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T. Reijers

Stratigraphy and sedimentology of the Niger Delta

During the Cenozoic, until the Middle Miocene, the Niger Delta grew through pulses of sedimentation over an oceanward-dipping continental basement into the Gulf of Guinea; thereafter progradation took place over a landward-dipping oceanic basement. A 12,000 m thick succession of overall regressive, offlapping sediments resulted that is composed of three diachronous siliciclastic units: the deep-marine pro-delta Akata Group, the shallow-marine delta-front Agbada Group and the continental, delta-top Benin Group.

Regionally, sediment dispersal was controlled by marine transgressive/regressive cycles related to eustatic sea-level changes with varying duration. Differential subsidence locally influenced sediment accumulation. Collectively, these controls resulted in eleven chronostratigraphically confined delta-wide mega-sequences with considerable internal lithological variation.

The various sea-level cycles were in or out of phase with each other and with local subsidence, and interfered with each other and thus influenced the depositional processes. At the high inflection points of the long-term eustatic sea-level curve, floodings took place that resulted in delta-wide shale markers. At the low inflection points, erosional channels were formed that are often associated, downdip, with turbidites in low-stand sediments (LSTs). The megasequences contain regional transgressive claystone units (TST) followed by a range of heterogeneous fine-to-coarse progradational or aggradational siliciclastic (para)sequence sets formed during sea-level high-stand (HST).

An updated biostratigraphic scheme for the Niger Delta is presented. It also updates a sedimentation model that takes into consideration local and delta-wide effects of sea-level cyclicity and delta tectonics. Megasequences were formed over time intervals of ~5 Ma within individual accurate megastructures that laterally linked into depobelts. The megasequences form the time-stratigraphic frame of the delta and are the backbone for the new delta-wide lithostratigraphy proposed here. Such a new lithostratigraphy is badly needed, in particular because of the vigorous new activity in the offshore part of the Niger Delta (not covered in this contribution). There, as well as in the onshore part of the delta, the traditional lithostratigraphic subdivision of the Cenozoic Niger Delta section into three formations is insufficient for optimum stratigraphic application; moreover, the various informal subdivisions that have been proposed over time are inconsistent.

Open access

T.J.A. Reijers

Abstract

Hydrocarbon exploration in The Netherlands has a chequered history from serendipitous oil shows via chance oil/ gas discoveries to finding the largest continental European oil field in 1943, followed by finding the largest gas field in the world in 1959. The present contribution traces the development of moderate to good porosity/permeability trends in depositional facies of Zechstein Stassfurt carbonates in a ‘gas play’ intermediate in significance between the above two plays but all in the northern part of The Netherlands. Various depositional facies in the Stassfurt carbonates were turned into ‘carbonate fabric units’ by diagenetic processes creating or occluding the porosity/permeability. This formed moderate to good gas reservoirs in barrier-shoal, open-marine shelf and proximal-slope carbonates in the subsurface of the province of Drenthe in the NE Netherlands. The diagenetic models forming these carbonate fabric units are linked to the variety of facies in a depositional model which shows explain and predicts the reservoir trends. Such depositional/diagenetic facies are ‘translated’ into characteristic petrophysical values recognisable on wire line logs in uncored wells, and in characteristic seismic expressions that show these trends in undrilled areas. This approach has been proven to be effective in delineating porosity trends, visualised by 3-D seismic in the Collendoornerveen field, and thus provides a new exploration ‘tool’ in hydrocarbon exploration .