Identification and Sedimentary Model of Shallow-Water Deltas: A Case Study of the Funing Formation, Subei Basin, Northeast China

Abstract

Shallow-water deltas are not only a hot spot for sedimentological research but also a key target for oil and gas exploration. In this paper, taking the third member (E₁f₃) of the Funing Formation in the Upper Jurassic as an example, based on observations made from core samples, well logging, cathode luminescence characteristics, and analytical assays, the development conditions, sedimentary characteristics, and sedimentary models of shallow-water deltas are summarized. These shallow-water deltas were deposited in conditions with the following characteristics: a gentle terrain platform, a subtropical climate with ample rainfall, an abundant source supply, strong hydrodynamic forces, shallow water bodies, and a frequently eustatic lake level. Shallow-water deltas are characterized by sediment deposition from traction currents, numerous underwater distributary channel scour structures, overlapping scouring structures, sand body distribution with planar features, underwater distributary channels as skeletal sand bodies, and undeveloped mouth bars. Based on these, it is believed that during the deposition period of E₁f₃, the Gaoyou Sag in the Subei Basin had favorable geological conditions for the development of shallow-water delta deposition. The shallow-water delta deposition that occurred during the sedimentary periods of the five major sand units in the Funing Formation is characterized by front subfacies, with underwater distributary channels as the framework for sand bodies, and multiple intermittent positive rhythms overlapping vertically with the Jianhu Uplift as the source of material supply. In this paper, a depositional model for shallow-water delta deposition during the E₁f₃ deposition period in the Gaoyou Sag is established, expanding the scope of oil reservoir exploration in the north slope region of the Gaoyou Sag and providing important geological evidence for the selection of favorable subtle zones.

1. Introduction

In recent years, nonmarine shallow-water deltas have been a hotspot in the field of sedimentological research and hydrocarbon geological exploration [1,2,3,4,5,6,7,8,9]. Fisk first proposed the concept of the “shallow-water delta” and divided fluvial-dominated deltas into deep-water and shallow-water types. In terms of sedimentary origin, Zhang et al. (2010) identified two kinds of shallow lake basin deltas with distinct characteristics, namely the dendritic distributary channel type and the continuous distributary bar type [3]. Zou divided lake basin deltas into six kinds of shallow-water deltas and three kinds of deep-water deltas based on the provenance system, combined with the gradient of the leading edge slope and the paleo-water depth. The slope of the sedimentary top surface of the carpet shallow-water fan delta front is gentle, and the characteristics of the forest layer are not obvious. The top surface of the Gilbert-type shallow-water fan delta front is steep; the front forest structure is obvious; the area is small, and flaky gravity flow deposits are developed. Gravity flow deposits are developed in deep-water fan delta fronts and slopes. The top surface of the sedimentary front of the blanket-type shallow-water braided river delta is gently inclined, and the progradation structure is not obvious. The top surface of the Gilbert shallow braided river delta front is steep; the progradation structure is obvious, and the front area is small. Deep-water braided river delta slopes and pro-deltas develop slump gravity flow deposits. The delta plains and front areas of blanket-type shallow-water meandering river deltas are large; the top surfaces of the front depositions are gently inclined; the progradation structures are not obvious, and early-developed estuary dams are easily transformed by later distributary channels. Gilbert-type shallow-water meandering river delta plain areas are large; the front areas are relatively small; the top surfaces of the front slopes are inclined; the mouth bars are developed and possesses a forest structure, and the preservation conditions are good. The front edge of deep-water meandering river deltas coincides with the slope area, and the slope and the front deltas develop slump turbidite fans [2]. Zhu suggested that with an increase in the intensity of river action, shallow-water deltas evolve from sheet-like to lump-like to dendritic. Therefore, it can be concluded that the source system plays a major role in controlling the sedimentation process and morphology of a delta. Therefore, according to different source systems, shallow-water deltas can be divided simply into shallow-water fan deltas, shallow-water braided river deltas, and shallow-water meandering river deltas [4].

