Hydrocarbon potential of the Meso-Cenozoic Turkana Depression,northern Kenya.I.Reservoirs:depositional environments,diagenetic characteristics,and source rock-reservoir relationships

摘要

Major oil exploration efforts started in the 70s in the Meso-Cenozoic Anza Rift and Cenozoic Turkana Depression of northern Kenya.Thick piles of fluvio-lacustrine sandstones and shales infill these different rift basins.West of Lake Turkana,the Auwerwer/Lomerimong Formation is part of the Palaeogene-middle Miocene age,7 km-thick fluvio-lacustrine infill of the Lokichar half-graben.East of Lake Turkana,the 220 m-thick Sera Iltomia Formation is of possible late Mesozoic-basal Palaeocene age,and comprises sandstones and mudstones with conglomeratic layers.The poorly dated Sera Iltomia Formation may represent either the early phase of Cenozoic East African rifting in northern Kenya or the Meso-Cenozoic Anza Rift.The sandstones of these two formations exhibit different sediment sources and consequent reservoir quality.The Sera Iltomia sandstones are immature and basement-derived.While the sources of clastic material from the Auwerwer/Lomerimong section originated from both volcanic and basement terrains.Palaeocurrent data for the Sera Iltomia and Auwerwer/Lomerimong basement-derived sandstones suggest a source to the south and south-east of Lake Turkana.The volcanic-derived clastic rocks forming part of the Auwerwer/Lomerimong section suggest a sediment source to the south-southeast of the Lokichar Basin,linked to the lower Miocene Samburu Basalts Formation.Evidence for significant burial diagenesis is absent in both.In the Auwerwer/Lomerimong sandstones,calcite-analcite precipitation and calcite cementation significantly reduced the porosity from initial values of 40-45% to values which ranges up to 15%.In the Sera Iltomia sandstones,different early diagenetic events are recorded by calcite,quartz or kaolin cements.Quartz overgrowths and kaolin precipitation are local phenomena,and did not induce significant porosity reduction.In some cases,calcite cementation completely occluded the initial porosity,but in other cases it has helped preserve significant porosity by limiting mechanical compaction.This cementation resulted in a higher porosity (up to 33%) than in the Lomerimong sandstones and a better potential to form reservoirs of good quality.Comparison with potential reservoirs encountered in the Anza Rift wells demonstrate that volcaniclastic-rich sandstones are more frequently affected by pore cementation rather than the arkosic-type sandstones.The complex stratigraphic and structural organization of the Turkana Depression/Anza Rift system illustrates the potential variability in sediment source,diagenesis and reservoir-source rock relationships when exploring for reservoirs in continental rift systems.