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.
Highlights
The Kuqa depression is a foreland depression experienced multistage tectonic evolutions during Mesozoic to Cenozoic periods, many high and steep thrust faults and fault-related folds were formed (Feng et al, 2018; Neng et al, 2018; Lai et al., 2019)
The lithologies of the Cretaceous Bashijiqike Formation in Kuqa depression include a wide ranges from conglomerate (Fig.2A), pebby sandstone (Fig.2B), fine-medium grained sandstone (Fig.2C-2D), siltstone (Fig.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 pore spaces include residual intergranular pores with irregular morphology (Fig.3A, 3B), intergranular and intragranular dissolution pores (Fig.3C, 3D) due to dissolved feldspar and rock fragment grains
Summary
The Kuqa depression is a foreland depression experienced multistage tectonic evolutions during Mesozoic to Cenozoic periods, many high and steep thrust faults and fault-related folds were formed (Feng et al, 2018; Neng et al, 2018; Lai et al., 2019). The dominant gas bearing Lower Cretaceous Bashijiqike Formation is buried to an ultra-deep depth of 5500-8000m (Lai et al, 2019). The ultra-deep burial depths, complex structure patterns and concentrated stress will result in complex diagenetic modifications and pore evolution histories (Laubach et al, 2010; Wu et al, 2019; Del Sole et al, 2020). Previous studies have individually unraveled the structural evolution, in situ stress, fracture as well as diagenesis of Bashijiqike Formation in Kuqa depression (Jia and Li, 2008; Lai et al, 2017a; Shen et al, 2017; Nian et al, 2018; Ju and Wang, 2018; Lai et al, 2019). Foreland fold-and-thrust belts are challenging for hydrocarbon exploration due to their structural complexity and heterogeneous reservoir quality distribution (Vandeginste et al, 2012). The structural complexity highly impacts fluid flow and diagenetic processes (Vandeginste et al, 2012). The comprehensive structural diagenesis analysis in Kuqa depression is of great scientific and practical significances
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