# Study on Sedimentary Facies of the Lower Eocene Basal Deposits in the Southeastern Part of the Jiyang Depression, Bohai Bay Basin, China

Authors: Ya Yang
Journal: Mobile Information Systems
 (2022)

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## Abstract

Based on the analysis of cores, logging, and seismic data, this paper identifies the sedimentary facies of the basal deposits from the lower Eocene in the southeastern part of the Jiyang Depression and studies its depositional law. On this basis, a sedimentary model of the basal deposits is established. The basal sediments of the lower Eocene in the Jiyang Depression are mainly red, brown, gray, and variegated clastic rocks, which are mainly divided into Kongdian and lower Shahejie-4 Formation. It is considered that the climate during the depositional period was relatively arid, and the depositional process was greatly affected by floods. Three types of sedimentary facies were identified in the study area: proluvial fan, fan delta, and beach-bar deposition. Proluvial fans are mainly developed at the exit of the paleo gullies in the south area, and fan deltas are mainly distributed in the large gentle slope belt in the northwest, while beach bars are isolated and scattered along the lake shoreline in shallow lakes. The fan-delta and beach-bar clastic rocks are well sorted, with high compositional and textural maturity and can be used as good oil and gas reservoirs. The basal deposits in the southeast of Jiyang Depression are mainly flood-controlled sedimentary models. During the flood period, a large amount of detrital material brought by the flood was deposited in the gentle slope zone, forming the deposition of proluvial fan and fan delta, and the sand bodies at the front of the fan delta formed beach-bar deposits on its front under the transformation of waves and coastal currents. However, during the inter-flood period, only small-scale proluvial fan deposits are formed, fan delta and beach-bar sand bodies are rare.


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## Body

## 1. Introduction

The Jiyang Depression is an important oil and gas enrichment depression of the Bohai Bay Basin in China. Previous studies have mainly focused on the Shahejie 1–3 and Dongying Formation in the middle and shallow parts [1, 2], and as the exploration of shallow and middle areas continues to mature, people have begun to focus on its basal deposits in deep areas. At present, there are few studies on basal sediments, and their exploration potential is huge; the proven reserves are 30 × 106 t [3]. Therefore, it is a very urgent task to study the sedimentary facies characteristics and sedimentary model of basal sediments in the southeastern part of Jiyang Depression.At present, people have a certain understanding of the basal sediments in the Jiyang Depression. Xu et al. studied its sedimentary features and found that the basal sediments in the southeast of Jiyang Depression were mainly composed of clastic rocks and rock salt, and the formation of rock salt was caused by evaporation [4]. Meng et al. believed that the basal deposition is widely developed from Kongdian to the lower Shahejie-4 Formation, and the sedimentary facies in the study area mainly include braided rivers, proluvial fan, and salt lake deposits, in which medium-coarse-grained clastic rocks have good storage performance [5, 6]. Cao et al. analyzed and evaluated the source rocks of the basement strata in the Dongying Depression and think that the study area has good oil and gas accumulation conditions [7, 8].However, there are still many problems, such as unclear reservoir characteristics, types, and distribution of sedimentary facies, inability to determine favorable exploration directions, and so on. Moreover, the previous research data were obtained by conventional methods [9, 10], which were relatively single, and the accuracy of the research results was low. The data for this study combine the 5G and Internet of Things (IoT) technology in the collection and processing process [11]. The data collection and processing instruments are highly sensitive and interconnected in real time, the obtained data are highly accurate, and the obtained results are highly credible.On the basis of synthesizing the previous research results mentioned above and the current one, the sedimentary facies are identified in the study area, and its distribution law is analyzed. On this basis, the sedimentary model of basal deposition is established. The research results of this paper have laid a foundation for the subsequent exploration of oil and gas reservoirs in the Jiyang Depression and guided the next exploration direction.


