The Forming Environment Of Fine-grained Sedimentary Rocks Of The Lower Third Member Of Paleogene Shahejie Formation,Bonan Sag

Recent success in the exploration and development of shale oil has inspired research on fine-grained rocks.However,few studies have been conducted on deposition mechanism and formation environment of fine-grained rocks.An example in the lower part of the third member of Shahejie Formation in Bonan Sag is presented in the paper to better understanding the nature of fine-grained rocks.The sedimentary characteristics of fine-grained rocks are analyzed with a combination of petrographic observation and geochemical analyses.The fine-grained rocks in the study area are composed of calcite,clay,quartz with a certain percentage of other minerals,such as feldspar,pyrite,dolomite and organic matter.Many laminae are preserved in these rocks.Moreover,some sedimentary structure modified by the event processes(e.g.turbidity current and storm flow)can be found.Based on the sedimentary characteristics of the fine-grained rocks,three different formations are found in fine-grained rocks,including distributed as muddy water deposition,the semi-muddy water deposition and the clear water deposition.Also the turbidite deposition and the storm resuspension deposition can be found in fine-grained rocks.The analysis of the Milankovitch cycle using the coring wells found the change of lithofacies of the fine-grained rocks is in accord with eccentricity and precession.The eccentricity and precessionhave obvious influence upon the change of terrigenous input intensity and composition of the fine-grained rocks vertical evolution.In an eccentricity cycle,the organic carbon content shows a low-high-low variation.In a precession cycle,the content of carbonate rocks shows a low-high-low variation,and the content of organic matter shows a high-low-high variation.Based on the sedimentary characteristics that are changed with astronomical cycles,five different depositional systems are found in fine-grained rocks,including distributed as the muddy water deposition system,the semi-muddy water deposition system,the clear water deposition system,the turbidite deposition system and the storm resuspension deposition system.In a precession cycle,the depositional systemshas different combinations,like "semi-muddy water deposition system-deposition system-semi-muddy water deposition system " or " muddy water deposition system-semi-muddy water deposition system-muddy water deposition system ".The storm resuspension deposition system often developes in the muddy water deposition system and semi-muddy water deposition systems.The turbidite deposition system is developed in the upside of lower part in the third member of Shahejie Formation,and it often develops in the muddy water deposition system.