Qi Zhong , Jin-Guo Zhng ,b,＊, Dong Tng , Jin-Hui Jing ,Jing-Jing Shen , Mu-Xin Ci , Pin-Xie Li

a School of Energy Resources, China University of Geosciences (Beijing), Beijing 100083, China

b Key Laboratory of Strategy Evaluation for Shale Gas,Ministry of Land and Resources,Beijing 100083,China

c Exploration and Development Research Institute of Shengli Oilfield Branch of Sinopec, Dongying 257015,Shandong Province, China

Abstract The sources, transportation and depositional processes of lacustrine mudrock are still poorly understood. Existing studies have demonstrated the controlling effect of astronomical forcing on lacustrine mudrock deposition, but its depositional mechanism and evolution are still not systematically investigated.Most research related to astronomical forcing exclusively highlights the sedimentation of carbonate rocks in deep-water lacustrine setting, with insufficient attention paid to the thick organic-rich, deep-lake mudrock.With the increasing interest in exploration and development of shale oil and gas accumulations,it is urgent to deeply understand depositional rules of lacustrine mudrock.This study reviews sediment sources,depositional mechanism and evolution process of mudrock through expounding the correlations between the periodic changes of astronomical forces,the parameters of Earth orbital and mudrock compositions.By investigating the existing literature and using some actual data of Jiyang Depression, Bohai Bay Basin in East China, this study expounds on the influence of astronomical cycles on the deposition of lacustrine mudrock.Moreover,efforts are made to analyze the effects of various orbital parameters(e.g.,precession,obliquity,and eccentricity with the periods ranging from tens of thousands years to million years) on the deposition of mudrock from small-scale(decimeters to meters) to large-scale (10s to 100s meters). Further, it is feasible to apply the high-precision isochronous stratigraphic correlation into clarifying the distribution of favorable shale oil and gas reservoirs.To conclude, this study enunciates the sedimentation of mudrock from a new perspective (astronomical forcing) and provides a direction for the research on sedimentation of fine-grained sedimentary rocks.

Keywords Astronomical cycles, Lacustrine mudrock deposition, Organic-rich mudrock, Research status and prospects

1.Introduction

Fine-grained sedimentary rocks are generally defined as those rocks with grain size finer than 0.0625 mm (Aplin and Macquaker, 2011; Jiang et al.,2013). As a typical representative of the fine-grained sedimentary rocks, mudrock has an extensive occurrence in the sedimentary basins,accounting for about two-thirds of the entire sedimentary rocks (Potter et al., 2005; Peng, 2020). Research on the deposition of mudrock is far from sufficient,which is additionally ascribed to its small grain size, the difficulty of observation, and the limited access to microscopic experimental conditions. Global breakthroughs in shale oil and gas exploration have revealed the urgent importance to understand mudrock sedimentation.

Mudrock has been regarded as an ideal carrier for astronomical cycles due to its high continuity of sedimentation. It is an effective way for astronomical cycles to unlock the sedimentary characteristics,depositional processes and mechanism of lacustrine mudrock, and thus the research has been thriving in recent years (e.g., Kemp, 2016; Meyers and Malinverno, 2018; Ma and Li, 2020). Specifically, the astronomical solutions provided by Jacques Laskar(i.e., La2004 solution and La2010 solution) make it possible to accurately calculate the Earth's orbital parameters (eccentricity, obliquity, and precession)(Laskar et al., 2004, 2011) and furtherly investigate their influences on mudrock deposition.

In addition, it should be noted that most of the current research emphasizes the “static” characteristics of lithological changes in sedimentary records.However,efforts should also be made to enunciate the relationship between the astronomical forcing and some “dynamic” features of the mudrock (e.g.,depositional process and depositional mechanism).For instance, the depositional mechanism of laminae is expected to be related to astronomical forcing. As an important controlling factor for the deposition of various laminae in lakes (Cooper et al., 2000), the climate can control the physically-deposited laminae by affecting the intensity of terrestrial input. It controlled the chemically-deposited laminae by influencing the salinity. It also controlled biologicallydeposited and biochemically-deposited laminae by impacting the nutrient condition of the lake. Since astronomical forcing fundamentally affects the climate (Wu et al., 2011), it is logical to speculate whether there are certain laws governing the distribution of all types of laminae. Therefore, the depositional hydrodynamics of mudrock is expected to be related to the astronomical forcing as well.

Based on the above analysis, the research on the depositional process and mechanism of astronomical cycle-controlled fine-grained rocks is still poorly understood.This paper explains how astronomical cycles affect the depositional process and mechanism of lacustrine fine-grained sediments, so as to facilitate the development of“fine-grained sedimentology”and guide the exploration and development of shale oil and gas (Zhang et al., 2022).