## 2. Materials and Methods

The research data in this paper are the logging, core, and seismic data of 31 wells obtained based on the differences in electrical conductivity, electrochemical properties, and elastic and nuclear physical properties of different rocks in the study area.Starting with the analysis of basic geological data with the help of core analysis and combining field macroobservations and indoor microobservations, this paper studies the properties of sedimentary facies markers in basal deposits, that is, composition, color, structure, sorting, maturity of rocks, and so on. On this basis, combined with the characteristics of fossils and biological relics in the study area, the paleo-sedimentary environment was analyzed.In this paper, the method of logging data interpretation is used to analyze the sedimentary environment. Digital processing was performed on data obtained with 7 different logging methods, including natural gamma ray, spontaneous potential, induction log, microlateral resistivity log, acoustic, density, and neutron, and different log sections are divided by the comparative analysis of different curves. By analyzing the curve characteristics of each logging section, its relationship with the sedimentary facies is established, and the depositional environment is identified.In addition, seismic data interpretation is also an important research method in this paper. Seismic data contain rich underground information, including lithofacies, depositional extent, depositional laws, and others. Through the amplitude, frequency, and continuity reflection parameters of the seismic reflection, the seismic facies are divided into a section of the study area. By analyzing the basic characteristics of seismic facies, they can be converted into corresponding sedimentary facies.Combining the above materials and methods, the main idea of this paper is to make full use of core and logging data, reproduce the paleo-sedimentary environment in the study area, and obtain the type of sedimentary facies. The distribution law of sedimentary facies in the study area is analyzed by analyzing the seismic profiles of multiple wells, and the sedimentary model of this period is established.Compared with traditional research data, the acquisition and processing of seismic and logging data in this study combine the 5G and IoT technology, and with their help, the collected seismic data are transmitted back in real time; the quality of the whole process of acquisition is controlled, and the data obtained are of higher quality. At the same time, using smart sensors, machine-machine connectivity, and big data analysis to collect and process data makes the results of the study more precise, reducing multisolutions.


## 3. Results

### 3.1. Sedimentary Facies Markers

The study of sedimentary facies markers is the basis for the sedimentary facies analysis and the study of lithofacies and paleogeography [12]. Sedimentary facies markers refer to some characteristic elements of sedimentary rocks, such as lithological, paleontological, and geochemical characteristics, among which lithological characteristics such as components, color, type of rocks, and its structure are mainly studied.


#### 3.1.1. Type of Rocks

Rock types developed in the study area are mainly conglomerate, sandstone, siltstone, gypsum, anhydrite, mudstone, and others.The conglomerate is mainly distributed at the edge of the study area, mainly fine conglomerate, with poor sorting and polishing, and the thickness of a single layer is mainly 1–2 meters, up to 5 meters. The grain size of the conglomerate varies within 1–10 mm. Sandstone is the most important oil reservoir, and it is widely distributed in the study area, the grain size of which is mainly distributed in the range from 0.1 mm to 2 mm and the maximum can reach 4 mm. The content of quartz and feldspar is relatively high. The type of sandstone in the study area is mainly arkoses, followed by quartz-feldspar and feldspar-quartz greywacke. The distribution of siltstone is far from the provenance area, its particle size is mainly distributed between 0.05 and 0.1 mm with high maturity and good sorting. There are also a large number of mudstone and rock salt distributed in the center of the depression. The formation of salt rocks is related to evaporation, common salt rocks such as gypsum and anhydrite usually appear in the form of bands and nodules.It is not difficult to find that the sediments in the gentle slope zone are mainly interbedded by sandstone, siltstone, and mudstone, while in the deep part of the depression, mudstone is intercalated with gypsum and anhydrite.


#### 3.1.2. Color of Rocks

The color of mud rock is a good indication of the depositional environment. Gray and dark gray represent a semideep lake in a humid environment; purple-red, red, brown-red, and mottled colors usually reflect the arid climate on land and coastal lakes; gray-green and green most often reflect deposits of coastal and shallow lakes in relatively arid conditions. The mud rocks of the lower Shahejie-4 and Kongdian formation in the southeast part of the Jiyang Depression are usually purple-red, red, brown-red, green, and variegated colors, reflecting its arid and oxidative depositional environment.


#### 3.1.3. Grain Size of Rocks

The particle size of sediments is controlled by factors such as transportation mode, medium, and environment; therefore, the study of particle size characteristics is also an important basis for judging the depositional environment [13]. The grain size of rocks in different parts of the study area varies greatly, at the edge of the basin, the grain size of the rock is larger, mainly between 0.2 and 5 mm, in the gentle slope zone, the grain size of clastic rock is generally between 0.1 and 2 mm, while the particle size of clastic rock near the center of the depression is basically less than 0.1 mm.


#### 3.1.4. Structural Features of Rocks

The structural features of the lower Eocene basal deposits in the southeastern part of the Jiyang Depression are diverse: massive, cross, corrugated, parallel bedding, and so on. Bioturbation is also very common (Figure1).Figure 1
Structural features of clastic rock of the basal deposits in the southeastern part of the Jiyang Depression. (a) Massive bedding, (b) parallel bedding, (c) cross-bedding, and (d) boring by an organism.
(a)(b)(c)(d)Massive bedding, reflecting the effect of gravity flows, is mainly developed on the edge of the gentle slope of the depression; they are characteristic of brown-red conglomerates, purple-red, mottled gravel sandstones, and others. Cross bedding is widespread in various parts of the basin, including trough-shaped, wedge-shaped, and wavy bedding, which reflects a strong hydrodynamic condition. Parallel bedding structure is mainly developed in gray-white or variegated siltstones and silty mudstones, which indicates a relatively immobile environment in which sediments accumulated. Bioturbation and biofossils are also frequently seen in cores.


