Laminae Characteristics and Their Relationship with Mudstone Reservoir Quality in the Qingshankou Formation, Sanzhao Depression, Songliao Basin, Northeast China

Abstract

Lamination is the predominant and widely developed sedimentary structure in mudstones. Similar to organic pores in shale gas reservoirs, the inorganic pores in the laminae of shale oil reservoirs are equivalently important high-quality reservoir spaces and flow channels. The laminae characteristics are strongly heterogeneous, being controlled by both deposition and diagenesis. However, the origin of this diversity is poorly understood. A detailed examination of cores, thin sections, and scanning electron microscopy analyses were conducted on the lacustrine mudstone of the Qingshankou Formation in the Songliao Basin to study the influence of deposition and diagenesis on laminae characteristics and their relationship to reservoir quality. Three types of laminae are mainly developed, namely thick siliceous laminae, thin siliceous laminae, and thin siliceous and argillaceous mixed laminae. Deposition controls the type and distribution of laminae. The thin siliceous and argillaceous mixed laminae are controlled by climate-driven seasonal flux variations. The thick siliceous laminae and thin siliceous laminae are controlled by bottom current or gravity-driven transport processes due to increased terrestrial input. The thin siliceous laminae have the optimum reservoir properties, followed by the thin siliceous and argillaceous mixed laminae, while the thick siliceous laminae have the worst properties. Diagenesis controls the pore evolution of the laminae. Different laminae have different paths of diagenesis. The thin siliceous laminae are mainly cemented by chlorite, preserving some primary porosity. The clay mineral content of the thin siliceous and argillaceous mixed laminae is high, and the primary pores are mainly destroyed by the strong deformation of the clay minerals during compaction. The thick siliceous laminae are intensely cemented by calcite, losing most of the porosity. The present study enhances the understanding of reservoir characteristics in laminae and provides a reference for shale oil exploration.

1. Introduction

Mudstone is not only a hydrocarbon-source rock but also an important oil and gas reservoir [1,2]. The mudstone reservoir is strongly heterogeneous in terms of composition, texture, and bedding characteristics [3,4]. This feature significantly impacts the reservoir’s pore characteristics, fluid flow, and mechanical characteristics [5,6,7,8,9,10,11,12,13,14]. Lamination is the dominant sedimentary structure in mudstone [15,16]. Seemingly homogeneous mudstone can be found to be extensively laminated upon close examination of a fresh surface of the core, thin section, or using scanning electron microscopy [16,17,18,19,20,21].

The development of lamination can significantly improve the reservoir properties of the mudstone. Previous studies have found that the pore structure of the laminated mudstone and massive mudstone (laminae are not developed) differed in terms of pore type, pore size distribution, and pore connectivity, and the reservoir properties of the laminated mudstone were better than those of the massive mudstone [19,20,22,23]. A laser scanning confocal microscopy study also revealed a higher oil content and lighter oil in the laminae compared to the mudstone matrix [24]. Therefore, the laminae can be a superior reservoir space and seepage channel for shale reservoirs. Unlike shale gas reservoirs, where organic matter pores provide high-quality connected pore space [2,25,26,27,28], shale oil reservoirs exhibit the lowest porosity in terms of diagenetic pore evolution [29,30,31,32]. Laminae with superior reservoir quality relative to the mudstone matrix may be the key potential high-quality reservoir space for shale oil exploration. In addition, hydraulic fracturing is the key to the efficient extraction of shale oil and gas [33]. Different geologic features will affect hydraulic fracturing parameter settings [34,35,36,37]. The development of laminae results in mechanical heterogeneity along the direction of the laminae at the mm to μm scale [19,20,38,39]. Studies have found that the tensile strength, cohesion, and internal friction angle of the laminae were significantly lower compared to the mudstone matrix [38,40,41]. The proper setting of hydraulic fracturing parameters in laminated mudstone also requires a detailed study of laminae characteristics.

The reservoir properties of the laminae are controlled by a combination of deposition and diagenesis. Depositional processes control the type and distribution of laminae [4,17,42,43]. Diagenetic processes control the evolution of porosity in laminae [19,44]. Recent studies have shown the diversity and complexity of the pore characteristics of different types of laminae. Wang et al. [45] found that laminae with varied densities and thicknesses have different pore characteristics. The optimum porosity in shale was found with a combination of low density of laminae and high thickness of individual lamina. Shi et al. [46] found that there are differences in the pore characteristics of laminae with various morphologies. The thin parallel laminae, thick parallel laminae, and wavy laminae mainly develop rigid mineral interparticle pores with better connectivity. The lenticular laminae, weak laminae, and sandy laminae mainly develop inter- and intraparticle pores of clay minerals with poor connectivity. Xin et al. [44] found that laminae with varied mineral compositions have different pore characteristics. The main pore types of the siliceous laminae and mixed laminae are interparticle pores. The dolomite laminae and calcite laminae also develop abundant intraparticle pores. These studies have revealed strong heterogeneity in the reservoir properties of laminae. However, the origins of the heterogeneity, especially the influence of deposition and diagenesis on reservoir characteristics, still lack understanding.

