Research on the Method of Recoverying Microtopography of Epeiric Carbonate Platform in Depositional Stage:A case study from the layer A of Jia 2 2 Member in Moxi Gas Field ,Sichuan Basin

Key words: 
• Epeiric Sea /
• carbonate platform /
• microtopography during depositional stage /
• grain shoal /
• Jialingjiang Formation /
• Triassic /
• Moxi Gas Field /
• Sichuan Basin

Abstract: The microtopography difference of the epeiric carbonate platform in the depositional stage is so tiny,as a result,it is too hard to identify.This paper is then,firstly,mainly based on the development dominating factors of the grain shoal within the platform,and then points out that the microtopography highland tends to be beyond the wave base in the relativeregression stage,where the grain shoal develops.The depositional rate is higher than any other microfacies area within the platform,hence,the topography difference was enhanced at that stage.Besides,after the sedimentation of the grain shoal,the grainsupported framework forms under the influence of physical compaction,where the compaction rate is much lower than the finer sediments,so the depositional thickness resulted from the topography difference of various microfacies areas is also further enhanced,which shows that the grainstone thickness within a certain period of time can be used, approximately,in recovering its microtopography prominence when it began to present, and that the compaction correction can be ignored when approximately recovering the microtopography within the platform at the depositional stage.It then further analyses the effects on the grain shoal construction and reservoir development by the exposure process during the depositional stage,and it divides the shoal within the epeiric platform into three genetic types:unexposed shoal,shortmediumterm exposed shoal and longterm exposed shoal, meanwhile,pointing out that the method of grainstone grossthickness which is used,approximately,in recovering the depositional palaeogeomorphology of epeiric carbonate platform is preferable for the unexposed and shortmediumterm exposed shoals,where the main characteristics of the shortmedium term exposed shoal can be concluded as follows(contrarily be longterm exposure ):①unconformity characteristic limited,palaeosol lacks;②irregular karren and corroded fissure lack;③intargranular solution extremely develops;④the freshwater cements in early stage and cement unconformity exist;⑤lensdiagenesis reservoir of atmospheric genesis positively associates with the grainstone grossthickness.Based on this,the paper then takes the view that microtopography prominence can be inverted by the reservoir study of the grain shoal.Followed by that principle,it also suggestes that the concrete study procedure can be like this:the isochronous geologic body to be chosen as the main body of grainstone deposition,the confirmation of reservoir genesis and exposured time,the topography recovering during depositional stage and the application of achievement verification.
Sichuan Basin was characterized by developing epeiric carbonate platform during the period of Jialingjiang in early Triassic.The basin was also hit by an fierce transgression in the period of early Jia 22, which made the salinity of the seawater normal on the whole.Moxi gas field locates in the south of gentleoblique tectonic zone of the palaeohigh in the central of the basin,where the tectonic body is about 280 km2, and the exploration wells penetrating the Jia 2 member sum up to 42(data by 2005),of which the total cored wells of Jia 2 member sum up to 16,besides, the distribution is relatively uniform.Hence,the gas field was used in the case study,where the result indicates that highland mainly develop in the well areas of eastern side of Mo 24Mo 205,southern side of Mo 206,the rim of Mo 207,Mo 151Mo 36,Mo 208 and its southern part,Mo 48,Northern part of Mo 202(7 in all),which distribute in the direction of southwestnortheast,swales mainly develop in Mo 12Mo 201,Mo 0052Mo 150,southern part of Mo 22 and eastern part of Mo 206.There is thick deposition of underwatergypsum(darkgray and massive gypsum,no exposed marks found) developing in the well areas of Mo 0052Mo 150,Mo 207,which has come into being by the shoal development around the microhighland ,resulting in the sealing restrict of the lowlying environment of interbank sea and the evaporation and concentation of the waterbody,the gypsum deposition therefore comes into being.It can be believed from the above that the recovery of the palaeogeomorphology by taking advantage of the grainstone thickness can be supported by the lithofacies deposition.
According to the graph of sedimentary facies compiled by actual cored data of B Formation,Jia 22 Submember, the palaeogeomorphology during early sedimentary stage carries on developing in the stage of Jia 22B.dominating the distributing framework of its sedimentation and reservoir.The result indicates that this sort of method used in recovering the microtopography during depositional stage is applied to the characteristics of lithofacies and so on.,besides,the recovered microtopography in the sedimentary stage can not only continue to successively develop over a long period of time and can also dominate the deposition of adjacent stratigraphy and the framework of reservoir distribution,furthermore,relatively speaking,its production application effect is good,which also indicates that dedicate description of sedimentation and reservoir is an effective and adjuvant measure for those gas fields at developing stage with high density of well pattern and that this kind of method should be widely popularized,especially for those maturing basin fields with high level of well exploration,data that is out of date, rare triporosity logging series,all of these are significant and also has referenced significance for the reservoir prediction and tapping the new potential formation.