| In recent years,Xixiangchi formation of the Cambrian on the south area of central Sichuan Basin,due to large oil and gas fields found in Longwangmiao formation of the lower Cambrian which was in the same tectonic conditions with Xixiangchi formation,has become one of the important exploration areas,while there are not too much previous researches on Xixiangchi formation of the Cambrian on the south area of central Sichuan Basin,so this study is based on the previous datas,taking the Sichuan south central area as the investigated area and the Cambrian Xixiangchi formation as the target formation,with comprehensive methods like core observation,thin section identification,geophysical(seismic and logging)methods test and pore permeability,to get the identification,classification of sequence stratigraphy and the types of the sedimentary facies as well as detailed distributional characteristics of Xixiangchi formation in the investigated area.The main research results are as follows:(1)In the aspect of sequence division,it is mainly divided into a three-grade sequence in Xixiangchi formation,namely SQ1,whose lower group is Gaotai formation without depositional hiatus and in conformable contact which is the bottom interface of SQ1;The interface between the upper group,Tongzi formation,which is in conformable contact with Xixiangchi formation.SQ1 contains 1 maximum flooding surface(mfs1),1 early-late HST interface((E-L)TS)within the high system tract.The SQ1 system consists of 3 domains,namely TST,EHST and LHST.The top and bottom interfaces of SQ1-TST are respectively the bottom surface of Xixiangchi formation and mfs1;The top and bottom interfaces of SQ1-EHST are respectively mfs1 and TS(E-L);The top and bottom interfaces of SQ1-LHST are respectively(E-L)TS and the top surface of Xixiangchi formation.The LST remained little due to weathering.In the SQ1,the changes of lithofacies,well loggings and seismic facies show 1 complete sedimentary cycle in transgressive and regressive environment.TST,EHST and LHST correspond to the Xi-1,Xi-2 and Xi-3 stages respectively.(2)The investigated area was generally in the restricted platform sedimentary environment.We identified three types of subfacies including the platform subfacies,the bank subfacies and the lagoon subfacies and five microfacies consisting of the sandy-muddy dolomitic flat,the dolomitic flat,the sand bank,the muddy-dolomitic lagoon and the dolomitic lagoon.On the distribution of sedimentary facies,the main part of the Xi-1 stage is on the development of the restricted platform subfacies while the bank subfacies is locally developed in the east.In the most part of the Xi-2 stage,the lagoon subfacies is mainly developed,except the large scale development of the bank subfacies in the middle and east areas.In the Xi-3 stage,the main developed subfacies is the platform subfacies excluding the large scale development of the bank subfacies in the eastern part of the investigated area.(3)The physical properties of lithologies in the investigated area are poor,and compared with the other two sedimentary facies belts,the bank subfacies has a better physical property,followed by the platform subfacies.While the lagoon subfacies is the worst.Here are three main reasons to explicate the ranks among three subfacies: the first one is the sedimentary environment for the bank subfacies was a turbulent environment of high energy which gave the assurance for the residual dolarenite being scrubbed clearly leading to a better primary porosity than those of the lagoon subfacies and platform subfacies which were in a environment of low energy over the same period with bank subfacies.the second reason is the late dissolution in deep depth causing positive alteration to the intergranular pores of the bank facies,of which the saturation of acidic fluids enlarged the primary pores ulteriorly;Last but not the least,the constructive fractures generating in the late tectonic movements got the intergranular pores and caves connected together which greatly improved the properities of the bank facies.(4)The favorable sedimentary facies in the investigated area are mainly distributed in the eastern part,and slightly in the Moxi structure in the western part of the investigated area which was subjected to the late stage tectonic denudation.Within the Xi-1 stage(SQ1-EHST),there are bank facies mainly including residual dolarenites and dolarenites around Well Gaoshi21,Well He12,Well Guangtan2 and Well Nanchong1 respectively.Within the Xi-2 stage(SQ1-EHST),there are bank facies mainly including residual dolarenites and dolarenites around Well Anping1,the Well Moxi21-Well Moxi23 Belt,the Well Gaoshi23-Well Gaoshi7-Well Gaoshi8 Belt,Well Gaoshi16,the Well Nvji-Well Nvshen5 Belt and the Well Gaoshi21-Well He12-Well Guangtan2 Belt respectively.As with the Xi-3 stage(SQ1-LHST),there are bank facies mainly including residual dolarenites and dolarenites around Well Anping1,the Well Baolong1-Well Nvji-Well Moxi23 Belt,Well Nanchong1 and the Well Gaoshi21-Well He12-Well Guangtan2 Belt respectively... |
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