2.Depositional process and formation mechanism of lacustrine mudrocks

2.1. Source and genesis of sediment components in mudrocks

It is generally more difficult to investigate the genesis of each individual component in mudrock than that in sandstone and carbonate rocks(Milliken et al.,2012; Taylor and Macquaker, 2014), which mainly results from the fine-grained size of mudrock that makes it challenging to characterize each component(Schieber et al., 2000) as well as the presence of various material components (McAllister and Taylor,2015). At present, it has been a hotspot to investigate the sources and genesis of carbonate minerals,felsic sediments, organic matter and clay minerals in the lacustrine mudrocks (Ball‵evre et al., 2012;Jarmo?owicz-Szulc et al., 2016; Zhang et al., 2021).Details of the sources and genesis of sediment components in mudrock are as follows.

2.1.1. Source and genesis of carbonate minerals

The source of carbonate minerals in marine environment is primarily seawater rich in Ca2+and CO32?(Tucker, 1991). In contrast, the source of carbonate minerals in lacustrine environment is more complex due to the complicated properties and provenance characteristics of lake water (Gierlowski-Kordesch,2010). For example, Jiang et al. (2007) found that the fine-grained carbonate rocks in the third member of Shahejie Formation of Shulu Sag in Jizhong Depression, Bohai Bay Basin, were formed by terrestrial input, which firstly experienced physical weathering and then were transported into the basin by mechanical action.

A series of transformations of carbonate minerals after deposition make it difficult to determine the characteristics during the depositional period. Specifically, the relatively unstable aragonite and highmagnesium calcite tend to be transformed into lowmagnesium calcite after deposition, inducing changes in mineral morphologies and chemical compositions.In addition, activities of microorganisms (e.g., methanogens)and fluids at the burial stage may also lead to the formation of carbonate rocks. For example, Jiang et al. (2014) and Zhang et al. (2016a) examined the genesis of the sparry calcite in the Cenozoic mudrock in eastern China,whose recrystallization is ascribed to the dissolution of existing carbonate minerals by organic acids that discharged during the organic matter maturation.

In addition, many studies have shown that the factors such as paleoclimate, water salinity and paleowater depth control the lake water stratification for carbonate deposition.The more significant the lake water stratification is,the more conducive it is to the formation of lacustrine fine-grained sedimentary foliation. Various algae, carbonate and other material components form carbonate laminae during the process of seasonal change(Anderson,1986;Anderson and Dean,1988;Larsen and MacDonald,1993;Wang et al.,2015).

2.1.2. Source and genesis of felsic sediments

The origin and formation of felsic minerals in mudrocks have significant implications for the interpretation of depositional environments and diagenetic pathways.Felsic minerals in continental lake basin are mainly terrigenous input. Felsic sediments mostly refer to terrigenous clastic components,which mainly occur in mechanical sedimentation and are typically non-viscous particles. When felsic clasts are transported into the lake basin from terrigenous rivers,they are controlled by the combined action of gravity,buoyancy,drag and uplift forces caused by the bottom bed shear. When the hydrodynamic force of the load weakens,the particle transportation speed decreases.The hydrodynamic force gradually weakens, and thus the particle size decreases, towards the center of the lake basin (Jin et al., 2020). Felsic sediments can be denuded and resuspended under the action of subsequent storm flow and underflow (density current,debris flow and turbidity current).

Some felsic minerals are authigenic, including overgrowth on detrital quartz, intragranular porefilling cement and intergranular clay-size microquartz cement dispersed throughout the rock matrix(Peng et al.,2020).For example,Schieber et al.(2000)found most quartz particles (50%-100%) were not transported and deposited from land in the Late Devonian marine mudrock of eastern United States;instead, they resulted from the dissolution and recrystallization of radiolarians or opals in the early diagenesis stage. Moreover, relatively unstable smectite tends to be transformed into more stable illite during the diagenesis, which provides silicon ions to form authigenic quartz (McAllister and Taylor, 2015).Therefore, more work should be done to reevaluate the quartz particles in the mudrock,especially those in the lacustrine mudrock. The source of quartz in lacustrine mudrock deposits includes terrigenous detrital and authigenic parts. Lacustrine mudrock differs from the marine counterpart due to differences in the formation environment and original mineral composition. For example, types and quantities of siliceous organisms in the marine and lake environments are very different (Wright, 1991), which also suggests that it is not proper to simply apply the understanding obtained from the marine mudrock to the research on the lacustrine mudrock.

