The Upper Devonian Buchan Formation in the Central North Sea is a typical terrestrial deposit and predominantly comprises fine to medium-grained sandstones with occasional conglomerates and mudstones. The Buchan Forma...The Upper Devonian Buchan Formation in the Central North Sea is a typical terrestrial deposit and predominantly comprises fine to medium-grained sandstones with occasional conglomerates and mudstones. The Buchan Formation has been previously described as being made up mostly of braided fluvial sandstones;however, this study confirms the presence and significance of aeolian sandstones within this fluvial-dominated sequence. Facies architecture is investigated through analogue outcrop study, well log curves and numerical facies modelling, and the results show contrasting differences between fluvial and aeolian facies. The fluvial facies is composed of multiple superimposed and sand-dominated fining-upward cycles in the vertical direction, and laterally an individual cycle has a large width/thickness ratio but is smaller than the field scale. However, the high channel deposition proportion (CDP, average value = 72%) in fluvial-dominated intervals means that it is likely all the sand bodies are interconnected. Aeolian facies comprise superimposed dune and interdune depositions and can be laterally correlated over considerable distances (over 1 km). Although the aeolian sandstones are volumetrically minor (approx. 30%) within the whole Buchan Formation, they have very high porosity and permeability (14.1% - 28%, 27 - 5290 mD) and therefore are excellent potential reservoirs. The fluvial sandstones are significantly cemented by quartz overgrowth and dolomite and by comparison with the aeolian sandstones are poor reservoirs. Aeolian sandstones can be differentiated from fluvial sandstones using several features: pin-stripe lamentation, good sorting, high visible porosity, friable nature and lack of muddy or conglomeratic contents;these characteristics allow aeolian sandstones can be tentatively recognized by low gamma ray values, high sonic transit time and low density in uncored wells. The thin, laterally correlatable and permeable aeolian sandstones within the Buchan Formation are effective reservoirs and could form important exploration targets when the Devonian is targeted elsewhere in the North Sea.展开更多
The Upper Devonian Buchan Formation reservoirs in the UK Central North Sea are litharenite/sublitharenite and were deposited in fluvial–aeolian settings. The grain-coating clays in the aeolian sandstones have effecti...The Upper Devonian Buchan Formation reservoirs in the UK Central North Sea are litharenite/sublitharenite and were deposited in fluvial–aeolian settings. The grain-coating clays in the aeolian sandstones have effectively inhibited quartz overgrowth. Hence, the reduction of reservoir quality is mainly due to mechanical compaction and early dolomite precipitation in both fluvial and aeolian sandstones; quartz overgrowth and kaolinite illitization in fluvial sandstones; and limited smectite illitization in aeolian sandstones. The carbon/oxygen stable isotopes of dolomite cements suggest a predominantly marine carbon source and precipitation temperatures between 25 and 58 ℃ indicating a shallow burial depth during dolomite precipitation. The temperatures and the dolomite distribution indicate that the cements originated from the overlying Upper Permian Zechstein carbonates. Extensive quartz overgrowths formed at 80 and 120 ℃ in the late and deep diagenetic burial history. The most probable silica source was from feldspar kaolinitization and pressure dissolution of quartz grains. Through detailed petrography and geochemical analyses, the burial–paragenesis–thermal history of the Buchan Formation has been constructed. Similar diagenetic processes are likely to have occurred in the Buchan Formation in other parts of the Central and Northern North Sea. This study may allow new petroleum plays to be considered in areas previously thought to have poor hydrocarbon potential.展开更多
Newly discovered remains of the early Middle Eocene (Lutetian) sirenian Protosiren (Protosirenidae) in shark tooth rich conglomerates from a coastal delta environment northwest of the European Rhenish Massif at Fü...Newly discovered remains of the early Middle Eocene (Lutetian) sirenian Protosiren (Protosirenidae) in shark tooth rich conglomerates from a coastal delta environment northwest of the European Rhenish Massif at Fürstenau (northwestern Germany), represent the most northerly occurrence of this genus whose global distribution was generally restricted to warm waters. Its presence of the remains so far north can be explained by seasonal inflow of warm Tethys surface water into the cool, upwelling-influenced, basin. The existence of two discrete centers of sirenian evolution can be explained by the opening of the Atlantic and the upwelling that separated the North American warm water faunal province from those of Africa and Eurasia. A slightly modified evolutionary model is presented in which the oldest Early Eocene manatee sirenians evolved in the Caribbean of Central America. Protosiren, however, appears to have developed polyphyletically along the African coastline of the Tethys, and represents the oldest known dugong ancestor. Younger (Oligocene) European sirenian skeletons of Halitherium and Anomotherium are included in the phylostratigraphic model in which sirenians had generally reduced their teeth by 28 Ma as an adaptation for feeding on sea-plants (macroalgae/seagrass). Teeth from early megatooth sharks, which preyed on sirenians, have been recorded from shallow marine Eocene and Oligocene coastlines of the southern proto-North Sea Basin, and shark bite marks have been found on sirenian skeletons.展开更多
