Introduction
Depositional cyclicity is a common geological feature. Existing at various scales (formation system, rocks, and fabric elements of rock), it can reflect the inherent regularity of sedimentation. It is attractive for numerous geologists [1⇓-3]. Cyclicity is particularly evident in basin-wide fine-grained sedimentary rocks with grain sizes less than 62 μm and fine clast content higher than 50%. Cyclic changes in mineral composition, sedimentary structures, and reservoir spaces can be observed at core, thin section and micro-nano scales [4⇓⇓-7]. Many scholars have made significant achievements in the mineral composition determination, lithofacies classification, sedimentary environment reconstruction, identification of reservoir space types, and analysis of diagenetic processes relating to fine-grained sedimentary rocks [8⇓⇓⇓-12]. These achievements have enriched the scientific theoretical system of sedimentology and deepened the understanding of fine-grained sedimentary rocks. Moreover, the rising exploration and development of shale oil and gas have posed higher demands on the basic geological research of fine-grained sedimentary rocks as both the source rock and reservoir rock. More and more scholars have realized that cycle characterization and cycle formation mechanism analysis are crucial to revealing the heterogeneity and shale oil/gas enrichment laws of fine-grained sedimentary rocks.
For lacustrine fine-grained sedimentary rocks with complex composition, rapid facies variation, weak diagenesis, and high heterogeneity, there are no systematic, quantitative, and operable methods for characterizing their cyclic features [13]. In studies of coarse clastic sediments, sequence stratigraphy is generally used for establishing the isochronous stratigraphic framework and analyzing the stratigraphic stacking patterns, and an establishment of high-precision stratigraphic framework is the basis for revealing the key geological features and genetic mechanisms [14-15]. However, the conventional identification marks and research methods based on sequence stratigraphy (e.g. the classic sequence stratigraphy theory proposed by Vail based on passive continental margins and 3D seismic data) are not applicable to the lacustrine fine-grained sedimentary rocks.
To achieve a better characterization of the cyclic features of lacustrine fine-grained sedimentary rocks, the study of sequence stratigraphy was believed necessary by integrating mineral composition, lithofacies, geochemistry, logs and paleontological data [16⇓-18]. Johnson [19] proposed the theory of sequence stratigraphy of transgressive-regressive (T-R) cycles based on the extensive T-R cycles in faulted lake basins, where a T-R cycle is defined as a sedimentary unit formed during the period from the beginning of one water deepening event to the beginning of the next water deepening event of the same level, and two systems tracts are divided in one T-R cycle, namely, lacustrine transgressive systems tract (LTST) and lacustrine regressive systems tract (LRST). LTST corresponds to the transgressive systems tract (TST) in the underlying sequence in the classic sequence stratigraphy, while LRST corresponds to the highstand systems tract (HST) in the underlying sequence plus the lowstand systems tract (LST) in the overlying sequence. Lacustrine sediments have small distribution and frequent lake level change, and the semi-deep to deep lacustrine sediments show T-R cycles owing to their sensitivity to climate and provenance. Therefore, the lacustrine fine-grained sedimentary rocks are qualified for the study of T-R sequence stratigraphy.