There are various methods for characterizing mudstone reservoirs [2,47,48]. Since the laminae can be so thin, mostly at the micron scale, it is difficult to identify the pore characteristics of a particular type of lamina by conventional quantitative methods such as mercury intrusion porosimetry and low-field nuclear magnetic resonance. Scanning electron microscopy can identify the pore type and characterize the pore features of a specific lamina at the micron scale. Therefore, in this study, the pore characteristics of different laminae were characterized by scanning electron microscopy.

The Songliao Basin is a large terrestrial oil-bearing basin in China. The Late Cretaceous mudstone of the Qingshankou Formation in the Songliao Basin is an organic-rich lacustrine argillaceous mudstone [49,50]. In recent years, major shale oil exploration and development have been carried out for the mudstones of the Qingshankou Formation, obtaining a daily oil production of 228 bbl. and a daily natural gas production of 456,120 scf. [51]. The current successfully developed shale oil is mainly produced from carbonate and siliceous interbeds, interbedded with shale and calcareous or siliceous mudstones, including the middle Bakken Formation of the Williston Basin [52], the Wolfcamp Formation of the Permain Basin [53], the Eagle Ford Shale of the Gulf Coast Basin [54], the Lucaogou Formation of the Junggar Basin [55], the Chang 7 Formation of the Ordos Basin [56], the Shahejie Formation of the Bohai Bay Basin [57], and others. In contrast, there is no precedent for the successful exploration of argillaceous mudstone reservoirs similar to the Qingshankou Formation in the Songliao Basin [58]. Abundant cores have confirmed the extensive lamination development in the Qingshankou Formation mudstones. The laminae may be the key reservoir spaces and flow channels for shale oil enrichment and production. Therefore, it is important to study the depositional and diagenetic controls on laminae characteristics and their relationship to reservoir quality. In this study, we (1) classify the types of laminae and study the petrological characteristics of the different laminae, (2) conduct an evaluation of pore structure characteristics and a comparison of different laminae based on scanning electron microscopy, (3) evaluate the depositional control on the type and distribution of laminae, and (4) investigate diagenetic control on pore development in different types of laminae. This study will improve the understanding of the influence of laminae on the mudstone reservoir and provide support for shale oil and gas exploration.

2. Geological Setting

The Songliao Basin, with an area of 260,000 km², is a large Middle Cenozoic terrestrial basin in northeastern China [59,60]. The Songliao Basin is rich in oil and gas resources and has produced more than 2.4 billion tons of oil cumulatively since 1959 [61]. Currently, the shale oil in the Qingshankou Formation is a new growth point for crude oil production in the basin, with great exploration potential [51]. The Songliao Basin is divided into six tectonic units: the northern plunge, the central depression, the northeastern uplift, the southeastern uplift, the southwestern uplift, and the western slope (Figure 1) [60]. The evolution of the Songliao Basin can be divided into three main stages: syn-rift, post-rift thermal subsidence, and structural inversion [60,62].

The Qingshankou Formation was deposited in the post-rift thermal subsidence stage (Figure 1). Paleoclimatic studies indicate that the Qingshankou Formation has a humid subtropical–tropical environment [63]. At the early stage of the deposition of the Qingshankou Formation, the lake reached its maximum area of 87,000 km² [60]. During this stage, mainly the first member of the Qingshankou Formation, thick organic-rich dark argillaceous laminated mudstones were deposited. In the second and third members of the Qingshankou Formation, the delta in the north and west gradually advanced to the lake, and the lake area decreased to 41,000 km² [59]. The exploration strata for shale oil in the Qingshankou Formation are concentrated in the first member of the Qingshankou Formation and the lower part of the second member of the Qingshankou Formation. Well E1, the key well in this study, is located in the central part of the Sanzhao Sag, one of the main sedimentary centers of the mudstones of the Qingshankou Formation (Figure 1).

3. Samples and Methods

The core section of the key well E1 in the Qingshankou Formation is 1927–2052 m, totaling 125 m. This core section contains the first member of the Qingshankou Formation and the lower part of the second member of the Qingshankou Formation, which is the main stratum for shale oil exploration at present. It is difficult to subdivide the mudstone of the Qingshankou Formation on the core, so equal-interval sampling was conducted in this study. A total of 251 samples were collected at 0.5 m intervals throughout the core section. The characteristics of the laminae were studied sequentially at multiple scales, from the fresh surfaces of the cores to the thin sections, and scanning electron microscopy. Specifically, the core and thin section observations were first carried out for each sample. This step investigated the characteristics and vertical distribution of the laminae and conducted the classification of the types of laminae. At the same time, each sample was analyzed for organic carbon content. Geochemical characteristics, especially the organic matter content, are very sensitive to the depositional environment and will aid in understanding the vertical distribution characteristics of the laminae. After that, the samples were selected according to the different types of laminae for scanning electron microscopy observation. This step focuses on analyzing the pore characteristics of different laminae. Finally, the effects of deposition and diagenesis on the reservoir characteristics of laminae in the mudstone were evaluated by combining the distribution and pore characteristics of different types of laminae.

The samples were cut perpendicular to the lamination for thin sections and scanning electron microscopy (SEM) observations. The samples for the total organic carbon analysis were taken from the same locations. The total organic carbon content was determined using a LECO CS230 elemental analyzer. Ar–ion beam milling was carried out for scanning electron microscopy sample preparation. The SEM observation of pore characteristics was carried out using a Zeiss crossbeam 540 microscope. The accelerating voltage ranged from 5 to 15 kV and the working distances ranged from 5 to 8 mm. The observation of laminae characteristics began with the examination of the macroscopic distribution of laminae at low magnification (<500×). Then, the pore development characteristics within individual laminae were evaluated at high magnification (>2000×). The surface porosity and pore size distribution in SEM images were quantified using the point counting method and threshold segmentation method of JMicroVision 1.3.4 software. A count of 1000–1200 points per image was carried out.

