## 6 Conclusions  
Given the total organic carbon, rock-eval pyrolysis, vitrinite reflectance data, and source rock quality, these results show that the organic matter composition of source rocks in the Huanghekou Depression, specifically in E\({}_{3}\)s\({}_{1-2}\), can be predominantly classified as Type II\({}_{2}\), while in E\({}_{2}\)s\({}_{3}\), the predominant organic matter type is a combination of Types II\({}_{1}\) and II\({}_{2}\) with a minor presence of Type I. Lastly, in E\({}_{2}\)s\({}_{4}\), the main organic matter type was identified as Type II\({}_{2}\) with a small amount of Type III. In terms of maturity, E\({}_{3}\)s\({}_{1}\) was in an intermediate position between the immature and mature stages, while E\({}_{3}\)s\({}_{2}\) and E\({}_{2}\)s\({}_{3}\) were situated between the low-maturity and mature stages, and E\({}_{2}\)s\({}_{4}\) transitioned into the stage of full maturity.  
The sources of organic matter at different layers in the study area were analysed by using biomarker methods. In these methods, \(n\)-alkane- and sterane-related parameters are effectively used to distinguish terrigenous higher plants from aquatic organisms. The inorganic geochemical method is applied to the sedimentary environment, water salinity, and palaeoclimate of the study area. After conducting the aforementioned analysis, we postulated that the sedimentary organic matter of E\({}_{3}\)s\({}_{1-2}\) in the Huanghekou Depression represents a mixed source, primarily of aquatic organisms, with a lesser contribution from terrigenous higher plants. It was deposited under strong reducing conditions, characterised by high water salinity and the presence of saline and semi-saline water bodies, which provided favourable preservation conditions for organic matter. The organic matter in E\({}_{2}\)s\({}_{3}\) represented a mixed source, primarily of aquatic organisms. The source rocks were deposited under reducing conditions with relatively low salinity, ranging from semi-saline to freshwater. In E\({}_{2}\)s\({}_{3}\), the climate during the sedimentary period was warm with abundant moisture, resulting in high palaeoproductivity.  
The organic matter of the Shahejie Formation was influenced by different depositional environments and climates in distinct ways. These findings indicate that the organic material in the Shahejie Formation of the Huanghekou Depression was predominantly derived from aquatic organisms and algal sources, as evidenced by the discrimination indices for distinguishing between aquatic organisms and higher plants of terrestrial origin. However, the contributions of aquatic organisms and algae were higher in E\({}_{2}\)s\({}_{3}\) than in E\({}_{3}\)s\({}_{1-2}\). We further developed an organic matter enrichment model for the Huanghekou Depression. Specifically, E\({}_{3}\)s\({}_{1-2}\) represented the preservation model, and E\({}_{2}\)s\({}_{3}\) represented the productivity model.  
Z.J.: conceptualization, methodology, original draft preparation, reviewing, and editing; D.H.: visualization, investigation, supervision, validation, reviewing, and editing; J.H.: methodology, software, and validation. All authors have read and agreed to the published version of the manuscript.  
This research was supported by the National Natural Science Foundation of China (Grant No. 41872131).  
Not applicable.  
The datasets presented in this study can be obtained upon request to the corresponding author.  
The authors would like to thank the Tianjin Branch of the China National Offshore Oil Corporation for providing samples and related documents.  
The authors declare no conflicts of interest.