| Puguang gas field is located in Xuanhan-Daxian area in the northeast of Sichuan Basin,with Qingxichang-Laojunshan tectonic belt in its east,and Tieshanpo gas fied in its north,and Dukouhe gas field and Luojiazhai gas field in its south.Puguang gas field is a packaged large carbonated gas field,and is composed of Changxing formation and Feixianguan formation.It is a reef-beach gas reservoir controlled by sedimentary facies.The strata drilled downwards in Puguang area are sequencely Jurassic,Triassic,Permian,Carboniferous and Silurian.Paleozoic lacks Upper Silurian,Devonian and most of Carboniferous.There is only Huanglong formation reserved in Carboniferous.Mesozoic lacks Upper Cretaceous.Cenozoic barely has sediments.Influenced by multi-period structural movements,there are often unconformity between strata.There are parallel unconformity between Silurian and Carboniferous,Carboniferous and Permian,Upper and Lower Permian.In the end of Triassic,affected by Indosinian movement,Sichuan Basin was uplifted and Yangzte platform was in a wide-range regression,so as to form the lacustrine,delta and fluvial sediments which mainly developed terrestrial clastic rocks.On the basis of the drilling cores and careful observation and description of outcrop sections,the sequence boundaries are identified from lithology,sedimentary facies,electrical property and diagenetic change.Feixianguan formation covers 6 million years from 251 to245 million years ago.Changxing formation covers 2 million years from 253 to 251 million years ago.Under the control of the sequence time scale,combined with lithological character and superposed style characters,Feixianguan formation in Puguang gas field is divided into 23rd-order sequences and 9 4th-order sequences and Changxing formation is divided into 23rd-order sequences and 5 4th-order sequences.Based on the division of sequences,the frameworks of sequence of Changxing and Feixianguan formation in the research area is built showing that the sequences are complete.Although the lithology and thickness differ in each layer,the sequence layers have a good comparison in the area.Each sequence is made up of a transgressive system tract and a highstand system tract,without lowstand system tract or condense segment.Based on the analysis of thin sections of cored wells,it is pointed out that Changxing formayion and Feixianguan formation have pure sediments,mainly carbonate rocks with few muds,gypsum rocks and transitional rocks,and carbonate rocks are mainly dolomites.According to the sedimentary environment and drilling data,it is believed that Changxing formation and Feixianguan formation mainly develop carbonate platform.Combined with Wilson sedimentation model,8 subfacies and more than 30 microfacies are identified,which are crossly distributed.Among these microfacies,the reef flat and oolitic beach in platform edge facies,the intraplatform shoals and reefs in open platform and restricted platform facies,the platform tidal-flat in restricted platform facies are beneficial to the development of reserviors.By comparison with sedimentary microfacies horizontal tracking,it is pointed out thatthe zone in late Changxing stage(sequence ? ~ ?)and the early Feixianguan stage(sequence ?~?)was shallow in the west and deep in the east.The Puguang main body was in platform-margin facies with the western water transited to the platform-margin slope~shelf facies.In Feisan stage(sequence ? ~early ?),the whole area turned into a carbonate platform with a open platform in the west and the restricted platform in the east as the the geomorphic difference in the east and west become smaller caused by the filling function;In Feisi stage(late sequence ?~?),the filling function continued and a large-scale regression occurred,turning the whole area into the plateau environment.According to the thickness of rock stratum thickness ratio,dolomite,such as shale content,has carried out four sequence sedimentary microfacies distribution under study. |
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