Role of bedding planes played in enhancing dissolution in sandstones

Abstract

Diagenesis exerts an important control on porosity evolution, and research of diagenesis and diagenetic minerals provides insights into reservoir quality evaluation and CO₂ storage. Thin section, XRD (X-ray diffraction), CT (Computed Tomography), scanning electron microscopy (SEM), and NMR (Nuclear Magnetic Resonance) tests were used to investigate composition, texture, pore spaces, and diagenesis of sandstones in Paleogene Dongying Formation in Bohai Bay Basin, China, with special aims to unravel diagentic dissolution along bedding planes. The oversized pores, remnants in feldspar-hosted pores, and kaolinite within feldspar grains indicate a high degree of dissolution the framework grains experienced during burial. The CO₂-rich or organic acids are responsible for the feldspar dissolution. Grain size plays the primary role in enhancing bedding dissolution process, and bedding planes in fine-medium grained sandstones with high content of feldspars are frequently enlarged by dissolution. The CT scanning image confirms dissolution pores are distributed discontinuously along the bedding planes. The dissolution pores along bedding planes have large pore size, and correspond to the right peak of the bi-modal T₂ (transverse relaxation time) spectrum. The laminated sandstones and siltstones, or sandstones with cross beddings help improve framework grain dissolution. These new findings help improve the understanding of diagenetic models, and have implications in reservoir quality prediction and resource assessments in sandstones.

Graphical abstract

Keywords

dissolution / sandstones / diagenesis / bedding plane / Dongying Formation

1 Introduction

Diagenesis exerts a strong control on reservoir quality and heterogeneity in sandstones (De Segonzac, 1968; Stonecipher et al., 1984; Al-ramadan et al., 2005; Higgs, et al., 2007; Morad et al., 2010; Lai et al., 2017; Bjørlykke, 2014; Liu et al., 2015; Garzanti, 2019; Jafari et al., 2020; Wang et al., 2021). Framework grain dissolution is a natural consequence of diagenesis due to increasing burial depth and temperature (Taylor et al., 2010; Lai et al., 2015). As an important diagenetic modification, dissolution can occur at shallow burial conditions due to meteoric water fluxing, and at deep burial environments because of organic acid-rich or CO₂-rich fluids (Rossi et al., 2001; Islam, 2009; Chen et al., 2017; Lai et al., 2020). Dissolution of framework grains significantly enhances porosity in sandstone (Bowen et al., 2011). Additionally, these secondary dissolution pores are hardly decreased by subsequent compaction (Henares et al., 2016; Saïag et al., 2016; Yue et al., 2018; Lai et al., 2020).

Cross-laminations are internal sedimentary structures in sedimentary rocks, and they are important for sedimentological interpretation (Tucker, 2003; Xu et al., 2015). The bedding planes in layered rocks are weak interfaces, and they are the main factors contributing to the branching of fractures (Flewelling et al., 2013; Chang et al., 2015; Lee et al., 2015; Tang et al., 2018; Weng et al., 2018). The sedimentary beddings can affect fracture growth and act as pathways for fluid flow especially when they are intersected by fractures (Chang et al., 2015; De Silva et al., 2016; Swanson, 2007; Zhou et al., 2018; Momeni et al., 2019). In fine-grained rocks, the lamina (beddings) are favorable for the formation of bedding parallel fractures, and they play important roles in hydrocarbon storage capacity and pore structure (Huang et al., 2018; Liu et al., 2019). The diagenetic changes at the lamina scale are important for hydrocarbon migration and accumulation (Liang et al., 2018). Diagenetic dissolution can make bedding planes more visible in outcrops (Bathurst, 1987), indicating the phenomenon of bedding enhanced dissolution. However, previous investigations into diagenetic enhancement of beddings have been restricted in lithologies in terms of lime mudstone/marl alternations (Bathurst, 1987), micrite (Feng et al., 2013), and salt/dolostone interfaces (Zhang et al., 2019). Among them, the calcareous and gypsum minerals are dissolved along the weak bedding planes (Bathurst, 1987; Feng et al., 2013; De Silva et al., 2016; Zhang et al., 2019), and bedding dissolution only occurred on layer interfaces within a limestone-siltstone or siltstone-salt interfaces (Feng et al., 2013; Zhang et al., 2019). However, whether the diagenetic dissolution in sandstones can occur along the layer structures (bedding planes) remain an open research question.

