川东北地区孤立台地飞仙关组优质储层形成主控因素——大气淡水淋滤及渗透回流白云石化

川东北地区在三叠系飞仙关组飞一段-飞二段沉积时期为孤立台地,发育了多个向上变浅的鲕粒滩沉积旋回,且飞二段沉积时期在孤立台地内部形成了膏质泻湖沉积.在孤立台地背景下发育的储层主要岩性为残余鲕粒泥-粉晶白云岩、残余鲕粒粉-细晶白云岩和晶粒白云岩.在向上变浅的鲕粒滩沉积旋回背景下,大气淡水淋滤导致每个向上变浅的旋回自上而下依次为:致密的泥晶白云岩→粒内溶孔(或铸模孔)型残余鲕粒泥-粉晶白云岩→残余粒间溶孔型残余鲕粒粉-细晶白云岩→晶间孔型晶粒白云岩.储层岩石学特征及地球化学分析表明,飞仙关组白云岩储层经历了2期主要的白云石化作用,第一期为渗透回流白云石化作用,第二期为埋藏白云石化作用；渗透回流白云石化增加了岩石强度、抑制压溶和胶结作用,有利于孔隙保存,而埋藏白云石化造成一定的减孔；埋藏溶蚀作用对早期孔隙进行扩溶,增加了储层渗透率,但并不大规模增加储层的整体储集空间.综合分析川东北地区沉积背景、储层岩石学特征、地球化学特征表明,大气淡水淋滤是优质储层形成的先决条件,渗透回流白云石化是优质储层保存的关键因素.

The Main Origins of High Quality Reservoir in Feixianguan Formation in Northeast Sichuan Basin:Atmospheric Water Eluviation and Seepage-reflux Dolomitization

The depositional environment was isolated restricted platform and a lot of depositional cycles of shallow upward oolite shoal existed at the stage from Member 1 to 2 of Feixianguan Formation of Lower Triassic in the northeast Sichuan basin,especially gypsum lagoon at the Member 2 of Feixianguan Formation in the isolated restricted platform.On the isolated restricted platform geologic setting,the main lithology of reservoir includes remnant oolitic dolomite and finely crystalline dolomite.On the shallow upward oolite shoal geologic setting,the atmospheric water eluviation results in the lithology sequence from top to bottom is tight micrite dolomite,intragranular pore or moldic pore residual oolith micritic or powder crystalline dolomite,intragranular pore residual oolith powder or finely crystalline dolomite,and intercrystalline pore dolomite.Petrology and geochemistry of reservoir show that the reservoir dolomite underwent two stages of dolomitization.The first stage is seepage-reflux dolomitization and the second is burial dolomitization.The seepage-reflux dolomitization make an important role in preservation of early pores and increase rock strength,inhibit pressure solution and cementation,but the buried dolomitization partially fill pore space.It can be recognized two types of buried dissolution.The first stage happened after seepage reflux dolomitization and before crude oil filling and it is correlated with dissolution by organic acid.The second occurred in the deep buried environment,it is correlated with acid generated by TSR.The buried dissolution can not generate reservoir porosity,but increases permeability.Based on comprehensive studies of geologic settings,reservoir petrology and geochemistry,it is concluded that the atmospheric water eluviation is prerequisite condition for reservoir porosity generation and the seepage-reflux dolomitization is necessary condition for reservoir porosity preservation.