Study On Petrology And Geochemistry Of Dengying Formation In Southeastern Sichuan

The dolomite of Sinian Dengying Formation?Z₂dn?in Sichuan Basin,which is an important exploration target horizon for carbonate reservoirs,is widely distributed with a large thickness.The dolomite of Z₂dn experienced multi-stage tectonic uplifting movements,and obvious replacement and diagenesis.For its genesis,there are many studies on the secondary dolomitization model.However,by detailed petrological work,it found that the microbial dolomite in the second and the forth section of Z₂dn in the southeastern Sichuan Basin preserved the signal of original sedimentary environment and seawater,which is altered in light degree.Based on the detailed work in petrography,geomicrobiology and geochemistry of the core samples of Well DS 1 and L 1 in the southeastern Sichuan Basin,the following findings are obtained:1)The dolomite of Z₂dn in Well DS 1 and L 1 are divided into three categories,microbial dolomite,grain dolomite and granular dolomite,and six types.Microbial dolomite is further divided into stromatolite dolomite and thrombolites dolomite.Grain dolomite occupies most of the core,which is dominated by putty to powder crystal dolomite,containing some medium to coarse crystal dolomite that is transformed by later diagenesis stronger.The number of granular dolomite is small,mainly composed of sparite granular dolomite and algal granular dolomite.The dolomite crystals in the microbial dolomite are small and micritic,which preserve the signal of the original sedimentary environment better.2)The trace elements of stromatolite dolomite of Z₂dn indicate that the salinity is relatively higher and sedimentary environment is relatively reduced,and the rare earth elements?REE?partitioning model is consistent with seawater.The average temperature of the microenvironment during dolomite precipitation was 28.7?,calculated from the oxygen isotope geological thermometer of the primary dolomite in the laboratory.3)In the dark layer of micritic stromatolite dolomite,there is spherical dolomite,chain-like or cluster-like crystals with a diameter of 250⁵00 nm,with a relatively small degree of crystallinity.Its morphology,structure and aggregation are similar to those of microbial-mediated primary dolomite in the laboratory,indicating that they are closely related to microbial activity.Analyzed by energy dispersive spectrometer?EDS?of high-resolution scanning electron microscopy?SEM?,the dolomite spherules contain a certain amount of Cr,which may be mediated into the dolomite lattice by microbial activities.These characteristics reveal the original sedimentary environment and the genetic model of microbial dolomite of the Dengying Formation dolomite in the southeastern Sichuan Basin,which provide guidance for the exploration of the target reservoir and become an important example of primary dolomite in the geological history.