As a typical lacustrine laminated shale system, the 2nd member of the Paleogene Kongdian Formation (Kong 2 Member or Ek2) in the Cangdong Sag of the Huanghua Depression in the Bohai Bay Basin features wide lateral distribution, large vertical thickness, and active oil and gas shows. Commercial oil flow has been achieved from a number of vertical and horizontal wells targeting the Kong 2 Member. The lithology, physical properties, oil potential, electrical property, brittleness, horizontal in-situ stress anisotropy and source rock features of the Kong 2 Member have been basically clarified through early systematic geological analysis. As high-frequency cycles have not been divided and analyzed thoroughly, the distribution of effective shale remains unclear [20⇓-22]. Moreover, organic matter plays an important role in the formation and evolution processes of fine-grained sedimentary rocks. On one hand, as a part of the rock matrix, organic matter is an important carrier for pore development and shale oil/gas occurrence. On the other hand, as a material basis for hydrocarbon generation, the content of organic matter determines the resource potential of oil and gas. Therefore, it is of great significance to divide high-frequency cycles, analyze their architectural features, and identify their impacts on organic matters, in order to guide the exploration, evaluation, and development of shale oil in the Sangdong Sag. Taking the Kong 2 Member deposited in the continental depressed lake basin in the Cangdong Sag as an example, a sequence stratigraphic framework with five-order high-frequency cycles was formed for the fine-grained sedimentary rocks in the Kong 2 Member, after identification of sequence boundaries and T-R turnround surfaces through division of sequences based on continental T-R cycles. Then, the high-frequency cycles of fine-grained sedimentary rocks were characterized by different methods and at different scales, and the influence of high-frequency cycles on organic matter were identified. The study results are expected to provide a basis for optimizing deployment of horizontal wells and designing the fracturing stages/ clusters in the Cangdong Sag and its adjacent areas.
1. Geologic setting
The Cangdong Sag is located in the south of the Huanghua Depression, the Bohai Bay Basin. It is separated by the Xuhei Fault from the Xuhe Bulge to the east, and by the Cangdong Fault from the Cangxian Uplift to the west, and neighbors the Dongguang Bulge and Kongdian Bulge to the south and north, respectively. The sag is elongated extending from southwest to northeast. Within the basin, there are two positive structural belts (Kongdian structural belt, and Shenusi faulted-nose belt) and three slope belts (and Kongxi slope, Kongdong slope, and Nanpi slope) [20].
During the deposition of the Kong 2 Member, the Cangdong Sag was a depressed, closed lake basin under a humid to semi-arid climate. With large size, deep water and weak faulting activity in surrounding areas, the sag was dominated by braided river delta facies, fan delta facies, shore shallow lake subfacies, and semi-deep lake to deep lake subfacies (Fig. 1 ). The Kong 2 Member is universally composed of gray-black fine-grained sedimentary rocks, which are deposits of inland lacustrine facies. It is divided into four submembers, Ek24, Ek23, Ek22, and Ek21, from bottom to top. The lower part of Ek24, also known as the lower sandstone interval, mainly consists of deposits of delta front and prodelta subfacies, with the lithology dominated by silty fine sandstones and upward-fining to silty mudstone and muddy siltstone. Ek23-Ek22, also known as the upper sandstone interval, mainly develop deposits of semi-deep lake to deep lake subfacies, with the lithology dominated by mudstone, shale, and dolomitic mudstone, and a set of light gray fine siltstone at the top of Ek22 formed due to the development of underwater fans. Ek21 varies gradually from fine-grained sediments of semi-deep lake to deep lake subfacies in the lower part to silty mudstone of shore shallow lake subfacies, with thin layers of sandstones.
Fig. 1. (a) Sedimentary facies and (b) composite stratigraphic column of Kong 2 Member, Cangdong Sag. |
In this study, the Ek23, Ek22, and lower Ek21 were typically investigated to understand the high-frequency cycles of fine-grained sedimentary rocks in the Kong 2 Member in the Cangdong Sag. A 500 m continuous core was taken from the Kong 2 Member in Well G108-8 for detailed description, and more than a thousand of samples were tested and analyzed for lithology, source rock features, physical properties, oil potential, and other properties (Fig. 2 ).