In this study, special attention is paid to whether the presences of weak bedding interfaces (bedding planes) can improve grain dissolution in the Paleogene Dongying Formation in Nanpu Sag, Bohai Bay Basin, China. Cores are described in terms of lithology, grain size and beddings (laminations and cross beddings). Thin sections, XRD (X-ray diffraction) and SEM (scanning electron microscopy) images are observed to unravel the composition, diagenesis, diagenetic minerals, beddings and pore systems distributed along the bedding interfaces. Then micro-CT (Computed Tomography) scanning is used to provide insights into the bedding dissolution pores. Additionally the pore size distribution of bedding dissolution pores is described using NMR tests, and effect of dissolution pores on the immobility of pore systems is unraveled. The question whether diagenetic dissolution in sandstones can occur along bedding planes is answered from thin section, CT and NMR analysis. The results will provide scientific guidance in the diagenesis model of sandstones, and have practical implications in reservoir quality evaluation and resource assessments. In addition, the insights into the diagenetic modification within lamina scales will provide guidance for shale oil exploration and development in fine-grained sedimentary rocks, in which the lamina are well developed (Huang et al., 2018; Liu and Liu, 2019; Liu et al., 2019).

2 Geologic settings

The Bohai Bay Basin (area of 20 × 10⁴ km²) is a Cenozoic rift basin, and it appears as a north-east-trending N-shaped geometry (Dong et al., 2010; Guo et al., 2016). The petroliferous Nanpu Sag is located at north-east Huanghua Depression within Bohai Bay Basin (Fig. 1) (Chen et al., 2016; Guo et al., 2016; Lai et al., 2020). The Nanpu Sag is a half-graben basin (Guo et al., 2013). The Cenozoic sedimentary successions consist of Eocene Kongdian (Ek), Eocene Shahejie (Es), Paleogene Dongying (Ed), Neogene Guantao (Ng), Minghuazhen (Nm) and Quaternary Pingyuan (Qp) Formation in an ascending order (Chen et al., 2016; Jiang et al., 2016; Zhang et al., 2017; Niu et al., 2020).

The depositional facies of Dongying Formation is interpreted as fan- braided delta (Dong et al., 2015; Liu et al., 2017; Lai et al., 2020). The thick lacustrine mudstones and black shales of the third Memberof Shahejie Formation (Ed₃) and the third Member of Dongying Formation (Es₃) are organic matter-rich source rocks (Xu et al., 2006; Guo et al., 2013; Lai et al., 2019). The Member 1 of Dongying Formation (Ed₁) is the principal reservoir rocks (Dong et al., 2015). The traps are dominantly structural traps or structure-related lithologic traps, and faults are the main hydrocarbon migration pathways (Guo et al., 2013).

3 Samples and methods

A total of 30 thin sections were cut perpendicular to cross beddings, and were analyzed using the Leica optical microscope at the State Key Laboratory of Petroleum Resources and Prospecting. Thin sections were impregnated with resin (blue) for pore system observation and stained with Alizarin red S and K-ferricyanide carbonate mineral identification.

Scanning electron microscopy (SEM) measurements were conducted on 8 gold coated, freshly broken plugs, and the FEI Quanta 200 was used to identify the micropores (<10 μm) as well as clay minerals (Table 1).

Thirty samples were used for X-ray diffraction (XRD) analysis, which was performed on whole-rock and clay fractions (<2 μm) to obtain the mineral content (clay minerals, quartz, feldspar, and carbonate, etc). The various minerals were identified from the diffractograms through referencing to the ICDD Powder Diffraction File (Yandoka et al., 2015).

Eight core plug samples (1 inch in diameter) were scanned using a μCT (micro-CT) instrument. The CT scanning provides two-dimensional (2D) images, and generates 3D grayscale images by overlapping 2D grayscale CT images (Cnudde and Boone, 2013; Zhang et al., 2020).

NMR T₂ spectrum was measured using a CoreSpec-1000 NMR instrument with a magnetic field frequency of 2M HZ on the eight samples. The movable water was removed using a centrifugal machine, and the T₂ distributions at saturated and centrifuged status were measured, respectively. The waiting time (Tw) and echo-time (Te) are set as 6000 ms and 0.2 ms (Lai et al., 2018, 2019; Wang et al., 2020).

