The Kingfisher Field, Uganda - A Bird in the Hand! S R Curd, R Downie, P C Logan, P Holley Heritage Oil plc *

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1 A Bird in the Hand! EXTENDED ABSTRACT The Kingfisher Field, Uganda - A Bird in the Hand! Heritage Oil plc * The Kingfisher Field lies on Uganda s western border beneath Lake Albert, situated in the western arm of the East African Rift Valley System, (Figures 1 and 2). The field lies wholly within Exploration Area 3A, and is operated by Heritage, who are partnered by Tullow Oil. The discovery well, Kingfisher 1 (and 1A) tested at a combined rate of 13,893 bopd from four reservoir intervals of Late Miocene- Early Pliocene age. The field has subsequently been appraised by three further wells and of those four wells, three have been suspended as future producers. The Kingfisher oil is a sweet low sulphur oil of c o API with some associated wax, being typical of oils sourced from a Type II lacustrine source rock. The oil has a moderate gas:oil ratio of approximately scf/bbl and it is thought that the reservoir has partly degassed. Reserves are estimated to be in the region of 200 mmbo and plans are in preparation for the development of the field. The Albert Basin is some 150 km long by 35 km wide and trends southwest northeast. The field is approximately 16 km long by 3.5 km wide and covers an area of 37.5 km 2 (Figure 3). The structure itself comprises a southwest-northeast trending compressional feature with dip closure to the north and west, closure to the southeast being created by the main bounding fault of the Albert Basin. The structure has two culminations, Kingfisher South, on which the three wells and sidetracks have been drilled, and Kingfisher North, which will be reached by offshore drilling. Although much of the field lies under Lake Albert, the Kingfisher South structural culmination lies under a narrow strip of land, some 10 km by 2 km, located adjacent to the basin bounding fault and which represents a surface expression of the Kingfisher structure. All three wells and sidetracks drilled to date have been sited on this area (Figure 3). The geological sequence in the Albert Basin is of Miocene Recent age, resting on metamorphosed Precambrian basement. The oldest sediments so far encountered have been of Late Miocene age. It is thought that approximately 6,000 metres of section were deposited in the central part of the basin, with some 3,000 metres present in the area of the Kingfisher Field. The sequence comprises a series of interbedded sandstones and shales, representing a mixture of low-stand events, when sedimentation was dominated by fluvial processes and flood or highstand events, when lacustrine deposition dominated. Structurally, the Kingfisher Field is located on the south-eastern margin of the Albert Basin. Sinistral strike-slip movement across the basin is thought to have caused compressional structures to form at restraining bends in the basin margins resulting in gentle folding of the sediments. The Kingfisher Field lies in one of these uplifted areas (Figure 4). The seismic database consists of an approximately 2 kilometre-spaced grid of 2D seismic data acquired in 2003, on which the structure was first identified. This grid was infilled to a 1 kilometre grid over the Kingfisher structure in Starting in April 2007, a 330 km 2 3D survey was acquired (Figure 6). Seismic data quality and the continuity of reflectors is fairly good for most of the 3D survey area, but over the crest of the Kingfisher structure, data quality becomes poor (Figure 6). This is thought to be due to the escape of dissolved gas from the Kingfisher reservoirs, through the gas-permeable top seal to form a gas cloud. This gas, held in the sands and shales, inhibits the transmission of seismic P wave energy resulting in very poor reflector continuity. The individual reservoir intervals in Kingfisher cannot be resolved on the seismic data due to the relatively low frequency content of the data. Figure 7 shows a time structure map on the top of the Early Pliocene sequence, illustrating the closure of the Kingfisher sequence against the basin-margin fault. A typical stratigraphy for Kingfisher is shown in Figure 8. An informal chronostratigraphy has been erected, based on high-resolution quantitative palynology and is also being used to recognise climactic variations in the basin history.