### 3.2. Sedimentary Facies Identification

Based on the above analysis of the lithological characteristics, grain size distribution, and sedimentary structure, it is believed that the sedimentary facies of the basal deposits in the southeast of the Jiyang Depression can be divided into three types: proluvial fan, fan delta, and beach-bar deposition, and each of them exhibits different sedimentary characteristics.


#### 3.2.1. Proluvial Fan

According to the distance from the provenance area, the proluvial fan can be divided into the distant, middle, and near parts [14]. There is a large difference in the grain size of rocks in different parts of the proluvial fan. The near and middle parts are mainly conglomerate and coarse-grained sandstone, while the distant part has more medium-grained sandstone and mudstone. Overall, the rock sorting is very poor, mainly massive bedding, with cross bedding visible. Both compositional and structural maturity are low.Debris flow deposits formed by massive accumulations are mainly developed at the bottom of proluvial fans and gradually change to braided river deposits upwards. The thickness of sand bodies in proluvial fan deposits generally varies from 70 to 300 meters. The size of gravel and sand grains varies from 0.1 to 2 mm with a predominance of 0.2–1 mm. Fossil or biological matter is very rare.


#### 3.2.2. Fan Delta

There are many types of sedimentary rocks in the fan delta, including variegated coarse-grained, fine-grained sandstone, interlayered sandstone containing brown-red mudstone and siltstone. The grain size of the rock is mainly 0.1–0.5 mm. The content of quartz in fan-delta sandstones is usually between 33.5% and 55.1%, the average here is 46.1%. The content of feldspar is usually between 32.9% and 40.3%, with an average of 36.2%. The content of debris in sedimentary rocks is usually between 14.3% and 20%, with an average of 17.7%, and among them, the content of metamorphic debris is much higher than that of magmatic and sedimentary rocks. Mineralogical maturity is usually 0.63–0.8, which indicates a relatively high degree of maturity of the rocks.The structural types of the fan-delta sediments are many, often with cross-bedding and parallel-bedding, reflecting the existence of fluvial action. There are various types of cross-bedding, including grooved, wedge-shaped, and wavy. The main cause of their formation is river and wave activity. In a single-depositional cycle, the sediment grain size is continuously finer from the bottom to the top (Figure2).Figure 2
Sedimentary facies profile of fan delta in the basal deposits in the southeastern part of the Jiyang Depression. (1) fine-grained conglomerate, (2) coarse-grained sandstone, (3) medium-grained sandstone, (4) fine-grained sandstone, (5) siltstone, (6) ostracod shell, (7) gastropod shell, and (8) cross-bedding.Fan delta can be mainly divided into the fan-delta front, fan-delta plain, and profan delta. Among them, the underwater channels and mouth bars of the fan-delta front are represented by fine-grained sandstones and siltstones, and they are strongly transformed by waves, showing good sorting and high maturity, which can be used as a good oil and gas reservoir. Besides, bioturbation, mollusk fossils, and traces are very common here.


#### 3.2.3. Beach Bar

Beach bars are ordinary sand bodies in the shallow part of the lake. Their formation is mainly controlled by wave activity and coastal currents. Beach bars develop parallel to the lake’s shoreline, usually in a belt shape. The material that forms the beach bars comes mainly from the nearby fan-delta sand bodies. Under the influence of waves, the material is well sorted. There is a high content of quartz and feldspars and a small amount of rock debris, and the mineral maturity of the rocks is high.The grain size of the beach bars is medium, and mainly medium-fine sandstones and siltstones vary from 0.01 mm to 0.25 mm. Affected by the transformation of waves in a single sedimentary cycle, the grain size of the rock continues to become coarser from the bottom to top. The thickness of the sand body is small, generally 2–10 meters.