4 Results

4.1 Lithology and pore spaces

The lithology of Paleogene Dongying Formation (burial depths 2500‒4500m) is dominantly fine-medium grained sandstone, and siltstone interbedded with gray-black mudstone (Fig. 2). Parallel bedding, cross beddings (tabular and wedge-shape), convolute beddings as well as wavy beddings are observed (Fig. 2). As for the mineral composition, the sandstones consists of 42 wt.%‒4 wt% quartz, 18wt.%‒44wt% feldspar, 9wt.%‒34wt% clay minerals, and 0wt.%‒15.8wt.% carbonate minerals according to XRD analysis.

The porosity of Paleogene Dongying sandstones ranges from 1.7% to 32.3% and permeability is in the range from 0.001 mD to 1819 mD (averaged as 108.7mD). Thin section shows that pore spaces of the sandstones in Dongying Formation contain a combination of primary intergranular pores (Figs. 3(a) and 3(b)), and secondary pores due to dissolution (partly to complete) of framework grains (mainly feldspars) (Figs. 3(c) and 3(d)). Micropores coexist with clay minerals (mainly authigenic vermicular kaolinite and webby illite/smectite mixed layers) (Figs. 3(e) and 3(f)).

4.2 Diagenesis and diagenetic minerals

As observed by thin section analysis, the dominance of point and point-long grain contacts and abundance of primary pores indicate sandstones in the Dongying Formation had only experienced moderate degree of mechanical compactions (Figs. 3(a)‒3(d)). Extensive compaction will significantly reduce porosity (Zhang et al., 2021). The dominant diagenetic minerals include abundant carbonates, various clay minerals and minor amounts of quartz (Guo et al., 2013; Chen et al., 2017; Lai et al., 2019, 2020). However, the framework grains are generally intensively dissolved, which can be evidenced by dominance of secondary dissolution pores (Figs. 3(c) and3(d)). Thin sections also indicate that the authigenic kaolinites are commonly coexisting with feldspar-hosted dissolution pores (Lai et al., 2018) (Figs. 3(c) and 3(d)). Additionally, SEM observations confirm that abundant feldspars are dissolved, and there are some feldspar remnants or even kaolinites within the partly dissolved feldspar dissolution pores (Figs. 3(g) and 3(h)). The feldspar grains can be dissolved in the early diagenetic stage due to meteoric water flushing, and during burial stages due to organic acids after hydrocarbon charge (Chen et al., 2017). Framework grain dissolution is pervasive in the sandstones (Fig. 3) (Chen et al., 2017; Lai et al., 2019, 2020).

5 Discussion

5.1 Insights from thin section and CT analysis

Cross beddings or laminations are commonly observed in cores, and the bedding planes are frequently enlarged by late burial dissolution, especially in those fine-medium grained sandstones with laminations and cross beddings (Figs. 2(a)‒2(e)). However, in some siltstones or very fine grained sandstones which are also abundant in cross beddings (wavy or convolute), no evident dissolution alone bedding planes are detected (Figs. 2(f)‒2(g)). Therefore grain size may play the primary role in controlling diagenetic dissolution process along the bedding planes (Fig. 2). Thin section petrography and XRD analysis confirm that these sedimentary beddings/laminations are mainly feldspar grains or rock fragments or even clay minerals (Fig. 4). The dissolution rate of feldspars is much higher than that of quartz at certain burial environments (Worthington et al., 2016). The laminated sandstones or sandstones with cross beddings have a relatively higher content of clay as well as feldspar content compared with the massive sandstones by XRD analysis (Figs. 4(a)‒4(c)). Presumably, the laminated sandstones or sandstones with cross-beddings originally contain more feldspars than are now present sine some of the preexisting feldspars are dissolved (Fig. 3). Laminations/beddings are commonly associated with fine-medium grained sandstones, in which dissolution pores are aligned along bedding planes (Figs. 4(a)‒4(b)). Crude oil/bitumen commonly invaded into the bedding planes (Fig. 4(b)). Conversely, the massive (no beddings) sandstones have high quartz content but low feldspar and clay content, which can be evidenced by the thin section observation (Figs. 4(d)‒4(f)).

Fractures (diagenetic in origin) are easily to be formed parallel to beddings due to changed rock properties and in situ stress associated with bedding interfaces (Zeng and Li, 2009; Chang et al., 2015). Additionally, diagenetic fractures can further improve the grain dissolution along fracture planes (Zazoun, 2013; Anders et al., 2014; Dashti et al., 2018).

