Study Of Reservoir Characteristics And The Formation Mechanism About Dolomitic Rock In Saline Lacustrine Basin

Carbonate tight oil is an important area of of oil and gas exploration in Qaidam Basin.The research on carbonate tight oil reservoir is the foundation of effective exploration work.This paper takes western Qaidam Eocene dolomitic rock for research,and studies deeply of reservoir characteristics and the formation mechanism of the dolomitic rock in saline lacustrine basin through petrological characteristics,physical properties,pore structure and geochemical characteristics.Through polarizing microscope and x-ray diffraction we draw a conclusion that the rocks in Lower Eocene Ganchaigou Formation in Western Qaidam Basin are mainly carbonate,at the same time developmenting part siltstone and mudstone interlayer.The rocks show obvious mixed deposits features.The rock‘s average porosity is 8.6%and the average permeability is0.108 m D,belongs to?medium porosity-ultra-low permeability?.The dolomite rock has the best physical properties.And the dolomite porosity increases with the increasing of dolomite content.The curve of capillary pressure test though mercury porosimetry turn out to be?narrow lower site-wide higher site?double platform structure features.This mainly because the reservoir space mainly contain macro-cracks and intergranular pore of dolomite which pore radius is small but the quantity is huge.This paper studies the geochemical characteristics of lacustrine dolomite in research area.The Fe O in the rock values ranges from 0.258%to 1.648%,and the average value is 0.942%.The Mn O values ranges from 0.003%to 0.137%,and the average value is 0.052%.It is similar to penecontemporaneous dolomite.The Si O₂ values ranges from 0.118%to 10.381%,and the average value is 4.493%.The Al₂O₃ values ranges from 0.101%?6.062%,and the average value is 2.071%.The rock has the characteristics of high Silicon and aluminum content and low content of iron and manganese,which indicating mixed sedimentary characteristics.Dolomicrite‘s discrete trace element‘s spider web diagram shows the diversity of the source material of this mixed deposited lacustrine dolomite,indicating the different source of the sediment which has mixed sedimentary characteristics;The trace element‘s characteristics turns out that the average value of Sr is 1202.7 ppm,and Sr/Ba is greater than 1,its average value is4.64.The rock‘s?REE is between n×10^-5?n×10^-4,and this value significantly higher than seawater,indicating saltwater environments.The?¹³C_PDB values ranges from-4.5‰to 0.2‰,and the average value is-2.4‰.The?¹⁸O_PDB values ranges from-6.1‰to 0.4‰,and the average value is-2.6‰.The carbon and oxygen isotope composition indicates the sedimentary environment is semi-open semi-closed environment.The average value of?Eu is 0.87,showed mild losses,which significantly different from enriched properties in hydrothermal rocks.So we can speculate that the dolomite‘s metasomatism occurred in penecontemporaneous period.Therefore,the dolomite in study area turns to be penecontemporaneous and low temperature hypothermia causes.All the research results above clarifiy the reservoir characteristics and formation mechanism of dolomitic rocks in western Qaidam Basin,and it has important implications for the oil and gas exploration.