### 3.3. Distribution Pattern of Sedimentary Facies

On the basis of clarifying the types of sedimentary facies, combined with the structure and paleogeographic features of the southeastern Jiyang Depression, the distribution law of the sedimentary facies of basal deposits was studied. From the Kongdian to the lower Shahejie-4 Formation, frequent floods occurred, and the supply source of the sediments was comparatively abundant.In the northern part of the study area, due to the intense activity of the Chennan fault, the depression sank on a large scale, forming a steep slope zone deposit and developing a subaqueous fan deposit, with a narrow distribution range and a large sedimentary thickness. In the southern and western regions, gentle slope belts are mainly developed, and a large number of proluvial fan and fan-delta deposits are distributed. The thickness of the sediments is relatively small, but the distribution range is wide. The sand bodies deposited in the gentle slope zone are the main oil and gas reservoirs in the study area.Three main provenance areas provide detrital material for the gentle slope belt: Wudi uplift in the northwest, Guangrao uplift in the southeast, and Luxi uplift in the southwest. The fault activity in the southern Luxi and Guangrao uplift is weak, and stable water bodies are not developed. A large area of proluvial fan deposits is developed at the exit of the paleo gullies in the south. Along the inner direction of the basin, the terrain is flat and wide, and a small area of fan-delta deposits developed in front of the proluvial fan in the southeast. The gentle slope belt in the northwest is widely distributed, when the flooding watercourse enters the depression, it suddenly widens, the slope suddenly becomes gentle, and the watercourse spreads forward in a large area, forming a tongue-shaped fan-delta sedimentary body on the plane. The sand bodies near the fan delta are transported by waves and coastal currents to form isolated and dispersed beach-bar deposits along the lake shoreline (Figure3).Figure 3
Distribution pattern of sedimentary facies of the basal deposits in the southeastern part of the Jiyang Depression. (1) Proluvial fan, (2) fan delta, (3) beach bar, (4) proluvial plain, (5) shallow lake, (6) semideep lake, (7) sub-aqueous fan, and (8) location of well.During the period of Kongdian and lower Shahejie-4 Formation, it was the initial stage of basin subsidence. Therefore, shallow lakes are mainly developed in the interior of the basin far from the provenance area, which were transformed into deep lakes during the flood period.


### 3.4. Sedimentary Model of the Basal Deposition

The sedimentary process of the basal deposition in the southeastern Jiyang Depression is relatively complex and is greatly affected by floods.During the flood period, the lake level rose rapidly, and the flood brought a large amount of detrital material, which accumulated on the edge of the gentle slope belt, forming a proluvial fan with a small distribution range, and in the interior of the gentle slope belt, a large area of fan delta formed. During the flood period, the sand bodies in the front of the fan delta were transformed by the waves and coastal current, and a large number of isolated beach-bar sand bodies were formed on the front side of the fan delta. At this time, mudstone and gypsum-bearing mudstone are mainly developed in the center of the basin (Figure4).Figure 4
Sedimentary model of the basal deposits in the southeastern part of the Jiyang Depression. (1) Proluvial fan, (2) proluvial plain, (3) fan-delta, (4) beach-bar, (5) sub-aqueous fan, (6) littoral lake, (7) shallow lake, (8) semideep lake, (9) sandstone, (10) mudstone, (11) conglomerate, and (12) gypsum mudstone.During the intermittent period of floods, the provenance supply was less, and proluvial fans were developed, but fan deltas and beach-bar sand bodies were less developed. At the same time, due to the strong evaporation during the flood intermittent period, gypsum and gypsum mudstone were deposited in the center of the depression.Vertically, the interbedded clastic rock and mudstone are mainly deposited at the edge of the gentle slope belt. The clastic rock is mainly formed during the flood period. It gradually becomes finer in grain size upward and eventually transforms into the mudstone formed during the interflood period. In the central area of the basin, mudstone, gypsum-bearing mudstone, anhydrite, and gypsum are interbedded.


## 3.1. Sedimentary Facies Markers

The study of sedimentary facies markers is the basis for the sedimentary facies analysis and the study of lithofacies and paleogeography [12]. Sedimentary facies markers refer to some characteristic elements of sedimentary rocks, such as lithological, paleontological, and geochemical characteristics, among which lithological characteristics such as components, color, type of rocks, and its structure are mainly studied.


### 3.1.1. Type of Rocks

Rock types developed in the study area are mainly conglomerate, sandstone, siltstone, gypsum, anhydrite, mudstone, and others.The conglomerate is mainly distributed at the edge of the study area, mainly fine conglomerate, with poor sorting and polishing, and the thickness of a single layer is mainly 1–2 meters, up to 5 meters. The grain size of the conglomerate varies within 1–10 mm. Sandstone is the most important oil reservoir, and it is widely distributed in the study area, the grain size of which is mainly distributed in the range from 0.1 mm to 2 mm and the maximum can reach 4 mm. The content of quartz and feldspar is relatively high. The type of sandstone in the study area is mainly arkoses, followed by quartz-feldspar and feldspar-quartz greywacke. The distribution of siltstone is far from the provenance area, its particle size is mainly distributed between 0.05 and 0.1 mm with high maturity and good sorting. There are also a large number of mudstone and rock salt distributed in the center of the depression. The formation of salt rocks is related to evaporation, common salt rocks such as gypsum and anhydrite usually appear in the form of bands and nodules.It is not difficult to find that the sediments in the gentle slope zone are mainly interbedded by sandstone, siltstone, and mudstone, while in the deep part of the depression, mudstone is intercalated with gypsum and anhydrite.