4. Results

4.1. Petrological Characteristics

The observation of cores and thin sections reveals that the lamination in the lacustrine argillaceous mudstone samples of the Qingshankou Formation is widely developed. The characteristics of the laminae of all 251 samples were counted. The percentage of laminated mudstone can be up to 80%. There are different degrees of variation in color, grain size, geometry, continuity, mineral composition, and organic carbon content among the different laminae. Therefore, the establishment of an appropriate laminae classification scheme is the basis of the study.

Previous research has formed various standards for the classification of laminae. The first category is based on the morphology of the laminae. Campbell [64] proposed classifying the laminae types based on the laminae shape (planar, wavy, curved), whether the laminae are continuous (continuous, discontinuous), and whether the laminae are parallel to each other (parallel, non-parallel). The morphology of the laminae is a visual description of the characteristics of the laminae. Different morphologies can reflect different fluid properties, hydrodynamic conditions, and deposition processes. The second category is based on the thickness of the laminae. Ingram [65] proposed dividing the laminae by the thickness of 1 cm and dividing the thick and thin laminae by 3 mm. Potter [66] refined Ingram’s classification scheme by dividing very thin laminae and thin laminae by 0.5 mm, and medium laminae and thick laminae by 5 mm. O’Brien [43] classified thin and thick laminae by the thickness of 1 mm in the Lower Jurassic Toarcian Shale, Yorkshire, England. It has been found that the thin laminae mostly have planar morphology, which is formed by suspension settling. In contrast, the thick laminae develop micro-scour and micro-cross-lamination features, which are formed by bottom flowing currents. The thickness of the laminae also represents a visual description of the laminae and is easier to quantify and measure than the geometry of the laminae. In addition, the thickness of the laminae reflects the different depositional processes and hydrodynamic conditions. The third category is based on the composition of the laminae. Different compositions not only reflect different depositional environments but also directly affect the pore types. Xin [44] divided siliceous, mixed, dolomite, calcite, and organic laminae based on a 50% composition content criterion in the Paleogene Kongdian Formation lacustrine mudstone. The siliceous laminae mainly develop interparticle pores, while the dolomite and calcite laminae also develop intraparticle pores.

A preferred laminae classification can not only reflect the depositional condition, but also facilitate the study of pore characteristics and allow for quantitative statistics. Therefore, in this study, we propose a classification standard that combines the thickness and mineral composition of the laminae. The thickness is divided into thick laminae and thin laminae, with 1 mm as the boundary. The mineral composition is divided into siliceous, calcareous, or argillaceous. The Qingshankou Formation in the Songliao Basin mainly develops three types of laminae (Figure 2), namely thick siliceous laminae (THSL), thin siliceous laminae (TSL), and thin siliceous and argillaceous mixed laminae (TSAL).

4.1.1. Thick Siliceous Laminae

The thick siliceous lamina is mainly developed in the lower part of the second member of the Qingshankou Formation (Figure 2). The thickness of the thick siliceous laminae is 1–10 mm (Figure 3). The lamina is mainly composed of quartz and feldspar, followed by carbonate minerals and clay minerals. The carbonate minerals are partly ostracods and partly intergranular calcite cements (Figure 3F and Figure 4A–C). The clay minerals are mainly chlorite as intergranular cement (Figure 4B–D). The grain size range of the quartz and feldspar within the laminae is 30–150 μm, mostly 30–100 μm (Figure 4). The shape of the laminae is mostly wavy or curved (Figure 3). The continuity of the laminae is relatively good. Scour, cross-lamination, and weak to moderate bioturbation are developed in the laminae (Figure 3A–D). The grain size becomes finer upwards inside the laminae. When the laminae contain ostracods, the ostracod shells are generally in the upper part of the siliceous laminae because the ostracod shells have a larger surface area and thus gain a greater lift force (Figure 3F).

4.1.2. Thin Siliceous Laminae

The thin siliceous lamina is the dominant type of laminae in the mudstone of the Qingshankou Formation. It is widely developed in the whole study part of the Qingshankou Formation, with greater abundance in the first member of the Qingshankou Formation (Figure 2). The thickness of the thin siliceous laminae is in the range of 20–1000 μm and can be as thin as a few grains (Figure 5). The lamina is mainly composed of quartz and feldspar, followed by clay minerals and pyrite. The clay minerals are mainly filling materials and cements, and the composition is dominated by chlorite, followed by illite and mixed-layer illite–smectite (I/S) (Figure 6B,D,F). The grain size of the quartz and feldspar within the lamina is 2–30 μm, mostly 2–15 μm (Figure 6B,D,F). The shape of the laminae is planar or wavy (Figure 5). The continuity of the laminae is relatively poor. Micro-scour and micro-cross-lamination can be found in the laminae (Figure 5C–F). The interior of the laminae generally does not show grain-size vertical variation, but upward grain-size fining can be recognized above the scour surface (Figure 5D). In addition, weakly bioturbation can be observed within the laminae (Figure 5C).