Fig. 2. Composite column of logging and experimental data of Kong 2 Member in Well G108-8, Cangdong Sag. |
2. Division and architectural features of high- frequency T-R cycles
2.1. Division of high-frequency cycles of fine-grained sediments of Kong 2 Member
The Kong 2 Member is a third-order sequence in the Kongdian Formation, the Cangdong Sag, and its top and bottom boundaries are in unconformable contact at the basin margin. The Kong 2 Member is subdivided into four fourth-order sequences, i.e., SQ1, SQ2, SQ3 and SQ4, from bottom to top, which correspond to four submembers [23]. Conventional tri-porosity logging data (Δt, ρ and ϕN) can be used for lithology identification of fine-grained sedimentary rocks. Specifically, from shale to carbonate rock, the interval transit time Δt decreases, the density ρ increases, and the neutron porosity ϕN decreases (Fig. 3a ). Accordingly, a vivid logging vectorgraph in red pattern is proposed. When the content of felsic and clay minerals is high, the area between the logging curves used for lithology determination is filled with red color, and a larger red area indicates a higher content of felsic and clay minerals. When the content of carbonate minerals is high, the area between the logging curves used for lithology determination is not filled with any color; and a larger colorless area indicates a higher content of carbonate minerals [24] (Fig. 3b ). Guided by the theory of sequence stratigraphy based on T-R cycles, high-precision stratigraphic framework was established and high-frequency cycles were divided by using the logging vectorgraph in red pattern. The fifth-order sequence boundary and T-R turnround surface are the transition interfaces between the "red" and "colorless" logging curves. The difference is that the sequence boundary is the transition interface from "colorless" to "red" from bottom to top, and the T-R turnround surface is a transition interface from "red" to "colorless" from bottom to top (Fig. 3 ). On this basis, combining petromineralogical, elemental geochemical and organic geochemical data, high-frequency cycles were divided for the organic-rich fine-grained sedimentary rocks in Ek23-Ek21 in Well G108-8, where the cumulative thickness of shales is 363.49 m.
Fig. 3. Logging vectorgraph in red pattern (modified from Ref. [24]). |
The fine-grained sedimentary rocks of the Kong 2 Member in the Cangdong Sag are subdivided into 11 fifth-order sequences, including SQ①, SQ②, SQ③, SQ④ and SQ⑤ in the fourth-order sequence SQ2 (i.e. Ek23) (Fig. 4 ), SQ⑥, SQ⑦, SQ⑧ and SQ⑨ in the fourth-order sequence SQ3 (i.e. Ek22), and SQ⑩ and SQ⑪ in the lower part of the fourth-order sequence SQ4 (i.e. Ek21). SQ⑨ is the upper sandstone interval. Each fifth-order sequence can be recognized with a T cycle and an R cycle. Due to the strong heterogeneity of fine-grained sedimentary rocks, there may still be lower-order variations within each T or R cycle. Nonetheless, according to the matching between research scale and data density, the cycles in the fifth-order sequence were characterized in this study.
Fig. 4. Architectural features of fifth-order cycles in fine-grained sedimentary rocks of Kong 2 Member in Well G108-8. |
2.2. Vertical evolution of sequences
The fifth-order sequences of the organic-rich fine- grained sedimentary rocks of the Kong 2 Member exhibit an evolutionary law in the vertical direction. This law controls the characteristics of fine-grained sedimentary rocks of submembers and also contributes to the exploration and prediction of shale oil. According to the variation of mineral composition and elemental geochemical characteristics, SQ①-SQ③ were deposited in the rapid transgression stage with expanding lake basin, humid climate and rapidly rising lake level, resulting in an increased content of felsic minerals. SQ④-SQ⑧ were deposited in a fluctuating transgression stage with humid-wet alternated climate; as sensitive to climate change, the basin deposited alternating felsic minerals-rich interval and carbonate minerals-rich interval with varying thicknesses. SQ⑨-SQ⑪ were deposited in a highstand stage with humid climate and predominant felsic minerals. In terms of organic geochemical characteristics and lamina development characteristics, laminae were relatively developed in the rapid transgression stage and the highstand stage, and the highest TOC value occurred in the highstand stage and at the end of the rapid transgression stage (SQ③).
