Milankovitch cycle of continental deep-water fine-grained sedimentary rocks in the lower submember of Es3 of Well FY1 in Dongying Sag and its significance for shale oil exploration

Abstract

China has recently faced significant difficulties in the exploitation of its shale oil and gas resources. An essential geological obstacle preventing the breakthrough of Chinese shale oil exploration is the precise identification of productive oil and gas pools and ideal formation. Therefore, it is crucial to examine the properties of shale reservoirs. Deep-water fine-grained sedimentary rocks in the lower member of the Well FY1 in Dongying Sag are analyzed using the Milankovitch cycle based on core, geochemical analysis, and gamma logging data. The findings demonstrate that: (1) The entire Milankovitch cycle is preserved in the Es3x of Well FY1 in Dongying Sag, and that the long Eccentricity of 405 ka and the Precession cycle of 23.2 ka are the key controlling factors in the deposition. (2) The “three-end-member” method is used to divide eight different types of lithofacies. The main vertical changes in these lithofacies are from organic massive gray mudstone to organic lamellar callitic mudstone to organic massive gray mudstone to organic lamellar gray mudstone to organic lamellar gray mudstone and back again. From shallow to deep to deeper, the entire water depth fluctuated. (3) Each of the four lengthy Eccentricity cycles has a half-cycle of warm, humid weather and cold, dry weather. Analysis was done on how the lithofacies and organic matter concentration changed with high and small eccentricities. The enrichment of biological materials in warm, wet, dry, and cold climates was hypothesized by examining the response of fine-grain sedimentary rocks to eccentricities and Precession periods. Larger Eccentricity is thought to be more suitable for storing shale oil.

Keywords

Shale Milankovitch cycle the lower submember of Es3 fine-grain sedimentary rocks Dongying Sag

Introduction

In recent years, the shortage of oil and gas energy has promoted shale oil and gas to become a new force of energy exploration and development in sedimentary basins (Chen et al., 2022a, 2022b, 2022c; Lai et al., 2022; Liang et al., 2017a, 2017b, 2022; Liu et al., 2022; Wu et al., 2022; Xu et al., 2022; Zeng et al., 2021; Zhou et al., 2022). The conventional oil and gas from Cenozoic strata in continental basins in China's east has reached the high water cut development stage. The exploration effect of conventional energy and the taste of resources are getting worse and worse (Zhang et al., 2022a, 2022b). A significant point of reference for our unconventional oil and gas industry and a source of inspiration and promotion for the development of shale oil resources is the commercial exploitation of shale oil and gas in North America and Australia (Li et al., 2020a; Lv et al., 2023; Wang et al., 2019). The estimated amount of domestic shale oil recoverable reserves is 5.0 × 10⁹ tons (Li and Zhu 2020b). However, due to the stark differences in the types of sedimentary basin, the eastern part of our country shale oil and gas exploration cannot completely replicate the successful experience of North America. It has established a base in the Ordos Basin, Songliao Basin, Bohai Bay Basin, and other locations to conduct extensive drilling in the oil shale business based on monitoring the global vanguard of shale oil and gas research (Ma et al., 2012; Pu et al., 2019; Song, 2019; Zhao et al., 2008). Lacustrine cycle stratigraphy has been studied in the past, an emphasis on unconventional oil and gas intervals. For example, Wu et al. (2011) established astronomical time scales for the Cretaceous Qingshankou Formation and Nenjiang Formation in the Songliao Basin, revealing the astronomical cycles’ control over sedimentary processes and demonstrating their potential to aid in the stratigraphic division and correlation of high resolution. Chen (2019) investigated the cycle of fine-grained source rocks in the Chang 7 Member of the Triassic in Ordos, as well as the duration and limit ages of each submember. She also looked at the enrichment mechanism of organic matter. In the Dongying Depression, relatively continuous and complete fine-grained sedimentary rocks were deposited during the Es3 and Es4 upper submembers, which recorded abundant paleoclimate and paleoenvironmental evolution information, and are ideal carriers for the study of cyclic stratigraphy (Yao et al., 2007). Although there have been previous studies on cyclic stratigraphy in this region, the high-frequency sequence division scheme and standard are not uniform, and the age of the stratigraphic border is still up for debate (Shi et al., 2019; Sun et al., 2017; Zhang and Jin, 2021, 2022a, 2022b; Zhang and Yao 2013).

