| The exploration of Dengying Formation in the Sichuan basin has a long history,and researchers have accumulated abundant research achievements about Dengying Formation.In recent years,the major breakthrough of oil and gas drilling and some theory in structure and karst alteration give the starting point for us to rethink.Such as Gao Shiti structure,Jinshi structure,the Tongwan movement tension groove theory,and series theory of dolomitization,hydrothermalism,dissolution and so on.Introducing wavelet-Fischer plot method and multiple methods including traditional lithologic sequence stratigraphy and seismic sequence stratigraphy,geochemical sequence stratigraphy and so on classify the sequence stratigraphy of Dengying Formation and recognize four Third-order Sequence of Dengying Formation.According to the Identification of each sequence interface on the the seismic,logging,carbon and oxygen stable isotopic response,establish the group of the sequence stratigraphic framework and analysis the variation characteristics of accommodation space in the different sequence system.Based on the distribution analyzes of sedimentary facies in sequence framework,A line beween well Anping-1 and well Zishen-1 in central Sichan exist the intraplatform sag.It formed by extension effect during I-stage of the Tongwan movement and it has certain inheritance in the time,space,genesis about the early stage of Cambrian intracratonic sag and the intraplatform sag of the period of Dengying.It is the Performance of Tongwan movement multi-stage.This article further summarizes the Sinian sedimentary facies evolution influenced by Tongwan movement multi-stage:Tongwan movement I-stage,SQ3 transgression formed the south-north intraplatform sag.The intraplatform sag deposited mudstone,Containing a small amount of dolomite.the edge of intraplatform sag mainly debeloped Algae set.Structural stable period:It is the structural stable period in Sichuan basin from SQ3 HST of Sinian to late SQ4 HST.Tongwan movement II-stage:the tension function of tectonic movemen made the line that form Mianyang to Lezhi and Changning to form a south-north intracratonic sag.The intracratonic sag deposited a thick layer of Maidiping Formation,and both sides of Dengying Formation was exposed and corroded.Analyzing the properties and forming time of minerals in Dengying Formation,the fluid characteristics is then studied and results can be concluded that six periods of dolomite fluid activities in Dengying Formation.In sedimentary period of Dengying Formation,sea water and meteoric water is included significantly.The main mechanism of dolomization of this period include penecontemporaneous Sabkha dolomization and seepage reflux dolomization,the extent of dolomitization related to sea level and the position of algae concentration.The main sediments including dolomicrite,algal dolomite,powder-fine dolomite surrounding rock,fibrous dolomite deposition related to sea fill by stitch?pore and holes,granulated sparry dolomite related to the biological activity and atmospheric water.The fluid in 542 Ma of semi-open environment in the early period of shallow burial period was still the sea,which got higher temperature influenced by hydrothermal process between 40.8 ? 100 ?.So this period had deposited the blade shape dolomite and the part of dolomite has became the blade quartz replaced by silica.The fluid of the middle age in shallow burial period was formation water,the Seam and hole were fill granulous fine crystal dolomite.Tongwan movement II-stage and caledonian movement made the Dengying Formation into Supergene dissolution stage.There is developed lamellar dolomite in the big seam and hole.Its edge had the overgrowth boundary edges which toward the central seam and hole and its band is bright red under the cathode ray.The last phase of caledonian movement,coarse grain dolomite had formed because of the effect of fluid in formation water in homogenization temperature between 150~175?.The fluid belongd to interlamination fluid and diagenetic fluid was low salinity formation water fluid.In the early of Emei taphrogeny,there developed saddle dolomite.Inclusions which formed by underground magma upwelling invasion by stress release in the process from squeezing action to tension action were high homogenization temperature(from 245 to 264?)and low salinity.The Dengying formation of Sinian has seven dissolutions.The multi-period of fluid had five different types of control functions to reservoir: Atmospheric fresh water effect in the early of penecontemporaneous period.In the 1st dissolution,intra--granular dissolved pore,mold pore,intergranular dissolved pore was often seen in grainstone of bank facies.The 2nd dissolution,the Karst reservoir reservoir was marked by the development of dissolution of seam?pore hole and botryoidalis dolomite;The 3rd dissolution,Atmospheric fresh water effect in the interim ofSyngenetic period,the Karst involved with unconformity surface of was the result of Tongwan movement I-stage;The 4th dissolution related to the top denudation caused by Tongwan movement II-stage,which distributed in two sides of the central Sichuan intracratonic sag and the east of platform edge;The 5th karstification in caledonian movement and the karstification of top denudation,which distributed in Leshan-Longnvsi palaeohigh district;The 6th dissolution related to hydrotherm,which identification mark was that the dissolution of seam incise botryoidalis dolomite and saddle dolomite and growing quartz fill in seam;The 7th mark was that the dissolution of saddle dolomite and quartz.The source rock was clay shale and related to organic acid fluid which generated by large number of hydrocarbon generation in last Dyas.The Dengying Formation of Sinian has three hydrothermal processes.The 1st hydrothermal process was activating in middle and end of Penecontemporaneous period of 536.3±5.5Ma Tongwan movement I-stage.The hydrotherm funneled eruption was caused by tension of seabed.The fluid was hydrothermal siliceous fluid which the temperature was 75-100? and the fluid led to the silica rock of sequence of SQ3,SQ4 develop into stratiform,stirpped,en plaque.In the 2nd hydrothermal process,basal magma upwelling along the fracture caused by stress released during caledonian movement transiting to Emei taphrogeny.Hydrothermal fluid transported along the Dengying formation and grew saddle dolomite mineral;In the 3rd hydrothermal process,the ultrahigh pressure caused by oil cracking during the end of Jurassic and the early of cretaceous of Yanshanian led gas pool being broken and eascaped,forming heavy minerals with formation water and its symbol was cawk,galena gathering.Through analyzing the tectonic evolution,sedimengtary facies,petrological char--acteristics,physical characteristics and diagenesis of Dengying formation of Sinian with Gaoshiti of central Sichuan,well Dingshan-1 in south east of Sichuan,well Jinshi-1 in south west of Sichuan and Summarizing the main control element of the reservoir of Dengying formation in south of Sichuan,the multiperiod superimposed karstification was the main control element which developed the reservoir of Deng--ying formation.At the same time,phase-controled reservoir was the basis of the Dengying formation reservoir development.From a regional scale,phase-controled reservoir thickness was still significantly to occupy in the total thickness,especially the intra-platform shoal patch?marginal-platform shoal of Dengying formation was also important reservoir space,shoal's types mainly include algal shoal,sand shoal,etc.,They were controlled by sequence,sedimentary facies.In addition,the seam and hole of botryoidalis dolomite concerned with atmospheric water dissolution in Syngenetic period is also the important reservoir space.Looked from the overall,the Krast transformaiton with Tongwan movement II-stage and caledonian movement was occurring in the traditional I-area in exploration district.The Krast transformaiton with Tongwan movement I-stage and caledonian movement was occurring in the traditional II-area in exploration district.The traditional III-area was concerned with Tongwan move--ment I,depositional stage,high level shoal phase.During the depositional stage,high level shoal phase which been transformed by the KrastThe location of Krast transformation transformed by Tongwan movement II-stage in high level shoal phase of depositional stage will become the important and beneficial area in exploration.The broad platform that controlled by sequence of high frequency is the quite beneficial area in future exploration. |
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