Sequence Stratigraphic Frameworks In Different Scales Of Lower-Middle Jurassic In The Northern Qiangtang Depression And Their Significances

Sedimentary strata of the Late Triassic to the Early Cretaceous was recorded in the Mesozoic Qiangtang Basin.A key of that is Lower Jurassic sedimentary succession,which recorded not only the nature and evolution of the Mesozoic Qiangtang Basin,but also the rapid expansion of the Meso-Tethys Ocean.The evolutionary history was recorded in the sedimentary strata in the Northern Qiangtang Depression.The QK-1 well,the first scientific drilling well is drilled in the middle of the Northern Qiangtang Basin,has recovered a continuous Upper Triassic to Middle-Upper Jurassic sedimentary succession,and allowed further studies of paleoclimate,sedimentary evolution,and petroleum geology of the Qiangtang Basin.Based on drillcores,well logging,and geochemical data of the QK-1 well and QZ-16 well,we recognize sequences of different scales within the Quemoco Formation.Within sequence stratigraphic frameworks of different scales,we study the sedimentary processes and main controlling factors of the Middle-Lower Jurassic Quemoco Formation,combined with the Early-Middle Jurassic sedimentary evolution in the Northern Qiangtang Depression and the mixed siliciclastic-carbonate sediments in the third member of Quemoco Formation.In addition,we discuss petroleum geology and its significance in the QK-1 well.The sedimentary environment of the Quemoco Formation in the QK-1 well include marginal lake,restricted platform,shelf,and tidal flat in ascending order.We recognize five third-order sequences,a long-term cycle,and several high-frequency cycles within the Quemoco Formation.A correlation of the long-term cycle in the Quemoco Formation with global sealevel curves shows clear similarity,which indicates that eustatic change is the main controlling factor on the long-term sequence of the sedimentary evolution in the Qiangtang Basin during the Early-Middle Jurassic.Based on the sedimentary succession of the Quemoco Formation,rifting and sea-level change affected the evolution of third-order sequences.Main controlling factors change from rifting early to sea-level change late.High-frequency mixed siliciclastic-carbonate sediments which were recorded within high-frequency cycles occurred near the sequence boundary and invariant lithology sediment occurred near the maximum flooding surface.High-frequency sealevel changes driven by Earth's orbital parameters and sea-level changes in scales of third-order sequences affect carbonate production and clastic inputs,and control the distribution of mixed siliciclastic-carbonate sediments within sequence stratigraphic framework.Several source rocks,reservoirs,and caps were discovered in the QK-1 well.Within sequence stratigraphic framework of the Quemoco Formation,source rocks occurred near maximum flooding surface,thick gypsum occurred in the transgressive system tract of the third-order sequences and long-term cycle,gas logging abnormal zones occurred near sequence boundary.Combined with the feature that reservoirs distribute in uplift and source rocks distribute in depression,spatial distribution characteristics of petroleum geological condition can be built.In addition,the degree of modification in the Northern Qiangtang Depression is weaker than that in the Southern Qiangtang Depression,which has petroleum exploration significance.