Sedimentary Characteristics,Origin And Hydrocarbon Accumulation Of Lacustrine Carbonate-bearing Fine-grained Sedimentary Rocks

Carbonate-bearing fine-grained sedimentary rock refers to rock with carbonate minerals accounting for more than 20²5%of the total composition,which is mainly composed of fine-grained particles?<62.5?m?.Carbonate-bearing fine-grained sedimentary rocks are widely distributed in rift lake basins in China and are important carriers of shale oil.However,carbonate-bearing fine-grained sedimentary rocks have complex composition and diverse material sources.Moreover,there is no systematic study of the rock on the unified classification scheme and the evaluation of hydrocarbon accumulation.Therefore,the sedimentology and petroleum geology research of this kind of sedimentary rock has the double significance of science and exploration and production.In this paper,the calcite-rich fine-grained sedimentary rocks in Es₄^U-Es₃^L Formation of Shulu Sag and Dongying Sag in the Bohai Bay Basin and dolomite-rich fine-grained sedimentary rocks in Qianjiang Formation of Qianjiang Sag in the Jianghan Basin are selected as the research objects to carry out the systematic analysis of their sedimentary characteristics,origin and hydrocarbon accumulation.The components of carbonate-bearing fine-grained sedimentary rocks include calcite,dolomite,quartz,feldspar,clay minerals,pyrite,evaporite minerals,and organic matter.Material sources include terrigenous input and authigenic production.The biological activity and organic matter evolution in the basin have important influences on authigenic mineral formation.The authigenic carbonate minerals are mainly formed by organic-inorganic coupling reaction.According to the material composition of fine-grained sedimentary rocks,the lithologic classification scheme is established with carbonate minerals,clay minerals,felsic minerals,and evaporite minerals as four end elements.Moreover,fine-grained sedimentary rocks are divided into low,medium and high organic matter types by bounds of 1%and 2%of TOC.According to the thickness and combination models of sedimentary layers within fine-grained sedimentary rocks,the description basis is established.The lithofacies classification scheme of carbonate-bearing fine-grained sedimentary rocks is established based on the above principles.Finally,based on mineral composition and sedimentary characteristics,combined with the sedimentary origin,depositional environment,and combination relationship between lithofacies,the lithofacies combination division scheme for carbonate-bearing fine-grained sedimentary rocks is established for the first time.The enrichment factors of organic matter,including the source of organic matter,salinity of water body and redox conditions,are determined based on the evaluation of the abundance,type and maturity of organic matter in each lithofacies combination and the analysis of depositional environments.Lithofacies combinations with materials mainly formed by organic-inorganic coupling reaction have more aquatic organic matter,high TOC content,and good preservation condition,having great hydrocarbon generation potential.The main reservoir space types of carbonate-bearing fine-grained sedimentary rocks include interparticle pores,intraparticle pores,organic pores and fractures.The pores between carbonate crystals are generally filled with bitumen material,suggesting they are effective hydrocarbon storage spaces.It is proposed that the forming factors of shale oil reservoirs include high-quality source rock,effective storage space,and effective hydrocarbon migration channel through the comparative analysis of each study basin,which are affected by depositional environments,origins of mineral and lithofacies.The distribution law of shale oil can be more accurately confirmed by using the division scheme of fine-grained sedimentary rocks based on sedimentary origins.