Many scholars have different views on the division of sedimentary facies and the distribution law of sand bodies in shallow-water deltas [6,10,11,12]. At present, the relatively unified understanding of shallow-water deltas is that they are formed in depressions or fault basins with a stable structure, gentle terrain platform, slow subsidence, and shallow water [13]. A shallow-water delta is dominated by fluvial action, and the sand body of the underwater distributary channel is regarded as a skeletal sand body, with few or no mouth bars and sheet sand [14].

This study reveals the sedimentary mechanisms of shallow-water deltas in the Gaoyou Depression of the Subei Basin under unique structural and climatic conditions and establishes a sedimentary facies model for the distribution of lake basin sand bodies. The deposition of E₁f₃ members in the Gaoyou Sag is considered to have taken place during the period of general delta deposition. In the literature, the northern slope of the study area during this period is described as having a gentle terrain, strong hydrodynamic force, sediment supply provenance, and a warm and humid paleo-environment [15]. These characteristics are conducive to the development of shallow-water delta deposition [16,17,18,19].

The development of a shallow-water delta in the study area, the Gaoyou sag, during this period is examined through a systematic and targeted in-depth analysis of the sedimentary characteristics of E₁f₃. This study addresses two key gaps. (1) Dynamic coupling of control factors: By integrating cathodoluminescence analysis, core facies combination, paleoclimate, and paleo-structural data, we demonstrate that the E₁f₃ shallow-water delta in the Gaoyou Depression is controlled by stable tectonic subsidence, subtropical climate, and frequent lake level oscillations, which collectively promote the development of extensive and thin sand bodies. (2) Redefining sand structure: Unlike traditional delta models dominated by river mouth dams, this study suggests that underwater distributary channels are the main sand skeletons. Core and logging data (such as a box-shaped gamma-ray response) confirm that sedimentary features exhibit intermittent positive rhythms in the vertical direction and have a universal scouring structure, indicating repeated erosion and sedimentation events. These findings provide new evidence for identifying shallow-water delta reservoirs. By summarizing the sedimentary development conditions and formation of the shallow-water delta during the E₁f₃ deposition period, the sedimentary development law in this period is summarized, and the sedimentary model of the shallow-water delta of E₁f₃ in the Gaoyou Sag is established by taking the five major sand formations of the three sub-members of the E₁f₃ member as the research unit, which provides a significant geological basis for the exploration and development of the study area; this provides a new research method for regions with similar sedimentary characteristics worldwide.

2. Geologic Setting

2.1. Tectonic Settings

Subei Basin, located in the northeast of Jiangsu Province, belongs to the onshore part of the southern Subei–South Yellow Sea Basin, with a general northeast trend. The basin as a whole has a tectonic pattern of one uplift and two depressions, namely the Yanfu Depression, the Jianhu Uplift, and the Dongtai Depression, ordered from north to south (Figure 1a). The Gaoyou Sag is located in the central part of the Dongtai Depression of Subei Basin. It is connected by the Wubao and Zhenwu Faults and Tongyang Uplift in the southeast, the southern Jiangsu Uplift in the south, the Lingtangqiao protrusion in the west, the Liubao Low Uplift in the northwest, and the Jianhu Uplift in the north (Figure 1b). The Gaoyou Sag was formed during the Late Cretaceous Taizhou Formation depositional period at the end of the Yizheng Movement [20] and consists of a semi-graben faulted lake basin formed by differential uplift. The southern part of the lake basin is controlled by faults, showing a steep fault boundary. The northern part is the gentle slope sedimentary area, and the sedimentary layer extends to the north. Overall, the lake basin has an asymmetric structure with a steep south and gentle north, which is a typical feature of semi-graben basins [21].From south to north, the depression successively develops a south fault terrace belt, a deep depression belt, and a north slope, presenting a tectonic pattern of fault–depression–slope. Three important tectonic events occurred in the tectonic evolution of the depression, which are, respectively, the Yizheng Event, the Wubao Event, and the Zhenwu Event (Figure 2). The tectonic environment of this study area is the fault depression stage of the Yizheng Movement [12]. No large-scale tectonic events occurred in the fault depression stage in the study area, so a gentle tectonic pattern was formed in the north, which provided the right geological conditions for the formation of shallow-water delta deposits in this period.