4.1.3. Thin Siliceous and Argillaceous Mixed Laminae

The thin siliceous and argillaceous mixed lamina is mainly developed in the lower part of the first member of the Qingshankou Formation (Figure 2). The lamina is composed of even silica-rich bright laminae alternating with dark laminae rich in clay and organic matter (Figure 7). The thickness of the bright laminae is 100–400 μm, and the thickness of the dark laminae is 20–250 μm (Figure 7). The bright lamina is mainly composed of quartz, clay minerals, and feldspar. The clay minerals are predominantly illite and mixed-layer illite–smectite (I/S), with minor chlorite (Figure 8B). The grain size of the quartz and feldspar is in the range of 1–8 μm (Figure 8B). The bottom of the bright laminae sometimes contains a thin siliceous lamina with micro-scour (Figure 7A and Figure 8A). The dark lamina is mainly composed of clay minerals, quartz, and organic matter. The clay minerals are predominantly illite and mixed-layer illite–smectite (I/S), with minor chlorite (Figure 8C,D). The grain size of the quartz is mostly 0.5–3 μm (Figure 8C,D). The base of the darker lamina tends to develop an organic-rich lamina (Figure 7B and Figure 8D). Usually, the contact between the bright and the dark lamina is indistinct. The shape of the laminae is mostly planar, with good continuity.

4.2. Diagenetic Characteristics

The main diagenetic processes in the laminae of the Qingshankou Formation mudstones are compaction, cementation, dissolution, and the thermal maturation of the organic matter.

4.2.1. Compaction

The different laminae were all subjected to intense compaction. The mineral grains in the thick siliceous laminae are mainly in long contacts (Figure 4A,B). The mineral grains in the thin siliceous laminae are mainly characterized by point and long contacts (Figure 6). The thin siliceous and argillaceous mixed laminae contain large amounts of clay minerals, with rigid mineral grains floating in the clay mineral matrix (Figure 8). During compaction, the clay minerals were strongly deformed around the rigid minerals (e.g., quartz) (Figure 8).

4.2.2. Cementation

The main cements in the laminae are calcite, authigenic clay minerals such as chlorite, pyrite, and authigenic micro-quartz. The cements in the different laminae vary significantly. The main cements of the thick siliceous laminae are calcite, with minor authigenic chlorite, and pyrite. The calcite fills the pores between the mineral grains with irregular shapes (Figure 4A–C). The authigenic chlorite further fills the pores remaining from calcite cementation between the mineral grains (Figure 4B–D). The pyrite mainly fills the dissolved pores of the feldspars (Figure 4E) or fills the interparticle pores of the mineral grains (Figure 4B). The main cements of the thin siliceous laminae are authigenic chlorite, followed by minor pyrite. The chlorite and pyrite fill the interparticle pores of the minerals and the dissolution pores of the feldspars (Figure 6B,D,F). The chlorite has no distinct compaction deformation, and its ends are generally orientated towards the rigid mineral grains, reflecting its diagenetic genesis (Figure 6B,D,F). In addition, the authigenic chlorite in dissolved feldspars generally grows along the dissolution margins, towards the center (Figure 6B). The cements of the thin siliceous and argillaceous mixed laminae are predominantly authigenic micro-quartz, with minor chlorite, and pyrite. The authigenic micro-quartz is found predominantly in the dark laminae, dispersed within the organic-rich clay mineral matrix (Figure 8D). This type of quartz displays distinct euhedral shapes. The chlorite and pyrite fill the dissolution pores of the feldspar, the interparticle pores, and the intraparticle pores of clay minerals (Figure 8B–D).

4.2.3. Dissolution

The feldspar was intensely dissolved. The degree of dissolution varies across different laminae. The feldspars in the thick siliceous laminae are partially dissolved, and their dissolution pores are mostly in the range of 100–500 nm (Figure 4B,D). The feldspars in the thin siliceous laminae and the thin siliceous and argillaceous mixed laminae are fully dissolved, and only the outline of the feldspar remains (Figure 6B and Figure 8B). The dissolution pores of the feldspar are filled with many authigenic minerals, but the pore sizes can still be as large as 5 μm, and the average pore size is more than 500 nm (Figure 6B and Figure 8B). No obvious dissolution is observed within the ostracod shells (Figure 4H).

4.2.4. Thermal Maturation of Organic Matter

The maturity of the mudstone samples in this study is at the peak oil stage (R_o = 0.76%–0.85%). Few secondary organic matter pores were found. The thermal maturation of the organic matter produced a large amount of solid bitumen and oil, which further blocked the original pore space. It can be observed that almost all types of pores are filled with bitumen (Figure 4, Figure 6 and Figure 8).

4.3. Reservoir Pore Structure Characteristics

Four types of pores are mainly developed in the different laminae of the mudstone of the Qingshankou Formation (Figure 9): the interparticle pores between the rigid minerals, the interparticle pores of the clay minerals around the rigid minerals, the intraparticle dissolution pores (feldspar and quartz), and the intraparticle pores of the clay minerals. Different laminae have varied pore types and characteristics.