### 3.1.2. Color of Rocks

The color of mud rock is a good indication of the depositional environment. Gray and dark gray represent a semideep lake in a humid environment; purple-red, red, brown-red, and mottled colors usually reflect the arid climate on land and coastal lakes; gray-green and green most often reflect deposits of coastal and shallow lakes in relatively arid conditions. The mud rocks of the lower Shahejie-4 and Kongdian formation in the southeast part of the Jiyang Depression are usually purple-red, red, brown-red, green, and variegated colors, reflecting its arid and oxidative depositional environment.


### 3.1.3. Grain Size of Rocks

The particle size of sediments is controlled by factors such as transportation mode, medium, and environment; therefore, the study of particle size characteristics is also an important basis for judging the depositional environment [13]. The grain size of rocks in different parts of the study area varies greatly, at the edge of the basin, the grain size of the rock is larger, mainly between 0.2 and 5 mm, in the gentle slope zone, the grain size of clastic rock is generally between 0.1 and 2 mm, while the particle size of clastic rock near the center of the depression is basically less than 0.1 mm.


### 3.1.4. Structural Features of Rocks

The structural features of the lower Eocene basal deposits in the southeastern part of the Jiyang Depression are diverse: massive, cross, corrugated, parallel bedding, and so on. Bioturbation is also very common (Figure1).Figure 1
Structural features of clastic rock of the basal deposits in the southeastern part of the Jiyang Depression. (a) Massive bedding, (b) parallel bedding, (c) cross-bedding, and (d) boring by an organism.
(a)(b)(c)(d)Massive bedding, reflecting the effect of gravity flows, is mainly developed on the edge of the gentle slope of the depression; they are characteristic of brown-red conglomerates, purple-red, mottled gravel sandstones, and others. Cross bedding is widespread in various parts of the basin, including trough-shaped, wedge-shaped, and wavy bedding, which reflects a strong hydrodynamic condition. Parallel bedding structure is mainly developed in gray-white or variegated siltstones and silty mudstones, which indicates a relatively immobile environment in which sediments accumulated. Bioturbation and biofossils are also frequently seen in cores.


## 3.1.1. Type of Rocks

Rock types developed in the study area are mainly conglomerate, sandstone, siltstone, gypsum, anhydrite, mudstone, and others.The conglomerate is mainly distributed at the edge of the study area, mainly fine conglomerate, with poor sorting and polishing, and the thickness of a single layer is mainly 1–2 meters, up to 5 meters. The grain size of the conglomerate varies within 1–10 mm. Sandstone is the most important oil reservoir, and it is widely distributed in the study area, the grain size of which is mainly distributed in the range from 0.1 mm to 2 mm and the maximum can reach 4 mm. The content of quartz and feldspar is relatively high. The type of sandstone in the study area is mainly arkoses, followed by quartz-feldspar and feldspar-quartz greywacke. The distribution of siltstone is far from the provenance area, its particle size is mainly distributed between 0.05 and 0.1 mm with high maturity and good sorting. There are also a large number of mudstone and rock salt distributed in the center of the depression. The formation of salt rocks is related to evaporation, common salt rocks such as gypsum and anhydrite usually appear in the form of bands and nodules.It is not difficult to find that the sediments in the gentle slope zone are mainly interbedded by sandstone, siltstone, and mudstone, while in the deep part of the depression, mudstone is intercalated with gypsum and anhydrite.


## 3.1.2. Color of Rocks

The color of mud rock is a good indication of the depositional environment. Gray and dark gray represent a semideep lake in a humid environment; purple-red, red, brown-red, and mottled colors usually reflect the arid climate on land and coastal lakes; gray-green and green most often reflect deposits of coastal and shallow lakes in relatively arid conditions. The mud rocks of the lower Shahejie-4 and Kongdian formation in the southeast part of the Jiyang Depression are usually purple-red, red, brown-red, green, and variegated colors, reflecting its arid and oxidative depositional environment.