As in Jiyang Depression of Bo-hai Bay Basin important oil-producing areas, has become the domestic shale oil exploration and development of the demonstration zone, the paleogene Shahejie three and four shale with a thickness larger and stable development, burial characteristics of medium and high organic matter abundance, is the main hydrocarbon source of conventional oil, is also the main target strata shale oil exploration and production (Liang et al., 2017a, 2017b; Yan, 2010; Zhang et al., 2003, 2012; Zou et al., 2012). These fine-grained sedimentary rocks’ lithofacies, sedimentation environment, sedimentary model and distribution law, reservoir microstructure characterization, and shale oil enrichment factors have all been the subject of fruitful discussions among academics in recent years (Chen et al., 2016; Du et al., 2016; Wang et al., 2016). The findings of these studies have helped advance shale oil exploration and development. Still unknown is the genetic mechanism generating fine-grained sedimentary lithofacies, particularly those that are advantageous for shale oil exploration. As a result, unconventional oil and gas exploration is beginning to pay attention to and adopt the high-precision cycle and sequence identification method based on the idea of astronomical periods, and the pertinent techniques and technologies are continually improving (Hao et al., 2014; Wang et al., 2013).

The major research object in this work is an organic-rich, fine-grained rock system in Es3x of Well FY1 in Boxing Sag, Jiyang Depression. The use of geochemical element analysis to determine the primary controlling factors of sequence formation and driving mechanism, the high precision, and other factors within the frame of quality when the development rule of shale reserves is based on normalized processing of the natural gamma curve of frequency spectrum analysis, wavelet transform, or data analysis (Shi et al., 2020; Yang, 2022).

Geological overview

Boxing Sag is a third-order negative tectonic unit in the southern overburden belt, while Jiyang Sag is a secondary negative tectonic unit at the southeast boundary of the Bo-hai Bay Basin, which is characterized by north-fault and south-overburden. Three and four on the section through sand under the period of sedimentation of thick, rich organic matter, fine-grained sedimentary rock became the most important oil shale exploration horizon. Fine-grain in the rock inclusions of turbidite sand bodies formed a large number of lithologic traps, which are also important to conventional oil and gas exploration. Jiyang Sag experienced a severe chasmic stage.

For the purpose of drilling in the three sections under the section with a dark gray layer containing calcareous mudstone, and dark gray block containing calcareous mudstone, and brown gray layer containing calcareous oil shale, the combination of deep lake-half deep lake sedimentary, it contains many rich shale formations. Well FY1 brougham and northern sub-sag (Figure 1), Fan Home oilfield blocks.

Figure 1.

Geological survey of Dongying Sag (Wu et al., 2016).

Methods and data

Identification methods of astronomical cycle

The three components of the earth's orbit, namely Eccentricity, Obliquity, and Precession, are where Milankovitch's theory mostly begins (Figure 2). One of the crucial orbital components affecting the earth's orbit around the sun is Eccentricity. Theoretically, there are long Eccentricity cycles of 405 ka and short Eccentricity cycles of 95 and 125 ka. The Obliquity, which has periods of 54, 41, and 39 ka, and a range of 22.1° to 24.5°, also varies continuously throughout the earth's revolution. It has a correlation with the average surface temperature of the earth. Because of the gravitational attraction between the earth and the sun on the earth's equator, Precession is the slow rotation of the earth's spin axis along a vertical ecliptic plane and through its center. Precession cycles typically last 24, 22, and 19 ka. The northern hemisphere experiences brief hot summers and lengthy cold winters when Precession is minimal. The summer is long and hot, and the winter is brief and chilly when the Precession is large. The three components of the earth's orbit cause the amount of sunlight it receives to change on a regular basis, which tends to cause changes in the earth's surface temperature. Fine-grained sediment accumulation is sensitive to the induction of the periodic change of the climate system and feedback to sedimentary processes, and it records the climate at various levels in the sedimentary stratum cycle. This change is caused by the earth's surface layers experiencing periodic changes in temperature (Bol'shakov, 2014).

Figure 2.

Schematic diagram of (a) eccentricity variation from 38 to 41 Ma; (b) obliquity variation from 38 to 41 Ma; and (c) precession variation from 38 to 41 Ma.

The following are the main steps in recognizing a Milankovitch cycle: choose alternate indications first. Well logging has been extensively employed in Milankovitch cycle analysis because of its high resolution, strong continuity, and accessibility of sampling data. Therefore, among the popular paleoclimate proxies employed in Milankovitch cycle research, well logging is the most suitable indicator. The preprocessing of the data comes next. Prewhitening, detrending, deminimization, interpolation, and resampling are a few examples of data preparation techniques. Since the examined well data are currently largely digital, interpolation processing is not frequently applied. The basic goal of noise reduction is to remove some nonorbital components that were retrieved to improve the data's authenticity; Detrending is used to remove extremely low-frequency or extremely high-frequency nonorbital periodic signals and to stop the entire data column from gradually increasing or decreasing due to trend variations. Third, analyze the spectrum and filters. A quantitative statistical technique called spectrum analysis is used to study periodic phenomena. Its basic idea is to turn the time series data that was extracted into various frequency signals. Spectral analysis will find some dominant frequencies with prominent peaks, extract the peaks of these dominant frequencies, and then determine the reciprocal of the dominant frequency peak to determine the corresponding thickness of the stratum. Through the corresponding proportional relationship, whether the target strata are driven by the astronomical orbit is tested, so as to test whether there is Milankovitch cycle in the stratum of the study area.