4.3.1. Thick Siliceous Laminae

The thick siliceous laminae mainly develop interparticle pores between the rigid minerals and the intraparticle dissolution pores of the feldspar and quartz (Figure 4). The shape of the interparticle pores is triangular or elongated. The pores are strongly cemented by the authigenic carbonate and chlorite (Figure 4B–D). The remaining pore space is very small, with pore sizes of tens to hundreds of nanometers, mostly less than 500 nm (Figure 4B–D). The shape of the dissolution pores of the feldspar is angular. The pore size of these pores is mostly less than 3 μm and more than 100 nm (Figure 4B,D,E). In addition, it can be observed that the dissolution pores of the feldspar are filled with pyrite to varying degrees (Figure 4D–E). The shape of the dissolution pores of the quartz is angular or round. The size of these pores can reach 2 μm, being mostly in the range of 100 to 500 nm (Figure 4F). In general, the abundance of dissolution pores in the feldspar is much higher than that in the quartz (Figure 4D,F). No distinct pores were observed within the shells of the ostracod (Figure 4G,H).

4.3.2. Thin Siliceous Laminae

The main pore types of the thin siliceous laminae are the interparticle pores between the rigid minerals, the dissolution pores of the feldspar and quartz, and the intraparticle pores of the clay minerals (Figure 6). The interparticle pore shape is mainly triangular or irregularly angular, with some being elongated (Figure 6B,D). The length of the triangular pores can be up to 2.5 μm, mostly in the range of tens to hundreds of nanometers. The long axis of the elongated pores can reach 3 μm. The interparticle pores are strongly reduced and filled with authigenic chlorite, pyrite, and bitumen (Figure 6B,D,F). The dissolution pores of the feldspar are fully dissolved, and only the outline of the feldspar remains. The dissolution pore space is filled with authigenic chlorite, bitumen, and pyrite (Figure 6D,F). The grain size of the feldspar is 5–12 μm, and the long axis of the pores in the unfilled part can be up to 5 μm (Figure 6B,D). Despite strong cementation, the pore sizes of the feldspars are mostly larger than 500 nm due to intense dissolution. The dissolution pores of the quartz are angular or round, with pore sizes ranging from a few tens to 200 nanometers (Figure 6B,F). The pores between the clay mineral sheets are mostly elongated or angular, with pore sizes of tens of nanometers. At the same time, the intraparticle pores of the clay minerals up to several hundred nanometers are also present. These clay minerals are mostly authigenic chlorite. Additionally, the ends of these pores of clay minerals are oriented toward the rigid minerals (Figure 6B,D). It is suggested that cementing the ends of the clay minerals near the rigid minerals prevents the collapse of these pores and forms a pressure shadow to protect the pores during compaction [67].

4.3.3. Thin Siliceous and Argillaceous Mixed Laminae

The thin siliceous and argillaceous mixed laminae develop all four types of pores. The pore structure characteristics differ in the bright and dark laminae. Among them, the interparticle pores are mainly pores formed by the ductile clay minerals around the rigid grains, followed by the pores between the rigid minerals. The former is the dominant pore type in both the bright and dark laminae (Figure 8B–D). The shape of the pores is angular or elongated. Angular pores have pore sizes of a few tens to hundreds of nanometers, mostly less than 300 nanometers. The elongated pores can be several microns in length, but they are mostly tens of nanometers in width. Most of the rigid minerals are isolated in the clay matrix of the dark laminae, so the interparticle pores between the rigid minerals are poorly developed. Due to the increased content of rigid minerals in the bright laminae, the interparticle pores between the rigid minerals are more developed. These pores are mostly angular, with pore size mostly less than 500 nm (Figure 8B). In addition, both types of interparticle pore spaces were observed to be filled with chlorite, pyrite, and bitumen. The bottom of the bright laminae sometimes contains a thin siliceous lamina with pore characteristics similar to those described in Section 4.3.2 (Figure 6).

The intraparticle pores mainly resulted from the dissolution of the feldspar and the pores between the clay mineral sheets. The pore characteristics of the bright and dark laminae are similar. The feldspar has suffered intense dissolution, and only the feldspar outline remains. The shape of the dissolved pores is angular, with chlorite cementation within them. The residual pores can be 0.5–5 μm in diameter (Figure 8B,C). The clay minerals suffer from strong compaction, and the intraparticle pores are poorly developed. The main shape of these pores is elongated (Figure 8B–D). The pore length is mostly below 1 μm, and the width is mostly tens of nanometers. The intraparticle pores of the clay minerals around the rigid grains can be relatively well preserved due to the pressure shadow of rigid minerals. These pores have an angular or elongated shape and the pore size can be larger than 100 nm (Figure 8B). In addition, the intraparticle pores of the clay minerals are widely filled with bitumen, further reducing the pore space. The organic matter is mainly developed in the dark laminae, forming organic matter-rich laminae (Figure 8D). Due to its ductility and low thermal maturation, the organic matter compacts and squeezes between mineral particles, blocking adjacent pore spaces (Figure 8D) [68]. The development of the organic matter pores in the dark laminae is generally poor and cannot provide effective storage space.