2.2. Stratigraphy

The Funing Formation is an important stratigraphic unit for deep depression exploration, overlying the Dainan Formation and underlying the Taizhou Formation. From bottom to top, there are four formations of Funing Formation, namely the E₁f₁ member, the E₁f₂ member, the E₁f₃ member, and the E₁f₄ member. As the most significant formation for exploration in the Gaoyou Sag, the E₁f₃ member belongs tectonically to the Subei Basin fault depression stage, which is represented by the Yizheng Event. The top interface of the E₁f₃ member is the marker layer of the first lacustrine flooding during the second stage of the Paleogene transgression, with lithologic characteristics of silty mudstone, argillaceous siltstone, and a mudstone section as the base. The bottom interface of the E₁f₃ member is the end of the maximum flood surface of the first stage of Paleogene lacustrine transgression, and the sedimentation of the reverse lacustrine cycle becomes the bottom boundary of the E₁f₃ member. The main lithology features are the dark gray and black mudstone section standard layer of semi-deep lacustrine (a lake of medium depth, generally between 5 and 15 m) facies. The E₁f₃ member is interbedded with sandstone, siltstone, and deep-dark mudstone, with a thickness of 220~350 m (Figure 2). According to the sedimentary cycle formed by the lake level change, the cyclical thickness method is used to divide the stratigraphic correlation. In the interval without an obvious cycle, the thickness variation rule is used to help divide the stratigraphic boundaries of the three members of the Funing Formation. Combined with the combination form of logging curves, the third member of the Funing Formation (E₁f₃ member) is further divided into three sub-sections. The first sub-segment (E₁f₃¹ member), the second subsegment (E₁f₃² member), and the Sanya subsegment (E₁f₃³ member) are divided from top to bottom. The E₁f₃¹ member consists of dark-gray mudstone, light-gray siltstone, argillaceous siltstone, and fine sandstone. The lithologic characteristics of the E₁f₃¹ member mostly comprise positive rhythm, inverted rhythm, or compound rhythm, combined with a formation thickness of about 100 m. The E₁f₃² member consists of dark-gray mudstone, light-gray siltstone, argillaceous siltstone, and fine sandstone, in positive and compound rhythms, with a formation thickness of about 150 m. The E₁f₃³ member is composed of gray mudstone interspersed with thin sandstone, which is composed of positive rhythms, anti-rhythms, or compound rhythms and has a formation thickness of about 80 m.

The three subsegments of Funing Formation are further divided into ten sand groups [22], namely the E₁f₃¹⁻¹–E₁f₃³⁻³ members (Figure 2), in which the E₁f₃³⁻³ members (hereinafter referred to as group a), the E₁f₃²⁻² members (hereinafter referred to as group b), the E₁f₃²⁻¹ members (hereinafter referred to as group c), the E₁f₃¹⁻² members (hereinafter referred to as group d), and the E₁f₃¹⁻¹ members (hereinafter referred to as group e), which are in important oil- and gas-rich stratigraphic positions [23,24]. This paper mainly focuses on the sedimentary characteristics of the above five major sand groups.

3. Data and Methods

The data for this study come from 234 wells in the Funing Formation of the Gaoyou Depression in the Subei Basin, including core data, logging data, heavy mineral data, cathodoluminescence data, seismic logging profile data, and grain size analysis data. The horizon of the core is mainly derived from five important groups: E₁f₃³⁻³ (hereinafter referred to as group a), E₁f₃²⁻² (hereinafter referred to as group b), E₁f₃²⁻¹ (hereinafter referred to as group c), E₁f₃¹⁻² (hereinafter referred to as group d), and E₁f₃¹⁻¹ (hereinafter referred to as group e). All data were provided by the Sinopec Jiangsu Oilfield Branch to ensure their accuracy and authenticity (

Table 1. Research data.

NO.	Research Data	Number
1	Coring well	43
2	Logging data	234
3	Cathodoluminescence sample	12
4	Seismic profile	2
5	Particle size analysis sample	40
6	Single-well facies analysis	2

).

