Albian to Turonian agglutinated foraminiferal assemblages of the Lower Saxony Cretaceous sub-basins – implications for sequence stratigraphy and paleoenvironmental interpretation

Albian to Turonian carbonate deposits at three different locations of the Lower Saxony Cretaceous and thereby of the European mid-Cretaceous epeiric shelf sea were investigated for their fossil agglutinated foraminiferal fauna. In this study, 71 samples from two quarries and three drill cores were treated with formic acid, which enabled the study of agglutinated foraminiferal assemblages even in highly lithified limestones. In total, 114 species were determined and classified as belonging to nine morphogroups. In general, four agglutinated foraminiferal assemblages are distinguished: (1) an uppermost Albian–lowermost Cenomanian assemblage from the Wunstorf drill cores, with the dominant taxa Bathysiphon spp., Nothia spp., Psammosphaera fusca, Reophax subfusiformis, Bulbobaculites problematicus, Tritaxia tricarinata, Flourensina intermedia, Vialovella frankei, Arenobulimina truncata, and Voloshinoides advenus; (2) a Cenomanian assemblage from the Baddeckenstedt quarry and Wunstorf drill cores, with Ammolagena clavata, Tritaxia tricarinata, Vialovella frankei, Arenobulimina truncata, and Voloshinoides advenus; (3) an assemblage related to the Cenomanian–Turonian Boundary Event in Wunstorf and Söhlde dominated by Bulbobaculites problematicus; and (4) a Turonian assemblage in the Wunstorf and Söhlde sections with high numbers of Ammolagena contorta, Repmanina charoides, Bulbobaculites problematicus, Gerochammina stanislawi, and Spiroplectammina navarroana. The latest Albian–earliest Cenomanian assemblage consists of tubular, globular, and elongate foraminiferal morphogroups which are typical for the low- to mid-latitude slope biofacies. All other assemblages are composed of elongate foraminiferal morphogroups with additionally globular forms in the proximal settings of Baddeckenstedt and Söhlde or flattened planispiral and streptospiral forms in more distal settings of Wunstorf. For these assemblages, a new agglutinated foraminiferal biofacies named “mid-latitude shelf biofacies” is proposed herein. Changes in the relative abundance of different morphogroups can often be referred to single features of depositional sequences. Furthermore, classical macro-bioevents, which are often depositional-related, of the Lower Saxony Cretaceous seem to have a micro-bioevent or acme equivalent of the agglutinated foraminiferal fauna.

Geologically based integrated approach for zonation of a Late Jurassic–Early Cretaceous carbonate reservoir; a case from Persian Gulf

In this study, our attempt is to integrate sedimentological and petrophysical data for reservoir evaluation in the sequence stratigraphic framework. Petrographic analysis of the Late Jurassic–Early Cretaceous Fahliyan Formation reservoirs of two oilfields in the northwest of the Persian Gulf led to recognition of twelve microfacies. They can be classified into four facies associations, including open marine, shoal, lagoon and tidal flat, which are deposited in a homoclinal ramp carbonate. Sequence stratigraphy of the studied successions led to the recognition of three third-order depositional sequences based on vertical changes in microfacies and gamma ray analysis. Except for the upper boundary of the third sequence, the other sequence boundaries are type I (SBT.1). Dissolution is the most important diagenetic feature that affected the lower depositional sequence which is caused by the development of subaerial exposure after the deposition of the Fahliyan Formation, whereas cementation is the main diagenetic feature affecting the second- and third depositional sequences, causing their lower reservoir quality. In order to identify the flow units, the flow zone index methods, porosity throat radius (R35) and modified Lorenz based on stratigraphy were applied. The key wells studied in this area have shown good correlation throughout the studied oilfields which may potentially be used for hydrocarbon exploration and field development in the Late Jurassic–Early Cretaceous deposits of the Persian Gulf. This study integrates geological and petrophysical data (rock typing) toward sequence stratigraphic framework.

Application of seismic stratigraphy in reservoir characterisation: a case study of the passive margin deposits of the northern Orange Basin, South Africa

The Barremian-Cenozoic depositional sequences in the northern Orange Basin, SW, South Africa, were investigated using the principles of seismic stratigraphy to understand the interplay of tectonics and sedimentary processes in the distribution of potential hydrocarbon reservoirs. A seismic stratigraphic workflow (seismic sequence, seismic facies and lithofacies analysis) was completed by utilising three seismic lines (L1, L2 and L3) tied to Wireline data (gamma, checkshots and sonic) in two exploration wells (A1 and A2). Seven depositional sequences were mapped followed by the creation of lithofacies log interpreted from the gamma-ray log (GR) by setting maximum GR value at 60 API for Sandstone, 60–100 API for Siltstone and above 100 API for Shale. Six seismic facies units are recognised based on internal geometry and configurations of the seismic reflectors; Tangential-Oblique (SF1), Hummocky (SF2), Wavy-Parallel (SF3), Chaotic (SF4), Sub-parallel/parallel (SF5) and Divergent (SF6). SF4 is dominant within the Barremian-Aptian sequence and expressed in an incised valley fill, suggesting mass transport deposition accompanied by strong hydrodynamic conditions. Evidence of sedimentary basins progradation is seen within the Late-Albian-Turonian sequences, because of the occurrences of SF2, SF6 and SF 4 facies. SF5 facies is prominent in the Maastrichtian/Campanian sequence, indicating that the deposition of sediments may have been accompanied by uniform margin subsidence after the Late-Cretaceous uplift of the Africa margin. The occurrence of SF1 and SF4 facies within the Cenozoic sequence indicates terrigenous pro-deltaic deposits and mass transport deposits, respectively. Further results from seismic-lithofacies modelling reveal that sand deposits of Barremian-Aptian (SF4 facies unit) and Albian sequences (SF2 and SF6 facies units) are potential stratigraphic reservoirs in this part of the basin.