2.1.3. Organic matter deposition in mudrocks

It is generally considered that organic matter is deposited in low-energy, anoxic deep-water environment,in which the generation rate of organic matter is high and the accumulation rate of sediments is low(Pedersen and Calvert, 1990; Tyson, 2001, 2005;Bohacs et al., 2005). The physical and chemical requirements must ensure the preservation of organic matter, which is: 1) high organic matter productivity,2) minimized decay rate of organic matter, and 3)minimized the impact of other rock components on organic matter dilution (Dean and Gardner, 1982;Arthur et al., 1987; Arthur and Dean, 1998; Algeo et al., 2004). These conditions guarantee the formation of organic-rich sediments (Salen et al., 2000;Katz, 2005).

2.1.4. Source and genesis of clay minerals

Clay minerals refer to hydrous aluminosilicate minerals with layered structure,and their particle size is mostly less than 2 μm(Ren,1992).Clay minerals are common in lacustrine mudrocks (Ren, 1992; Xu et al.,2003).The types of clay minerals and their diagenetic transformation are diverse,which has a vital impact on the physical properties of reservoirs (Fu, 1995; Chen et al., 2009; Xie et al., 2010). Clay minerals are mainly imported from land,and a small part is formed by the dissolution and alteration of feldspar and rock debris during diagenesis. Clay minerals can combine and adsorb various substances due to their fine-grained nature, large surface area and charge property.

Organic matter can be divided into two types.The first is polar organic molecules, mainly organic acids.The second is neutral molecules, mainly various lipid compounds. They can combine and adsorb clay minerals in different ways. At the same time, they also affect the properties of clay minerals. There is an interactive relationship between clay minerals and organic matter (Cai, 2003).

2.2. Interactions among various components in lacustrine mudrocks

Sediment components in mudrock are generally believed to be in the form of “single particles” that constantly settle to the bottom of the water column through continuous “drizzle” (Katz, 2005). In this context, there are no interactions among various mudrock components. However, interactions among various organic and inorganic components are proven to be present by recent studies(Cai et al.,2019;Liu et al.,2019). Therefore, it is of critical significance to investigate the interactions between inorganic minerals and organic matter to gain a better understanding of the depositional process of mudrock(Cai et al.,2015).

Inorganic minerals include quartz, feldspar,calcite, dolomite and clay minerals, etc. Organic matter,coming from living organisms,is featured by its strong colloidal property, solubility, ionization, and other chemical activities. Since both inorganic minerals and organic matter are chemically active, a series of interactions occur during the deposition of finegrained materials (Lash and Blood, 2004). For example, “marine snow” (larger floats that stick together after collisions with suspended particles in the marine) and organic clay complexes are found in many mudrocks (Macquaker et al., 2010a), and they are believed to be the products of flocculation, aggregation,and deposition between organic matter and inorganic minerals during the depositional process.

2.3. Sedimentary genesis of mudrocks

Since most mudrock is composed of laminae(Anderson and Dean,1988),the formation mechanism of laminae is the key to studying the sedimentary genesis of mudrock. Meanwhile, many breakthroughs have been recently made in understanding the sedimentary hydrodynamics of mudrock (Schieber et al.,2007), which has attracted widespread attention.

2.3.1. Genesis of laminae

There are generally four types of laminae according to the mineral composition, namely the detrital (e.g., quartz, feldspar, and some clay)laminae,the biological debris(e.g.,organic matter as well as calcareous and siliceous biological debris)laminae, the authigenic (e.g., calcite, gypsum, and zeolite) laminae, and mixed laminae (Zolitschka et al., 2015). These different types of laminae can be vertically coupled to form various couplets,including the carbonate mineral-organic matter couplet, the carbonate mineral-clay couplet, the carbonate mineral-diatom couplet, the terrigenous debris-organic matter couplet, and the clay-organic matter couplet (Glenn and Kelts, 1991).

