Structural diagenesis in ultra-deep tight sandstones in the Kuqa Depression, Tarim Basin, China

The Lower Cretaceous Bashijiqike Formation of the Kuqa Depression is made up of ultra-deeply buried sandstones in fold-and-thrust belts. Few researches have linked diagenetic processes with structure. To fill this gap, a comprehensive analysis integrating diagenesis with structure pattern, fracture and in situ stress is performed following a structural diagenetic approach. The results show that the pore spaces include residual intergranular pores, intergranular and intragranular dissolution pores, and micro-fractures. The sandstones experienced a high degree of mechanical compaction, but compaction is limited in well-sorted rocks or abundant in rigid quartz grains. The most volumetrically important diagenetic minerals are calcites. The framework grains experienced a varied degree of dissolution, and intergranular and intragranular dissolution pores are formed. Special attention is paid on the dissolution associated with the fracture planes. Large numbers of natural fractures are cemented by carbonate cements, which limit fluid flow. In addition, the presence of fracture enhances dissolution and the fracture planes are enlarged by dissolution. Cementation and dissolution can occur simultaneously in fracture surfaces, and the enlarged fracture surfaces can be cemented by late-stage cements. The in situ stress magnitudes are calculated using well logs. The horizontal stress difference (Delta sigma) determines the degree of mechanical compaction, and rocks associated with low Delta sigma experienced a low degree of compaction, and these contain preserved intergranular pores. Natural fractures are mainly related to the low Delta sigma layers. The presence of intergranular and intragranular dissolution pores is mainly associated with the fractured zones. The high-quality reservoirs with intergranular pores or fractures are related to low Delta sigma layers. The structural diagenesis researches above help the prediction of reservoir quality in ultra-deep sandstones and reduce the uncertainty in deep natural gas exploration in the Kuqa Depression.

Automated summary

This study comprehensively analyzes the relationship between diagenetic processes and structural pattern, fractures, and in situ stress in the Lower Cretaceous Bashijiqike Formation of the Kuqa Depression. The results reveal the presence of various pore spaces, mechanical compaction, dissolution associated with fractures, and the influence of in situ stress on the formation of intergranular pores and fractures. These findings contribute to predicting reservoir quality and reducing uncertainty in deep natural gas exploration in the Kuqa Depression.