## 3.1.3. Grain Size of Rocks

The particle size of sediments is controlled by factors such as transportation mode, medium, and environment; therefore, the study of particle size characteristics is also an important basis for judging the depositional environment [13]. The grain size of rocks in different parts of the study area varies greatly, at the edge of the basin, the grain size of the rock is larger, mainly between 0.2 and 5 mm, in the gentle slope zone, the grain size of clastic rock is generally between 0.1 and 2 mm, while the particle size of clastic rock near the center of the depression is basically less than 0.1 mm.


## 3.1.4. Structural Features of Rocks

The structural features of the lower Eocene basal deposits in the southeastern part of the Jiyang Depression are diverse: massive, cross, corrugated, parallel bedding, and so on. Bioturbation is also very common (Figure1).Figure 1
Structural features of clastic rock of the basal deposits in the southeastern part of the Jiyang Depression. (a) Massive bedding, (b) parallel bedding, (c) cross-bedding, and (d) boring by an organism.
(a)(b)(c)(d)Massive bedding, reflecting the effect of gravity flows, is mainly developed on the edge of the gentle slope of the depression; they are characteristic of brown-red conglomerates, purple-red, mottled gravel sandstones, and others. Cross bedding is widespread in various parts of the basin, including trough-shaped, wedge-shaped, and wavy bedding, which reflects a strong hydrodynamic condition. Parallel bedding structure is mainly developed in gray-white or variegated siltstones and silty mudstones, which indicates a relatively immobile environment in which sediments accumulated. Bioturbation and biofossils are also frequently seen in cores.


## 3.2. Sedimentary Facies Identification

Based on the above analysis of the lithological characteristics, grain size distribution, and sedimentary structure, it is believed that the sedimentary facies of the basal deposits in the southeast of the Jiyang Depression can be divided into three types: proluvial fan, fan delta, and beach-bar deposition, and each of them exhibits different sedimentary characteristics.


### 3.2.1. Proluvial Fan

According to the distance from the provenance area, the proluvial fan can be divided into the distant, middle, and near parts [14]. There is a large difference in the grain size of rocks in different parts of the proluvial fan. The near and middle parts are mainly conglomerate and coarse-grained sandstone, while the distant part has more medium-grained sandstone and mudstone. Overall, the rock sorting is very poor, mainly massive bedding, with cross bedding visible. Both compositional and structural maturity are low.Debris flow deposits formed by massive accumulations are mainly developed at the bottom of proluvial fans and gradually change to braided river deposits upwards. The thickness of sand bodies in proluvial fan deposits generally varies from 70 to 300 meters. The size of gravel and sand grains varies from 0.1 to 2 mm with a predominance of 0.2–1 mm. Fossil or biological matter is very rare.


### 3.2.2. Fan Delta

There are many types of sedimentary rocks in the fan delta, including variegated coarse-grained, fine-grained sandstone, interlayered sandstone containing brown-red mudstone and siltstone. The grain size of the rock is mainly 0.1–0.5 mm. The content of quartz in fan-delta sandstones is usually between 33.5% and 55.1%, the average here is 46.1%. The content of feldspar is usually between 32.9% and 40.3%, with an average of 36.2%. The content of debris in sedimentary rocks is usually between 14.3% and 20%, with an average of 17.7%, and among them, the content of metamorphic debris is much higher than that of magmatic and sedimentary rocks. Mineralogical maturity is usually 0.63–0.8, which indicates a relatively high degree of maturity of the rocks.The structural types of the fan-delta sediments are many, often with cross-bedding and parallel-bedding, reflecting the existence of fluvial action. There are various types of cross-bedding, including grooved, wedge-shaped, and wavy. The main cause of their formation is river and wave activity. In a single-depositional cycle, the sediment grain size is continuously finer from the bottom to the top (Figure2).Figure 2
Sedimentary facies profile of fan delta in the basal deposits in the southeastern part of the Jiyang Depression. (1) fine-grained conglomerate, (2) coarse-grained sandstone, (3) medium-grained sandstone, (4) fine-grained sandstone, (5) siltstone, (6) ostracod shell, (7) gastropod shell, and (8) cross-bedding.Fan delta can be mainly divided into the fan-delta front, fan-delta plain, and profan delta. Among them, the underwater channels and mouth bars of the fan-delta front are represented by fine-grained sandstones and siltstones, and they are strongly transformed by waves, showing good sorting and high maturity, which can be used as a good oil and gas reservoir. Besides, bioturbation, mollusk fossils, and traces are very common here.


