Core descriptions will reflect the centimeter-scale to millimeter-scale changes in sedimentary structures, while thin section will provide insights into the micrometer-scale variations of lamina combination (Jiang et al., 2017). In terms of lamina characterization, conventional (full suite) logs are limited due to their low vertical resolution of meter-scale. ECS and NMR logs are also in the decimeter scale, and therefore have no visible ability to capture the variation of lamina. Consequently, advanced image log data and high-resolution image processing methods are required.
4.1. Core-scale lamina assemblage
Lamina is the smallest megascopic layer in a sedimentary succession without internal layers (Campbell, 1967; Lazar et al., 2015). According to lamina thickness, the fine-grained sedimentary rocks can be divided into massive, layered (bedded) and laminated (Fig. 3-Fig. 5) (Lazar et al., 2015; Liu et al., 2019; Wang et al., 2021; Lin et al., 2021). From massive (per thickness > 10 cm) to bedded (> 1 cm) and then to laminated (< 1 cm) sedimentary structures, the thickness of individual lamina is gradually decreased (Liu et al., 2019). Massive rocks are recognized as no internal lamina structure in 10 cm scale or thicker (Fig. 3). Bedded or layered rocks are categorized as rocks with layer thickness of 1–10 cm (Fig. 4), while the laminated rocks are characterized by dense lamina development with lamina thickness less than 1 cm (Fig. 5) (Liu et al., 2019). The layered and laminated structure is generally formed in the conditions with seasonally cyclical changes of sedimentary environments and hydrodynamics (Shi et al., 2020).
Massive rocks therefore show no interior lamina structure, and the slab images are bright or yellow, and the button conductivity curve is not frequently fluctuated, and only show uniform bright to yellow color patterns, indicating no evident changes in grain size and mineral composition (Fig. 3).
There are a total of 82 numbers of laminae in 51.5 cm core intervals in the layered rocks in Fig. 4, and the maximum thickness of individual lamina is 11.2 mm, while the minimum is 2.4 mm (Fig. 4). The slab images are alternating bright or yellow layers in millimeter scales. In addition, the button conductivity curve is frequently fluctuated with dark long layers and bright but short layers (Fig. 4).
There are a total of 133 numbers of laminae in 50.0 cm core intervals in the laminated rocks in Fig. 5, and the maximum thickness of individual lamina is 15 mm, while the minimum is 3.3 mm (Fig. 5). There are a total of 57 laminae recognized in a 9 cm interval in the layers with the highest density of lamina, and the minimum thickness of individual lamina is 1.6 mm (Fig. 5). The slab images are very frequently alternated bright or yellow layers in millimeter scales. Additionally, the button conductivity curves are very frequently fluctuated with dark and bright laminas in millimeter scales (Fig. 5).
4.2. Thin section-scale lamina structure
At the scale of thin sections, the micrometer scale laminae will be detected (Yawar et al., 2017). There are mainly five types of laminae in the fine-grained sedimentary rocks: carbonate lamina, silty lamina, clay mineral lamina, tuffaceous lamina and organic matter lamina (Fig. 6) (Burke and Kemp, 2002; Gan and Scholz, 2013; Liu et al., 2018; Liu Dongdong et al., 2019). Dark laminae include clay mineral lamina and organic matter lamina. Light laminae consist of silty lamina, while the bright laminae include tuffaceous lamina and carbonate lamina (Gan and Scholz, 2013; Shi et al., 2022). Color contrasts between dark and light laminae are distinct (Gan and Scholz, 2013). The bright carbonate and tuffaceous laminae are banded with dark clay and organic matter laminae as well as light silty lamina, forming rhythmic alternations (Fig. 6) (Shi et al., 2020). The color patterns (dark-yellow-light-bright) and conductivity values (the width of the curves) of the button conductivity curves can unravel the variations in composition and texture of individual lamina (Fig. 6).
The silty laminae are composed of quartz and feldspar, with minor amounts of clay minerals, and silty laminae are formed by high energy fluids (turbidity currents) (Lei et al., 2015; Liu Dongdong et al., 2019). Silty laminae are commonly grayish-white and are distinct from the dark organic or clay laminae in grain size and mineral composition (Fig. 6) (Lei et al., 2015; Yawar et al., 2017; Nzekwe et al., 2018). The silty lamina is recognized as yellow color patterns and normal conductivity values (Fig. 6).
