Reservoir Study of an Underperforming Lacustrine Turbidite Field in West Africa

• Main Author: Lefeber, T. (author)
• Other Authors: Luthi, S.M. (mentor)
• Format: Master Thesis
• Language: English
• Published: 2002
• Subjects: Melania; South Atlantic Ocean
• Online Access: http://resolver.tudelft.nl/uuid:3a2c1249-b5fc-4eea-ba61-282b4fec5d6b

The objective oil field is located offshore south Gabon. It was discovered in 1969 and first put it on production in 1980. The present operator bought 90% of the field in 1994. To date, under primary recovery, only 6% of the currently estimated STOIIP has been recovered. Heterogeneity of the deposits and poor reservoir quality as a result of diagenetic alteration upon burial are the main reasons for the low production rates. The main goal of this study was to investigate how production in this underperforming reservoir could be improved. For this purpose a detailed geologic model was developed for the B-area of the field. Then the performance of a hypothetical well with a horizontal trajectory into a selected target layer was analyzed. Literature studies pointed out that the oil-bearing Melania sands were deposited in a rifting phase preceding the opening of the South Atlantic Ocean. Seismic interpretation was performed by the present operator and indicated that the Melania Formation is nowadays found in a series of rotated fault blocks. The Top Melania Reservoir surface that was interpreted by the present operator is used in this study. With the help of spectral analyses on gamma ray log curves a tentative well correlation was established. The oil-bearing Upper Melania Unit was hereby subdivided into three subunits: A, B and C. Petrophysical analysis of wireline logs provided clay volume, porosity, water saturation and permeability curves. Based on the interpreted surfaces and the petrophysical evaluation, a 3D static model of the Upper Melania Unit was created. This model included several property grids. Literature studies on sedimentation in half-grabens and the results of seismic interpretation, well correlation and petrophysical analyses lead to a plausible geologic model. The Upper Melania Unit is thought to comprise amalgating fan deltas or turbidite fan lobes, or a combination of these. Sub-aqueous channels transported most sediment into the area from at least two principal directions. Syn-sedimentary ...