Laminae have been widely analyzed to reconstruct the paleoclimate and paleo-sedimentary environments (e.g., Brauer, 2004; Elrick and Hinnov, 2007).However, there are rare studies that systematically investigate the depositional process and mechanism of laminae in the lacustrine mudrock (Wang and Zhong, 2004). Currently, there are five known depositional mechanisms for laminae in mudrock (LeVay and Gilbert, 1976; Ricketts, 1994; Pearson et al.,2004; Haugaard et al., 2016; Zhang et al., 2016b;Lehrmann et al., 2019; Peng, 2020): 1) biological action, 2) chemical action, 3) biochemical action, 4)mechanical action, and, 5) gravity flow action. Biological action refers to the bloom of a large number of organisms in a certain season due to temperature,salinity, and/or nutrient content changes in the water; laminae formed by the biological action have thin thickness, generally 0.1 mm-0.2 mm. For example, in stratified lakes, the seasonal circulation and mixing of water bodies can cause the bottom lake water rich in nutrients to rise to the surface, causing seasonal blooms of algae (Hay and Honjo, 1990).Chemical action mainly occurs in lacustrine basins with strong evaporation and high salinity. For example, periodic changes in the evaporation intensity can form gypsum laminae at the bottom of the lake(Last, 2001). The thickness of laminae formed by chemical action is in the range of 0.2 mm-0.5 mm.Biochemical action mainly refers to the metabolism of plankton microorganisms (such as blooming algae),destroying the carbon balance in the lake water,and/or the binding and adsorbing effects of plankton microorganisms, thereby promoting the precipitation of carbonate minerals (e.g., Thompson et al., 1997;Piper and Calvert, 2009). The thickness of laminae formed by biochemical action is also in the range of 0.2 mm-0.5 mm. For example, some researchers believe that the layered couplets composed of carbonate mineral layers and organic matter layers are related to biochemical processes(Wang and Liu,1993;Stiller and Nissenbaum, 1999). Specifically, the blooming of organic matter induces the precipitation of carbonate minerals to form carbonate laminae,while the demise of organic matter leads to the formation of organic laminae (Xu et al., 1997).Meanwhile, the components deposited by the mechanical action are mainly detrital minerals (e.g.,quartz, feldspar, and some clays; Gierlowski-Kordesch, 2010), and the thickness of laminae formed by mechanical action/physical process is generally 0.5 mm-0.8 mm. In addition, carbonate minerals can also be transported to the basin by the mechanical action to form carbonate mineral laminae, such as some of the laminae in the marl of the third member of Shahejie Formation in the Shulu Sag, Bohai Bay Basin (Kong et al., 2016). Under the influence of gravity flow, the laminae are transported and deposited in a turbulent environment in the form of aggregated particles (Yang et al., 2018), with relatively large thickness, generally in a range of 0.5 mm-1 mm.

Determination of the genesis of laminae directly affects the identification of the annual laminar assemblage, thereby affecting the paleoclimate reconstruction(Park and Fursich,2001).Moreover,the genetic mechanism of laminae also controls the distribution and petrophysical characteristics of the laminae, thereby affecting shale oil and gas exploration and development (Aplin and Macquaker, 2011).Although the five depositional mechanisms of laminae have been clarified theoretically in the above analysis,it is still difficult to distinguish the genesis of the laminae in practical work. For example, there are multiple interpretations when it comes to the genesis of the laminae composed of detrital minerals. Some researchers believe that the laminae composed of detrital minerals are caused by the temperature jump during the winter when the water body is stratified(H°akanson and Jansson,1992;Liu et al.,2001)or even when the water surface is frozen (Glenn and Kelts,1991). In contrast, other researchers argue that after the energy weakens at the end of a turbidity current,relatively coarse-grained sand-grade materials deposit, and the remaining fine-grained materials in the water body get settled in a suspended state, which could also form detrital layers (Potter et al., 2005).Moreover, Soyinka and Slatt (2008) found that the lofting effect of the hyperpycnal flow formed by the flood can also contribute to the formation of detrital laminae in the Lewis mudrock of the Cretaceous in Wyoming. Therefore, it is safe to conclude that the differences in genetic mechanisms of detrital laminae are actual indicators of different sedimentary environments, paleoclimates, and occurrence rules,thereby bringing about different understandings concerning the shale oil and gas exploration and development. However, there is currently a lack of a set of standards to identify laminae with different geneses in outcrops and cores.

In addition, the research on the depositional mechanism of laminae in lacustrine basins has been dominantly implemented on the Quaternary sediments(Zolitschka et al.,2015),and understandings based on these relatively new sediments are then applied to interpret the more ancient strata.However,there are various differences between the Quaternary and the more ancient periods in the atmospheric characteristics and biological types, which will inevitably affect the deposition of the laminae (Schimmelmann et al.,2016).