Here in this study, it is confirmed that the sedimentary beddings in sandstones can also enhance dissolution along the bedding interfaces. The 2D slices indicate the presences of laminations in the fine-grained sandstones (Fig. 5(a)). The extraction of pores reveals that several aligned pore system layers are observed, and the (dissolution) pores are distributed discontinuously along the bedding planes (Fig. 5(b)). In addition, the 3D reconstruction of pore networks confirms the bedding planes with traces of dissolution, and the pore size of dissolution pores range from tens of micrometers to 200‒300 μm (Fig. 5(c)). The microscopic thin section photomicrographs confirm the presences of aligned dissolution pores (100‒300 μm in pore sizes) along the bedding planes (Fig. 5(d)). The dissolved framework grains are mainly feldspars, forming partly dissolution pores or even moldic pores (Fig. 5(d)). The white quartz grains have a directional alignment (Fig. 5(d)).

The more evident in bedding interfaces, the more dissolution pores are observed. For instance, the bedding planes are obviously observed by 2D CT image (Fig. 6(a)), and the 3D reconstruction of pore networks show abundant dissolution pores (200‒300 μm in pore sizes) are obviously distributed along the bedding planes (Figs. 6(b) and 6(c)). Additionally, two sets of aligned dissolution pore are observed along the bedding interfaces by thin section analysis (Fig. 6(d)). The framework grains (mainly quartz) are directionally aligned (Fig. 6(d)). Conversely, there are no evident bedding planes detected by the 2D CT slices (Fig. 7(a)), and consequently no obvious aligned dissolution pores (only slight signs of dissolution along the bedding planes) are detected by the 3D reconstruction of pore networks (Figs. 7(b)‒7(c)). In addition, the dissolution pores only show weakly aligned under microscopic observation, and are very discontinuously distributed (Fig. 7(d)).

Thin sections confirm that the dissolution pores along bedding planes are mainly associated with the fine to medium grained sandstones which are abundant in feldspar and rock fragments (Figs. 5(d), 6(d), and 7(d)). Therefore both the CT images and thin section photomicrographs confirm that the effects of sedimentary beddings on improving framework grain dissolution. The presence of bedding planes will behave as flow pathways of acid-rich fluids, and therefore result in the formation of secondary dissolution porosity (Momeni et al., 2019).

5.2 NMR responses of dissolution pores

The reservoir spaces in sandstones include nanoscale (intercrystalline pores within authigenic clay minerals), microscale (micropores, intragranular dissolution pores), millimeter scale (large intergranular pores and moldic pores) (Lai et al., 2018, 2020). The dissolution of framework grains can form microscale or even millimeter scale pores along the bedding planes (Figs. 5‒7). NMR test is commonly used to infer pore size distribution (Coates et al., 1991; Daigle et al., 2017). The presences of large dissolution pores along bedding planes (200‒300 μm) or intergranular pores refer to the large T₂ components (>100 ms), which relate to the right peak of a bimodal T₂ distribution ((Dillinger and Esteban, 2014) (Fig. 8(a)). Conversely, the micropores are associated with short T₂ components (left peak of the T₂ spectrum) (Rezaee et al., 2012; Daigle et al., 2014; Lai et al., 2016). The bi-modal T₂ behavior indicates the coexistence of micropores and large intergranular pores or even moldic pores (Fig. 8(a)), which is also evidenced by thin section observations (Fig. 8(b)), additionally the SEM image further confirms the presences of dissolution pores (Fig. 8(c)). The T₂ spectra at the saturated and centrifugal status in Fig. 8 are not evidently deviated, which implies that abundant pore systems are immovable, and actually the irreducible water content determined from the NMR test is as high as 62.45%, which is attributed to abundance of dissolution pores (Fig. 8). Therefore the secondary dissolution pores are not always connected by effective pore throats (Dutton and Loucks, 2010; Lai et al., 2017), and will a high irreducible water content from NMR tests in encountered (Fig. 8).

6 Conclusions

The moldic pores, remnants in dissolution pores, and clay minerals within feldspar-hosted dissolution pores indicate that the sandstones are heavily dissolved. Bedding planes in fine-medium grained sandstones with high feldspar content are frequently enlarged by dissolution, while in the very fine-grained sandstones or clay rich sandstones, no bedding dissolution is observed. The diagenetic dissolution processes parallel to bedding planes varied greatly from that vertical to bedding planes. Layer structures (bedding planes) help improve the bedding-enhanced dissolution.

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