Firstly, to determine the provenance system and direction of the E₁f₃ member in the Gaoyou Sag, the clastic component characteristics and heavy minerals were analyzed using a CITLCL8200MK5-2 cathode luminescence instrument and an ICPMS 2030 inductively coupled plasma mass spectrometer. A total of 12 sandstone samples were sent off for cathodoluminescence testing. A total of 14 sandstone samples were sent off for heavy minerals testing. We determined the source of the parent rock in the sedimentary rocks through cathodoluminescence technology. The cathodoluminescence characteristics of different rock-forming minerals such as quartz and feldspar correspond to different geological conditions. The cathodoluminescence of igneous quartz is mainly blue-purple, reflecting the temperature of quartz crystallization. Quartz in deep metamorphic rocks typically emits blue light, while quartz in shallow metamorphic rocks typically emits brown light. Quartz in sedimentary rocks does not emit light. Generally speaking, authigenic feldspar does not emit light; alkaline feldspar mainly emits blue light, and plagioclase is mainly red, yellow, etc. The luminescent color of feldspar in low-temperature metamorphic rocks is influenced by the metamorphic process, and it emits brown light or almost no light at all. Based on the cathodoluminescence discrimination results, the attribution of the parent rocks were determined as were the directions of primary and secondary biomass sources.

Secondly, stratigraphic correlation was carried out using the cycle thickness method. This method is a quantitative technique used to analyze and interpret sedimentary cycles in stratigraphic sequences. It reveals the regularity of sedimentary processes by measuring the thickness of individual sedimentary cycles or repeated patterns in stratigraphic columns and studying their vertical and lateral variations. Observations were carried out on core samples from 43 coring wells, and the color of the cores was used to determine whether the sedimentary environment was an oxidizing or reducing environment. The sedimentary environment was inferred based on the rock types found in the core samples. For example, conglomerates are commonly found in high-energy environments such as alluvial fans; sandstone is commonly found in sedimentary environments such as rivers and deltas, and mudstone is commonly found in low-energy environments such as lakes and deep seas. Special sedimentary structures found in the core samples were used to determine the sedimentary environment; for instance, parallel bedding indicated a low-energy environment; cross-bedding indicated a high-energy environment, and biological disturbance indicated shallow water bodies. Shallow water bodies enrich the sedimentary environment with oxygen, allowing organisms to survive. At the same time, the sedimentary characteristics of the study area were elucidated using core samples, and the shallow-water delta sedimentary facies belt of the study area was further systematically divided.

In addition, a seismic profile was mainly used to identify different paleo-geomorphological characteristic parameters caused by tectonic movements. In addition to sand transport channels, ancient river valleys are often the preferred areas for sand body filling. Slope break zones play an important role in the differentiation and aggregation of sand bodies. By observing seismic profiles, it is possible to effectively identify the relationships between different strata and elucidate the types of ancient river valleys and slope break zones.

Grain size analysis was conducted on 40 sampling points from 4 wells, and the sorting and flow characteristics of sediments were determined by comparing C-M values and through morphological analysis. By using the jump amplitude of the cumulative probability curve of granularity, the hydrodynamic conditions were determined in order to help identify the sedimentary characteristics of shallow-water deltas.

Finally, we selected core wells with relatively complete data; collected the typical sedimentary characteristics of shallow-water delta; synthesized GR (gamma ray), SP (spontaneous potential), RT (resistivity), and RILM (resistivity induction logging measurement) of 243 wells; and identified shallow-water delta sedimentary subfacies based on different lithofacies and sedimentary characteristics. To clarify the temporal and spatial distribution of sedimentary systems, we summarized the sedimentary evolution law and established the sedimentary model. Various single factors that could reflect sedimentary facies were comprehensively considered, such as rock fabric and paleontological characteristics. Then, according to the single factor analysis and multi-factor comprehensive mapping method, plane facies analysis was carried out.

4. Results and Discussion

4.1. The Characteristics of Detrital Compositions

Abundant resources, gentle terrain, humid and rainy climate, shallow water depth, and frequently changing lake levels are favorable geological conditions for the development of shallow-water deltas. The main detrital components in sandstone, namely quartz, feldspar, rock debris, and heavy minerals, are of great significance in the study of provenance recovery. This study was based on sandstone debris composition data and counted and analyzed the quartz, feldspar, and rock debris components in the sandstone of twelve wells in the third section of the Funing Formation (

Table 2. Classification table of rock composition of the third member of the Funing Formation in the Gaoyou Depression.