4.3.4. Quantitative Analysis of Porosity and Pore Size Distribution of Different Laminae

The porosity and pore size distribution of different laminae vary significantly. The thin siliceous laminae have the highest porosity (average 10.71%), followed by the thin siliceous and argillaceous mixed laminae (average 5.37%, 6.13% for the bright laminae and 4.61% for the dark laminae). The thick siliceous laminae have the lowest porosity (average 4.32%). The average pore size of the thin siliceous and argillaceous mixed laminae (average 49 nm) is smaller than that of the thin siliceous laminae (average 313 nm) and thick siliceous laminae (average 204 nm) (Figure 10). Moreover, among the thin siliceous and argillaceous mixed laminae, the bright laminae have a wider range of pore sizes, while the dark laminae have smaller pore sizes. The pore size distributions of the thin and thick siliceous laminae are similar, but the pores in the thin siliceous laminae have a higher abundance (Figure 10). Pores larger than 1 µm are mainly the dissolution pores of the feldspar, mostly in the thin siliceous laminae and the bright laminae within the thin siliceous and argillaceous mixed laminae (Figure 6 and Figure 8). The dissolution pores of the feldspar in the thick siliceous laminae are mostly in the hundreds of nanometers (Figure 4).

5. Discussion

5.1. Depositional Control of Laminae Types and Distribution

Significant vertical variability was found in the laminae types in the mudstones of the Qingshankou Formation, which can be divided into three sections: the upper, middle, and lower sections (Figure 2). The lower section is in the lower part of the first member of the Qingshankou Formation, which mainly develops thin siliceous laminae and thin siliceous and argillaceous mixed laminae. The middle section is in the upper part of the first member of the Qingshankou Formation, mainly developing thin siliceous laminae. The upper section is in the lower part of the second member of the Qingshankou Formation, which mainly develops thick siliceous laminae and thin siliceous laminae.

The variation in different laminae types is closely related to their depositional conditions. Thin siliceous and argillaceous mixed laminae are concentrated in the lower section and are associated with a high organic carbon content (Figure 2). The alternating bright and dark laminae may represent the seasonal cycle of sedimentation, also known as varve [69,70]. In spring and summer, the sedimentary flux was high, and silica-rich bright laminae were deposited. In contrast, the sedimentary flux was low in autumn and winter, depositing dark laminae, rich in clay and organic matter. Previously, the sedimentation rate of the Qingshankou Formation mudstone was calculated to be in the range of 5.6 cm/kyr to 14.4 cm/kyr by establishing an astronomical time scale (ATS) [71]. The thickness of a single thin siliceous and argillaceous mixed lamina is slightly lower than the low value of the calculated annual sedimentation rate (Figure 7). Considering the slow deposition rate of organic-rich mudstones, it can be concluded that the thin siliceous and argillaceous mixed laminae represent seasonal deposition. The occurrence of varve suggests that the depositional process was controlled by climate-driven seasonal fluxes, reflecting a long-term stable anoxic environment.

A variety of mechanisms may exist for the formation of siliceous laminae, including suspension, eolian input, gravity-driven transport processes, and bottom currents [3,15,42,43,72]. The extensively developed scour and cross-lamination at different scales suggest that bottom currents or gravity-driven transport processes may be the dominant mechanism (Figure 3 and Figure 5). Recently, flume experiments revealed that when a mixture of quartz silt and clay flowed through the flume, the coarse quartz silt would separate from the clay and form siliceous laminae [73]. This result further supports the idea that the siliceous laminae of the present study, especially the micrometer-thick thin siliceous laminae, may not be dominated by the early recognition of suspension genesis. The deposition of siliceous laminae is controlled by terrestrial input, reflecting a seasonally oxidizing bottom water environment.

In addition, the flume experiment predicts a shift in the types of siliceous laminae with enhanced flow velocity and sedimentation rate, as shown by the interlaminated silt and mud to laminated silt, or ripple-laminated mud to rippled silt [73]. The transition from thin siliceous laminae to thick siliceous laminae in the present study reflected an enhanced sedimentary rate. It can be observed that thick siliceous laminae have larger grain size, greater lamina thickness, and better continuity (Figure 3 and Figure 5). This is also illustrated by the enrichment of organic matter, which is relatively low in thick siliceous laminae and relatively high in thin siliceous laminae (Figure 2). It can be inferred that the transition from thin to thick siliceous laminae was accompanied by a significant increase in terrestrial input.

Overall, the distribution characteristics of the laminae reflect an upward increase in grain size, lamina thickness, deposition rate, and oxygenation level, as well as an upward decrease in the organic carbon content from the lower section to the upper section (Figure 2). The mudstone of the Qingshankou Formation can be interpreted as a rapid transgressive systems tract (in the lower section) and a long-term highstand systems tract (the middle and upper section). The transgressive systems tract has a weak terrestrial input, stable anoxic bottom water, and high organic matter flux. Such depositional conditions favor the formation of thin siliceous and argillaceous mixed laminae and thin siliceous laminae. At the beginning of the highstand systems tract, there is a gradual increase in terrestrial inputs and oxygenation of the lake bottom water. The predominant laminae type shifts to thin siliceous laminae. With the increasing terrestrial input, the laminae types are dominated by thick siliceous laminae and thin siliceous laminae.

5.2. Diagenetic Control of Pore Development in Different Laminae

Diagenetic evolution profoundly affects pore development and reservoir quality. The effects of diagenesis are discussed separately according to physical and chemical aspects. Physical processes cause strong compaction and deformation. Compaction is the most significant process in the early stages of diagenesis, greatly reducing mudstone porosity [3,15,74]. Compaction is affected by mineral composition and grain size [75,76,77]. The higher the clay mineral content, the stronger the compaction that occurs. As a result, compaction has a much stronger effect on thin siliceous and argillaceous mixed laminae than on siliceous laminae (Figure 4, Figure 6 and Figure 8), severely destroying the primary porosity.