2.3.2. Depositional hydrodynamic characteristics of mudrock

Mudrock has been traditionally thought to be deposited slowly in still-water environments (Tyson,1995; Katz, 2005). However, such a thought has been faced with great challenges with the deepening of research (Macquaker and Bohacs, 2007). Schieber et al. (2007) confirmed that the flocs formed by the aggregation of fine particles can also be deposited under strong hydrodynamic conditions in their flume experiment. Particles finer than 0.01 mm are usually deposited in the form of flocs (Lash and Blood, 2004),and the hydrodynamic conditions corresponding to such floc deposition are sufficient to transport particles of medium-grained sand (Schieber, 2011). Signals of turbidity currents, storms, and undercurrents that control or affect mudrock deposition have been reported in the marine mudrock of many basins,including the Longmaxi (Lower Silurian) mudrock in the Sichuan Basin of China (Liang et al., 2016); the Devonian mudrock (Wilson and Schieber, 2015), the Carboniferous Barnett mudrock (Loucks and Rupple,2007), the Cretaceous Mancos mudrock (Macquaker et al., 2007), the Cretaceous Mowry mudrock(Macquaker et al., 2010b), and the mudrock in the Modern Gulf of Mexico (Tripsanas et al., 2004) in the New York State; the Carboniferous Edale mudrock(K¨onitzer et al., 2014) and the Jurassic Whitby mudrock (Ghadeer and Macquaker, 2011) in Britain;the Carboniferous mudrock in western Canada (Plint et al., 2012) and the Cretaceous mudrock in Alberta(Plint, 2014).

Compared with marine mudrock, lacustrine mudrock affected by water flows has been less reported. However, this does not mean that the sedimentary environment of lacustrine mudrock is“a pool of stagnant water”. For example, turbidity currents are very common in lake basins, and Section E of the Bouma Sequence is mudrock. Our previous work has demonstrated the presence of two main genetic mechanisms for the Eocene massive mudrock in the Jiyang Depression,Bohai Bay Basin:1)mudrocks in the shallow water environment were transported to the deep water area in the form of block sliding and slumping, and 2) Section E of the Bouma Sequence deposited at the end of the turbidity currents (Zhang et al., 2016b). Similarly, turbidity currents are proven to exist in the organic-rich marlstone in the Shulu Sag of the Bohai Bay Basin (Kong et al., 2016).Moreover, as found by the “Large Lake Observation Laboratory”of the University of Minnesota,storms can act on the deep lake area to suspend, transport, and re-deposit the pre-deposited materials. For example,centimeter-level“pits”are observed through sonar at the bottom of Lake Ontario where the water depth is greater than 100 m after the storms have eroded the bottom of the lake (Halfman et al., 2006). Similarly,some sediments deposited at a depth of 30 m-60 m in Lake Michigan are observed to be eroded by storms and transported to deeper areas for deposition (Hawley and Lee, 1999; Schwab et al., 2006). Through deploying “sediment traps” at different water depths in Lake Subil, Urban et al. (2004) collected finegrained redeposited sediments at a depth of 120 m-220 m and interpreted them as products of storm events. Specifically, the storm can exert pressure on the surface of the water body, and such pressure can be transmitted to the bottom of the lake water body at a certain oblique angle, leading to greater pressure at the bottom of the lake.It has been demonstrated that the storm can increase the pressure on the lake bottom by 20% even at a depth of 90 m(Churchill et al., 2004). Such an increase in pressure leads to the instability of lake bottom sediments,which can be suspended,transported,and redeposited(Lou et al.,2000).In addition to the Great Lakes region of the United States, the suspension, transportation,and redeposition of the above-mentioned fine particles have also been observed in the deep water of some small lakes,such as the Lake Rehtij¨arvi in Finland(lake area of about 0.5 km2) (Horppila and Niemisto,2008).

The water content of fine-grained materials could be as high as 90 vol.% at the beginning of the deposition, and the resistance of the fine-grained materials to compaction is weak (Potter et al., 2005). In this context,these fine-grained materials are subjected to pore water loss and compaction,which tend to flatten the deposits and destroy the original sedimentary structures (Schieber and Southard, 2009). Therefore,it has been difficult to identify mudrock that is affected by hydrodynamics. There have been no systematic schemes at this moment that can identify various types of mudrock affected by sedimentary hydrodynamics,and there is a greater lack of research on the characteristics of mudrock occurrences. However, it is very important to distinguish the mudrock that experiences strong sedimentary hydrodynamics from that results from static-water deposition, since they have very different textures and structures(Ghadeer and Macquaker,2011),which directly affect the development of fractures and the flow characteristics of shale oil and gas during the hydraulic fracturing process in horizontal wells. Therefore, there is still a huge gap between current understanding and actual needs in exploration development practice.

