The Study On The Sequence,the Sedimentary Facies Of The Devonian Guanwushan Formation,NW Sichuan Basin

The Devonian system is absent in most parts of the upper Yangtze affected by the Caledonian movement,northwest margin of the upper Yangtze platform.is the only record of Devonian marine carbonate.Guanwushan Formation is the first set of carbonate rocks after Pingyipu Formation's clastic rock deposition which is important geological records that the sedimentary environment changes from muddy water to clean water.There are three different views on sedimentary facies types and its distribution: carbonate slope,lagoon facies and reef-bank complex.These differences restrict the understanding of the Neopaleozoic sedimentary evolution of the upper Yangtze and the prediction of carbonate reservoir distribution.In recent years,several exploratory wells implemented by CNPC in the study area drilled the Guanwushan Formation which bring a lot of new information.The thesis put the Devonian Guanwushan Formation carbonatite in NW Sichuan Basin as the research object.Using sequence stratigraphy,sedimentary geologynd lithology paleogeographic.Combined with core observation,outcrop,section drilling logging to synthetically analyze.Carried out the research with the area's Devonian Guanwushan Formation carbonatite sequence and sedimentary.In order to division of sequence interface ccuratelly,Macroscopically,through analysis by three field outcrops,drilling corecore and new geophysical logging data.Combined Microcosmic,through identifying the sequence of diagenesis,especially dolomitization,to discover the hidden sequence boundaries.Thus,the Devonian Guanwushan Formation is divided into three 3 order sequences(SQ1,SQ2,SQ3).The marine flooding surface of the study area has a very obvious sequence of accretion sequence structure below as well as retrogradation sequence structure above,and the maximum flooding surface is a set of thin and rich biologically mudstones.SQ1 and SQ2 are divided into TST and HST,in which SQ1-TST develops 2 symmetrical quasi sequences,SQ1-HTS develops 1 symmetrical types and 3 upward deepening asymmetrical accretion sequences.SQ2-TST develops 1 symmetrical types and 3 upward deep asymmetric asymmetrical sequences.SQ2-HST develops 1 symmetrical types and 2 upward shallowly asymmetrical quasi sequences.Because of the incompleteness of geological records,SQ3 only developed TST.According to the development of the Devonian strata from the west to the East,the increase of the loss in the bottom strata and the previous research results,it can be concluded that the early stage of the Devonian sea water came from the Ganzi-Litang Ocean on the western margin of the upper Yangtze.Facing the Ganzi-Litang Ocean,the western margin of the upper Yangtze is a epeiric sea.During the sedimentary period of the Yangwu formation,the western margin of the upper Yangtze's sedimentary environment is a epeiric sea carbonate platform.The region in the east of Beichuan-Nanba,in the west of Tongkou-Majiangba-Yanmenba is the marginal area of the platform margin face,further,bank and reef of platform margin type can be divided in Beichuan and Ganxi.The region in the east of Tongkou-Majiangba-Yanmenba,in the west of Zhitong-Jiange is the marginal area of the open-restricted platform,further,bank of intra-platform type can be divided in m Majiaoba,Hejiangliang and Shuangtan-3.It comes to the conclusion by comparison of sedimentary facies in sequence framework: SQ1 is widely distributed in the region with good lateral contrast.and typical characteristics of developing reefs.Among them,the reef limestone in Guixi outcrop has a huge scale,and the accumulative interval can reach 100 m which consist of 5 reef cycles,strongly dolomitization on reef limestone is a common phenomenon in HST,also be found in Hejialiang and Shuangtan-3,it's the main ban and reef-forming period in the region.The transverse thickness varies greatly in SQ1,.The thickness of the Tangwangzhai syncline is the largest but to the Shuangyu area is only about 40 ~ 50 m.The SQ2 sequence is well developed in the front of Longmen mountain with only TST system tract remaining in the Shuangyu area.The maximum flooding surface is developed in Guanwushan Formation which show black mudstone.The SQ3 sequence is well developed in the front of Longmen moutain and has been eroded completely in the basin.