Study On Deposition Characteristics And Model Of Fine Grains In Salt-lake Basin

In this study,Lucaogou Formation in Jimsar Sag,located in Junggar Basin,was selected as the research study area.Utilizing stratigraphy,sedimentology,and other theoretical frameworks.Regarding the unclear mechanism of multi frequency cycles in the Lucaogou Formation and the unclear response mechanism of fine-grained reservoir "sweet spot" sedimentary dynamics.Sediment simulation technology was employed as the primary research methodology.Building upon previous research results,the main types of sedimentary facies and their characteristics were identified,along with the combination of lithofacies in the study area.The sedimentary dynamic process was then reconstructed,and the factors influencing sedimentary response were analyzed.Furthermore,a model of fine sediment deposition in saline lake basins was discussed,aiming to provide valuable insights into lacustrine fine-grained sedimentology and serve as a reference for oil and gas exploration and development in similar saline lake basins.1)Based on core observation from five coring wells,one outcrop dissection,and sedimentary origin analysis of the study area,the lithofacies combination and types of sedimentary facies were determined.It was observed that the enrichment of relatively coarse grains in favorable reservoirs is attributed to long-distance transportation by density flow.The predominant lithologies in the study area consist of clastic rock,organic rich mudstone,volcanic tuff,and carbonate rock.The sediment sequences within Lucaogou sedimentary body exhibit composite rhythms,particularly in layers where these rhythms are evident.Some layers display erosion surfaces in the middle,resembling typical sedimentary sequences formed by hyperpycnal flow.The sedimentation processes include sudden contact relationships,blocky and clumpy sedimentation,and suspended sedimentation.The research area provides favorable conditions for the development of hyperpycnal flows,especially during temporary floods under relatively dry climate conditions.The significant carbonate content indicates relatively high water salinity during sedimentation,indicating a dry climate at that time.In such arid climate,seasonal floods have a considerable impact,as they transport long-term weathered sediment from the ground into the river channels,resulting in a higher proportion of debris and facilitating the formation of density flow.Additionally,predominantly gray to dark gray sediments are observed without exposed oxidation or erosion transformation features.This suggests that the the lake basins water depth has consistently remained shallow to semi-deep relative to the lake basin itself,limiting the basins depth and reducing the occurrence of slope instability-induced collapses during sedimentation.Consequently,long-distance transportation of coarse sediment occurs without significant collapse sedimentation.Given the limited sedimentary background of the lake basin in the study area,deep-water turbidite development conditions are absent,and the impact of lake waves is not obvious.Lake current also have limited effect.Instead,the primary influence is attributed to river processes,suggesting that the sediments are predominantly derived from hyperpycnal flow deposits.2)Based on the characteristics observed in rock cores and outcrops within the study area,a sedimentary genesis analysis was conducted.To further investigate the sedimentary dynamic processes,control factors,and genetic mechanisms of terrestrial debris and endogenous chemical sedimentation,four sets of experimental simulation equipment were developed.The focus was on studying the overall evolution process and internal flow pattern changes of hyperpycnal flows.A glass circular water tank,spanning a total length of 27 meters,was established to facilitate visual observation and image monitoring of sediment distribution within the water flow.To monitor and analyze changes in sediment content during the flow of hyperpycnal flows and after sedimentation,a high-speed camera,a multi-point turbidimeter system,and a particle size sampling device were employed.These instruments provided measurements of flow velocity,sediment content,and particle size distribution at various positions and water depths in the flume.To address the experimental requirements for different particle sizes and sediment compositions,a mud and sand screening device was used to ensure accurate particle size selection and sand supply.Additionally,a quantitative water supply system was implemented to enable multi-source water supply and variable flow rates for investigating the spatial distribution of density flow.To analyze the characteristics of sediment distribution,FARO 3D scanners were utilized to obtain precise three-dimensional elevation data of sedimentary surfaces.This approach facilitated the acquisition of data and images related to sedimentary landforms,allowing for the determination of net sediment thickness increase during each experimental period.Furthermore,a track vertical camera device was employed to overcome the limitations of manual photography,such as potential bending in photos,enabling better observation and recording of the morphology of each experimental exhibition.To explore the spatial relationship between salt deposition processes and sand salt interlayers,a wind channel deposition simulation device was developed and equipped with corresponding temperature and humidity monitoring equipment.3)To investigate the sedimentary evolution process and characteristics of the saline lake in the research area,the following physical simulation experiments were conducted:(1)Early-stage of smooth flow lake environment: In this phase,rivers outside the basin(hyperpycnal flow)supplied abundant terrestrial debris to the lake basin.By changing sedimentary dynamic conditions and sediment compositions,the formation conditions of hyperpycnal flows,factors influencing long-distance transportation,and hyperpycnal flow sedimentary models were explored.The main controlling conditions for hyperpycnal flow formation were identified,highlighting the concentration of sediment and the duration of river water as key factors governing hyperpycnal flow development.It was concluded that hyperpycnal flows have the capacity to transport sediment over long distances.(2)Short-term open and closed saline lake environment: During this period,multiple rivers flowed into the lake basin,depositing coarse sediments and transforming them into various hypopycnal flow.Simulation experiments were carried out to analyze the separation of fine and coarse sediments caused by this phenomenon,as well as different sedimentary processes and formation conditions of different hypopycnal flow.The experiments also explored the redistributive effect of lake waves on fine sediment within lake basins.(3)Water retention enclosed saline lake environment: Based on investigations conducted in Manas Lake,a supersaturated solution was prepared,and sodium carbonate was used as a substitute for rocksalt in the salt lake.This enabled the simulation of the precipitation process of salt sediment and the exploration of the similarities in structure,spatial distribution,and sedimentary mechanism of salt sediment.4)According to the sedimentary sequence in outcrop cores and verified through flume simulation facies,this study compares existing lacustrine fine-grained sedimentary models described by previous researchers.These models include the organic matter enrichment model,lithofacies distribution model,and "source sink" system genetic model.However,it is noted that these sedimentary models exhibit strong regional characteristics due to variations in basin geological conditions,making them challenging to promote and apply universally.Furthermore,regarding the fine sediment model of saline lake facies,the contemporary approach still relies on the type and distribution of fine sediment petrology as the basis of coupling,with a lack of analysis on the genetic mechanism of fine sediment.Building upon previous models,this paper explores the sedimentary dynamic mechanisms centered around the formation of relatively coarse sediment in the "sweet spot" of fine-grained sediment in saline lake facies.This formation is attributed to hyperpycnal flows,while the relatively fine sediment is formed through the attenuation of these hyperpycnal flow.Considering the complex evolutionary background of the saline lake basin,the study summarizes the evolutionary characteristics observed in different stages of the saline lake basin.One significant difference from previous research results is the explanation of the origin of the delta in the study area as being associated with the formation of hyperpycnal flows.Additionally,dolomitic flat and mixed flat deposits are classified as the deposition of the hyperpycnal flows and floating light flows,while beach bar sheet sand is categorized as the deposition of the lobules,channels,and channel edges within the hyperpycnal flow.