Well Name	Quartz (%)	Feldspar (%)	Rock Fragments (%)
L1	63	19	18
WN1	70	17	13
SH16	65	17	18
WA1	62.5	20	17.5
SHX34	65	18	17
SH6	66	18	16
Z1	72	16	12
JA3	62	18.5	19.5
D1	67.5	15.5	17
SH9	65.5	16	18.5
SH12	65	17	18
SHX21	67	18	15

). Specifically, twelve microscopic sections were prepared and analyzed to determine the parent rock properties of this section during the sedimentary period. Based on the analysis data, the rock composition classification was obtained (Figure 3). The lithology of the sediment in the study area is mainly composed of lithic feldspar sandstone and feldspar lithic sandstone, which indicates that the sediment transport distance is short.

After normalizing the rare earth element content of six samples from the third section of the Funing Formation using chondrite meteorites, a standardized distribution curve of rare earth elements was plotted (Figure 4). It can be seen that the standard curve of chondrite meteorites in this section is characterized by the enrichment of light rare earth elements, the relative loss of heavy rare earth elements, the strong fractionation of light rare earth elements, the weak fractionation of heavy rare earth elements, the rightward shift in light rare earth elements, and flat L-shaped heavy rare earth elements. The enrichment of light rare earth elements is usually related to the weathering products of felsic rocks or ancient sedimentary rocks. In shallow-water delta environments, sediments mainly come from terrestrial debris transported by rivers, which are usually dominated by felsic materials and conform to the characteristics of light rare earth enrichment. The relative depletion of heavy rare earth elements may be related to sediment sorting or chemical weathering processes. In shallow-water delta environments, where hydrodynamic conditions are strong, sediments undergo sorting, resulting in a relative depletion of heavy rare earth elements. The strong fractionation of light rare earth elements indicates that sediments are affected by chemical weathering, exposed to the water atmosphere on the surface, and also indicates shallow-water sedimentary environments.

Furthermore, we analyzed the characteristics of cathode luminescence of different rock-forming minerals, such as quartz and feldspar, deposited during the deposition of E₁f₃ members in the Gaoyou Sag and obtained from 16 wells. A total of 12 sandstone samples were sent for cathodoluminescence testing. As shown in Figure 5, the parent rocks are mainly ancient sedimentary rocks, followed by shallow metamorphic rocks (DA1 Well, 1931.83 m) and a small number of magmatic rocks (WN1 Well, 3026.36 m). The parent rock is composed mainly of ancient sedimentary rock, which means that the study area has experienced multiple sedimentary events and accumulated a large amount of detrital material. This detrital material is eroded and transported during subsequent geological activities, becoming a source of new sediments. Therefore, these ancient sedimentary rocks provide a rich material basis for the supply of material sources. Shallow metamorphic rocks are formed through the metamorphism of sedimentary or volcanic rocks under low-temperature and low-pressure conditions. They have low degrees of metamorphism and still retain some of the structure and composition of the original rock. The formation of shallow metamorphic rocks is often related to regional tectonic activity, which promotes rock fragmentation and erosion, making the transportation of these rocks to sedimentary environments quite easy. Therefore, the presence of shallow metamorphic rocks also indicates that there are more rocks available for erosion in the region, providing additional material sources.

4.2. Characteristics of Shallow-Water Delta

The characteristics of shallow-water delta sedimentation can be identified in various ways, such as through lithofacies type, grain size characteristics, sedimentary structures, and logging curve response. The lithological types are mainly sandstone, siltstone, and mudstone. Sandstone is well sorted, and mudstone is often interbedded with sandstone, reflecting a low-energy sedimentary environment. The granularity features are characterized by a positive rhythm and moderate-to-good sorting. Sedimentary structures include cross-bedding, parallel bedding, etc. Cross-bedding and erosion-filling structures are commonly found in high-energy river environments, while biological disturbances are more developed in low-energy plain environments, which can reflect frequent changes in shallow-water delta water bodies. On the logging curve, the sand body exhibits negative anomalies in the natural potential, high resistivity, and low gamma ray values, while the bell-shaped curve indicates river sedimentation. These pieces of evidence collectively reveal the typical characteristics of shallow-water delta sedimentation.