Theoretical period data and astronomical cycle recognition

Theoretical orbital period data

The deposition time of Es3x was 38.8 to 40.9 Ma (Yao et al., 2007), during which the mean paleolatitude of Jiyang Depression was 38°N, and the paleoclimate belonged to the subtropical region (Li et al., 2007). The theoretical astronomical period model proposed by Laskar et al. (2004) was comprehensively used to analyze the theoretical astronomical period and the amount of surface sunshine. It is determined that the earth orbit parameter period during the deposition of Es3 in Jiyang Depression is 405, 124, 96.2, 51.2, 39.6, 38.1, 23.2, 21.9, and 18.7 ka. Among them, 405 ka (E), 125 ka (e1), and 96.2 ka (e2) belong to Eccentricity cycle, 51.2 ka (O3), 39.6 ka (O2), and 38.1 ka (O1) belong to Obliquity cycle. And 23.2 ka (P3), 21.9 ka (P2), and 18.7 ka (P1) belong to the Precession cycle, the cycle ratio was 21.66: 6.68: 5.14: 2.73: 2.12: 2.04: 1.24: 1.17: 1 (Figure 3). The wavelet energy spectrum also shows the frequency band with high correlation coefficient energy. The period value of these earth orbit parameters can be used as the standard to identify the Milankovitch cycle in Es3x of the Boxing Depression.

Figure 3.

(a) Theoretical orbital period from 38 to 41 Ma and (b) sunshine amount from 38 to 41 Ma.

Recognition of the Milankovitch cycle in the Es3x

Due to weathering and sedimentation, which affect the amount of sunlight and cause environmental changes like temperature and rainfall due to fine-grained sedimentary rock on the earth's surface caused by astronomical cycles (Chen et al., 2022a, 2022b, 2022c), the sedimentary records in the strata periodically appeared and recorded the relevant information. A perfect tool for the cycle analysis of fine-grained rocks is the natural gamma curve, which is sensitive to the lithology characteristics and can detect the content of organic matter and argillaceous matter in fine-grained rocks as well as their feedback of paleoclimate and sedimentary environment changes. As the astronomical period analysis data in this study, the GR data of Es3x (3051–3251 m) in the Well FY1 of Boxing Depression was used. Spectrum analysis and wavelet analysis were performed after normalization and the elimination of event influence.

Result

Milankovitch cycle recognition

Spectrum analysis results show that the main interval peak cycle thickness were 48.82, 12.26, 4.85, 2.86, and 2.26 m (Figure 4), of the ratio between main peak value is about 21.6: 5.39: 2.14: 2.09: 1.26: 1. With the theory of orbital period (E: e2: O3: O2: P3: P1) ratio is 21.66: 5.14: 2.12: 2.04: 1.24: 1. It accords well with those value. The Eccentricity, Obliquity, and Precession also show strong and continuous signals on the wavelet chromatogram, which indicates that the Milankovitch cycle in the sedimentary strata of the Es3x in the study area is driven by the earth's orbital period and preserved relatively well. Gaussian bandpass filtering was selected to process the long Eccentricity, Obliquity, and Precession period of the target layer respectively, and the astronomical cycle distribution diagram of Es3x subsegment was obtained (Figure 5), including four long Eccentricity cycles, 16 short Eccentricity cycles, 41 Obliquity cycles, and 70 Precession cycles. The deposition time of the whole interval is the sum of cumulative astronomical cycles of the same level (Lin et al., 2021). The Eccentricity deposition time of the Es3x is 1.539 Ma, the Obliquity deposition time is 1.623 Ma and the Precessional deposition time is 1.624 Ma, and the average deposition time is 1.598 Ma. That is, the total deposition time of the Es3x is 1.6 Ma.

Figure 4.

Results of (a) spectrum analysis of GR data in Es3x of Well FY1 and (b) wavelet transform analysis of GR data in Es3x of Well FY1.

Figure 5.

Milankovitch cycle stratigraphic framework in Es3x of Well FY1.

