Sedimentary Characteristics And Formation Mechanisms Of Fine Grained Rocks In The Xiguayuan Formation Of The Luanping Basin

Luanping Basin is a Mesozoic continental faulted basin in China,in which Luanye-1 well has made a breakthrough in oil and gas exploration recently.In order to study the sedimentary characteristics and formation mechanism of fine grained rocks in the Luanping Basin,this paper takes the material components of fine grained rocks as the first element,and according to their sources,the mineral components in the basin can be classified as endogenic,terrigenous,volcanic and hydrothermal sources to analyze the characteristics and sources of the main components.Combined with core observation and a small amount of whole-rock diffraction data,the rock types of Watermelon-Yuan Formation can be divided into four categories:volcanic rock,mudstone,carbonate rock and siliceous rock.Subsequently,16 lithologies were identified according to the microscopic features of the thin sections,and 12 of the main lithologies were characterized.Through the analysis of geochemical data,the sedimentary environment in the study area is recognized as follows:1.From deep to shallow vertically,the reductivity of Watermelon-Yuan Formation is weakened,the productivity and salinity both increase first and then decrease,the water depth gradually decreases,and the overall palaeo-weathering intensity is weak,which is a dry climate condition.Among them,the?member has the strongest reducibility and the deepest water depth,and the palaeo-productivity and salinity of the section are the highest,while the shallower depth and salinity of the section are lower.2.The source area is mainly the tectonic background of continental island arc and has the character of active continental margin.3.The hydrothermal activities in the study area are identified and the REE composition characteristics of the hydrothermal fluids are unique to the lake environment,such as high?REE content,enrichment of light REE,depletion of heavy REE,strong negative Eu anomaly and weak positive Ce anomaly.The organic matter content of?segment of Watermelon Garden Formation is the highest among the three segments,and according to TOC and S₁+S₂ relation diagram,the samples are mainly distributed in the best-best range,followed by the upper segment and the lower segment.Based on the analysis of the main controlling factors of Watermelon Garden Formation,it is found that strong reductivity,relatively humid climate,higher salinity in the suitable range,deeper water depth,certain intensity of volcanic and hydrothermal activities are conducive to the enrichment of organic matter.The?section of the watermelon formation is the most frequent volcanic activity,and the input of volcanic ash is beneficial to the initial productivity and preservation conditions of organic matter,but the excessive pyroclastic material will lead to volcanic poisoning effect,and the death of a large number of microorganisms requires longer recovery time to reproduce,so the productivity of this section is not particularly strong.The brackish water environment may be due to the fact that volcanic ash is usually covered with soluble salts.The volcanic activity in?section is weakened and the hydrothermal activity at the bottom of the lake is enhanced.The release of nutrients and minerals from the hydrothermal fluid provided a sufficient source of organic matter accumulation,and the fall of a small amount of volcanic ash in this section also caused a fertile effect and the initial productivity reached its peak.Hydrothermal fluid increases water temperature and salinity,which creates good conditions for the preservation of organic matter.The hydrothermal activity in?section was weakened,and the last intense volcanic activity resulted in volcanic poisoning effect.The productivity was weak,the climate continued to be dry,the water level of the lake declined,and a large amount of terrigenous detrital was imported.The depositional environment of the lake was weak and reductive,which was not conducive to the enrichment of organic matter.