Organic matter lamina is recognized are dark in thin section (Fig. 6) (Liu et al., 2018). The organic-rich laminae consist of amorphous organic matter and clay minerals, as well as minor amounts of quartz and pyrite (Liu Dongdong et al., 2019; Xi et al., 2020). The organic matters are mainly formed in deep water and reduced environment (Liu Dongdong et al., 2019). The rapid changes from silty lamina to organic matter laminae result in a rapid variation of color patterns from yellow to bright (Fig. 6). Organic matters have low conductivity (narrow conductivity values), and therefore the color pattern is bright (Fig. 6).
The clay mineral laminae are commonly dark-grey under plane polarized light and cross polarized light (Fig. 6). The clay mineral laminae, which are supposed to be deposited in suspension settling, are composed of clay mineral particles (illite, illite and smectite mixed layer) with minor amounts of felsic minerals (Fig. 6) (Liu Dongdong et al., 2019). Consequently the clay lamina is easily to be recognized on the image logs due to their dark color patterns and high conductivity values (Fig. 6). There is a rapid changes of color patterns at the silt-clay lamina contacts (Fig. 6).
Tuffs may occur as thin beds ranging from 3 to 10 cm thick or appear as individual laminae ranging from 1 to 10 mm thick in thin sections (Fig. 6) (Jiao et al., 2020). Both tuffaceous and organic matter laminae are resistive, and therefore they are recognized by bright color patterns and low conductivity values (Fig. 6).
The carbonate laminae, which are formed in arid and high salinity setting or diagenetic cements, occur in the form of microcrystalline or sparry calcite (Liang et al., 2018; Liu Dongdong et al., 2019). Carbonate lamina can easily be identified due to reddish stained color and their higher white interference color under cross polarized light (Fig. 6). Light carbonate laminae also have light color patterns and moderate conductivity values (Fig. 6). At the silt-carbonate lamina contact, the button conductivity curves are gradually changed from light to yellow (Fig. 6).
The lamina couplets can be classified according to the mineral assemblage: silt-organic, tuff-organic, silt-clay and carbonate-silt, clay-organic couplets, in some case silt-clay-organic triplets can be detected (Fig. 6) (Zhao et al., 2019). The combination of laminae couplets or triplets show rhythmic alternations, forming the laminated or layered rocks.
4.3. Multi-scale lamina characterization using well logs
Fluorescent-light images will show more details than normal light (Keim et al., 2020). Core and thin section observation show that the mineral particles are homogeneously distributed in massive rocks (Fig. 7), whereas layered and laminated rocks are characterized by frequently alternating bright and dark laminae (Fig. 8; Fig. 9) (Li et al., 2020).
Massive rocks have no visible lamina in 10 cm intervals under both normal and fluorescent-light (Fig. 7). No evident laminae but instead disorganized, chaotic detrital grains are detected in thin section images (Jiang et al., 2017). The slab images have yellow-bright color patterns, while the button conductivity curves are stable in the color patterns and conductivity values (Fig. 7).
In layered rocks, both cores and thin sections reveal the carbonate and silty laminae as well as organic matters in the millimeter scale (Fig. 8). Core observation show that there are a total of 33 numbers of lamina in 15 cm core interval (Fig. 8). In addition, the fluorescent-light images highlight the presences of fluorescent silty and carbonate laminae (Fig. 8). Thin section confirms the occurrence of calcite (stained reddish) and grayish-white silty lamina interbedded with very thin (micrometer scale) laminae of organic matters (Fig. 8). The slab images show the presence of high density of lamina, and the button conductivity curves are characterized by frequently alternated yellow and white bands showing the rhythmic alternations of silty and carbonate laminae (Fig. 8).
In laminated rocks, the core observation shows the millimeter-scale silty lamina and clay mineral lamina. Core observation show that there are a total of 47 numbers of lamina in 19 cm core interval (Fig. 9), and the fluorescent-light core images further confirm the fluorescent silty lamina (Fig. 9). Additionally thin sections highlight the occurrences of micrometer-scale silty and clay mineral lamina, forming silt-clay couplets (Fig. 9). The slab images have low angles, which are in consistent with the core images. The button conductivity curves are very frequently fluctuated, in which the light color patterns indicate the silty lamina, while the dark color patterns reveal the clay mineral lamina, and they are in consistent with the silt-clay couplets as observed from thin sections (Fig. 9).
The multi-scale lamina structure (individual lamina, and lamina assemblage including silt-clay couplets, silt-clay-organic triplets, etc) determined from core and thin section observations can be calibrated with geophysical logs to establish a predictable model for lamina structure in fine-grained sedimentary rocks. Image logs and the related processed slab images as well as button conductivity curves, which have vertical resolution of 2.5 mm, can reflect the minerals composition and thickness of individual lamina and their assemblage, and therefore can characterize the multi-scale lamina structure in fine-grained sedimentary rocks.