Paleoenvironment and Main Controlling Factors of Source Rocks in the Shahezi Formation,Xujiaweizi Fault Depression
[Objective]The study make clear the main controlling factors of source rock in Shahezi Formation,and provides geologic basis for the optimal selection of gas exploration in deep layer of Songliao Basin.[Methods]We re-covered the sedimentary facies and water property in the Shahazi Formation,based on core observation,organic geo-chemical analysis,and major and trace elements testing,determining fault activity,sedimentary facies,water prop-erties,and their control on source rock quality.[Results]Two source rock,mudstone and coal,were identified in the Shahezi Formation. The faulting during the deposition of the first source rock in the Shahezi Formation was charac-terized by strong activity,generating a small lake with great water depth,high salinity,and strong reducing proper-ties. These source rocks were mudstone in a semi-deep lake. The faulting activity during the deposition of the second source rock gradually weakened,with the lake size increasing while water depth decreased. Salinity and reducibility were also weakened,and the coal in shore lake was the primary source rock.[Conclusions]The source rock type was mainly controlled by fault activity,sedimentary facies,and water properties. The downthrown side become impor-tant storage for mudstone because of the fault activity,which determined the thickness of mudstone. The sedimentary facies controlled the source rock type through their impact on organic matter import. The difference between paleoen-vironments,which was determined by paleosalins,paleowater depth,and redox,is the key factor for the enrichment and preservation of organic matter,and the main controlling factor for source rock quality distribution.
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