A doubtful reservoir: biostratigraphy of a Douala Basin exploration well
An exploration well in the Douala Basin reached 2,850 m. Cuttings were described every 10 m. Between 1,150 and 1,480 m, planktonic foraminifera (including Globigerinoides and Praeorbulina) and calcareous nannofossils are abundant; gamma-ray values are 45–60 API. From 1,480 to 1,520 m, a mixed assemblage contains shallow-water benthic foraminifera together with reworked Cretaceous dinoflagellates; gamma-ray values peak at 90 API, and the interval is a calcareous shale. From 1,520 to 1,780 m, fine- to coarse-grained sandstone has 18–24% porosity and 50–300 mD permeability, with gas shows of 2–5%; foraminifera are almost absent except for rare agglutinated forms. A 20 m polymict conglomerate with mudstone clasts lies at 1,780–1,800 m. Below that, kaolinitic sand and coal seams are barren of foraminifera.
1. Identify the stratigraphic break and explain the evidence for it. Interpret the depositional environments above and below the break.
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The break is at 1,480–1,520 m. The key evidence is the mixed assemblage: shallow-water benthic foraminifera and reworked Cretaceous dinoflagellates occur together within a Miocene section whose planktonic foraminifera above include Globigerinoides and Praeorbulina. Reworking of older fossils into younger sediment, a gamma-ray peak and a calcareous shale are typical of a transgressive lag or unconformity surface. The 20 m polymict conglomerate at 1,780–1,800 m may be a lower lag or a channel base, not the same surface.
Environments: above 1,480 m, abundant planktonic forams and nannofossils indicate open-marine, probably outer-shelf to upper-slope deposition. From 1,520 to 1,780 m, clean sandstone with rare agglutinated forams and gas shows suggests a delta-front or shoreface setting. Below 1,800 m, kaolinitic sand and coal with no forams indicate continental or fluviatile deposition.
2. Assess the reservoir–seal pair and charge risk for the sandstone at 1,520–1,780 m. What additional data would you request?
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The sandstone has good reservoir quality: porosity 18–24% and permeability 50–300 mD. The calcareous shale at 1,480–1,520 m is a potential seal, and the gas shows indicate charge. But the seal is thin and may be breached by the unconformity; the conglomerate below could be a thief zone or a lateral migration path. Charge risk depends on whether the underlying continental section contains a mature source rock; the coal seams are a possible source but are thin and barren of marine indicators. The unconformity may also cause top-seal leakage.
Request: core plug analysis, mercury-injection capillary pressure, biostratigraphic reworking quantification, seismic mapping of the trap, and geochemical analysis of the coal and shale. A drill-stem test would confirm connectivity.