Genesis Of The Middle Devonian Dolomite Guanwushan Formation In Ganxi Section Of Northern Sichuan

The Paleozoic marine carbonate oil and gas is a potential reservoir in the Sichuan basin,explore the Devonian reservoir of geological event displays,the dolomite of the middle Devonian dolomite Guanwushan Formation can be a high-quality reservoirs.The Ganxi section is one of the typical standard sections of the Devonian,and it is also a research focus of the Devonian in the Beichuan,Longmenshan area.In order to open a new situation of the Paleozoic exploration from oil and gas in the Sichuan basin,fundamental geological research is particularly important way of the middle Devonian dolomite Guanwushan Formation.The dolomite genesis of the middle Devonian dolomite Guanwushan Formation in the Beichuan,Longmenshan area is studied based on previous research,combined with rock structure analysis,trace element,major element,geochemical characteristics of rare earth element of dolomite.Based on the measured data of Ganxi section in Beichuan,Longmenshan area,the sedimentary facies of Guanwushan Formation of Middle Devonian in Ganxi,Beichuan are analyzed.The bottom of lithology is clastic rocks,two main sedimentary types are identified,including fluvial facies and barrier free coast of shoreland facies;The upper of lithology is carbonate,two main sedimentary types are identified,including carbonate platformand,carbonate ramp and platform facies,with the sea level of rises and falls periodically,the carbonate ramp and carbonate platform can be transformed into each other in a particular sedimentary environment,of which the most direct identification mark are the emergence of breccia and sliding deformation structure.There are two types of subfacies including platform edge reef and foreslope in carbonate platform and two types of subfacies including shallow ramp and deep water ramp in carbonate ramp.It is found that bioherm limestones are developed in the early and late stage of Guanwushan Formation,but not in the middle stage.Sedimentary environment of the middle Devonian Guanwushan Formation was controlled by two basically continuous Transgression-Regression cycles in Beichuan-Liangshuidong,Longmenshan area.Classification scheme based on the genesis-structure of dolomite,combined with rock structure analysis including grain size,crystal fabric,crystal form and interface distribution characteristics,study of fillings,three types of Guanwushan Formation dolomite are identified,the dolomite whose original structure was destroyed to varying degrees,the original structure disappeared from the dolomite,the broken dolomite with structural fracture occurred.The Guanwushan Formation dolomite of Mg/Ca ratios have no obvious difference,it indicats that the different types of dolomite has a strong inheritance.Trace element analysis results show that different types of the Guanwushan Formation dolomite have lower ratios of w_Th/w_U and w_V/w_V+Ni.Characters of the Guanwushan Formation dolomite trace elements which is rich in Fe,Mn and poor in Sr,Na.it indicats that relic structure dolomite happens relatively open system,and this type of dolomite is affected by the external fluid which rich in Fe and Mn.Characters of the Guanwushan Formation dolomite trace elements which is poor in Fe,Mn and rich in Sr,Na.it indicats that grain structure dolomite happens relatively sealing system.Rare earth element analysis results show that the diagenesis fluid is congenetic under different dolomitization step,and the diagenesis fluid is related to sea water.Characters of the Guanwushan Formation dolomite trace elements which in?La>1,?Ce<1,?Eu>1,rare earth distribution patterns of positive?La anomalous,weak negative?Ce anomalous and positive?Eu anomalous,it indicats that Guanwushan Formation the dolomitizationin a hot and reducing environment.The dolomites of the Middle Devonian Guanwushan Formation in the Beichuan Ganxi Formation is the burial genesis dolomitization,which can be further divided into three stages:buried dolomitization in the early diagenetic stage,dolomite in the metaphase diagenetic stage,and dolomitization in the late diagenetic stage.