2.3. Architectural features of high-frequency T-R cycles
2.3.1. Minerals
The mineral composition of fine-grained sedimentary rocks of the Kong 2 Member in the Cangdong Sag is complex, and it cannot be effectively identified by conventional core and thin-section observations. For identifying the variation of mineral content, whole-rock X-ray diffraction (XRD) analysis was conducted on densely sampled cores from the organic-rich fine-grained sedimentary rocks of the Kong 2 Member in Well G108-8. The analysis results show that the Kong 2 Member mainly contain felsic minerals, carbonate minerals and clay minerals, in addition to small amounts of analcite, pyrite and siderite (Fig. 5 ). The felsic minerals mainly include fine silt-sized and mud-sized quartz and feldspar, with an average content of 44.1% approximately. The carbonate minerals are mainly dolomite and calcite, with an average content of 34.1% approximately. The dolomite has a higher abundance and a wider distribution than the calcite, with an average content of 26.1%. The clay minerals mainly include illite, illite/smectite (I/S), and a small amount of chlorite, with an average content as low as 14.2%. Among the special minerals, analcite has a relatively high content (up to 6.8% on average). The felsic minerals and clay minerals in the study area are mainly originated from terrigenous clasts, collectively referred to as terrigenous clastic minerals, while the carbonate minerals are mainly intrabasinal minerals. The fine-grained sedimentary rocks of the Kong 2 Member have apparent features of mixed sediments, without obvious dominant minerals, and show a waxing-and-waning trend between the content of terrigenous clastic minerals composed of felsic minerals and clay minerals the content of carbonate minerals, reflecting the changes in the intensity of source supply.
Fig. 5. Main mineral characteristics of fine-grained sedimentary rocks of Kong 2 Member, Cangdong Sag. (a) Well G108-8, 2982.78 m, felsic minerals in slightly layered distribution, with graded bedding, under polarized light; (b) Well G108-8, 2945.15 m, felsic minerals in dispersed distribution, under polarized light; (c) Well G108-8, 3202.62 m, laminated calcite, under perpendicular polarized light; (d) Well G108-8, 2925.99 m, calcite in dispersed distribution, under polarized light; (e) Well G108-8, 2985.97 m, analcite laminae interbedded with carbonate mineral laminae, under perpendicular polarized light; (f) Well G108-8, 3054.55 m, quartz grains in sporadic distribution, under polarized light; (g) Well G108-8, 2978.73 m, felsic laminae, under polarized light; (h) Well G108-8, 3069.62 m, massive micritic dolomite, under polarized light; (i) Well G108-8, 3321.11 m, calcite filling fractures, under perpendicular polarized light; (j) Well G108-8, 3016.09 m, radial pyrite, under polarized light. |
Comparison reveals that the contents of terrigenous clastic minerals and carbonate minerals in the fine-grained sedimentary rocks change very frequently in vertical direction, making their regularity difficult to identify. However, statistics show that the content of terrigenous clastic minerals in the T cycle ranges from 43.37% to 67.17%, with an average of 50.57%, while that in the R cycle ranges from 37.67% to 54.39%, with an average of 46.22%. Moreover, the content of carbonate minerals in the T cycle ranges from 26.07% to 37.91%, with an average of 33.37%, while that in the R cycle ranges from 33.35% to 44.83%, with an average of 37.51%. SQ⑨ is mainly composed of silty-fine sandstones, and is not included in the statistics. The content of terrigenous clastic minerals is relatively dominant in the T cycle, while the content of carbonate minerals is relatively dominant in the R cycle (Fig. 4 ).
