## 6 Conclusions  
1. Based on the mineral composition and grain size variation of the Fengcheng Formation reservoir in the Mahu Depression, four lithofacies were delineated, each exhibiting distinct pore and fracture development characteristics. Silstone interlayers displayed the most extensive porosity development, characterized by fewer macropores but abundant micro-pores, with sizes ranging from 100 nm to 4000 nm, primarily intergranular, followed by felsic shale, while clay-bearing felsic shale exhibited the poorest development, dominated by interparticle pores with the smallest pore diameters. Dolomitic felsic shale featured the most developed fractures, predominantly comprising interbedded and structural fractures, with a fracture porosity reaching 10.37%, which was significantly higher than other lithofacies. Silstone interlayers ranked next,  
Figure 13: Porosity variation with depth and diagenesis diagram. (**a**) Diagram of porosity variation with depth; (**b**) 4755.67 m, Dissolution; (**c**) 4859.57 m, Dissolution.  
with felsic shale and clay-bearing felsic shale exhibiting similar fracture development patterns, albeit with lower macroscopic fracture densities; the poorest connectivity was observed in clay-bearing felsic shale.
2. Among the four lithofacies, siltstone interlayers exhibited the most favorable overall pore and fracture development conditions, with pore and fracture porosities showing little discrepancy but relatively high values overall. In contrast, clay-bearing felsic shale displayed the poorest overall pore and fracture development. Dolomitic felsic shale featured the highest proportion of fractures in its pore-fracture system, boasting superior connectivity, whereas felsic shale exhibited the least developed internal fractures and contributed the lowest fracture porosity. Overall, siltstone interlayers were deemed the most favorable reservoir among the four lithofacies, with dolomitic felsic shale showing distinct advantages among the three shale types.
3. The development of pores and fractures is primarily influenced by sedimentary and diagenetic factors. Sedimentation primarily occurred in semi-deep lake, shallow lake, and lakeshorse subfacies. A higher carbonate mineral content during sedimentation resulted in increased shale brittleness and the enhanced propensity for fracture formation. Structural processes mainly led to the formation of large-scale macroscopic fractures, while diagenetic processes, including compaction, pressure solution, dissolution, and replacement, played a crucial role in pore formation, which his closely associated with pore genesis.  
Conceptualization, X.T.; data curation, Y.J., X.T. and C.L.; formal analysis, Z.J. and L.Y.; funding acquisition, W.H. and L.H.; investigation, Y.J., L.H. and Z.J.; methodology, X.T., Z.J. and L.Y.; project administration, W.H. and Z.J.; resources, X.T. and W.H.; validation, X.T., W.H. and L.H.; visualization, Y.J. and C.L.; writing--original draft, Y.J. and C.L.; writing--review and editing, X.T., L.H. and L.Y. All authors have read and agreed to the published version of the manuscript.  
This work is financially supported by the Strategic Cooperation Technology Projects of CNPC and CUPB (ZLZX2020-01-05).  
Not applicable.  
The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.  
The authors would like to express their sincere thanks to the PetroChina Xinjiang Oilfield Company for their assistance in providing the information and for their technical input to this work.  
Authors [PERSON] and [PERSON] were employed by Xinjiang Oilfield Company, PetroChina. The remaining authors declare that the re-search was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.