玛南地区风城组富有机质页岩成因机制——来自有机岩石学、生物标志物及同位素的证据OA

[Objective]The Mahu Sag,a pivotal,hydrocarbon-rich depression within the Junggar Basin,hosts the Permian Fengcheng Formation,a primary source rock interval widely regarded as a key"sweet spot"target for shale oil exploration.Unlike the well-studied depocenter and northern slope of the Mahu Sag,the Southern Mahu Sag represents a marginal lacustrine facies.Although possessing distinct source rock quality,organic matter(OM)occurrence,and paleo-environmental evolution,the underlying genetic mechanisms remain poorly constrained.[Methods]To address this,this study integrates organic petrology,molecular geochemistry,and stable carbon isotope analyses on 33 mudstone and shale samples from 12 wells in the Southern Mahu Sag.Systematic tests were conducted to comprehensively evaluate the geochemical characteristics and hydrocarbon generation potential.These included determination of the total organic carbon(TOC)and total sulfur(TS)content,Rock-Eval pyrolysis,measurement of vitrinite reflectance(Ro),chloroform bitumen"A"extraction,gas chromatography–mass spectrometry(GC–MS)of saturated and aromatic hydrocarbons,and analysis of the carbon isotope compositions of extracts and fractions.[Results]The results indicate that the OM abundance of the Fengcheng Formation source rocks in the Southern Mahu area generally reaches"good"to"excellent"levels.However,affected by the dilution of terrigenous detritus and transport effects,the OM is predominantly mixed Type II–III kerogen,contrasting significantly with the Type I–II OM typically found in the Northern Mahu area.The samples are generally within the peak oil-generation window(Ro>0.8%).[Conclusions]After rigorously assessing and calibrating the thermal maturity effects on source-related parameters(e.g.,Pr/Ph,β-carotane,and ETR),multi-proxy analysis confirms that the OM mainly originates from lower aquatic organisms,such as algae and bacteria,with limited input from higher terrestrial plants.Regarding the depositional environment,the study area primarily evolved in a saline lacustrine slope setting,characterized by dual control of endogenous chemical precipitation and exogenous terrigenous detrital input.The water column exhibited weak reducing and high-salinity conditions.[Significance]This study(i)elucidates the material basis of the Fengcheng Formation source rocks as a"saline mixed sedimentary"shale oil enrichment zone in a marginal facies,(ii)reveals the depositional heterogeneity compared to the sag center and northern slope,and(iii)provides a theoretical foundation for expanding shale oil exploration in the Junggar Basin.