### 3.2.3. Beach Bar

Beach bars are ordinary sand bodies in the shallow part of the lake. Their formation is mainly controlled by wave activity and coastal currents. Beach bars develop parallel to the lake’s shoreline, usually in a belt shape. The material that forms the beach bars comes mainly from the nearby fan-delta sand bodies. Under the influence of waves, the material is well sorted. There is a high content of quartz and feldspars and a small amount of rock debris, and the mineral maturity of the rocks is high.The grain size of the beach bars is medium, and mainly medium-fine sandstones and siltstones vary from 0.01 mm to 0.25 mm. Affected by the transformation of waves in a single sedimentary cycle, the grain size of the rock continues to become coarser from the bottom to top. The thickness of the sand body is small, generally 2–10 meters.


## 3.2.1. Proluvial Fan

According to the distance from the provenance area, the proluvial fan can be divided into the distant, middle, and near parts [14]. There is a large difference in the grain size of rocks in different parts of the proluvial fan. The near and middle parts are mainly conglomerate and coarse-grained sandstone, while the distant part has more medium-grained sandstone and mudstone. Overall, the rock sorting is very poor, mainly massive bedding, with cross bedding visible. Both compositional and structural maturity are low.Debris flow deposits formed by massive accumulations are mainly developed at the bottom of proluvial fans and gradually change to braided river deposits upwards. The thickness of sand bodies in proluvial fan deposits generally varies from 70 to 300 meters. The size of gravel and sand grains varies from 0.1 to 2 mm with a predominance of 0.2–1 mm. Fossil or biological matter is very rare.


## 3.2.2. Fan Delta

There are many types of sedimentary rocks in the fan delta, including variegated coarse-grained, fine-grained sandstone, interlayered sandstone containing brown-red mudstone and siltstone. The grain size of the rock is mainly 0.1–0.5 mm. The content of quartz in fan-delta sandstones is usually between 33.5% and 55.1%, the average here is 46.1%. The content of feldspar is usually between 32.9% and 40.3%, with an average of 36.2%. The content of debris in sedimentary rocks is usually between 14.3% and 20%, with an average of 17.7%, and among them, the content of metamorphic debris is much higher than that of magmatic and sedimentary rocks. Mineralogical maturity is usually 0.63–0.8, which indicates a relatively high degree of maturity of the rocks.The structural types of the fan-delta sediments are many, often with cross-bedding and parallel-bedding, reflecting the existence of fluvial action. There are various types of cross-bedding, including grooved, wedge-shaped, and wavy. The main cause of their formation is river and wave activity. In a single-depositional cycle, the sediment grain size is continuously finer from the bottom to the top (Figure2).Figure 2
Sedimentary facies profile of fan delta in the basal deposits in the southeastern part of the Jiyang Depression. (1) fine-grained conglomerate, (2) coarse-grained sandstone, (3) medium-grained sandstone, (4) fine-grained sandstone, (5) siltstone, (6) ostracod shell, (7) gastropod shell, and (8) cross-bedding.Fan delta can be mainly divided into the fan-delta front, fan-delta plain, and profan delta. Among them, the underwater channels and mouth bars of the fan-delta front are represented by fine-grained sandstones and siltstones, and they are strongly transformed by waves, showing good sorting and high maturity, which can be used as a good oil and gas reservoir. Besides, bioturbation, mollusk fossils, and traces are very common here.


## 3.2.3. Beach Bar

Beach bars are ordinary sand bodies in the shallow part of the lake. Their formation is mainly controlled by wave activity and coastal currents. Beach bars develop parallel to the lake’s shoreline, usually in a belt shape. The material that forms the beach bars comes mainly from the nearby fan-delta sand bodies. Under the influence of waves, the material is well sorted. There is a high content of quartz and feldspars and a small amount of rock debris, and the mineral maturity of the rocks is high.The grain size of the beach bars is medium, and mainly medium-fine sandstones and siltstones vary from 0.01 mm to 0.25 mm. Affected by the transformation of waves in a single sedimentary cycle, the grain size of the rock continues to become coarser from the bottom to top. The thickness of the sand body is small, generally 2–10 meters.


