Formation Mechanism Of The Organic-rich Shales In The 7^th Member Of Yanchang Formation,Ordos Basin

In Ordos Basin,the Chang 7 organic-rich shale has high total organic carbon?TOC?content and and it is the most important source rock of Mesozoic in this basin.Howerer,the formation mechanism of Chang 7 organic-rich shale is still ambiguous.In order to solve this problem,the basic characteristics and distribution of organic-rich shale,the palaeoproductivity,bottom-water redox conditions and deposition rate during the formation of shale were studied,and the main controlling factors of organic accumulation were analyzed.Organic geochemical analysis shows that the mean TOC contents of Chang 7₁,Chang 7₂ and Chang 7₃ is 4.41%,5.49%,11.09%,respectively,with a trend of gradual decrease from Chang 7₃ to Chang 7₁.The types of organic matter are mainly I and II₁,indicating its source is simple,and it may dominantly come from lake primary organic matter.The distribution rang of Chang 7 organic-rich shale is also decreased gradually from Chang 7₃ to Chang 7₁.The enrichment of organic matter is influenced by palaeoductivity,bottom-water environment and deposition rate.The tectonic activity of basin margin was strong during the depostioin of Chang 7organic-rich shale of Yanchang Formation,leading to frequent volcanic and hydrothermal activites,which brought a large amount of nutrients to lake water and promoted the gowth of surface organisms.The qualitative and quantitative evaluation of the palaeoproductivity of Chang 7 period shows that the lake was characterized by extremely high nutrient content and had high primary productivity,with the highest value in Chang 7₃ period and the lowest value in Chang 7₂ period,which provided a rich material basis for the enrichment of organic matter.The high value area was in the center of basin and mainly distributed in NW-SE direction,in addition,the highest value occurred in the southeast area.Some redox proxies,including the size distribution of framboid pyrite,C_org:P molar ratio and the degree of pyritizion?DOP_T?,are used to identify the bottom-water environment of Chang 7 period,the results show that the bottom-water redox conditions at the time of deposition of the Chang 7 organic-rich shales were dominantly oxic-suboxic and intermittently anoxic.Study on the relationship between the deposition rate and the abundance of organic matter shows that when the deposition rate is less than 1.35 cm/ka,they have no obivious correlation,indicating that deposition rate had no significant effect on the enrichment of organic matter which might be mainly controlled by palaeoproductivity and preservation conditions;when the rate is greater than 1.35 cm/ka,the TOC content has a trend of gradual decrease with the rate increase,indicating the deposition rate had a certain control on the TOC content and mainly for the dilution of the organic matter,no matter the effect of productivity and preservation conditions.In the three factors above,the lake palaeoproductivity has the maximum contribution because it provided a rich material basis for the enrichment of organic matter.Howerer,the long time oxic-suboxic bottom-water conditions was not conductive to the preservation of organic matter.The deposition rate of Chang 7₃ and 7₂ periods contributed little but mainly played a role of dilution during Chang 7₁ period.Therefore,the formation of Chang 7 organic-rich shale was dominantly affected by lale palaeoproductivity.