Lithofacies division

The lithofacies division scheme is obtained based on the investigation of mineral properties, main and trace elements, and sedimentary structure. The research area's shale was discovered to contain mostly feldspar, quartz, and other felsic minerals, as well as clay minerals, carbonate minerals, and only trace amounts of siderite and pyrite. It was discovered that Al, Ba, Na, Ca, Sr, and other elements have large concentrations of major and trace elements. Laminar structures were less developed than sedimentary structures, which were mostly huge and stratified formations. Eight different forms of lithofacies can be classified using carbonate minerals, clay minerals, and felsic minerals as the three-end-members and 2% TOC as the upper limit (Table 1). The lithofacies largely undergo changes from organic massive callitic mudstone to organic-layered callitic mudstone to organic-rich massive callitic mudstone, according to observations of vertical lithofacies alterations. The total water depth moved from shallow to deep to deeper, then to the organic-rich stratified gray mudstone, then to the organic-rich stratified gray mudstone, and then to the organic-rich stratified gray mudstone.

Table 1.

Rock type classification scheme of Es3x in boxing depression.

TOC	Sedimentary structure	Mineral composition	Lithofacies type
	Dense lamellar	Argillaceous content ≥ 50%, 25%≤Carbonate content<50%	Contains organic dense lamellar gray mudstone
≤2%	Lamellar	Argillaceous content ≥ 50%, 25%≤Carbonate content<50%	Contains organic lamellar gray mudstone
	Massive	Argillaceous content ≥ 50%, 25%≤Carbonate content<50%	Contains organic massive gray mudstone
	Dense lamellar	Argillaceous content ≥ 50%, 25%≤Carbonate content<50%	Organic-rich dense lamellar gray mudstone
	Lamellar	Argillaceous content ≥ 50%, 25%≤Carbonate content<50%	Organic-rich lamellar gray mudstone
＞2%		Argillaceous content ≤ 25%, Carbonate content ≥ 75%	Organic-rich lamellar limestone
	Massive	Argillaceous content ≤ 75%, Carbonate content ≥ 25%	Organic-rich massive mudstone
		Argillaceous content ≥ 50%, 25%≤Carbonate content<50%	Organic-rich massive gray mudstone

Sedimentary environment

It is concluded that this region has a good Paleozoic source and depositional environment, indicating the significance of the element geochemical index, based on the comparative analysis of the chemical element composition characteristics, paleoclimate, paleosalinity, paleowater depth, and oxidation reductiveness of the samples at different depths of the Es3x. Es3x is primarily composed of lacustrine deposits, and clastic sandstone is one of the most significant reservoir types here. The majority of the rocks in the study area are fine-grained sedimentary rocks that were formed in a transitional environment of semideep lacustrine facies to deep lacustrine facies, brackish water to brackish water, with typical strong reduction-weak reduction paleoenvironmental conditions, and went through the evolution process of water from shallow-deep to shallow-deep, climate from dry cold to warm wet, and terrigenous supply from variable.

Discussion

Lithofacies division of fine-grained sedimentary rocks

In this study, the mineral composition, sedimentary structure, and organic matter content are taken into account to classify the fine-grained sedimentary rocks into lithofacies. First, TOC was classified into two categories: rich organic matter and poor organic matter, based on earlier research (Jiang et al., 2013). The mass fraction of unique minerals (anhydrite) and dense lamellar, lamellar, and massive sedimentary structures is then mentioned. On this basis, the fine-grained sedimentary rock lithofacies in the study region were separated according to the mass fraction of the principal minerals, the three-end-members of sandy rocks, argillaceous rocks, and carbonate rocks, and the borders of mass fraction 25%, 50%, and 75%. According to the lithofacies categorization approach indicated above, eight lithofacies types are primarily developed in the research region.

Paleoclimate and sedimentary environment

The development of high-quality unconventional oil and gas reservoirs is also constrained and exhibits cyclic development, and these changes in the type, abundance, and geochemical information of organic matter in the stratigraphic record are closely related to the recurring evolution of the earth's surface climate system and the resulting changes in the sedimentary environment (Lv et al., 2022). The fine-grained rocks in the lowest third member of the Boxing Depression exhibit frequent variations in the cyclic stratigraphic framework as well as organic geochemical values and mineral composition.

