Depositional environmental evolution and reservoir controlling effects of the P₂x formation in the Mahu Sag: Insights from the PCA-HCA coupled analysis

The conglomerates of the Xiazijie (P₂x) Formation in the Mahu Sag are dominated by dissolution pores of zeolite cement as the primary hydrocarbon reservoir spaces. Consequently, the storage characteristics of conglomerates determine that the depositional environment is one of the key factors controlling reservoir quality. To address the lack of clarity regarding the types of paleo-depositional environments of the P₂x Formation and their controlling effects on reservoir characteristics, this study integrated elemental geochemical analysis with machine learning and other analytical approaches. A dynamic mapping relationship was established among elemental assemblages, depositional environments, and reservoir characteristics to elucidate the evolutionary features of the paleo-depositional environments of the P₂x Formation and their controlling effects on reservoir physical properties. The results indicate that the paleo-depositional environment of the P₂x Formation is subject to the coupled control of provenance supply and paleoclimatic conditions, exhibiting significant spatial heterogeneity. Spatially, the depositional water depth increased gradually from north to south, accompanied by a synchronous decrease in oxidizing condition of the environment. Meanwhile, the region featured a dual provenance supply system consisting of terrigenous debris and intermediate-mafic magmatic materials. The input intensity of terrigenous debris showed clear regional heterogeneity, accompanied by the differential supply of intermediate-mafic magmatic components. Influenced by peripheral tectonic uplift induced by plate collision, moisture transport to the basin center was obstructed. This factor resulted in the formation of an anomalous paleoclimatic distribution pattern characterized by increasing aridity trend from north to south. These regional variations in the depositional environment further control the spatial differentiation of zeolite cement types. In the Mabei area, Na- and Al-rich volcanic glass derived from terrigenous debris and intermediate magmatic materials, under the synergistic influence of a semi-humid climate and phasic salinization events, promoted the development of cements dominated by analcime and heulandite. The Manan area developed a dysoxic, open, and low-salinity water environment, where zeolite cement is dominated by laumontite. In contrast, the Maxi area developed a two-stage depositional environment evolution, resulting in the balanced development of the three types of zeolite cement. The spatial differentiation of zeolite cement and their coupling with the depositional environment further determined the variations in dissolution intensity. Ultimately, this characteristic resulted in a distribution pattern of reservoir physical properties in the P₂x Formation conglomerates that follows the distribution pattern of “Maxi > Mabei > Manan”.