Chemical processes include the re-equilibration of mineral components and the maturation of organic matter. Calcite cement occurs mainly in sandstone reservoirs and significantly reduces the reservoir’s porosity and permeability [78,79,80,81,82,83]. The calcite cements in the present study mainly developed in thick siliceous laminae, strongly filling the intergranular pore space (Figure 4A–C). The initial porosity of the thick siliceous laminae may be much higher than that of the thin siliceous laminae, but its reservoir properties are destroyed by strong calcite cementation. In addition, calcite is the main cement of the thick siliceous laminae, which shows basal cementation. Previous studies have indicated that this type of calcite cement is mostly produced in early diagenesis, with the source of the calcite cement coming from the adjacent mudstones [81,82,83]. In addition, it was found that sandstones along the interfaces of sandstones and mudstones (mainly less than 1.5 m from the mudstone) are always tightly cemented by calcite [81]. Therefore, consistent with our observations, the thick siliceous laminae have the worst reservoir properties.

The organic acids released by the maturation of organic matter led to the dissolution of the feldspars [84,85,86]. The dissolution of the feldspar occurred in all types of laminae. The dissolution is more intense in the thin siliceous and argillaceous mixed laminae and in the thin siliceous laminae, where only the outline of the feldspar remains (Figure 6 and Figure 8). This may be due to their closer proximity to organic matter and a greater susceptibility to the effect of organic acids. Thus, even though the dissolution pores of the feldspars still suffer from later cementation, they have good reservoir properties.

The dominant form of pyrite in the different laminae is euhedral crystals. Previous studies have shown that euhedral pyrite precipitates from pore water that is oversaturated with pyrite but not saturated with iron monosulfides [87,88,89]. This morphology of pyrite is mostly formed in the early diagenesis [87,90]. The pyrite fills the intergranular pores, but can also resist compaction. The pyrite within the dissolution pores of the feldspar in thick siliceous laminae is mostly anhedral crystals (Figure 4E). The precipitation of the pyrite seems to be limited by the morphology and pore size of the dissolution pores of the feldspar. In addition, the formation of such pyrite occurs in the middle and late diagenesis and plays a mainly destructive role in the reservoir.

With increasing burial depth, clay mineral transformation becomes a key diagenetic process affecting reservoir quality. This process converts smectite to illite or chlorite [91,92]. The conversion reaction of smectite to illite requires a potassium source, which is usually derived from the decomposition of potassium feldspar [92]. Because smectite is a more silica-rich clay mineral, illitization also releases significant amounts of silica [92]. The silica released by this process is thought to be dispersed as an authigenic microcrystal quartz in the clay matrix of the mudstone [93,94]. This type of quartz cement occurs mainly in the dark laminae of the thin siliceous and argillaceous mixed laminae (Figure 8). Cathodoluminescence studies on the same formation from the study area also confirmed that quartz cements are mainly produced from clay mineral transformations and are distributed in the mudstone clay matrix [95]. The precipitation of such authigenic quartz can form additional interparticle pores of clay minerals around rigid grains, thus improving reservoir quality. In addition, the clay mineral transformation also leads to dehydration and a strong orientation of the clay minerals [96,97,98,99], which may further reduce porosity.

Chlorite is widely distributed in all types of laminae and is the main cement of the thin siliceous laminae (Figure 4 and Figure 6). Multiple mechanisms exist for chlorite formation, and all require iron and magnesium sources [91,100,101,102,103]. However, in any event, there is broad consensus that chlorite formation can effectively inhibit quartz overgrowth cementation during diagenesis [79,103]. Chlorite cementation reduces reservoir porosity but retains a certain amount of pore space, which may be the key to the optimal reservoir properties of thin siliceous laminae (Figure 6).

The maturation of organic matter likewise has an important effect on pore development [25,27,31,104,105,106]. Previous studies have confirmed that a large number of organic matter pores are formed during gas windows, which are key reservoir spaces for high hydrocarbon production in numerous shale formations, such as the Barnett Shale [2], the Woodford Shale [25], the Marcellus Shale [26], the Longmaxi Shale [28], the Eagle Ford Shale [27], and many others. However, during the oil window, few organic matter pores are developed [25]. In addition, previous studies have found that at this stage, the solid bitumen and oil produced by the maturation of organic matter fill the original pore space, greatly reducing the porosity and pore connectivity of the shale [31,107]. The filling of primary pores by solid bitumen is widespread in all types of laminae (Figure 4, Figure 6 and Figure 8). In addition, few organic matter pores develop in the organic matter-rich thin siliceous and argillaceous mixed laminae (Figure 8). Thus, the maturation of organic matter plays a disruptive role in pore development in all types of laminae.