Paleoclimate

Under paleoclimate conditions, there are great differences in weathering, migration and enrichment of different geochemical elements, so the geochemical elements recorded in the strata can reveal the paleoclimate conditions during the sedimentary period. Some major elements have different climate sensitivity: Na is easy to dissolve and migrate along the current water during weathering, while Al is more difficult to dissolve and migrate (Jiang, 2010; Lv et al., 2017), and is often transported along with debris or mineral particles after mechanical crushing of parent rock. Under the condition of dry and cold climate, mechanical, chemical weathering is not complete, the most easily dissolved chemical Na element in the first place to the overlying water sedimentary area precipitation, chemical weathering is not complete, and Al element in clastic particles as the carrier of a small amount of carrying, overlying water sedimentary area strong evaporation, water salinity increase and Na element enrichment, overall Na/Al ratio is larger; under the condition of warm and wet climate, the chemical weathering is relatively complete, Na element is still transported in the dissolved state and the Al element is transported in large quantities as fine mineral particles or colloids, and the Na/Al ratio is small at this stage. Therefore, a high Na/Al value indicates a dry and cold paleoclimate, while a low Na/Al value indicates a warm and wet paleoclimate. In addition to Na/Al, the Ca/Mg ratio can also indicate paleoclimate characteristics. The higher the Ca/Mg ratio is, the more dry and cold the paleoclimate is. From the vertical changes of Na/Al and Ca/Mg values (Figure 6), it can be seen that the Na/Al value of Es3x in Dongying Sag varies from 0.05 to 0.30 on the whole, with an average value of 0.09. Ca/Mg values varied from 1.91 to 30.19, with an average value of 12.90. In the first layer (3052–3117.76 m) below Es3, the minimum Na/Al value is 0.05, the maximum is 0.16, and the average value is 0.08. The overall change trend shows first increasing and then decreasing, so the climate gradually changes from warm and wet to dry and cold. In the second layer (3117.76∼3162.67 m), the minimum Na/Al value is 0.05, the maximum is 0.11, and the average value is 0.08. The overall change trend is still increasing first and then decreasing. Therefore, the climate change in the second layer gradually changes from warm and wet to dry and cold. In the third layer (3162.67∼3211.66 m), the minimum Na/Al value is 0.06, the maximum is 0.30, and the average value is 0.12. The Na/Al value in this stage is relatively high, and the changing trend is still increasing first and then decreasing. Therefore, the climate change in the third layer is warm and wet—relatively dry and cold. In the fourth layer (3211.66–3250 m), the minimum Na/Al value is 0.08, the maximum is 0.15, and the average value is 0.11. The climate change in the fourth layer is warm and wet-comparatively dry and cold. Although the overall change in Na/Al value in this stage tends to be constant, it can still be noticed that there is a modest trend of first increasing and then reducing. The internal temperature of each horizon changed from being warm and wet to being somewhat dry and chilly in accordance with the climatic shift of the aforementioned four horizons. As a result, the Es3x paleoclimate in the Dongying Depression offers a nice cycle.

Figure 6.

Comprehensive evolution characteristics of the sedimentary environment in Es3x of Well FY1 in Dongying Sag.

Paleosalinity

Salinity is a measure of how many dissolved compounds are present in seawater per mass. It can be used to tell a freshwater environment from a saltwater one apart. It is a crucial metric for assessing both the marine and terrestrial environments. Because it can't be estimated directly in sedimentary strata, indirect analysis can be done using a variety of replacement indices. Paleosalinometers can be found in sedimentary rocks as a number of trace elements. In this work, paleosalinity will be restored via modifications in the Sr and Ba components. While Sr has considerable chemical activity and is simple to move in water, Ba has stable chemical characteristics and is difficult to migrate. In a saltwater environment, the Sr/Ba number is relatively high, but in a freshwater environment, it is relatively low. In specific studies, the size of Sr/Ba is often used to distinguish saltwater environment (Sr/Ba value >1) and fresh-brackish water environment (Sr/Ba value <1). According to the vertical variation of Sr/Ba value (Figure 6), the overall Sr/Ba value of Es3x in Dongying Sag varies from 0.42 to 7.33, with an average value of 2.19. The Sr/Ba value of the tested samples is basically >1, indicating that the stratum of this member was deposited in the saline environment as a whole. In the first layer (3052–3117.76 m) of Es3x, the minimum Sr/Ba value is 0.67, the maximum is 2.19, and the average value is 1.51. The Sr/Ba value of this layer is relatively small on the whole, and the fluctuation range is small but the trend is gradually increasing, which indicates that the sedimentary environment of this layer is brackish water and salt water. In the second layer (3117.76∼3162.67 m) of Es3x, the minimum Sr/Ba value is 1.48, the maximum is 3.34, and the average value is 2.39. The overall variation trend is increasing. Therefore, the water medium of this layer was briny water when it was deposited. In the third layer (3162.67∼3211.66 m) of Es3x, the minimum Sr/Ba value is 0.42, the maximum is 7.33, and the average value is 2.56. The fluctuation range of Sr/Ba value in this layer is large, and the overall data gradually increases, indicating that the water salinity is high at this time. In the fourth layer (3211.66–3250 m) of Es3x, the minimum Sr/Ba value is 1.87, the maximum is 3.53, and the average value is 2.78. The variation range of the Sr/Ba value is small. At this time, the water salinity is relatively reduced, but it is still a briny environment. Therefore, the paleosalinity of the sedimentary water was brackish-salt water during the deposition of Es3x.

