Origin And Reservoir Characteristics Of Dolomite Of The Upper Permian In Coqen Basin,Tibet

Marine carbonate and clastic rocks sedimentary from ancient Palaeozoic to Cretaceous developed in Coqen Basin which was the second largest marine sedimentary basin in Qinghai Tibet Plateau.Found of sucrosic dolomite oil seepage in Zhanongema Formation of Silurian and Mujiucuo Formation of Permian proved that hydrocarbon migration and accumulation process has existence in Paleozoic.And this area has good prospects for oil and gas exploration.Based on the previous data collection,field geological work and indoor test analysis,this thesis summarizes the sucrosic dolomite of the Paleozoic Coqen Basin.Dolomite of different texture was divided into three types based on microscopic observation.They respectively are A-type(nonplanar)dolomite,S-type(subhedral)dolomite and E-type(euhedral)dolomite.Furthermore,based on the existence of zoned texture,there are single zones and multiple zones exhibited in dolomite,and the single zoned dolomite can be divided into two types as dolomite with foggy core and bright edge,and with bright core and foggy edge,respectively.A detailed study about the S-type dolomite with foggy core and bright edge,S-type dolomite with bright core and foggy edge,E-type dolomite with multi-zoned and E-type curved saddle dolomite was done through cathode luminescence,fluorescence microscopy and electron microprobe analysis.Experimental results showed that Micro fluorescence recognition accuracy of dolomite with foggy core and bright edge was higher than that of cathode luminescence.A plurality of fluorescence bands could be observed in part of crystal.Based on the result of EPMA,the content of SrO was relatively high,and the content of Fe O and MnO was low in the core of dolomite.Multi-zoned dolomite and saddle dolomite have a characteristic of extremely high content of FeO,and the high value was centered on the inner band.The ratios of Mg/Ca of dolomite from the edge to the core have characteristics of “low-high-low”,but the edge of saddle dolomite had a high ratio of Mg/Ca.The homogenization temperature of dolomite mainly range from 120? to 180?,and the average salinity was much higher than that of modern sea water,which indicated that the formation of dolomite through an evolutionary process of high temperature and high salinity.The results of microprobe analysis of isotopic showed that ?13C value of the zoned dolomite ranged from-0.18‰ to 0.40‰,?18O value ranged from-2.30‰ to-4.46‰ and ?13C value of the multi-zoned dolomite ranged from-0.22‰ to-2.38‰,?18O value ranged from-2.58 to-7.75‰.The ?18OSMOW value of dolomitized inclusions ranged from +6‰ to +16‰ and inclusions formed the zoned dolomite came from the mixture of native evaporation residue of deposited sea water and underground brine according to oxygen isotopic fractionation equation.The ?18OSMOW value of multi-zoned dolomite was close to that of magma water,and it might be reformed by the deep hydrothermal or fresh water.Dolomite of Mujiucuo formation experienced two phases of dolomitization: The first phses was reflux seepage model that seawater permeated into calcium carbonate deposit and limestone formation through intercrystalline pore or intergranular pores and formed the first generation crystallite-powder dolomite by ion exchange during diagenetic process.The second phase burial dolomitization play a dominant role in this stage,and formed the second generation dolomite characterized by powder-fine grain with structure of foggy core,bright edge and bright core,foggy edge,meanwhile,the third generation multi-zoned dolomite deposited in where fluid action strongly.Evaluation of reservoir indicates that dolomite reservoir was of strong heterogeneity,dissolution holes and fractures developed which formed fracture-dissolution porosity reservoir system.Constructive diagenesis composed of dolomitization,dissolution and cataclasis.Two phases of oil and gas inclusions developed in dolomite.The first phase which was mainly liquid hydrocarbon-based inclusions distributed in calcite veins,and the homogenization temperature of salt water inclusions with hydrocarbon associated with it was 130.8?.The second phase which was mainly gaseous hydrocarbon inclusions distributed in dolomite,and the homogenization temperature of salt water inclusions with hydrocarbon associated with it was 148.2 ?.Large scale oil and gas filling occurred after burial dolomitization and the pool-forming pattern may belong to hydrocarbons generated by young source rocks accumulate in old reservoirs.