文摘The Upper Devonian Buchan Formation in the Central North Sea is a typical terrestrial deposit and predominantly comprises fine to medium-grained sandstones with occasional conglomerates and mudstones. The Buchan Formation has been previously described as being made up mostly of braided fluvial sandstones;however, this study confirms the presence and significance of aeolian sandstones within this fluvial-dominated sequence. Facies architecture is investigated through analogue outcrop study, well log curves and numerical facies modelling, and the results show contrasting differences between fluvial and aeolian facies. The fluvial facies is composed of multiple superimposed and sand-dominated fining-upward cycles in the vertical direction, and laterally an individual cycle has a large width/thickness ratio but is smaller than the field scale. However, the high channel deposition proportion (CDP, average value = 72%) in fluvial-dominated intervals means that it is likely all the sand bodies are interconnected. Aeolian facies comprise superimposed dune and interdune depositions and can be laterally correlated over considerable distances (over 1 km). Although the aeolian sandstones are volumetrically minor (approx. 30%) within the whole Buchan Formation, they have very high porosity and permeability (14.1% - 28%, 27 - 5290 mD) and therefore are excellent potential reservoirs. The fluvial sandstones are significantly cemented by quartz overgrowth and dolomite and by comparison with the aeolian sandstones are poor reservoirs. Aeolian sandstones can be differentiated from fluvial sandstones using several features: pin-stripe lamentation, good sorting, high visible porosity, friable nature and lack of muddy or conglomeratic contents;these characteristics allow aeolian sandstones can be tentatively recognized by low gamma ray values, high sonic transit time and low density in uncored wells. The thin, laterally correlatable and permeable aeolian sandstones within the Buchan Formation are effective reservoirs and could form important exploration targets when the Devonian is targeted elsewhere in the North Sea.
基金EnQuest PLC for supporting this research through access to internal data and financial support
文摘The Upper Devonian Buchan Formation reservoirs in the UK Central North Sea are litharenite/sublitharenite and were deposited in fluvial–aeolian settings. The grain-coating clays in the aeolian sandstones have effectively inhibited quartz overgrowth. Hence, the reduction of reservoir quality is mainly due to mechanical compaction and early dolomite precipitation in both fluvial and aeolian sandstones; quartz overgrowth and kaolinite illitization in fluvial sandstones; and limited smectite illitization in aeolian sandstones. The carbon/oxygen stable isotopes of dolomite cements suggest a predominantly marine carbon source and precipitation temperatures between 25 and 58 ℃ indicating a shallow burial depth during dolomite precipitation. The temperatures and the dolomite distribution indicate that the cements originated from the overlying Upper Permian Zechstein carbonates. Extensive quartz overgrowths formed at 80 and 120 ℃ in the late and deep diagenetic burial history. The most probable silica source was from feldspar kaolinitization and pressure dissolution of quartz grains. Through detailed petrography and geochemical analyses, the burial–paragenesis–thermal history of the Buchan Formation has been constructed. Similar diagenetic processes are likely to have occurred in the Buchan Formation in other parts of the Central and Northern North Sea. This study may allow new petroleum plays to be considered in areas previously thought to have poor hydrocarbon potential.
文摘Newly discovered remains of the early Middle Eocene (Lutetian) sirenian Protosiren (Protosirenidae) in shark tooth rich conglomerates from a coastal delta environment northwest of the European Rhenish Massif at Fürstenau (northwestern Germany), represent the most northerly occurrence of this genus whose global distribution was generally restricted to warm waters. Its presence of the remains so far north can be explained by seasonal inflow of warm Tethys surface water into the cool, upwelling-influenced, basin. The existence of two discrete centers of sirenian evolution can be explained by the opening of the Atlantic and the upwelling that separated the North American warm water faunal province from those of Africa and Eurasia. A slightly modified evolutionary model is presented in which the oldest Early Eocene manatee sirenians evolved in the Caribbean of Central America. Protosiren, however, appears to have developed polyphyletically along the African coastline of the Tethys, and represents the oldest known dugong ancestor. Younger (Oligocene) European sirenian skeletons of Halitherium and Anomotherium are included in the phylostratigraphic model in which sirenians had generally reduced their teeth by 28 Ma as an adaptation for feeding on sea-plants (macroalgae/seagrass). Teeth from early megatooth sharks, which preyed on sirenians, have been recorded from shallow marine Eocene and Oligocene coastlines of the southern proto-North Sea Basin, and shark bite marks have been found on sirenian skeletons.