Paleowater depth and oxidative reducibility

The depth of a lake's water is a key indicator of its paleoenvironment, and the aggregation of organisms as well as the differentiation and deposition of different elements depend significantly on the depth of the lake's paleowater. As a result, the presence of distinctive components and organic material is frequently utilized as a key signal to determine the depth of paleowater. In the study of ancient lakes, TOC content is frequently employed to describe the depth of the ancient water. The higher the value, the greater is the depth of the ancient water. Despite its flaws, this approach has some reference value in the semideep lake to deep lake region, where the river supply has little impact. Some distinguishing characteristics that depend on the depth of the water can also be utilized to determine the depth of ancient waters, such as Fe and Mn. Mn element is stable and often occurs in deep water environments, while Fe element is active and prone to chemical reactions and near shore accumulation. Therefore, Fe/Mn value can be used as an effective index to measure ancient water depth. A larger Fe/Mn value indicates a shallow water environment, whereas a smaller Fe/Mn value indicates a deep water environment (Zhang and Yao, 2013). As can be seen from the vertical variation of Fe/Mn value (Figure 6), the overall Fe/Mn value of Es3x in Dongying Sag varies between 0.19 and 1.62, with an average value of 0.46. In the first layer of Es3x (3052–3117.76 m), the minimum Fe/Mn value is 0.23, the maximum is 1.61, and the average value is 0.48. The Fe/Mn value in this horizon is relatively small on the whole, and the fluctuation range is small, indicating that the water in this horizon is shallow. In the second layer of Es3x (3117.76∼3162.67 m), the minimum Fe/Mn value is 0.19, the maximum is 0.98, and the average value is 0.36. Compared with the first strata, the water in this layer becomes deeper. In the third layer of Es3x (3162.67∼3211.66 m), the minimum Fe/Mn value is 0.22, the maximum is 1.62, and the average value is 0.61, indicating that the water in the strata is gradually shallower. In the fourth layer of Es3x (3211.66–3250 m), the minimum Fe/Mn value is 0.23, the maximum is 0.59, and the average value is 0.38. The water in this layer becomes deeper gradually. Therefore, during the sedimentary period of Es3x, the water body changed from shallow–deep–shallow–deep.

The REDOX characteristics of lake water are determined by the amount of oxygen present, and the elements and their valence states in the sediment likewise clearly respond to this characteristic. The oxidation and reduction of water affect the two elements V and Ni. In contrast to Ni, which is easily enriched in an oxygen-rich environment (Ikan, 1993; Jia et al., 2013), V is easily enhanced in an anoxic environment. As a result, the ratio V/(V + Ni) is frequently employed to describe the oxidation and reduction properties of lake water. An oxygen-rich sedimentary environment is one where the V/(V + Ni) ratio is <0.4; an anaerobic sedimentary environment is one where the ratio is larger than 0.5; and an oxygen-poor sedimentary environment is one where the ratio is between 0.4 and 0.5. From the vertical variation of the V/(V + Ni) ratio (Figure 6), it can be seen that the V/(V + Ni) value of Es3x in Dongying Sag varies from 0.37 to 0.94 on the whole, with an average value of 0.69. In the first layer of Es3x (3052 to 3117.76 m), only one minimum value of V/(V + Ni) is 0.36. The maximum value is 0.88, and the average value is 0.64. The V/(V + Ni) value of this layer is relatively small, so the water in this layer is a reducing environment. In the second layer (3117.76∼3162.67 m), the minimum V/(V + Ni) value is 0.46, the maximum is 0.93, and the average value is 0.72. The water in this layer is a reducing environment. In the third layer (3162.67∼3211.66 m), the minimum V/(V + Ni) value is 0.55, the maximum is 0.94, and the average value is 0.71, which indicates that the water body in this layer is a strongly reducing environment. In the fourth layer (3211.66–3250 m), the minimum V/(V + Ni) value is 0.60, the maximum is 0.79, and the average value is 0.72, indicating that the water in this layer is a strongly reducing environment. Therefore, there was a reduction-strong reductive environment in Es3x.

