DEVELOPMENT POTENTIAL SEEN IN BREDASDORP BASIN OFF SOUTH AFRICA

Malcolm Wood
Soekor (Pty.) Ltd.
Cape Town

The Bredasdorp basin is situated off the south coast of the Republic of South Africa, southeast of Cape Town and west-south-west of Port Elizabeth (Fig. 1)(83457 bytes).

The basin covers about 18,000 sq km in generally less than 200 m of water.

Production from F-A gas field, 85 km south of Mossel Bay, began in 1992 with gas and condensate being piped from a fixed platform to an onshore facility for conversion to liquid fuels.

The reservoir for this field is in the upper part of the synrift shallow-marine succession. Two small satellite fields lie up to 25 km to the northwest of the F-A platform along the northern flank of the basin (Fig. 2)(38223 bytes).

E-M gas field 40 km west-northwest and E-FO gas field 30 km southeast of F-A field indicate a fairway of more than 70 km in length along this northern flank.

Reservoir sandstones also occur within the drift succession in submarine turbidite fan-channel complexes that were deposited during Aptian and Albian lowstands. These complexes host several small oil and gas fields with development potential (Fig. 2)(38223 bytes), and it is these fields that are the focus of this article.

DISCOVERIES

Exploration in the drift succession began in 1986 with the drilling of borehole E-AA1 at 122 km southwest of Mossel Bay. This discovered oil in Albian turbidites in the 14A sequence and gas and condensate in deeper sequences. The generalized chronostratigraphy is shown in Fig. 3.(124356 bytes)

Four years later E-BT field was discovered some 11 km to the southwest, the discovery well flowing 8,730 b/d of 42 gravity crude from a 27 m thick reservoir interval. This field is now at an early stage of development.

Also in 1990 borehole E-BD1 was drilled 27 km due west of E-AA1 and discovered oil in Aptian turbidites in the 13A sequence; flow rates were in excess of 8,500 b/d of oil. This was followed by borehole E-CE1, 4 km east of E-BD1, and this intersected a 51 m thick turbiditic sandstone containing a 31 m gas cap overlying a 20 m oil column to the base of the reservoir. E-CE field has recently been appraised and a number of development scenarios proposed.

Borehole E-CB1, 26 km southeast of E-AA1, was drilled in late 1993 and discovered oil in Albian turbidites in the 14A sequence, flowing 5,600 b/d of oil. Boreholes E-BK1 and E-AJ1 some 8 km and 16 km east-northeast of E-CB1, respectively, tested gas and oil in similar reservoirs. These fields are all in an early stage of appraisal.

E-BT OIL FIELD

After the discovery borehole E-BT1 was drilled in 118 in of water the field was delineated by three more boreholes, two of which were water bearing (Fig. 4)(59865 bytes).

In March 1991 E-BT2 found poorly developed, water-bearing sandstones below the level of the oil-water contact established by E-BT1. In December 1991 E-BT3 encountered poorly developed sandstones with no hydrocarbons.

A year later E-BT01P, directionally drilled from the E-BT1 location, intersected the southern flank of the reservoir at 2,347 in below mean sea level, 0.8 kin southwest of E-BT1. This borehole tested 9,620 b/d of oil from a full column of oil in a 29 in thick reservoir.

The seismic coverage over E-BT field comprises shot lines from a reconnaissance 3D survey acquired in 1989 and 2D seismic lines acquired in 1990, 1991, and 1993. These data have been processed together as one set to improve the reliability of the ultimate time and amplitude maps.

A subset of the conventional seismic data was selected for inversion and Geostack (amplitude versus offset) 1 2 processing to assist in the delineation of the reservoir and in the siting of potential injector boreholes. Soekor performed all seismic data processing in-house. The seismic response of the reservoir can be seen on Fig. 4 (59865 bytes) at about 1.8 sec.

The top and base of the reservoir sandstones were interpreted conventionally on an interactive workstation and two-way time, isochron, depth, amplitude, and acoustic impedance maps were generated.

The amplitude and impedance maps were used in conjunction with core and geophysical log data to compile the geological model. The depositional model is one of a deep-marine channel-levee complex whose channels are confined to the central portion of a predominantly mud-rich "fall." The porosity and permeability values of the reservoir determined by core analysis range from 11-23% and from 602,700 md, respectively'.

The reservoir was divided into three zones and property maps were drawn for each using the calibrated amplitude and impedance values to constrain the maps. Further constraints were provided by the pressure history match of borehole E-.BT01P. Although there is extensive 2D seismic coverage of the field, its interpretation is complicated by tuning effects within the reservoir interval. Reflecting this problem, together with the stratigraphic nature of the field and the difficulty experienced in its delineation, the estimated oil in place has a relatively broad range from 32-72 million st-tk bbl. The primary reserves are estimated at 5-30 million bbl. A final development plan is now being put together for the production of E-BT oil field.

