The Origin And Accumulation Patterns Of The Tight Oil Within The Lucaogou Formation In The Jimusaer Sag,the Junggar Basin

The middle Permian Lucaogou Formation located within the Jimusaer Sag of the southeastern Junggar Basin contains both significant source rocks and an important tight oil reservoirs and has formed one of the highest potential and largest tight oil fields in China.However,the studies on the characteristics and origin of tight oil reservoirs and the tight oil accumulation mechanisms within the Lucaogou Formation are still unclear.In this work,samples from typical drilling wells were investigated.Multidiscipline researches are concentrated on them including petrology,mineralogy,stable isotope geochemistry and organic geochemistry.Three preliminary conclusions were draw below.(1)The Lucaogou Formation is a series of lacustrine mixed sequences composed by carbonate minerals along with terrigenous clastic and volcaniclastic sediments.The types of reservoir rocks are complex and diverse,including dolomite,siltstone and tuff.They can be further divided into sandy dolomite,dolarenite,dolomicrite,dolomitic siltstone,dolomite-bearing siltstone,tuffaceous dolomite and tuff.Among them,the former three dolomites,especially the sandy dolomite,are most important.Dolomite and feldspar are the two major mineralogical compositions of the tight oil reservoir rocks.The Lucaogou Formation were deposited in a highly saline and evaporative environment evidenced by well-developed gypsum pseudomorphs and abnormally high?13C values of whole-rock carbonates.(2)The reservoir space within the formation can be divided into three types:primary intergranular and intercrystalline pores(e.g.,primary intergranular pores in dolarenite and intercrystalline pores in dolomicrite);secondary intergranular and intragranular dissolution pores(e.g.,feldspar dissolution pores within sandy dolomite units and dissolution intragranular pores within intraclasts and clastic feldspars);and fractures(e.g.,dendritic and high-angle fractures).Secondary dissolution pores form the major reservoir spaces within the tight reservoirs.The diagenetic system within the Lucaogou Formation was relatively closed,and organic fluids have significantly affected the reservoir rocks under this system.A large amounts of organic acid were consumed by carbonate minerals within the source rocks resulted in the formation of sparry calcites while organic CO2 entered into the reservoir rocks and dissolved carbonate and feldspar leading to secondary pores as well as the precipitation of secondary carbonates.Secondary carbonates commonly account for?15%of the modal abundance of carbonates within the formation and cause heterogeneous offsets in ?13C values,with ??13C values that generally range between 1.8‰ and 4.6‰.In addition,the correlation between ?13C and ?18O also decreased as a result of the influence of these secondary carbonates.Although interactions between organic fluids and carbonates(i.e.,dolomite)do not directly improve reservoir quality,they catalyze the dissolution of feldspar,which can enable further generation of secondary porosity.The?13C and ?18O of secondary carbonates and their influence on whole-rock ?13C can be used to geochemically identify the effect of organic fluids on carbonate-rich reservoirs in closed diagenetic system.(3)The geochemical characteristics of source rock varies much vertically which can be generally divided into six groups(A-F,from top to bottom).Multiple parameters(e.g.gammacerane index,?-carotane/Cmax and C24/C26)indicate that source rocks in all sections contain both abundant terrigenous and aquatic organic matter with different relative amount.And,the source rocks of the Lucaogou Formation were generally deposited in saline and stratified water columns under anoxic conditions(except sections B and F).Similarly,four groups of tight oil were identified by the geochemical profile(1-4,from top to bottom)according to the differences of geochemical characteristics.It is indicated that aquatic organic matter and terrigenous organic matter both contribute much to the tight oils.The results of oil-source rock correlation suggest that crude oil generally origin from the source rocks within the formation without outside source.The tight oils accumulated in three relatively independent subsystems with two accumulation patterns:oil only originated from interbedded mudstones without mixing(NA)and oil originated from deep depths via bedding migration with mixing of different thin bedded mudstone(VLMA).Generally,oil in upper "sweet point"accumulated in pattern VLMA while oil in the lower "sweet point" accumulated in pattern NA.