## 3.3. Distribution Pattern of Sedimentary Facies

On the basis of clarifying the types of sedimentary facies, combined with the structure and paleogeographic features of the southeastern Jiyang Depression, the distribution law of the sedimentary facies of basal deposits was studied. From the Kongdian to the lower Shahejie-4 Formation, frequent floods occurred, and the supply source of the sediments was comparatively abundant.In the northern part of the study area, due to the intense activity of the Chennan fault, the depression sank on a large scale, forming a steep slope zone deposit and developing a subaqueous fan deposit, with a narrow distribution range and a large sedimentary thickness. In the southern and western regions, gentle slope belts are mainly developed, and a large number of proluvial fan and fan-delta deposits are distributed. The thickness of the sediments is relatively small, but the distribution range is wide. The sand bodies deposited in the gentle slope zone are the main oil and gas reservoirs in the study area.Three main provenance areas provide detrital material for the gentle slope belt: Wudi uplift in the northwest, Guangrao uplift in the southeast, and Luxi uplift in the southwest. The fault activity in the southern Luxi and Guangrao uplift is weak, and stable water bodies are not developed. A large area of proluvial fan deposits is developed at the exit of the paleo gullies in the south. Along the inner direction of the basin, the terrain is flat and wide, and a small area of fan-delta deposits developed in front of the proluvial fan in the southeast. The gentle slope belt in the northwest is widely distributed, when the flooding watercourse enters the depression, it suddenly widens, the slope suddenly becomes gentle, and the watercourse spreads forward in a large area, forming a tongue-shaped fan-delta sedimentary body on the plane. The sand bodies near the fan delta are transported by waves and coastal currents to form isolated and dispersed beach-bar deposits along the lake shoreline (Figure3).Figure 3
Distribution pattern of sedimentary facies of the basal deposits in the southeastern part of the Jiyang Depression. (1) Proluvial fan, (2) fan delta, (3) beach bar, (4) proluvial plain, (5) shallow lake, (6) semideep lake, (7) sub-aqueous fan, and (8) location of well.During the period of Kongdian and lower Shahejie-4 Formation, it was the initial stage of basin subsidence. Therefore, shallow lakes are mainly developed in the interior of the basin far from the provenance area, which were transformed into deep lakes during the flood period.


## 3.4. Sedimentary Model of the Basal Deposition

The sedimentary process of the basal deposition in the southeastern Jiyang Depression is relatively complex and is greatly affected by floods.During the flood period, the lake level rose rapidly, and the flood brought a large amount of detrital material, which accumulated on the edge of the gentle slope belt, forming a proluvial fan with a small distribution range, and in the interior of the gentle slope belt, a large area of fan delta formed. During the flood period, the sand bodies in the front of the fan delta were transformed by the waves and coastal current, and a large number of isolated beach-bar sand bodies were formed on the front side of the fan delta. At this time, mudstone and gypsum-bearing mudstone are mainly developed in the center of the basin (Figure4).Figure 4
Sedimentary model of the basal deposits in the southeastern part of the Jiyang Depression. (1) Proluvial fan, (2) proluvial plain, (3) fan-delta, (4) beach-bar, (5) sub-aqueous fan, (6) littoral lake, (7) shallow lake, (8) semideep lake, (9) sandstone, (10) mudstone, (11) conglomerate, and (12) gypsum mudstone.During the intermittent period of floods, the provenance supply was less, and proluvial fans were developed, but fan deltas and beach-bar sand bodies were less developed. At the same time, due to the strong evaporation during the flood intermittent period, gypsum and gypsum mudstone were deposited in the center of the depression.Vertically, the interbedded clastic rock and mudstone are mainly deposited at the edge of the gentle slope belt. The clastic rock is mainly formed during the flood period. It gradually becomes finer in grain size upward and eventually transforms into the mudstone formed during the interflood period. In the central area of the basin, mudstone, gypsum-bearing mudstone, anhydrite, and gypsum are interbedded.


## 4. Conclusion

(1)
By analyzing the composition, type, color, and structure of clastic rocks in the basal sediments in the southeast of Jiyang Depression, three types of sedimentary facies can be identified in the study area: proluvial fans, fan deltas, and beach bars. Proluvial fan deposits are mainly developed at the exit of the ancient gullies in the south area. Fan deltas are mainly distributed in the large gentle slope belt in the northwest, while beach bars are isolated and scattered along the lake shoreline in shallow lakes. Among which the fan-delta fronts and beach bars are well sorted and have higher compositional and structural maturity.(2)
The basal deposits in the southeastern part of the Jiyang Depression are mainly flood-controlled sedimentary models. During the flood period, a large amount of detrital material carried by the flood was deposited in the gentle slope zone, forming the deposition of proluvial fans and fan deltas. At the same time, the waves and coastal currents were strong, and the sand bodies in the fan-delta front were transformed by waves to form beach bars, which are distributed in the shallow lake area at the front of the fan-delta. During the interflood period, the provenance supply was less, and only proluvial fan deposits developed; fan deltas and beach-bar sand bodies were rare. At this time, due to strong evaporation, a large number of gypsum and anhydrite were formed in the center of the lake basin. Vertically, the sediments in the gentle slope zone are mainly clastic rock formed during the flood period interbedded with mudstone deposited during the interflood period, while in the center of the depression, they are mainly mudstone intercalated with anhydrite and gypsum.



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*Source: 2959031-2022-09-05.xml*