The porosity of the different laminae reduces as the burial depth increases, but the paths vary dramatically (Figure 11). The thick siliceous laminae were tightly cemented by calcite during the early diagenesis, losing most of their interparticle porosity. The dissolution of the feldspar provided secondary porosity as the main reservoir space. The thin siliceous and argillaceous mixed laminae have a high clay mineral content. Extensive deformation of the clay minerals occurred during early compaction, and the primary pores suffered significant damage. The main remaining primary pores are associated with the clay minerals, including the interparticle pores of the clay minerals around the rigid minerals and the intraparticle pores of the clay minerals. The authigenic microcrystalline quartz was generated in the thin siliceous and argillaceous mixed laminae during the middle diagenesis, and it improved the reservoir space by forming additional interparticle pores. The dissolution of the feldspar also enhances porosity. The thin siliceous laminae were cemented largely by chlorite during the middle diagenesis, inhibiting authigenic quartz overgrowth cementation while retaining some primary interparticle porosity. The interparticle pores of the thin siliceous laminae are relatively well preserved among the three types of laminae. In addition, the feldspar dissolution provides additional secondary pores. Thus, under the control of diagenesis, the thin siliceous laminae have the best reservoir properties, followed by the thin siliceous and the argillaceous mixed laminae, while the thick siliceous laminae display the worst properties.

5.3. Integrated Depositional and Diagenetic Processes Control on Pore Characteristics in Different Laminae

The depositional process controls the type and distribution of the laminae (Figure 12). The Qingshankou Formation mainly develops three types of laminae: thick siliceous laminae (THSL), thin siliceous laminae (TSL), and thin siliceous and argillaceous mixed laminae (TSAL). The three sections are divided according to the distribution of different laminae. In the lower section, the climate is the main control, mainly forming the TSAL and TSL. The middle and upper sections are mainly controlled by the increasing terrestrial input. The middle section mainly develops TSL. In the upper section, THSL and TSL are developed.

During the diagenesis process, different laminae had varied evolutionary paths and developed different pore characteristics (Figure 11 and Figure 12). The THSL mainly underwent physical compaction, calcite cementation, and the dissolution of the feldspar. Their pores were primarily destroyed by calcite cementation in the early diagenetic stage. The TSL mainly experienced physical compaction, chlorite filling, the dissolution of the feldspar, and solid bitumen filling. Their pores were widely filled by chlorite in the mesodiagenetic stage, but they retained a certain number of primary pores. The TSAL mainly underwent physical compaction, clay mineral transformation, authigenic quartz generation, the dissolution of the feldspar, and solid bitumen filling. Their pores were mainly destroyed by physical compaction in the early diagenetic stage. The authigenic quartz formed during the mesodiagenetic stage improves the reservoir. The dissolution of the feldspar formed good secondary pores in all kinds of laminae.

The main pore types, the average pore size, and porosity vary across different laminae (Figure 10 and Figure 12). The main pore types of the THSL are the interparticle pores between the rigid minerals and the intraparticle dissolution pores of the feldspar and quartz. The average pore size is 204 nm, and the average porosity is 4.32%. The main pore types of the TSL are the interparticle pores between the rigid minerals, the dissolution pores of the feldspar and quartz, and the intraparticle pores of the clay minerals. The average pore size is 313 nm, and the average porosity is 10.71%. The main pore types of the TSAL are the interparticle pores of the clay minerals around the rigid minerals, the intraparticle dissolution pores (feldspar and quartz), and the intraparticle pores of the clay minerals. The average pore size is 49 nm, and the average porosity is 5.37%. This results in spatial differences in the reservoir’s physical properties (Figure 12). The lower section has a small average pore size and low porosity. The well-connected interparticle pores are poorly developed and may result in poor permeability. The middle section has a large average pore size and high porosity. The well-connected interparticle pores are widely developed and may have good permeability. The upper section has a relatively large average pore size, but the lowest porosity. The well-connected interparticle pores are poorly developed and may result in poor permeability.

6. Conclusions

The laminae are widely developed in the lacustrine argillaceous mudstone of the Qingshankou Formation in the Songliao Basin, China. There are three main types of laminae, namely thick siliceous laminae, thin siliceous laminae, and thin siliceous and argillaceous mixed laminae.

Depositional processes control the type and distribution of laminae. The formation of thin siliceous and argillaceous mixed laminae is controlled by climate-driven seasonal flux variations. The formation of thin and thick siliceous laminae is controlled by bottom flow or gravity-driven processes, under the control of increased terrestrial inputs. The dominant laminae type in the mudstones of the Qingshankou Formation changes from thin siliceous and argillaceous mixed laminae to thin siliceous laminae to thick siliceous laminae. This change represents an early transgressive systems tract and later highstand systems tract.

Diagenetic processes control the pore development in different laminae. The thin siliceous laminae have the highest porosity, followed by the thin siliceous and argillaceous mixed laminae, while the thick siliceous laminae exhibit the worst porosity. Different laminae have varied diagenetic pathways. The thin siliceous laminae mainly suffered chlorite cementation in the mesodiagenetic stage and preserved a certain number of primary pores. The thin siliceous and argillaceous mixed laminae mainly suffered from physical compaction. The clay minerals are intensely deformed, with a large number of primary pores destroyed. The authigenic quartz formed by clay mineral transformation improves the reservoir by supporting the pore space. The thick siliceous laminae are strongly cemented by calcite in the early diagenetic stage, losing most of the primary interparticle pores. The dissolution of the feldspars provides additional secondary porosity in all laminae. Deposition and diagenesis combine to control spatial variation in laminae reservoir quality. The laminae in the mudstone are important high-quality reservoir spaces and flow channels, which are also crucial for the exploration of shale oil and gas in other areas around the world.