2.3.2. Element geochemistry
The formation of fine-grained sedimentary rocks is a comprehensive process involving the supply, transportation and deposition of sediments, which is accompanied by migration and accumulation of elements. Based on the variation in element content or ratio, the sedimentary conditions, such as paleo-salinity, paleo-water-depth, paleo-redox, paleo-climate, and terrestrial input, can be reconstructed [25-26]. In order to determine the sedimentary environment during the formation of fine-grained sedimentary rocks of the Kong 2 Member in the Cangdong Sag, major and trace elements were analyzed for the samples of the Kong 2 Member. The sedimentary environment of the Kong 2 Member in Well G108-8 reconstructed from the element geochemical data shows the characteristics of brackish water to salt water, semi-deep water, anoxia, and arid-humid climate. The paleo-water depth and redox properties are relatively stable, with a value of δU in the range of 0.87-1.33 (avg. 1.08). However, the paleo-salinity (expressed by Sr/Ba) and paleo-climate (expressed by Rb/Sr) vary frequently in the vertical direction. The Sr/Ba value of the fifth-order T cycles ranges between 0.79 and 1.11, with an average of 0.95, and that of the fifth-order R cycles ranges between 0.95 and 1.37, with an average of 1.10. The Rb/Sr value of the fifth-order T cycles is 0.09-0.89, with an average of 0.24, and that of the fifth-order R cycles is 0.10-0.15, with an average of 0.12. For all sequences, except SQ⑩, T cycles show a lower paleo-salinity and a more humid paleo-climate than R cycles (Fig. 4 ).
The content of terrestrial clastic minerals is negatively correlated with Sr/Ba, and positively correlated with Rb/Sr, while the content of carbonate minerals is the opposite, indicating that the sedimentary environment has a remarkable impact on the characteristics of sediments (Fig. 6 ). In addition, eight core samples of fine-grained sedimentary rocks from the Kong 2 Member in Well G108-8 were analyzed by three-dimensional X radial fluorescence (2D-XRF). The results show that the distribution and composition of elements in fine-grained sedimentary rocks are characterized by alternating or mixing of continental clastic minerals and carbonate minerals. The elements representing continental clastic minerals, such as K, Al, and Si, have relatively high contents, and are alternated with those representing intrabasinal autogenic carbonate minerals, such as Ca, Fe, and Mg, in the fifth-order T cycle with well-developed laminated structure (Fig. 7 ). The elements representing carbonate minerals, such as Ca, Fe, and Mg, have relatively high contents, and are mixed with elements representing terrestrial clastic minerals, such as K, Al, and Si, in the fifth-order R cycles, with poor laminated structure (Fig. 8 ). It was concluded that the T cycles were mainly formed in the sedimentary environment with relatively low salinity, relatively deep water, and relatively humid climate, when terrestrial input was sufficient with a high content of felsic mineralss. However, the R cycles were formed in the sedimentary environment with a relatively high salinity, a relatively water depth, and a relatively arid climate, when terrestrial input was insufficient with a high content of carbonate minerals. During the deposition of the Kong 2 Member in the Cangdong Sag, as a closed lake basin with weak tectonic activity, the sedimentary environment was sensitive to the changes in climate and other environmental factors due to the small scale of lake basin. These changes were recorded in the fine-grained sedimentary rocks by high content of terrestrial clastic minerals and clay minerals during humid climate, due to the sufficient material supply with the increased amount of precipitation and surface runoff, and high content of carbonate minerals during arid climate, due to greater water evaporation than replenishment and the increased paleo-salinity.
