure 3). Both Carigali and EPMI are contractors to Petronas, the National Oil Company of Malaysia and operate under production sharing contracts (PSC's). The OOIP is in the order of 700 MMSTB. The field came on stream in March 1991 and is at present producing in excess of 30,000 BOPD. Cumulative oil produced amounted to about 60 MMSTB at the end of 1994. The crude gravity averages about 390 API. A summary of the range of reservoir and fluid properties is shown on Table 1. The associated gas production is about 43 million standard cubic feet per day and it contains more than 30% CO2. P. 161

Geochemistry of Crude Oils from Dulang Field: Some New Exploration and Development Insights into Source, Migration and Distribution of Hydrocarbons in the Malay Basin. Abstract Detailed geochemical data of oils from the Dulang field reveal the presence of three oil families, all of which come from very similar sources. The differences within the families reflect (1) fractionation processes operative during vertical migration and (2) minor contamination, especially by biomarkers extracted from coals by the migrating hydrocarbons. The deepest oils, which represent heavy residua from the fractionation process, are highly waxy, whereas the shallowest oils have low wax contents. The presence of free gas in association with the low-wax oils and its absence with the high-wax oils suggests that vertical migration may have occurred in the gas phase, with later separation of the light oil in the reservoir. Both associated and free gases contain a large amount of CO2, which is derived from a basement source. The vertical and lateral distribution of oil and gas properties, in combination with other geological and geophysical data, suggest that hydrocarbon emplacement occurred along faults. In some cases, separate reservoir compartments can be identified on the basis of geochemical signatures of the oils. These conclusions have important implications for exploration and field development in the Malay Basin. Introduction The Dulang field contains in excess of one billion barrels of oil and a substantial amount of gas, making it one of the largest fields located offshore Peninsular Malaysia (Fig. 1). The faulted, anticlinal trap is some 25 km long by 3-4 km wide and contains stacked Miocene sandstone reservoirs interpreted to have been deposited in a tropical tidal to sub-tidal environment. At least two episodes of faulting and several significant stratigraphic pinchouts complicate the field geometry. A typical well log over the production section is shown in Fig. 2. The field has been under development from 1990 until late 1996, with data from 118 development wells from four platforms and 15 exploration/appraisal wells attesting to the large number of reservoir compartments. Producing Group D and E reservoirs in Dulang are for the most part normally pressured (1500-1850 psig) and lie between 1100 and 1500 m TVDss. Overpressure increases rapidly below approximately 1450 m, reaching in excess of 5000 psig at around 1850 m TVDss in the deeper Group F sands. Both undersaturated and saturated oils are produced, with a range of gas cap sizes. The oils are predominantly waxy (7-55%), with considerable variation in physical properties (Table 1). The gas contains very large amounts of CO2 (40-75%). Geochemical techniques offer a useful, rapid and low cost set of tools to augment the more traditional geophysical, geological, petrophysical and engineering methods to learn more about vertical and lateral reservoir communication, migration pathways and gravity segregation within reservoirs. Such insights could have an impact on field depletion plans. The 1995 discovery of oil and gas in the overpressured Group F sands in Dulang stimulated further study to identify the source for those hydrocarbons and any relationships with the overlying oils. The results of the study reported here may lead to new exploration plays within Dulang as well as in other parts of the Malay Basin, particularly within the huge volume of untested overpressured section. P. 503^

TX 7508:Y-3836, U.S.A.. fax 01-214-952-9435. AbstractA process is demonstrated for building a deterministic reservoir model, using 3D acoustic impedance inversion and seismic-guided log property distribution. The extraction of fluid and pore volumes at proposed drilling locations assists the qualification of reservoir development scenarios. Fluid flow simulation assists the field development.