The Genesis Of The Upper Permian Changxing Formation Dolomite In The Huanglongchang Area,northeast Sichuan

In recent years,some scholars have carried out related research on the mechanism of dolomite in Changxing Group in different areas of northeastern Sichuan and have gained more understanding.However,there is a clear controversy about the genesis of dolomite.With the deepening of the exploration and development of the Changxing Formation gas reservoir in the northeastern Sichuan Basin,the dolomite genesis has become a key factor in defining the reservoir formation mechanism and distribution law and guiding the exploration and development of natural gas in the next step.Based on this,this paper selects the Changxing Formation dolomite in the Huanglongchang area of Northeastern Sichuan Province to carry out core description and sample collection,and analyzes rock composition such as indoor thin section identification,X-ray diffraction,cathodoluminescence,scanning electron microscopy,and trace elements,C and O.And Sr isotope geochemical analysis,trying to reveal the petrological and mineralogical characteristics of dolomite in the Changxing Formation in the area,to clarify the original sedimentary environment of dolomite,and to clarify the genetic mechanism of dolomite,in order to further analyze the dolomite on the east side of the Kaijiang-Liangping trough The genesis of the reservoir and the exploration and development of the next phase of the Changxing Formation of Huanglongchang provide an important basis.The paper has achieved the following four points:?1?The Changxing Formation dolomite in the Huanglongchang area can be divided into two types of quasi-synonymous dolomite and buried dolomite.According to the petrological type,it can be divided into:?1?quasi-synchronous micro-powder crystalline dolomite?Rd1?;?2?Early dolomite burial dolomite?Rd2?;?3?diagenetic burial dolomite?Rd3?;?4?night diagenetic grain dolomite?Rd4?;?5?night diagenetic fragmentation dolomite?Bd?.?2?The Changxing Formation in the Huanglongchang area mainly develops open platform,platform marginal reefs,front slopes and troughs,and established a comprehensive model of sedimentary facies.It is believed that the original sedimentary environment of microcrystalline dolomite is a reef with a marginal reef.The basement and the front slope of the platform;the original lithology of the reef dolomite and the dolomite are reef limestone and clastic limestone,and the original sedimentary environment of the reef dolomite is the bio-frame reef and reef flat,and the original sedimentary environment of the dolomite It is a reef shoal,a reef base,a reef nucleus,and a shoal on the edge of the platform.?3?It is speculated that the dolomitization of different types of dolomite in the Changxing Formation in the Huanglongchang area of Northeastern Sichuan Province occurs in a relatively closed burial diagenetic environment,and it is impossible to participate in the atmospheric water fluid with high radioactive crust source,and the dolomitization fluid may For the large-scale distribution of the Early-Middle Triassic,due to the dissolution of evaporite,the salinity of the seawater pore water with high salinity has a high?¹³C value,a high ⁸⁷Sr/⁸⁶Sr ratio,a low Mn content,and a low The characteristics of Fe content and high Sr content may be mixed with the raw brine in the Changxing period.?4?It is speculated that the same-generation brine in the Changxing period forms a quasi-synaptic dolomite through the near-surface environmental evaporative pump,and the high-salinity burial cycle source derived from the Early-Middle Triassic evaporite is dissolved along the pre-existing Fractures or cracks and TP unconformity face the micro-facies of the Changxing Formation platform reef or reef cover beach and some adjacent micro-phase limestones of the skeleton reef,which are superimposed to form different types of buried dolomite.