4.2.1. Lithofacies Types

Sedimentary facies and their sedimentary characteristics can be identified from the core descriptions of lithofacies to build a shallow-water delta sedimentary system [25,26]. Five types of principal lithofacies were identified from fine core descriptions in this paper (

Table 3. Lithofacies classification and interpretation of E₁f₃.

Typical Coring Well	Lithology	Color	Sedimentary Structure	Sedimentary Interpretation
SH36 DA1 SH6	Fine-grain sandstone	Light gray, grayish-white	Scour-and-fill structure, parallel bedding, and climbing cross-bedding	The sandstone migrates in the flat sand bed and has a poor lateral extension of fine layers.
L15 W19	Siltstone sandstone	Brown-yellow, light gray	Wavy cross-bedding	The wavy cross-bedding is a wavy curved surface on the interface of the strata, which is widely developed in the shallow-water delta facies deposits.
L15 W1	Muddy siltstone	Brownish-yellow, light gray	Flaser bedding	Typical sand–shale sedimentary composite bedding, in the case of frequent alternating strong and weak hydrodynamic environments.
JA3	Silty mudstone	Light gray	Flaser bedding	Sandstone and mudstone are formed by interactive deposition.
L5 SHX23	Mudstone	Brown-yellow, light gray	Massive bedding	Commonly found in shallow-water environments or exposure environments with the pedogenesis process in hot climate.

). The sedimentary environment and sedimentary process of different parts of the delta are reflected in different lithofacies assemblages in delta sedimentary rocks [27,28,29]. The lithological types are mainly sandstone, siltstone, and mudstone, which are closely related to the development of shallow-water delta sedimentation. The good sorting of sandstone reflects the water flow screening effect of the high-energy environment in the river channel, while the interbedded mudstone and sandstone reflect the low-energy intermittent sedimentation of the delta front. Sandstone represents distributary channel sedimentation, while siltstone and mudstone reflect the static water sedimentation of delta bays. This lithofacies combination is a typical feature of shallow-water delta sedimentation, reflecting frequent changes in water energy and indicating the alternating changes in the sedimentary environment in time and space, which is consistent with the coexistence of high-energy channels and low-energy bays in shallow-water delta sedimentary systems.

4.2.2. Grain-Size Characteristics

According to the grain-size analysis of 40 samples in the deposition period of the E₁f₃ member in the Gaoyou Sag, the standard C-M diagram of Pasega traction flow is illustrated (Figure 6a). The C value in the C-M diagram of the E₁f₃ member in the study area increases proportionally to the M value and is parallel to the C=M baseline. In addition, the maximum agitation index (CS) at the bottom of the flow is 500 μm, with a small difference between the C and M values of the samples, all indicating that the Gaoyou Sag has a well-sorted shallow-water delta sedimentary environment, created during the deposition period of the E₁f₃ member. The C-M diagram generally states the characteristics of tractive flow deposition during this period. The cumulative relative curve of the particle size is characterized by two jumps and one suspension (Figure 6b), in which the rolling part does not develop. The jumping component accounts for a relatively heavy portion, and two jump sub-populations are developed at about 60%~90%, with the slope ranging from 45° to 70°. The intercept points of the jump population and the suspension population are both in the range of 4Φ~5Φ, and the suspension population after the sample point 5Φ exhibits a low slope. These characteristics also indicate the characteristics of the traction current with strong sedimentary hydrodynamic conditions and are dominated by jump transportation. The jump population with a steep slope and high content indicates that the clastic particles are well-sorted, which is the specific manifestation of the microfacies of underwater distributary channels in shallow-water deltas with strong hydrodynamic conditions and fast migration speeds.

