Formation Mechanism And Development Model Of The Carboniferous Source Rocks In The Northeastern Junggar Basin

The Carboniferous source rocks in the Junggar Basin are important oil and gas source rocks,and exploration practice shows that they have good exploration prospects.However,previous studies on this set of hydrocarbon sources mainly focused on source rock evaluation and oil-source correlation,and the depositional environment of this set of source rocks,the biological source of organic matter,and the controlling factors for organic matter enrichment are still unclear.For the above problem,we use the principles and methods of molecular geochemistry,organic petrology and elemental geochemistry to analyze the results of organic petrology,saturated hydrocarbon and aromatic hydrocarbon chromatography-mass spectrometry analysis,major and trace elements and iron-speciation.The parent rock properties,weathering degree,and biological sources of organic matter of source rocks were analyzed in detail,the sedimentary paleoenvironment was restored,and the main controlling factors of organic matter enrichment in Carboniferous source rocks were clarified and the corresponding sedimentary patterns of source rocks were established.The research results show that the source rocks of Jiangbasitao Formation are developed in the Suosuoquan sag,which is relatively thick（not drilled through）.The Bashan Formation and Jiangbashi Formation（not drilled through）source rocks are developed in Shibei sag,and the Bashan Formation source rocks are relatively thin.The Bashan Formation source rocks are developed in Shishugou sag,eastern Junggar Basin,and their thickness is larger than that of Shibei area.The Shiqiantan Formation source rocks are developed in Shiqiantan sag,and the thickness of the source rock is larger than1000m.The source rocks of the Jiangbasitao Formation in the Suosuoquan sag are poor-fair source rocks with high maturity.Most of these sampes were in the post-mature stage,and the organic matter type is mainly type ⅡI.The source rocks of the Bashan Formation in the in the Shibei sag and Shishugou sag are good-excellent with relatively low maturity.Most of these samples were in the mature stage,and the type of organic matter is mainly type Ⅱ₂.The source rocks of the Shiqiantan Formation in the Shiqiantan Sag are fair-good source rocks with relatively high maturity.Most of these samples are in the mature to high-mature stage and the organic matter type is mainly type ⅡI.The parent rocks of Carboniferous samples in the study area are mainly quartz diorite and granodiorite,and the source rock samples of Shiqiantan Formation in Shiqiantan Sag and Bashan Formation in Shibei Sag suffered moderate weathering,while some Bashan Formation source rock samples in the Shishugou Sag suffered strong weathering.The Carboniferous source rocks in the study area were deposited in a warm and humid paleoclimate,but during the deposition of the Jiangbasuo Formation and Shiqiantan Formation source rocks,intermittent dry conditions may occur.Analysis results of organic carbon isotopes,organic petrology,and saturated and aromatic hydrocarbons show that the organic matter in the source rocks of the Bashan Formation is mainly derived from higher plants,the main plants are tree ferns and pro-gymnosperms.The source rocks of the Jiangbasitao Formation and Shiqiantan Formation have mixed source characteristics,the main plants are pteridosperms.Tropical rainforest-type plants flourished during the Carboniferous,which is controlled by the ropical paleoclimate zone.The flourishing of these tall plants is also the material basis for the formation of coal and organic-rich carboniferous mudstone in the Carboniferous swamp facies in the study area.The redox conditions of the Bashan Formation were mainly oxic,and the pore water may have changes in dysoxic,ferruginous or euxinic.During the deposition of Jiangbasitao Formation,the bottom water was stable well-oxgenated.The redox condition was relatively turbulent during the deposition of Shiqiantan Formation source rocks,it changes from euxinic condition,oxic condition,ferruginous condition,oxic condition,to possibly ferruginoud condition.The gammacerane indexs and Sr/Ba values show that the source rocks of the Jiangbasuo Formation in the Suosuoquan sag and the source rocks of the Shiqiantan Formation in the Shiqiantan sag were deposited in a brackish water environment,while the source rocks of the Bashan Formation was deposited in a freshwater environment.After comprehensive analysis on factors of organic matter enrichment such as productivity,preservation conditions and terrigenous inputs,combining the changes of organic matter orgins and sedimentary environments in typical wells,we can find that the main controlling factor for organic matter enrichment in Bashan Formation source rocks is the flourishing of higher plants while that for the Shiqiantan Formation source rocks is good preservation conditions.Therefore,the enrichment model of the“terrigenous OM input controlling with mix OM”in Jiangbasitao Formation source rocks in the Suosuoquan Sag and Shibei Sag,“the higher plants flourishing and in-situ accumulation”in Bashan Formation source rocks in Shibei Sag and Shishugou Sag,and“the preservation condition”in Shiqiantan Formation in Shiqiantan Sag were established.