Paleoresource

There are two types of terrigenous detrital input and endogenous detrital input in lake sediments. Terrigenous detrital input includes clastic particles, lithic particles, and clay minerals, etc. Terrigenous detrital input is a comprehensive reflection of physical transport and deposition of provenance supply. The endogenous sediments are mainly carbonate deposits, which are the result of chemical transport and deposition under the condition of insufficient provenance supply. Relevant studies have pointed out that calcite content and the Th/U ratio of radioactive logging are used to characterize the provenance supply, and the mass fraction of clay minerals and quartz is also used to characterize the ancient provenance (Chen, 2012). In this study, mass fractions of clay minerals and quartz were used, with high values indicating high terrigenous input intensity. From the vertical variation of the mass fraction of clay minerals and quartz (Figure 6), it can be seen that the mass fraction of clay minerals and quartz in Es3x in Dongying Sag varies from 12% to 86%, with an average value of 47%. In the first layer of Es3x (3052–3117.76 m), the mass fraction of clay minerals and quartz is 19%, 85%, and 51%, respectively. The mass fraction values of clay minerals and quartz in the second layer of Es3x (3117.76∼3162.67 m) range from 20% to 79%, with an average value of 48%. The mass fraction values of clay minerals and quartz in the third strata of Es3x (3162.67∼3211.66 m) range from 12% to 86%, with an average value of 47%. The mass fraction values of clay minerals and quartz in the fourth layer (3211.66∼3250 m) under Es3x range from 12% to 78%, and the mean value is 41%. The above data show that the mass fraction of clay and quartz is small in the lower part of Es3x, indicating that the terrigenous input is small in this sedimentary period, which mainly depends on internal biomass such as carbonate rocks for deposition. The mass fraction of clay and quartz minerals gradually increases in the middle to the upper part of the sedimentary period, indicating that the terrigenous input is constantly increasing and relatively stable in the warm and wet climate. In dry and cold climates, the deposition reaction mainly depends on biomass.

Geological response characteristics of the Milankovitch cycle

Geological response of long Eccentricity cycle

Four long Eccentricity cycles have been identified in Es3x of Well FY1. Long cycle and large Eccentricity value are the characteristics of a long Eccentricity cycle, which reflects the warm and humid climate in this region, but it is greatly affected by the surface and sunshine amount, and has obvious seasonal variation characteristics of warm and wet and drought. Therefore, a long Eccentricity cycle can be divided into two half-cycle cycles of dry and cold climate and warm and wet climate. While the input of clay and quartz minerals increased along with the strengthening of continental weathering throughout the semiperiodic cycle of warm and wet environments, the amount of carbonate minerals was comparatively low. At the same time, the ancient water depth was deep, which was easier to preserve organic matter, and the TOC content was relatively high, which was conducive to the development of organic-rich fine-grained sedimentary rocks. In the half-cycle cycle of dry and cold climate, the decrease of atmospheric precipitation leads to the weakening of the promotion of continental weathering, the coarsening of clay and quartz lithology, the reduction of provenance supply, and the increase of carbonate mineral content. However, the ancient water depth is shallow, the organic matter is difficult to preserve, and the TOC is relatively low (Figure 7). In the semiperiodic cycle of warm and humid climate, due to high temperature, high humidity, and obvious seasonal differences, is conducive to the formation and evolution of the lake basin, and also provides abundant water source and a large number of terrigenous debris and other nutrients for surface runoff.

Figure 7.

Comprehensive analysis of Milankovitch cycle in Es3x of Well FY1 in Dongying Sag.

The sedimentary period of the study area was in the warm and humid climate zone, and the provenance was from the southeast, fine sediment. The lithologic assemblage is characterized by fine sandstone interspersed with silty mudstone. In terms of the paleoenvironment of Es3x, climate change played a crucial role in the change of the sedimentary environment, with the climate gradually changing from warm and wet to dry and cold, water from deep to shallow, and the reduction gradually weakened. From the perspective of lithofacies, the control of long Eccentricity cycle on lithofacies is regular. As can be seen from Figure 7, the shale lithofacies will also change with different cycles. The lithofacies controlled in warm and wet climate are mainly organic-rich massive mudstone, organic-rich massive gray mudstone, organic-rich lamellar gray mudstone and organic-laminated gray mudstone. The lithofacies controlled in cold and dry climate are mainly organic-laminated gray mudstone and organic-laminated massive gray mudstone.

Geological response of Precession cycle

Since the Precession cycle curve is significantly regulated by the long Eccentricity cycle curve, the characteristics of the geological response of the Precession cycle are analyzed under the half-cycle background of dry cold and warm wet climate in the long Eccentricity cycle. The results show that there is a positive correlation between long Eccentricity and warm–wet semicycle. When the long Eccentricity is larger, it is in the semicycle of warm and wet climate. At this time, the input of terrigenous matter increases, and TOC content is higher (Figure 8). Under the background of a warm and wet climate, the Precession changes from small to large in a Precession cycle, and the climate gradually changes from dry and cold to warm and wet. As a result, the input of clay and quartz increases and so does the content of organic matter, but the content of carbonate decreases relatively. At this time, the climate is relatively dry and cold in winter, and the peak of Precession corresponds to the aphelion. When Precession changes, the corresponding climate is also relatively dry and cold, and the peak of Precession corresponds to perihelion.