Fig. 6. Correlation between indicators of sedimentary environment and contents of minerals, Cangdong Sag. |
Fig. 7. Fluorescence signal intensity distribution of elements in typical T cycles (Well G108-8, 3115.90 m, felsic shale). (a) Photo of core; (b) Intensity distribution of Ca; (c) Intensity distribution of Fe; (d) Intensity distribution of Al; (e) Intensity distribution of K; (f) Intensity distribution of Si. |
Fig. 8. Fluorescence signal intensity distribution of elements in typical R cycles (Well G108-8, 2983.89 m, limy-dolomitic shale). (a) Photo of core; (b) Intensity distribution of Ca; (c) Intensity distribution of Fe; (d) Intensity distribution of Al; (e) Intensity distribution of K; (f) Intensity distribution of Si. |
2.3.3. Laminae
Laminae are widely developed in the fine-grained sedimentary rocks of the Kong 2 Member in the Cangdong Sag. Based on their compositions, laminae are divided into dolomite lamina, felsic lamina, clay lamina, organic lamina, or mixture of two of the aforesaid. Based on structures, they are divided into continuous lamina, lenticular lamina and wavy lamina. Different types of laminae are frequently superimposed vertically, forming the most common fine-grained sedimentary rocks in the Kongdong sag. After cutting and polishing the 500 m core taken from Well G108-8, the polished surface was scanned with ordinary microscope and fluorescence microscope, and the microscopic photos were utilized for quantitative analysis of laminae. The laminae with a thickness of single layer larger than 0.1 m were classified as massive structure, those with a thickness of single layer smaller than 0.1 m were classified as layered structure, and those with soft-sediment deformational structures were classified as bioturbate structure. To facilitate comparison, based on the statistical data of the laminae in 336 m core of the organic-rich fine-grained sedimentary rocks in Well G108-8, the quantitative characterization parameters for centimeter-size laminae such as density and thickness ratio were proposed. Within the same fifth-order sequence, the number, thickness ratio and density of laminae in the T cycle are all significantly greater than those in the R cycle, indicating that the fine-grained sedimentary rocks with a higher content of felsic minerals have better developed laminae. The T cycle was formed under humid climatic conditions where the water depth increased and the water was stratified. Laminae are more easily formed under quiet conditions at the bottom of the water (Fig. 4 ).
3. Influence of high-frequency cycles on organic matters
3.1. High-frequency T cycle is conducive to the flourishment of primary productivity and the occurrence of organic matters
The lake basin was in an anaerobic environment during the deposition of the Kong 2 Member, providing favorable conditions to the preservation of organic matters. The TOC values of all fifth-order cycles in the fine grained sedimentary rocks are distributed in the range of 2.28%-6.00%, with an average of 3.67%. In the rapid transgression stage which SQ①-SQ③ correspond to, the TOC values increased significantly from bottom to top, and were higher in the T cycle than in the R cycle, indicating that rapid transgression was accompanied by rapid input of source materials under humid climate, which brought in the nutrients that motivate the massive reproduction of algae and improve the primary productivity of the lake. The rapid rising lake level created an anaerobic-reducing environment conducive to the preservation of organic matters. In the fluctuating transgression stage which SQ④-SQ⑧ correspond to, the TOC values fluctuated with climate, but were relatively stable as a whole. They were higher in the T cycle than in the R cycle, indicating a significant influence of climate change. In the highstand stage which SQ⑨-SQ⑪ correspond to, the TOC values were relatively stable with little variation. However, compared to the fluctuating transgression stage, the highstand stage exhibited higher TOC values in both the T and R cycles, indicating an increased source input due to the humid paleo-climate (Fig. 4 ). It can be concluded that the fine-grained sedimentary rocks of the Kong 2 Member have undergone three evolution stages from bottom to top, namely, rapid transgression, fluctuating transgression, and highstand, which macroscopically controlled the vertical development of organic matters.
Based on the organic geochemical data from Well G108-8, among the fifth-order cycles in the fine-grained sedimentary rocks, the T cycles have higher values of TOC and S1 than the R cycles, indicating that the T cycle has better shale oil exploration potential. Combined with the analysis results of laminae of different sizes, the T cycles commonly develop laminar structures and these laminae with different compositions are superposed vertically as observed in cores, and these laminae appear dense and flat as observed in thin sections, while the R cycles develop massive structures as observed in cores, and cryptic laminar structures with lower density and larger thickness of single layer as observed in thin sections. In conclusion, the T cycles were formed in deep and still water, which led to laminar structures formed at the lake bottom. Deep water depth is conducive to water stratification, and the anoxic-reducing environment formed at the bottom of the water is conducive to the preservation of organic matter.
