川东南地区下奥陶统桐梓组白云岩成因机制与控制因素OA

The genetic type of dolostone is a key factor affecting the formation of dolostone reservoirs.The dolostone of the Lower Ordovician Tungtzu Formation is widely developed in southeastern Sichuan Basin,but its genetic mechanism is still controversial and the controlling factors are still unclear.Starting with an analysis of petrological characteristics,combined with order degree,carbon,oxygen,and strontium isotope,and trace and rare earth element data,this study systematically analyzed the genetic mechanism and controlling factors of the dolostone in the Tungtzu Formation.The results show that micritic dolostone,silty crystal dolostone,silty fine crystal dolostone,peloidal dolostone,oolitic dolostone,sandy dolostone,silty fine dolomite,and heteromorphic dolomite cement developed in the Tungtzu Formation.The geochemical features of the different types of dolostone are similar and can be compared with coeval limestone,reflecting that the genetic dolomitization fluids originated in seawater.The negative δ18O values,heavy rare earth depletion,and positive Eu anomalies of the dolomite cement indicate that it formed in a burial environment,while the δ13C and 87Sr/86Sr values,similar to those of wall-rock or limestone,provide additional evidence that the dolomitization fluid originated in seawater.Based on an analysis that considered the paleoclimate,palaeogeography,and sea-level changes,it was concluded that the dolostone of the Tungtzu Formation has a(pene-)contemporaneous concentration-seepage reflux dolomitization origin,where some penecontemporaneous granular dolostone underwent recrystallization during the burial stage,in which dolomite cements directly precipitated via a magnesium-rich pore fluid.It was found that the palaeogeographic pattern combined with cyclic sea-level changes were the most important factors controlling the formation and spatial distribution of these large-scale dolostone.