Figure 8.

Internal changes of the Precession cycle framework under the background of a warm and humid climate cycle.

When the long Eccentricity is small, it is in the half-cycle of dry and cold climate, and the input of terrigenous material decreases. In this context, in a Precession cycle, the Precession changes from large to small, and the climate gradually changes from relatively warm and wet to dry and cold, resulting in a decrease in the input of clay and quartz, a decrease in the content of organic matter, and a relative increase in the content of carbonate. In Precession, organic-rich lamellar gray mudstone and organic-rich massive gray mudstone are mainly developed, and the alternating rhythm appears. At this time, the sedimentary structure types are stratified and massive alternately, indicating that the lithofacies development is controlled by Precession cycle. When Precession is small, the input of terrigenous materials is more, resulting in turbidity of sedimentary water, high content of clastic minerals and relatively low content of carbonate minerals. When Precession is large, the autogenous biomass is more, which is mainly manifested as more carbonate minerals and less clastic minerals. At this time, the climate is warm and wet, and the peak of Precession corresponds to perihelion. When Precession changes, the corresponding climate also changes from warm and wet to dry and cold, and the peak of Precession corresponds to aphelion. In the warm and wet climate, terrigenous materials are dominant and TOC content is higher, which is conducive to the preservation of organic matter. Therefore, the warm and wet climate with large Eccentricity is more conducive to the storage of shale oil (Figure 9).

Figure 9.

Internal changes of Precession cycle framework under the half-cycle background of dry and cold climate.

Because Precession can be regulated on a small scale, Precession is often regulated by Eccentricity, so climate change will first show on Eccentricity, and then related to the change of Precession. As can be seen from Figures 8 and 9, when the long Eccentricity is large, it is in the half-cycle of warm and humid climate, and organic-rich massive mudstone and organic-rich lamellar callitic mudstone are mainly developed in the interior of a year's lapse grid. The sedimentary structure gradually changes from massive structure to lamellar structure, and laminaceous structure is less developed. When the long Eccentricity is small, it is in the half-cycle of dry and cold climate. In the Precession, organic-rich lamellar callitic mudstone and organic-rich massive callitic mudstone mainly develop, and the alternating rhythm appears. At this time, the sedimentary structure types are stratified and massive alternately, indicating that the lithofacies development is controlled by Precession cycle. When Precession is small, the input of terrigenous materials is more, resulting in turbidity of sedimentary water, high content of clastic minerals and relatively low content of carbonate minerals. When Precession is large, the autogenous biomass is more, which is mainly manifested as more carbonate minerals and less clastic minerals.

Conclusions

The theoretical orbital period during the deposition period of Es3x in the Well FY1 was obtained by analyzing the astronomical orbital parameters, and the GR logging curve of Es3x in Well FY1 was extracted. The ratio of Eccentricity, Obliquity and Precession period was close to the theoretical earth orbital parameter period ratio obtained. Therefore, there is a relatively complete Milankovitch cycle in Es3x of Well FY1 in Dongying Sag, and the deposition of Es3x in Well FY1 is mainly controlled by the long Eccentricity of 405 ka and the Precession cycle of 23.2 ka.

Lacustrine sediments are mainly developed in Es3x, among which the clastic sandstone is one of the most important reservoir types in this area. With carbonate minerals, clay minerals and felsic minerals as three-end-members and 2% TOC as the limit, eight types of lithofacies can be divided. In the vertical direction, the lithofacies mainly go from organic massive callitic mudstone to organic layered callitic mudstone to organic-rich massive callitic mudstone. Then it goes to the organic-rich stratified calcareous mudstone, then to the organic-rich stratified calcareous mudstone and then to the organic-rich stratified calcareous mudstone. The overall water depth changed from shallow–deep–deeper.

Four long Eccentricity cycles have been divided into the sedimentary strata under Es3 in well FY1, and each cycle contains a half-cycle of warm wet climate and dry cold climate respectively. When the long Eccentricity is large, the climate is warm and wet, the fluvial action is strong, a large number of terrigenous detritus carries nutrients into the lake basin, and the primary productivity is high. On the other hand, the precipitation is large and the lake level is high, which is conducive to the preservation of organic matter. When the long Eccentricity is small, the climate is dry and cold, the fluvial action is weak, the nutrient input is low, the primary productivity is low, and the evaporation is large. At this time, the lake level is low, which is not conducive to the preservation of organic matter. The main development is organic-rich layered callitic mudstone and organic-rich massive callitic mudstone. Warm and wet climates are more favorable for shale oil storage than cold and dry climates.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the National Natural Science Foundation of China (grant number 41872112).

ORCID iD

Guangxu Wang

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