The analysis data of mineral petrology, elemental geochemistry, and organic geochemistry suggest that the T cycles are characterized by a high content of terrigenous clastic minerals, a low paleo-salinity, a slightly humid paleo-climate, high values of TOC and S1, and a high laminar density, while the R cycles are characterized by a high content of carbonate minerals, a high paleo-salinity, a relatively dry paleo-climate, lower values of TOC and S1 than the T cycles, and a low laminar density (Fig. 4 ).
3.2. High-frequency cycles effectively control the types of organic matters
According to the relevant theories of coal petrology and organic macerals of source rocks, with reference to the classification of vitrinite, inertinite, liptinite, and huminite proposed by the International Committee for Coal and Organic Petrology (ICCP) (referred to as the "ICCP System 1994") and the recommended classification of organic macerals under the Maceral Identification and Statistical Methods on Polished Surfaces of Whole Rocks (SY/T 6414-2014) [27⇓⇓⇓-31], the organic macerals of source rocks of the Kong 2 Member in Well G108-8 in the Cangdong Sag are divided into four categories: vitrinite group, inertinite group, exinite group, and sapropelinite group. The sapropelinite group mainly includes amorphous sapropelite, alginite, and bituminite. The amorphous sapropelite in the whole rock occurs in the forms of bituminite and mineral bitumen matrix. It does not have obvious morphological and structural characteristics and is generally formed by polymerization and condensation of the decomposed products of lower aquatic organisms or fungi. It was illustrated by the distribution of organic macerals of the Kong 2 Member in Well G108-8 that in addition to the formed organic matters, a large amount of small organic matters are also developed [32]. They are often accompanied with fine clay minerals, forming organic-clay complexes, and are classified as the amorphous mineral bitumen matrix due to their amorphous characteristics for quantitative analysis of organic macerals, as limited by the resolution of organic petrologic microscopic images of thin sections.
A large number of continental organic components of different sizes, including inertinites (mainly fusinite and semifusinite) (Fig. 9a-9d ), some sporinites (Fig. 9c , 9d), and vitrinites (Fig. 9e -9h ), are observed in the Kong 2 Member. These components are abundant in all samples, reflecting the strong continental input.
Fig. 9. Typical organic macerals of Kong 2 Member, Cangdong Sag. (a), (c), (e) and (g) show organic macerals reflecting the white light observed under oil immersion lenses; (b) (d), (f), and (h) show organic macerals reflecting fluorescence observed under oil immersion lenses. |
The aquatic algites mainly include telalginite and lamalginite (Fig. 9e -9h ). Telalginites generally derived from algae colony and thick-walled, single-celled algaes. They have clear group shapes, serrated edges, and honeycombed or sponge-like surfaces. Sometimes their group is composed of several hundred tubular single cells with hollows or gaps in the middle. Lamalginites are derived from small single-celled algaes, thin-walled planktonic algaes, or benthic algaes, and are formed by the aggregation of very dense linear algae residues. Their formation is often associated with seasonal algal blooms, representing a high level of productivity in the water. Algites are endogenous organic matters in the lake, representing endogenous sources in the water, while vitrinite, inertinite, and exinite are originated from the organic matter input from terrestrial higher plants.
In this study, the endogenous aquatic organic matters and terrigenous organic matters from external input were statistically analyzed respectively, measurement and statistics of the areas and sizes of organic macerals were performed using an image processing software (Fig. 10 ). The quantitative statistical results of the microscopic images of organic macerals for each cycle show that the type and content of terrestrial organic matters in the whole rocks exhibit the variation characteristics with some regularity (Fig. 11a -11d). The relative content of terrestrial organic matters generally decreases from SQ① to SQ⑤, while increases from SQ⑥ to SQ⑦ and then gradually decreases. According to the variation of the relative contents of organic macerals (Fig. 11c , 11d), regular changes were identified in the high-frequency T cycles and R cycles. The T cycles have relative higher contents of terrestrial organic matters than the R cycles, while the relative contents of endogenous organic matters are just the opposite.
