Application of 9-component S-wave 3D seismic data to study sedimentary facies and reservoirs in a biogasbearing area:A case study on the Pleistocene Qigequan Formation in Taidong area,Sanhu Depression,Qaidam Basin,NW China

To solve the problems in restoring sedimentary facies and predicting reservoirs in loose gas-bearing sediment,based on seismic sedimentologic analysis of the first 9-component S-wave 3D seismic dataset of China,a fourth-order isochronous stratigraphic framework was set up and then sedimentary facies and reservoirs in the Pleistocene Qigequan Formation in Taidong area of Qaidam Basin were studied by seismic geomorphology and seismic lithology.The study method and thought are as following.Firstly,techniques of phase rotation,frequency decomposition and fusion,and stratal slicing were applied to the 9-component S-wave seismic data to restore sedimentary facies of major marker beds based on sedimentary models reflected by satellite images.Then,techniques of seismic attribute extraction,principal component analysis,and random fitting were applied to calculate the reservoir thickness and physical parameters of a key sandbody,and the results are satisfactory and confirmed by blind testing wells.Study results reveal that the dominant sedimentary facies in the Qigequan Formation within the study area are delta front and shallow lake.The RGB fused slices indicate that there are two cycles with three sets of underwater distributary channel systems in one period.Among them,sandstones in the distributary channels of middle-low Qigequan Formation are thick and broad with superior physical properties,which are favorable reservoirs.The reservoir permeability is also affected by diagenesis.Distributary channel sandstone reservoirs extend further to the west of Sebei-1 gas field,which provides a basis to expand exploration to the western peripheral area.

Discovery and significance of Quaternary aqueously deposited aeolian sandstones in the Sanhu area, Qaidam Basin, China

Influenced by neo-tectonic movement, the Sanhu area of China＇s Qaidam Basin formed in the Quaternary in response to migration of its depocentre. A study of core material from the Qijia 1 Well reveals novel Quaternary aqueously deposited aeolian sandstones in the Qigequan Formation of the Sanhu area. Here we report the sedimentary petrology and geochemistry data of these deposits that constrain their depositional history. Evidence for aeolian influence during deposition includes： sorting and roundness, pure quartz sand with single mode grain size distribution and few suspension materials,very fine sand grain size distribution indicating sorting by saltation and suspension transport, dish-shaped and crescentshaped pits in grain surfaces indicating aeolian transport and chemical composition similar to that observed in active or recent dune deposits, namely enrichment in Si O_2, Na_2 O, and Mg O relative to the sand dam sediment in the lakeshore.Identification of these aqueously deposited aeolian sandstones expands the range of sedimentary deposit types found in the Sanhu area and improves understanding of its paleoclimatic history.

Geological implications of elements of the Pleistocene mudstone with different organism compositions and enrichment environments in the Qaidam Basin,China

Trace elements and rare earth elements(REEs)of two kinds of organic facies samples representing marginal and more basin-center deposits from Pleistocene lacustrine mudstones in the central Qaidam Basin were studied to understand the provenance,palaeotectonic setting,hydrothermal activity,palaeoredox conditions and sedimentary rate.The results show that the lacustrine mudstones were mainly derived from felsic sources with little contribution from ancient crustal sediments and no ultramafic(ophiolitic)source.The mudstones were deposited in a continental island arc tectonic setting,which is consistent with the tectonic evolution of the Cenozoic basin.Both two organic facies samples were hydrothermal in origin based on the ternary diagram of Ni–Zn–Co and normalized REE patterns.However,this does not mean that the water column in paleolake was affected by hydrothermal fluids in situ.This signal might indicate hydrothermal origins from hot springs related to active faults around the basin rather than the deep hydrothermal fluids entering the sediments via deep faculties based on the comprehensive analyses of normalized REE patterns,negative E_(uanom)(Eu anomaly),Y/Ho,Sm/Yb,and Eu/Sm.Redox proxies including U/Th,Ni/Co,and Mnanom values,are more sensitive for the studied samples indicating that most of the organic facies A samples were deposited under an oxygen-depleted condition,while the organic facies B samples were deposited under oxygen-rich conditions.Redox proxies of Ceanom values are unavailable for the organic facies B samples due to hypersaline environments,and V/C_(r)and V/(V+Ni)are invalid for the organic facies A samples,possibly because of their organism composition.The low Lan/Ybn values indicate high sedimentation rates,which is consistent with the average sedimentation rates of approximately 0.43 to 1.1 km/Ma.However,the La_(n)/Yb_(n) is more likely affected by the provenance of the studied samples,so it should be used with caution.