Sedimentary System And Source Rock Distribution Of Yuertus Formation In Tarim Basin

The source of oil and gas in deep and ultra-deep strata in Tarim Basin has been widely concerned by researchers,and the Yuertusi Formation,as an important set of pre-salt source rocks,has become the focus of attention.Therefore,the refined study of the Yuertusi Formation can provide a certain basis for the solution of the source of oil and gas in deep strata.Therefore,this paper will take the learned theory as the guidance and combined with the data we have,to carry out high-precision sequence stratigraphic analysis,sedimentary system study and source rock distribution characteristics and main control factors analysis of the Yuertusi Formation in Tarim Basin.Based on 9 field profiles and 17 Wells,the sedimentary system of the Yuertusi Formation in the Tarim Basin and its restriction on hydrocarbon source rocks have been systematically studied and analyzed.After detailed correlation,the Lower Cambrian Yuertusi Formation in Tarim Basin can be divided into two third-order sequences(SQ1,SQ2)and five quaternary sequences(sq1-sq5),among which the third-order sequence SQ1 can be divided into three quaternary sequences and SQ2 can be divided into two quaternary sequences.SQ1 occurred in the early Cambrian,with a slow transgression and a relatively limited impact area,while SQ2 was relatively rapid and had a larger impact area.The fourth order sequence sq1 occurred at the beginning of the first transgression,and the water was shallow.The subtidal deposits of purplish red dolomite and graying black arenaceous dolomite were mainly developed,which played a role of filling and supplementing.For sq2 and sq4 occurred in transgressive regressive-transformation period,is two sets of deep-water sedimentary,for sq2 shelf deposit,mainly for the deep sq4 mainly for shallow shelf deposit,the lower the lithology in black shale and black mudstone layer with argillaceous dolomite,siliceous shale,local siliceous rock bands,for sq2 and sq4 upper mudstone and banded dolomite alternating layers;sq3 and sq5 developed in the regressive process,and are mainly deposited in the subtidal zone,mainly composed of gray dolomite and algal dolomite,with local thin layers of argillaceous.And utilization of Mn,Fe,Rb,Sr,Ba and Cu,K,U,V,Mo and other elements and their ratio to sedimentary environment analysis shows that for sq2 period,deep water,warm and moist climate,low salinity,be helpful for biological growth and reproduction,and its vulcanization of oxygen oxidation reduction environment is extremely beneficial to the preservation of organic matter;In sq4 period,the water depth was relatively large,the salinity was relatively small,and the climate was relatively warm and humid,which was more suitable for the survival of organisms.In addition,the environment of vulcanization and hypoxia combined with secondary oxidation was more suitable for the preservation of organic matter.The source rocks of Yuertusi Formation have the characteristics of plane distribution,which is centered on the shelf facies in Manjiaer sag,overlaid in the south to the central uplifting zone,and gradually thinning in the north to the basin facies.In addition,the source rocks located in Manjiaer sag have the largest thickness and the highest organic matter abundance.For the vertical aspect,sq2 and sq4 are two sets of better source rocks.On the basis of previous studies,this paper holds that the source rocks of Yuertusi Formation are not only controlled by hydrothermal events,rising ocean currents and periodic hypoxia,but also affected by high-frequency sea level changes and topography.High frequency sea level change and topography affect the distribution and quality of source rocks in the Yuertusi Formation.The quality of source rocks deposited in sq2 is better than SQ4,but its distribution range is smaller than sq4.