To conclude, the sedimentary environment of the lacustrine mudrock is not “a pool of stagnant water”.Instead, its sedimentary environment has a certain amount of hydrodynamic energy. At present, studies are still lacking on the identification of the sedimentary hydrodynamic conditions of lacustrine mudrock and the distribution characteristics of lacustrine mudrock affected by sedimentary hydrodynamics.

3.Deposition of lacustrine mudrocks constrained by astronomical cycles

3.1. Lacustrine mudrock deposits recorded significant Milankovitch signals

Mudrock has been considered as the most ideal carrier for astronomical signals due to its relatively uninterrupted and long-lasting sedimentation process.Research on the astronomical forcing has been greatly promoted since the proposal of several astronomical solutions by a scientific research team led by Jacques Laskar (Laskar et al., 2004, 2011). Specifically, the La2004 solution improves the calculation model of precession and the tidal action model of the Earth-Moon system and provides the orbital parameters of the Earth (Milankovitch cycle) since the past 250 Ma (Laskar et al., 2004). Recently, the La2010 solution is proposed (Laskar et al., 2011), which improves the initial calculation conditions of the La2004 solution and uses the new astronomical ephemeris INPOP08 for calculation (Fienga et al., 2009),providing more accurate eccentricity curves since the past 250 Ma. The Earth's orbital parameters (i.e., eccentricity, obliquity, and precession) have been calculated very accurately since the past 250 Ma,especially the past 50 Ma,which provide a theoretical basis for astronomical analysis.

As early as 1926, Swedish scientist De Geer began to work with colleagues to find patterns in the thickness changes of the annual varve(annual lamina in lake basin) in lacustrine basins throughout the world, with an attempt to find the records of astronomical signals in lacustrine basins (De Geer, 1937; Zolitschka et al.,2015). Much progress has been made in identifying Milankovitch cycles in the lacustrine deposits since the 1990s, including the Green River Formation in the United States (Machlus et al., 2008), the Mudurnu-G¨oynük Basin in Turkey(Ocakoglu et al.,2012),and the Teruel Basin in Spain(Abels et al.,2009a;2009b),all of which are younger than Eocene.Meanwhile,significant Milankovitch signals have also been identified in the older Permian, Triassic, and Cretaceous lacustrine strata of many areas,including the Newark Basin in the eastern United States (Olsen and Kent, 1999), the Central European Basin in Germany (Szurlies, 2007),the Colorado Plateau in North America(Tanner,2000),the Carnarvon Basin in western Australia(Lever,2004),the Songliao Basin in China (Wu et al., 2012), and the Jiyang Depression in the Bohai Bay Basin in China(Zhang et al., 2016a; 2016b; Figs. 1 and 2).

Although lacustrine mudrock deposition is also affected by tectonics and terrestrial input, astronomical cycle is a crucial factor affecting lacustrine mudrock deposition. This paper focuses on revealing the sedimentary process and mechanism of lacustrine mudrock from the perspective of astronomical cycles.It is generally believed that the lacustrine environment is relatively more susceptible to tectonics and terrestrial input, which is not conducive to recording astronomical signals. However, significant astronomical signals have been found in lacustrine mudrock deposits. Moreover, existing astronomical analyses of Permian-Triassic strata are mostly carried out in lacustrine deposits (Wu et al., 2011). Astronomical cycle is the main control factor of climate change in the 104years to the 105years grade(meters to tens of meters) (Fig. 3).

3.2. Deposition of lacustrine mudrocks constrained by astronomical forcing

Changes in the Earth's orbital parameters cause periodic changes in the amount of insolation received on the Earth's surface and thus lead to periodic changes in climate,which further directly or indirectly controls weathering, transportation, and sedimentation processes (Wu et al., 2011). Astronomical cycle affects climate change by controlling climate cycle,and then affects precipitation cycle, wind field cycle,terrigenous input cycle, water circulation cycle and lake level rise and fall cycle, furthering controls the deposition (Fig. 4). There are mainly three orbital parameters: 1) eccentricity (with periods of 100 ky,400 ky, 1.3 My, and 2.0 My), which characterizes the trajectory of the Earth's movement around the sun;2)obliquity(with periods of 39 ky and 41 ky),which is the inclination angle of the Earth's rotation axis (varying between 22°02′and 24°30′) and can be expressed as the angle between the ecliptic plane and the equatorial plane;and,3)precession(with periods of 19 ky and 23 ky), which refers to the wobbling of the Earth's rotation axis around the ecliptic axis (Chen, 2000). In addition,the sun's radiation also changes periodically,which is manifested as sunspot cycles that have periods of tens to hundreds of years (Jia, 2011).

