Palaeogeographical development of two merging delta systems(Eocene Shahejie Formation)in the Bohai Bay Basin,E China and implications for hydrocarbon exploration

Two deltas developed simultaneously during the Eocene on the eastern side of a large lake that existed in the Dongying Sub-basin,which forms part of the Bohai Bay Basin in eastern China.The rivers that built the deltas had different catchment areas,which resulted in sediments with different permeability and porosity,due to differences in sorting and mud content.Both deltas prograded,and mass flows that originated frequently on their fronts formed lobes that expanded laterally.This eventually led to merging of both deltas,a feature that has rarely been described from ancient deposits thus far.Core analysis and seismic reflection data show that the merging of the two deltas took place in nine phases,determined by phases of slower progradation or even temporary retrogradation in between.The alternation of sediments from both deltas and their eventual mixing makes the architecture of the merged deltas much more complex than that of'classical'single deltas.This affects the predictability of the spatial distribution of possible reservoir characteristics significantly,but detailed core analysis shows that the best hydrocarbon reservoirs consist of sand bodies formed in distributary channels on the delta plains,and of sandy turbidites formed in the deep-lacustrine environment in front of the merged deltas.

Diagenetic mineralogy and its effect on the reservoir properties of the sandstones of the Permian of S120 block(Sulige gas field),Ordos Basin,NW China

The characteristics of diagenetic minerals and their effects on reservoir quality of the tight sandstones of the Permian in Sulige gas field of the Ordos Basin were studied through observations on thin sections,scanning electron microscopy,energy dispersive spectroscopy analysis and electron microprobe analysis.Diagenetic minerals in the Permian sandstones consist of illite,kaolinite,chlorite,siliceous and calcite cements.Large amounts of intercrystalline pores between kaolinite and illite provide channels for acidic fluids flow and thereby were conducive to the formation of clastic solution pores,intergranular solution pores and composite pores.Authigenic chlorite occurs in the form of three morphotypes as grain-coating,pore-lining and pore-filling.Grain-coating and pore-lining chlorite with different crystal shapes occur as coatings on the framework grains.Pore-filling chlorite precipitated as discrete flaky plates in pore spaces.Quartz microcrystals developed but quartz overgrowth did not develop because of the occurrence of porelining chlorite.This,in turn,led to the preservation of primary pores by occupying potential nucleation sites for quartz overgrowth and thereby preventing quartz microcrystals from merging into quartz overgrowth.This process is regarded as the most important for influencing the quality of the lithophysical properties.Calcite cement was mainly precipitated during a late diagenetic stage and has a negative effect on the reservoir quality.This study provides important insights into analyzing the relationship between diagenetic minerals and reservoir quality and the results are directly applicable to the exploration and development of tight sandstone reservoirs all over the world.

Evidence for fault activity during accumulation of the Furongian Chaomidian Formation(Shandong Province,China)

The Late Cambrian(Furongian)Chaomidian Formation accumulated in an epeiric sea that covered a large part of the North China Plate and extended from China to Korea.The depositional environment of the formation is commonly considered to have been affected by storms that broke up numerous limestone layers,but that was tectonically quiet.It is here argued,however,that some features of the formation-more in particular the many dozens of breccia layers and the occurrence of slid-down limestone blocks that are embedded in autochthonous oolites-can be explained satisfactorily only if some significant fault activity took place during its accumulation.The faulting may have been due to differential loading and subsidence,but an endogenic origin seems more probable.This implies that the structural history of the study area in Shandong Province may need re-consideration.

The control of Pleistocene palaeogeography on the distribution of sandy patches in a silty Holocene lagoon(central Netherlands)

The brackish lagoon in the central part of The Netherlands was closed by a dike in 1932 and gradually changed into a lake.Parts of this lake were reclaimed and the surficial sediments of one of the polders(Noordoostpolder),reclaimed in 1942,has been investigated in detail by mapping of the walls of some1500 km of drainage ditches approx.1.4 m deep.It appeared that the sediments consist of an uncommonly large amount of silt,to different degrees mixed with reworked peat that had developed during the Holocene transgression.Some sandy deposits occur locally,at places that are at first sight distributed in a haphazard way.The various sandy patches have different characteristics(grain-size distribution,rounding of the grains,mineral composition).These sands cannot have been introduced from the sea,nor can they have been supplied by the rivers that discharged into the lagoon,so they must have a local origin.It is found that several types of Pleistocene sandy or diamictic deposits below the Holocene peat and lagoonal sediments had an irregular topography and became eroded during the Holocene by wave activity.This resulted in sandy deposits around these Pleistocene highs,which consisted of glacial tills(boulder clays),river dunes(formed by aeolian activity along rivers under dry permafrost conditions)and coversand ridges,formed by aeolian activity in a belt between the ice margin and the more distal loess belt.This finding implies that palaegeographical interpretations of local grain-size anomalies in a specific deposit should not only consider facies changes due to changes in the sediment supply,but should also consider local erosion leading to the exposure of previously deposited material with a divergent composition.