Sedimentary Evolution Of The Lower Cambrian Longwangmiao Formation In Central Sichuan And Its Control On Reservoir Development

According to drilling and core data,the Longwangmiao Formation in the Gaoshiti Moxi area is a continuous and thick dolomite deposit.Based on previous research results,guided by theories such as sedimentology,sequence stratigraphy,geochemistry,and petroleum geology,using element and stable isotope testing,combined with reservoir physical characteristics,this paper systematically studied the sedimentary evolution model,paleoceanographic conditions,and main controlling factors for reservoir development within the high-frequency sequence of the Longwangmiao Formation in central Sichuan,and achieved the following three achievements and understandings:Using core and thin section data,the sedimentary evolution of the Longwangmiao Formation in central Sichuan was studied.It is believed that the Cambrian Longwangmiao Formation in Sichuan Basin mainly develops four sedimentary microfacies,namely,cloudy lagoon,intertidal sea,granular beach,and Yunping.Two fourth order sequences(sq1 and sq2)can be clearly identified,and each fourth order sequence can be divided into multiple high-frequency sequences,establishing a planar facies distribution model.It is believed that the sedimentary water body of the SQ1 stage of the Longwangmiao Formation slowly retreated towards the southeast,and each sedimentary subfacies(microfacies)showed a trend of migration towards the southeast;The sq2 sedimentary period inherited the sedimentary pattern of the sq1 sedimentary period.Through the testing of carbon and oxygen isotopes,major and trace elements in the whole well section coring,sea level and paleoclimate changes have been restored.Longwangmiao Formation δ 13 C has two cycles of change,indicating two sea level rises and falls,consistent with paleowater depth changes reflected by changes in Al2O3,Ti O2,B,and V elements.The good correlation between the Z value and temperature of ancient sea water estimated by carbon and oxygen isotopes,as well as Mg and Ca,reflects both the arid and hot climate and the subtropical marine environment;The regular changes of Mg O/Ca O,Mn O2,and Fe2O3/Mn O2 indicate that the early climate of the two sequences transited briefly to warm and humid;The Sr/Ba value reflects a significant increase in seawater salinity during late sedimentation.It is generally believed that the transgressive system tract was formed during the warm and humid climate and the rapid rise of sea level;When the climate is hot and dry,the rapid deposition of granular beach and cloud flat constitutes a high level system tract.The Longwangmiao Formation in central Sichuan mainly develops arenaceous dolomite and fine coarse crystalline dolomite,and the reservoir space is composed of intercrystalline(solution)pores,intergranular solution pores,and solution cavities.Based on core,thin section,and major and trace element data,it is believed that the main controlling factors for dolomite reservoirs in the Longwangmiao Formation of the Lower Cambrian in central Sichuan are sedimentary facies(grain beach facies),dolomitization,and quasi syngenetic karstification.The particles and intergranular cements of the Longwangmiao Formation have similar Sr and Ba contents,reflecting that they were formed in the early diagenetic stage.The flat REE content of pore filled powdered dolomite D1 reflects the formation of quasi syngenetic dolomitization.The depletion of LREE and HREE,relatively high Fe/Mn,and low Sr/Na contents jointly reflect that the D1 stage dolomite filled with holes may have been formed during quasi syngenetic dissolution and influenced by atmospheric fresh water leaching.Based on comprehensive analysis,the sedimentary evolution model and favorable reservoir development and evolution model of the Longwangmiao Formation have been established.