南羌塘依桑日剖面中侏罗统布曲组沉积相与氧化还原条件OA北大核心

The Middle Jurassic Buqu Formation in the Qiangtang Basin represents marine carbonate sedimentation and is regarded as a promising reservoir.However,ongoing debates surround its earlier depositional environment and the ancient seawater redox conditions.This study focuses on the lower strata of the Buqu Formation in the Yisangri section of the South Qiangtang Basin,conducting a thorough analysis of lithofacies,sedimentary facies,and ancient water body redox conditions.The results are as follows:(1)In the Yisangri section of the Biluo Co area in the South Qiangtang Basin,the lower strata of the Buqu Formation reveal a distally steepen carbonate ramp deposition system,delineated into inner ramp,mid ramp,and outer ramp sedimentary subfacies.(2)Five lithofacies associations are identified,encompassing tempestite lithofacies,oncolite limestone lithofacies,slumping lens-body limestone lithofacies,nodular limestone lithofacies,and calcareous silt mudstone lithofacies.Additionally,11 microfacies types are identified,including calcareous mudstone(MF1),mudstone(MF2),pelletoid mudstone(MF3),ooid wackstone(MF4),fecal pellet wackstone(MF5),oncolite wackstone(MF6),intraclast and pelletoid packstone(MF7),bioclastic packstone(MF8),intraclast grainstone(MF9),intraclast calcareous limestone(MF10),and intraclast rudstone(MF11).(3)The overall paleowater conditions in the Yisangri section of the Buqu Formation indicate a dysoxic-oxic environment,accompanied by a relatively low TOC content.The low TOC values may be associated with redox conditions,but they are more likely influenced by water nutrient levels and primary productivity.The TOC values in the lower part of the Buqu Formation suggest that the oil and gas of the dolomite reservoir did not originate from contemporaneous deep-water hydrocarbon source rocks but may have originated from deeper strata.