Controlling effect of sedimentary architecture on microscopic reservoir characteristics: Sangonghe Formation, Moxizhuang Area, Junggar Basin

The Junggar Basin is a typical intracontinental petroliferous basin in Central Asia, and the Moxizhuang area, located in the hinterland of the basin, is a key target for Jurassic hydrocarbon exploration. The Sangonghe Formation serves as the major petroliferous reservoir interval in this region, yet the coupling relationship between sedimentary architecture  and reservoir microscopic characteristics  remains unclear, which restricts the accurate evaluation of reservoir quality and efficient hydrocarbon exploration. This study aims to investigate the spatial assemblage relationship of sedimentary microfacies and reveal the controlling mechanism of sedimentary architecture on reservoir microscopic characteristics.

Based on the core samples, rock thin sections, and logging data of 12 wells in the Moxizhuang area, a multi-index comprehensive research method was adopted.The research methods include: (1) Sedimentary framework analysis: Through core logging and well log correlation, the types of sedimentary microfacies associations were identified, and the sand body distribution characteristics and geometric morphology were delineated.(2) Reservoir microscopic characterization: Cast thin section observation, scanning electron microscopy  analysis, and mercury intrusion capillary pressure  experiments were carried out to quantitatively characterize the pore types, pore-throat distribution, and reservoir physical parameters.(3) Diagenesis analysis: By means of rock thin section identification and scanning electron microscopy-cathodoluminescence  observation, the diagenetic processes and their impacts on reservoir quality were clarified.

The Sangonghe Formation in the study area is dominated by the estuary bar microfacies and subaqueous distributary channel microfacies of the delta front. A total of four sedimentary microfacies association patterns have been identified in the study area, namely ，superimposed subaqueous distributary channels, truncated subaqueous distributary channels, channel overlying reworked estuary bar, and superimposed estuary bars. These patterns reflect distinct sedimentary environments and hydrodynamic conditions.There are significant differences in reservoir characteristics under different sedimentary microfacies association patterns. Specifically, the sand bodies in the superimposed subaqueous distributary channel pattern exhibit superior overall porosity, permeability, and pore connectivity compared with those in the other patterns.(The conclusion supported by quantitative analytical data.) The sedimentary framework is the dominant factor controlling the microscopic reservoir characteristics of the Sangonghe Formation in the Moxizhuang area.The sedimentary framework controls reservoir quality by regulating diagenetic processes: the superimposed subaqueous distributary channel sand bodies formed in high-energy environments are well-sorted, characterized by intense feldspar dissolution and the development of abundant secondary pores; the superimposed estuary bar sand bodies formed in low-energy environments undergo significant carbonate cementation, resulting in substantial pore loss;In the truncated subaqueous distributary channel pattern, multiple phases of channels are vertically superimposed in an erosional manner, leading to the widespread development of erosion surfaces. The poorly sorted glutenites are mostly cemented by calcite crystals in a mosaic form, resulting in generally poor physical properties.

By identifying the types of sedimentary microfacies associations and comparing the differences in reservoir characteristics among various types, this study establishes the coupling relationship between the sedimentary framework and reservoir quality. The findings hold important significance for the fine evaluation and exploration of hydrocarbon reservoirs in the hinterland of the Junggar Basin and analogous intracontinental basins.

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