Prospectivity and Petroleum Systems Modelling of the Offshore Lamu Basin, Kenya: Implications for..
While recent years have seen major discoveries along the East African margin, the Lamu Basin in Kenya remains underexplored. Located in northern Kenya and covering both onshore and offshore, the Lamu Basin forms part of the Kenyan passive continental margin. The geology is controlled by tectonic movements that led to the break-up of Gondwana in the Jurassic and by Cretaceous activity on the Anza rift.
Large discoveries in Tanzania and Mozambique indicate hydrocarbon potential in East Africa, as do heavy oil deposits on the conjugate margin in Madagascar. Exploration in the offshore Lamu Basin is limited, with only seven wells, but 52 m net gas pay was discovered in channel and turbidite sands at Mbawa-1 in 2012. This indicates an active petroleum system, as do hydrocarbon shows in a number of wells and brightening
on seismic data. 2D seismic interpretation will be combined with analysis of regional gravity data to constrain the tectonic history and structural geology of theoffshore Lamu Basin. The survey covers from 1–4 km water depth, and comprises 1035 km of regional 2D lines, giving an overview of the basin and intersecting DSDP well 241. 9600 km of new 2D seismic data currently being acquired also will be included. Seismic attributes constrain stratigraphic zones, including shale intervals providing potential source rocks, and sand-rich deposits which may act as reservoirs.
Hydrocarbon plays are interpreted to range from pre-rift Karoo deposits, found in tilted fault blocks; through Jurassic syn-rift units terminating against faults and in drape and rollover anticlines; and into Cretaceous and Tertiary stratigraphic plays, reefs, turbidites, and fans. The maturity and nature of source rocks in the Lamu Basin remains a key question.
This study will utilize the petroleum systems concept, incorporating the presence of an active source rock a key element along with reservoir,trap and seal, with development of the system controlled by the timing of these elements. A selected regional line traversing the northern offshore Lamu Basin from west to east is modelled and source rock intervals are identified, based on regional stratigraphy, constraining. of generation. Combining regional studies with seismic data and the petroleum systems concept enables improved understanding of the prospectivity of the offshore Lamu Basin and indicates that all the required components of a petroleum system are present.
Introduction While recent years have seen major discoveries along the East African margin, the Lamu Basin in Kenya remains underexplored. Located in northern Kenya and covering both onshore and offshore, the Lamu Basin forms part of the Kenyan passive continental margin. The geology is controlled by tectonic movements that led to the break-up of Gondwana in the Jurassic and by Cretaceous activity on the Anza rift.
Large discoveries in Tanzania and Mozambique indicate hydrocarbon potential in East Africa, as do heavy oil deposits on the conjugate margin in Madagascar. Exploration in the Lamu Basin is limited, with only thirty wells, but 19 of these are associated with hydrocarbon shows. Of the eight offshore wells, 52 m net gas pay was discovered in channel and turbidite sands at Mbawa-1 in 2012, and a 14 m gross oil column was encountered by Sunbird-1 in Miocene reefs in 2014, placing the Lamu Basin as an emerging hydrocarbon province. These discoveries indicate at least one active petroleum system, as do hydrocarbon shows in a number of wells and brightening on seismic data (Mbede 1987). 2D seismic interpretation will be combined with analysis of regional gravity data to constrain the tectonic history and structural geology of the offshore Lamu Basin. A petroleum systems model is then constructed to analyze the maturity and prospectivity of the basin. Interpretation of the prospectivity and petroleum systems of the Lamu Basin will be constrained by 9600 km of 2D broadband seismic data acquired in 2013- 2014, and 1035 km of legacy 2D data (Figure 1). This study focuses on the deep water Lamu Basin, with the data lying between 2000-4000 m water depths. The seismic data intersects Deep Sea Drilling Project well 241 and the Pomboo-1 well, which are used to tie the interpretation.
Tectonics and Regional Gravity Interpretation
The hydrocarbon potential of offshore Kenya can be understood in terms of the tectonic history of the East African margin. The major tectonic events in the region influence the formation, uplift, and subsidence of the basin and therefore the environment of deposition. The Lamu Basin in Kenya is one of several basins along the margin that relates to the separation of Madagascar, India, Antarctica, and Australia from Africa and
America during the break up of Gondwanaland at the culmination of Jurassic rifting (Coffin and Rabinowitz 1987). Other basins related to this rifting event include the Somali Basin, the Tanzanian Pemba and Mafia Basins, the Rovuma Basin in Mozambique, and the Morondova Basin in Madagascar (Mbede and Dualeh 1997). These basins share a similar rift history with common structural and stratigraphic characteristics, and some are highly prospective with large discoveries in recent years. The Lamu Basin is classified as belonging to a passive continental margin, and is unusual in that it lies in a transitional position between a rifted margin to the north in Somalia and a transform margin to the south.
Figure 1 shows the position of the basin and the seismic data that will be used to illustrate the interpretation of the area. The history of the basin is dominated by the break-up of Kenya from Madagascar, and can be divided into pre-rift (late Proterozoic to Triassic), syn-rift (Triassic to Late Jurassic), and post-rift phases (Late Jurassic to Holocene).The major structural features of the offshore Lamu Basin can be seen on the gravity data (Figure 2). The Davy-Walu Ridge is the termination of the Davie Fracture Zone, and is visible as a gravity high, as is the Pemba-Simba Ridge (Rais-Assa 1987). The Tembo and Maridadi Troughs.
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