E-BD/E-CE FIELDS

Although the high initial flow rate from E-BD1 of 8,500 b/d of 38 gravity crude dropped during testing there was sufficient encouragement to appraise the area. A further four boreholes were drilled, one being dry the other three intersecting gas and oil reservoirs (Fig. 5)(28797 bytes).

In December 1990 borehole E-BD2 intersected a 25 m water-bearing sandstone at 2,552 m below mean sea level. Borehole E-CE1 was drilled in April 1991 in 102 m of water and confirmed a 51 m thick massive, amalgamated channel sandstone at 2,633 m below mean sea level, with a 31 m gas cap overlying a 20 m oil column.

On testing the oil zone flowed 6,000 b/d of 40 gravity crude while the gas zone flowed 10 MMscfd with a GOR of 7,000 scf/st-tk bbl The E-BD and E-CE reservoirs are not in hydraulic communication (Fig. 5)(28797 bytes). Boreholes E-CE2 and E-CE3 confirmed both the geological model and the extent of the two fields.

The seismic coverage over the E-BD and E-CE fields comprises 2D data from 1990 and 1991 and a 3D survey acquired in late 1992. Selected 2D lines were inverted and Geostack (AVO) processed to assist in the delineation of these reservoirs.

The top and base of both reservoirs were interpreted, and together with geophysical modeling of the amplitude variations of both the conventional and inverted data were used to produce a series of reservoir property maps.

The proposed depositional model is one of retrogradational basin floor fans within the confines of a fault-controlled erosional valley. These basin floor fans are characterized by stacked deep-marine channel deposits and form combination traps through stratigraphic pinchouts and localized inversion tectonics.

The average porosity and permeability values of the E-BD reservoir are 18% and 380 md, respectively, while those of the E-CE reservoir are 18% and 410 md, respectively. The total oil in place from both the E-BD and E-CE fields is estimated to lie within the range of 43-90 million st-tk bbl. However, the integrated interpretation of the 3D seismic with the borehole data is currently being evaluated and the volume range may be adjusted. Development scenarios are now being formulated.

E-CB OIL FIELD

Borehole E-CB1 was drilled in 135 m of water in late 1993. This borehole was to test the updip extent of deep marine turbidites in the 14A sequence, previously encountered in boreholes E-AJ1 and 2 and E-BK1 (Fig. 6)(156624 bytes). At E-CB1 a 34 m sandstone was intersected at 2,488 m below mean sea level with a 14 m oil column that flowed 5 MMscfd of gas and 5,800 b/d of 43 gravity crude.

Borehole E-AJ1 had been drilled in 1989 and intersected an oil-bearing reservoir in Albian deep marine turbidites in the 14A sequence. A step-out borehole, E-AJ2, encountered poorly developed sandstones in the same reservoir at 2,671 m below mean sea level that flowed 8 MMscfd of gas and 1,100 b/d of condensate.

In 1991, 8 km east-northeast of E-CB1, borehole E-BK 1 intersected a 52 m thick sandstone within this same sequence at 2,598 m below mean sea level with a 28 m hydrocarbon column. On testing this flowed 26 MMscfd of gas with 4,130 b/d of condensate.

A 3D seismic survey covering E-CB, E-BK, and E-AJ areas was acquired in 1994 and will be available for interpretation in early 1995. The existing 2D seismic data were inverted and Geostack AVO processed to aid in the delineation of these reservoirs (Fig. 6)(156624 bytes).

The geological model is one of stacked deep-marine gravity-flow sandstones, deposited from the west-southwest as lowstand basin-floor fans, The porosity and permeability values of the E-CB1 reservoir average 17% and 100 md, respectively.

Initial results from the 3D survey indicate the existence of additional fields and field extensions in this most promising hydrocarbon fairway.

CONCLUSIONS

Significant development potential exists in Block 9 in the Bredasdorp basin, where numerous oil and gas fields have been discovered, including South Africa's first producing field.

The recent discovery of oil at E-CB illustrates the existence of further exploration potential in the basin.

The second South African offshore licensing round opened in October 1994. All offshore blocks were offered for sublease except Block 9 in the Bredasdorp basin, where existing discoveries are due to be developed, and Block 11A in the Pletmos basin, where further gas discoveries have been made. Inquiries about participation both in these developments and in the further exploration of these blocks would be welcomed.

Acknowledgments

I thank my colleagues at Soekor for their support in providing the data for this article and the management of Soekor for their permission to submit it for publication.

REFERENCES

Smith, G.C., and Gidlow, P.M., Weighted stacking for rock property estimation and detection of gas, Geophysical Prospecting Vol. 35, 1987, pp. 993-1,014.
Fatti, J.L., Vail, P.J., Smith, G.C., Strauss, P.J., and Levitt, P.R., Detection of gas in sandstone reservoirs using AVO analysis: A 3D seismic case history using the Geostack technique, Geophysics, Vol. 59,1994, pp. 1,362-76.