Fig. 2 Correlation between the 23 ky precession and the lithology in Well Niuye 1 of Jiyang Depression, Bohai Bay Basin, East China. The wavelength of a sine wave represents a 23-ky precession cycle. Within a precession cycle, the contents of carbonate minerals,quartz+feldspar,clay,and TOC regularly change,indicating the controlling effect of the precession cycle on lithology.Symbol:GR=Gamma Ray Log (Unit for GR curve: API); AC = Acoustic Log (Unit for AC curve: μs/m); in the Lithology column: m = mudstone; l = lime mudstone.

Fig.3 Different factors control climate change at different time(Ruddiman,2014).Astronomical cycle is the main control factor of climate change in the 104 years to the 105 years grade (meters to tens of meters). Dansgaard-Oeschger cycle refers to the cyclical events on the millennium and centennial scales within the ice age.

Fig. 4 Schematic diagram of deposition constrained by Milankovitch cycle forcing, showing that the astronomical cycle controls climate cycles and impacts climate change, and then impacts precipitation cycle, terrigenous input cycle, lake level fluctuation cycle, wind field cycle, evaporation cycle and water circulation cycle, which furtherly controls deposition.

Astronomical forcing of different periods controls the sedimentation on the Earth in different ways.The value of eccentricity controls the distance between the Earth and the sun, thereby affecting the surface temperature and the intensity of weathering on the Earth (Olsen and Kent, 1999). The value of obliquity affects seasonality in different latitudes and different seasons on the Earth. For example, when obliquity is zero, the equator will always receive the vertical incident solar rays, and thus there will be no seasonal variation on the Earth. As the value of obliquity increases, seasonality gets enhanced. Precession is modulated by eccentricity, and the wobbling of the Earth's rotation axis results in the differentiated energy received on the Earth while it spins around the sun. For instance, in the precession minima, the northern hemisphere experiences hotter summers and colder winters, while in the precession maxima, the northern hemisphere experiences not hotter summers and not colder winters, the seasonal variation is not obvious. As for the sunspot cycles,there are mainly two episodes within a cycle according to the observation of the Earth's climate changes in the past 100 years, namely a dry episode with the presence of more sunspots and a wet and stormy episode with the presence of fewer sunspots(Wang, 2014). All these parameters jointly control the sedimentation in the basins, and of course, their impacts are different in various basins, depending on the latitude and geological setting,and various levels of astronomical cycles have the characteristics of step-by-step constraints (Fig. 5). Different regions have different causes of laminae, which are controlled by the influence of climate system and astronomical cycle, and there is variation in the thickness or composition of mudrock laminae(affected by climate and latitude). The thickness of laminae is diverse under the influence of climate system. When under the influence of astronomical cycles,the laminae thickness will vary with latitudes.The development of laminae thickness in high latitudes is mainly controlled by obliquity, and the development of laminae thickness in low latitudes is mainly affected by precession and obliquity.

Extensive efforts have been made to investigate the relationship between astronomical forcing and deposition of the Cenozoic lacustrine mudrock in Western Europe by researchers from the University of Utrecht in the Netherlands and the University of Barcelona in Spain (Table 1). Moreover, the effects of astronomical forcing on the lithology and sedimentary environment have also been observed in the oceanic and continental realms (Table 1). Table 1 mainly summarizes the examples of the impact of astronomical cycles on lacustrine mudrock deposition, and also summarizes some cases of oceanic and continental deposition for comparison and reference.

Fig.5 Step-by-step constraint pattern of astronomical cycles.The green curve in the figure represents astronomical cycles of 2.4 Ma,400 ka,125 ka, and 48 ka respectively from left to right. Curves of “organic matter” represent abundance.

Table 1 Case studies of astronomically controlled sedimentation.

3.3. Advantages of astronomical cycles in studying the depositional evolution of mudrocks

Since the research on the sedimentary evolution of lacustrine mudrock has just started (Potter et al.,2005), many problems have yet to be resolved. For example, there are multiple lithofacies in lacustrine mudrock,and we need to clarify how these lithofacies are distributed horizontally from the lacustrine basin margin to the center, and further how one lithofacies transitions vertically to another in the stratigraphy.

