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

Abstract

Abstract. The Lower Cretaceous Bashijiqike Formation of Kuqa depression is ultra-deeply buried sandstone in fold-and-thrust belts. Few researches link 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, and 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 aims are paid on the dissolution associated with the fracture planes. Most natural fractures are cemented by carbonate cements, which limit fluid flow. In addition, presences of fracture enhance 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 (Δσ) determines the degree of mechanical compaction, and rocks associated with low Δσ experienced a low degree of compaction, and there contain preserved intergranular pores. Natural fractures are mainly related to the low Δσ layers. The presences of intergranular and intragranular dissolution pores are mainly associated with the fractured zones. The high quality reservoirs with intergranular pores or fractures are related to low Δσ 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 Kuqa depression.

Comment on se-2021-85

“…2E-2F), and mudstone (Fig. 2G-2H), indicating a fan-braided deltaic environment (Jia and Li, 2008;Wang et al, 2013;Lai et al, 2018).…”

“…The lithologies include a wide range from siltstone, fine-medium grained sandstone, to conglomerate and pebby sandstones, and intergranular, intragranular pores as well as fracture constitute the main reservoir pore spaces (Nian et al, 2018;Lai et al, 2019;Nian et al, 2021). The depositional subfacies evolved from fan delta plain in K 1 bs 3 to braided delta front subfacies in K 1 bs 2 and K 1 bs 1 members, and the main depositional microfacies recognized include distributary channel, mouth bar and distributary bay (Wang et al, 2013;Lai et al, 2017a;Nian et al, 2018).…”