Research On The Sedimentary System Of Yan'an Formation In The Southern Of The Ordos Basin

Ordos basin is the second largest basin in our country, and the largest basin contained three energy sources of petroleum, gas and coal. The Yan'an formation in south of Ordos basin is the most important oil-bearing series in Jurassic,it was formed after the estably settling period of indosinian movement,the fluvial lake delta sedimentary system with steady thickness and distinct characteristics was formed.This paper by way of sedimentology and sequence stratigraphy, based on outcrop observation,single log facies analysis,combined with the rock types,sedimentary cycle,depositional texture structure,etc.There are three types delta sedimentary:the fluvial system,the lake system and the delta system.There are three provenances in the Yan'an formation in the south of Ordos basin,as the northeast ,the northwest and the southwest margin uplift area,but the sediment mainly come from the northwest and the northeast of the basin.The west uplift area of the basin was eroded,and the palaeo-current directions were from west to east,the products of denudation entered into the basin that made the northwest become a important provenances;The same to northeast area,the northeast uplift area also was eroded that made the northeast area become an another main provenances.The sedimentary system of Yan'an formation were fluvial system and fluvial lake system.Yan 10 substage was typical fluvial sedimentary system, from Yan 9 to Yan 4+5 substage,there was the fluvial lake sedimentary system,and the main subfacies were the delta front and the delta plain.There were two stages in the lithofacies palaeogeography of Yan'an formation,that were the fluvial system in Yan 10 substage and the fluvial lake system from Yan 9 substage to Yan 6 substage.The source rock and the container rock have important relation with the sedimentary facies and the distribution of sand body.The source rock of lake facies is the main oil generation,the stream channel sand body,deltaic plain distributary channel sand body and deltaic front sunaqueous distributary channel sand body are good reservoirs.Through analysis of sedimentary facies,the distribution of sand body,the character of structure and the property of reservoir,etc,the favorable exploration area can be found out in the main oil-bearing beds in Yan'an formation.