Sequence stratigraphy has been used as an effective means to characterize the distribution and sedimentary evolution of sandstone and carbonate formations(Catuneanu et al.,2009;Jiang,2010),and sequence boundaries are identified according to unconformities and corresponding conformities with reference to the rise and fall of the base level.However,mudrock is not very sensitive to water depth change, and it presents relatively homogeneous features, which makes it difficult to identify sequence boundaries of various levels in seismic, well logging,and core data. Alternatively, trace elements (Davies and Pickering, 1999) and TOC content (Algeo et al.,2004) are proposed for identification of sequence boundaries in mudrock, which, however, requires systematic high-density sampling and testing. In this context, these methods (sequence stratigraphy)cannot be widely applied to shale oil and gas exploration due to their high costs. Therefore, there is an urgent need to strengthen the study of the sedimentary evolution of lacustrine mudrock, which is challenging for the traditional sequence stratigraphy theory. Astronomical cycles provide the way to characterize the depositional evolution of mudrock,which is low-cost and easy to operate. Significant astronomical cycle signals will be recorded during the deposition of mudrocks. The astronomical cycle framework of the strata can be established by using the processed logging data (e.g., Gamma Ray) and combined with core, sedimentary environment. This method can be widely extended to shale oil and gas exploration.

4.Conclusions

The current research on the sedimentation of lacustrine mudrocks is still insufficient, and there are many problems to be solved, including the source and genesis of material components of mudrock, the depositional mechanism of mudrock, and the sedimentary evolution of mudrock. Existing studies have demonstrated the controlling effect of astronomical forcing on the lacustrine mudrock sedimentation, but the depositional processes are not systematically explored. Although there are already many case studies that demonstrate the control of astronomical forcing on deposition in the lacustrine basins, most of these studies are implemented in carbonate rocks,with little attention paid to thick, organic-rich mudrock in deep lakes. With the ever-increasing exploration and development practice in shale oil and gas accumulations in recent years, it is of both scientific and practical significance to have a deeper understanding of the relationship between astronomical forcing and mudrock sedimentation.

The influences of different orbital parameters(e.g.,precession, obliquity, and eccentricity) on mudrock sedimentation can be comprehensively analyzed from the perspective of astronomical forcing,which contributes to a better understanding of the depositional mechanism and process of mudrock from small scales(decimeters to meters) to larger scales (10s to 100s meters). In this context, the current fragmented understandingofmudrocksedimentationcanbeimproved,which provides a direction for the research on the sedimentation of fine-grained sedimentary rocks.

Astronomical cycles can be widely used in the study of mudrock, because the deposition of mudrock is continuous, and mudrock is ideal carrier for the research of astronomical cycles. However, its limitation is that it is difficult to adapt to large sets of sandstones, because sandstone deposition is “instantaneous”,not formed through periodic deposition year after year, and cannot record significant and continuous astronomical cycle signals. At present, most of the astronomical cycle responses used for basin studies are recorded in carbonate rocks, and the mudrockrecorded responses are only used for dating, while their application in shale oil and gas exploration is still in infancy. Moreover, the high-resolution stratigraphic framework established by astronomical cycles in the mudrock can also help achieve high-precision isochronous stratigraphic correlation in shale oil and gas exploration and development.

The application of astronomical cycle responses can play a great role in the establishment of stratigraphic framework (the precision of sag level can reach 10 m-40 m, and the accuracy of deep-water area in sag can reach 1 m-2 m) and the lithofacies division.It can significantly help to summarize the rule of sedimentary evolution, predict favorable reservoirs, alleviate the difficulty in stratigraphic comparison caused by the homogeneity of mudrock,provide a reasonable genetic source of deep-water mudrock,and also provide a theoretical basis for the exploration and development of shale oil and gas.

Abbreviations

Funding

The study was co-funded by the China National Key Research Project (Grant No. 2017ZX05009-002), the National Natural Science Foundation of China (Grant Nos. 41772090, 41802130), and the Foundation from Shandong Key Laboratory of Depositional Mineralization and Sedimentary Mineral, Shandong University of Science and Technology (Grant No. DMSM20190024).

Availability of data and materials

The data that support the findings of this study are available on request from the corresponding author.The data are not publicly available due to privacy or ethical restrictions.

Authors' contributions

All the authors have actively participated in the preparation of the manuscript.QZ contributed to data curation, software operation and original draft preparation. JGZ contributed to the conceptualization,methodology, visualization, investigation and revision.DT provided materials. JHJ contributed to software operation and data validation.JJS contributed to software operation and draft supervision. MXC and PXL contributed to revision. All authors read and approved the final proof.

Conflicts of interest

We declare that we have no conflict of interest.

Acknowledgements

We are grateful to the Geological Research Institute of Shengli and Yanchang Oil Companies for permission to access to the in-house database. We appreciate the help from Dr. Gregory Nadon at the Department of Geological Sciences,Ohio University for editing and improving the language through the manuscript.