Fig. 10. Schematic diagrams illustrating sizes of macerals in source rocks of Kong 2 Member in the same view field (at 3277.38 m in Well G108-8). |
Fig. 11. Characteristics of organic matters in high-frequency cycles in fine-grained sedimentary rocks of Kong 2 Member in Well G108-8. |
Vitrinite, as a representative of terrestrial organic matters, reflect the intensity of input of terrestrial organic matter. The size of vitrinite particles also reflects the strength of water transport capacity. Under humid climate, the rainfall is large, and the transport capability of surface runoff is strong. Especially the seasonal floods can easily transport large organic matter particles to the center of the lake. While in dry seasons, the surface runoff decreases significantly, and its transport capacity declines correspondingly; as a result, fewer large particles and only fine particles can be transported to the center of the lake. Statistical analysis of vitrinite particles in thin sections (Fig. 11e ) shows that the early stage is dominated by large particles, while the late stage is dominated by smaller particles, and the grain size varies by cycles. In the rapid transgression stage which SQ①-SQ③ correspond to, the humid climate and frequent seasonal floods led to the formation of relatively large vitrinite particles, and vitrinite particles are longer in the T cycle than in the R cycle. In the fluctuating transgression stage which SQ④-SQ⑧ correspond to, the cyclic wet-dry climate played a dominant role, resulting in generally smaller particles. As the transport pattern changed to have little correlation with hydrodynamic intensity conditions, the length of vitrinite particles exhibits no significant relationship with the T or R cycles. In the highstand stage which SQ⑨ and its overlying sequences correspond to, the climate changed to humid again, and the water was relatively stable, leading to still small vitrinite particles.
4. Conclusions
The fine-grained sedimentary rocks of the Kong 2 Member in the Paleogene Cangdong Sag are divided into 4 fourth-order sequences, and further into 11 fifth-order sequences. The fifth-order sequences experienced the rapid transgression in the early stage, the fluctuating transgression in the middle stage, and the highstand stage in the late stage, and each develops transgressive and regressive cycles.
The cycles are very different in mineral content, elemental geochemistry, and laminae. The T cycles record a high content of terrigenous clastic minerals, a high density of laminae, a low paleo-salinity, a relatively humid paleo-climate and high values of TOC and S1, while the R cycles display a high content of carbonate minerals, a low density of laminae, a high paleo-salinity, a relatively dry paleo-climate and low values of TOC and S1.
High-frequency cycles effectively control the abundance and types of organic matters. The Kong 2 Member exhibits generally high TOC values. Especially the rapid input of source materials in the rapid transgression stage led to the enrichment of felsic minerals, and also brought in a large amount of terrestrial organic matters. In the fluctuating transgression stage, the TOC values fluctuated with climate change but remained relatively stable as a whole. In the highstand stage, the TOC values increased with the wetting paleo-climate and strengthening terrestrial input. In addition, the nutrients brought in by terrestrial input promoted the proliferation of algae. Therefore, the T cycles exhibit relatively high TOC and abundant endogenous organic matters in water, in addition to terrigenous organic matters, implying a high primary productivity of lake for the generation and enrichment of shale oil.
Nomenclature
L1—distance from the acoustic log to the baseline, normalized interval transit time, dimensionless;
L2—distance from the density log to the baseline, normalized density, dimensionless;
Δt—interval transit time, s/m;
Δt1—minimum scale of acoustic log, μs/m;
Δt2—maximum scale of acoustic log, μs/m;
ϕN—compensated neutron porosity, %;
ϕN1—minimum scale of compensated neutron porosity log, %;
ϕN2—maximum scale of compensated neutron porosity log, %;
ρ—density, g/cm3;
ρ1—minimum scale of density log, g/cm3;
ρ2—maximum scale of density log, g/cm3.