2 A Bird in the Hand! The Kingfisher reservoirs comprise four, fluvial sandstone units of Late Miocene Early Pliocene age. The uppermost Zone 1 sand is some 7 metres thick, and of limited areal extent, although when tested in Kingfisher-1, it flowed at more than 4,000 bopd. The principal reservoirs in Zone 2 comprise three sandstones, termed the Kingfisher, A, B and C sands, within a gross interval of some 100 metres. They lie at a depth of approximately 2,100 metres below surface (-1,500 m TVDSS) and may be broadly correlated over the field, together comprising c. 50m net sand. Palynology has shown that the C sand is of Late Miocene age, while the A and B sands are of Early Pliocene age. The Late Miocene earliest Pliocene sequences encountered in each of the four wells penetrating this section on the Kingfisher South culmination are shown in Figure 9. Examination of cores from Kingfisher-3A shows that all three reservoir units comprise a series of cross-bedded sandstones, interbedded with varicoloured, frequently red-brown mudstones as illustrated by Figure 10. The sandstones are generally fine to coarse, predominantly medium grained, moderately to poorly sorted and are dominated by the presence of well developed trough cross-bedding. Subordinate planar bedding is present in places and rootlet (burrow) structures are locally visible. The sequence is interpreted as having been deposited in a Distributary Fluvial System (DFS) that is thought to have derived from a point-source some 10 km south of the field, fed by a major river, formerly part of a Late Tertiary proto-congo drainage system, as illustrated in Figures 11 to 13. It is believed that prior to a major phase of inversion, possibly as recently as 500,000 years ago, a large part of western Uganda s drainage system fed into a point source on the southeast margin of the Albert Basin, just south of where Kingfisher lies today (Figure 13). The petrology of the reservoirs accords with this hypothesis, the sands showing a compositional maturity that is very different from that of the rocks which make up the flanks of the Albert Basin. The sands comprise 40-70% fine-medium quartz, sub-angular - sub-rounded, the other principle constituents being feldspar (mainly potassium) and lithic fragments. The reservoir sequence in Kingfisher-3A also shows evidence of long periods of sub-aerial exposure, with colonisation by plants, interspersed with periods of drowning when lake level rose, resulting in the deposition of lacustrine shales. Top seal to the reservoir sequence is provided by such a lacustrine shale unit which is approximately 15 metres thick and bottom seal by a similar shale unit, some 21 metres in thickness. The A sand is not in communication with the lower B and C sands, but has a separate oil-water contact. The B and C sands are thought to share a common oil-water contact and to be in communication. All three sands are thought, however, to share a common aquifer. The fact that the three sands can be correlated between the wells over a distance of approximately 6 km, indicates that reservoir development is likely to be consistently good over the area of the field and that a high level of connectivity is to be expected in both the horizontal and vertical directions, as intervening shales are likely to be breached by down-cutting channels in the DFS. Log analyses show the formation to have good to excellent porosities with layer averages typically in the range 22-24%. Initial core analysis on Kingfisher-3A indicates that reservoir permeabilities are excellent with values typically in the range ,000mD. The arithmetic mean horizontal permeability is 1,769mD. These permeabilities are commensurate with those indicated from tests on wells Kingfisher-1A and Kingfisher-2. When tested in Kingfisher-2, the three main reservoir sands flowed at an aggregate rate of 14,364 bopd. The source rock for the Kingfisher oil has yet to be encountered during drilling, which is not surprising given that the wells drilled to date are confined to the basin margins. It is probable that the source rock is age equivalent to the Kingfisher reservoirs, and was deposited during a period of lake high-stand(s). Geochemical analysis of the oils found to date indicate a typical type II source-rock, with algal material providing the principal source component, with land-plants providing a minor contribution. Original oil in place for Kingfisher is currently estimated to be in the region of 600 mmbo, while recoverable reserves are expected to be c. 200 mmbo.

3 A Bird in the Hand! In conclusion, the Kingfisher Field represents the first major discovery to be made in this new frontier basin and as such presents the geoscientists working on the project with a number of interesting challenges. The cooperation of Tullow Oil in the publication of this abstract is gratefully acknowledged. * Heritage Oil plc Technical Services Office 34 Park Street London W1K 2JD

4 Figure 1 Exploration Area 3A Location Map

5 Figure 2 Kingfisher Field Location Map

6 Figure 3 Kingfisher Well Locations

7 Figure 4 Geometry of Basin Margin Fault at Reservoir Depth showing Restraining Bend

8 Key To Horizons PL2 Late Pleistocene PL1 Early Pleistocene P3 Late Pliocene P2 Mid Pliocene P1 Early Pliocene Figure 5 Northwest-southeast Seismic Dip Line through Kingfisher-1

9 Figure 6 Interpreted Gas Cloud on Seismic Data

10 Figure 7 Top Early Pliocene P1 Time Structure Map over the 3D Survey Area

11 Figure 8 Kingfisher Field Generalised Stratigraphic Column

12 Figure 9 Correlation of Kingfisher Reservoir Units

13 The Kingfisher Field, Uganda Figure 10 Kingfisher-3A Core Photographs

14 Figure 11 Paleogeographic Reconstruction of Proto-Congo Drainage Pattern

15 Figure 12 Paleogeographic Reconstruction of Kingfisher DFS

16 Figure 13 Restored catchment area of Kingfisher DFS