4.2.3. Sedimentary Structural Characteristics

The analysis of the sedimentary environment is based on the study of sedimentary hydrodynamic conditions, and the sedimentary structures are the key to the analysis and judgment of sedimentary hydrodynamic conditions [30,31]. Through the observation of the core in the E₁f₃ member, obtained from 43 wells, it was found that the sedimentary structures from this deposition period are extremely rich but are generally reflected as tractive flow bedding characteristics, which is consistent with the results of the C-M map (Figure 6). A large number of ragged erosional surfaces were formed as a result of the increased velocity of the water body and the scouring of the underlying sediments by the water flow (Figure 7a,b) In addition, cross-bedding, tabular cross-bedding, overlying scour, and wavy cross-bedding were developed (Figure 7). The scouring and filling structure indicates a sedimentary environment in the shallow-water delta, as it reflects the alternating characteristics of high-energy water flow and low-energy sedimentation, especially in the distributary channels. River water flow has a strong erosion ability and can form scouring surfaces, which are then filled with sediment when the river channel migrates or the water flow weakens. In the study area, these structural features reflect a turbulent flow and a strong hydrodynamic force during the deposition period of the E₁f₃ member as well as the sedimentary characteristics of shallow-water delta which were influenced by fluvial action during the deposition.

4.2.4. Identification of Sand Bodies Through Wireline Log Responses

The combination, amplitude, and shape of logging curves can reflect the lithology of sedimentary rocks to varying degrees in order to identify sedimentary facies types. In this study, different logging curves were identified and analyzed. By identifying GR and SP curves, which are sensitive to sedimentary materials [32], and summarizing their curve shapes, a logging facies deposition pattern was established, as shown in Figure 8. This research method was used to summarize the sedimentary environment reflected by different logging facies. The following types of logging curves are identified in the study area (Figure 9): bell-shaped (composite bell-shaped), box-shaped (composite box-shaped), and funnel-shaped (composite funnel-shaped).

Bell-shaped (compound bell-shaped): The shape of the logging curve is characterized by a low amplitude in the upper part, a high amplitude in the middle and lower parts, and a decrease in the amplitude of the logging curve from the bottom to the top, creating a bell-like shape with a wide bottom and a narrow top which indicates that the supply of sediments in this period has changed from sufficient to weakened. Taking the Hua X11 well as an example (Figure 8), the GR and SP logging curves are bell-shaped with depths of about 3050~3055 m. The lithology gradually changes from mudstone to siltstone, which generally represents the flank or front end of the underwater distributary channel.

Box-shaped (compound box-shaped): The shape of this kind of logging curve is consistent in the upper and lower parts, while the middle part is high, exhibiting a box-like shape with similar upper and lower widths, indicating that the supply of sediments in this period is sufficient and stable, that the grain size of sediments is uniform, that the sorting is good, and that the hydrodynamic conditions are strong. For example, Well Sha 2 (Figure 8), with a depth of about 2690–2702 m, has a box-shaped SP logging curve, and the lithology gradually changes from mudstone to siltstone and then back to mudstone, which generally represents the deposition of the center of the underwater distributary channel under strong hydrodynamic conditions.

Funnel-shaped (compound funnel-shaped): The logging curve is characterized by high amplitude in the upper and middle parts and low amplitude in the lower part, resulting in a funnel-like shape with a narrow bottom and a wide top, reflecting the supply of sediments, the strong hydrodynamic conditions, and the coarse sedimentary particle size in this period. Taking the Hua X11 well as an example (Figure 8), in the area with a depth of about 3058~3060 m, GR and SP logging curves show a funnel-like shape, and the lithology changes from mudstone to siltstone from bottom to top. This type of logging curve shape is not common in the study area and is generally seen in Gilbert-type delta front subfacies, mouth bar microfacies, shallow lake subfacies, and beach bar microfacies. Estuary bars are one of the most important indicators of the development of Gilbert deltas and have a three-layer structure (topset, fore-deposit, and bottomset), but a shallow-water delta does not have a three-layer structure, which is one of the differences between shallow-water deltas and normal delta deposits, and as a result, funnel-shaped curves are not common in shallow-water delta fronts.

5. Discussion

5.1. Identification of Shallow-Water Deltas

5.1.1. The Characteristics of a Gentle Terrain

The Gaoyou Sag is an early stage of fault depression from the Late Cretaceous–Paepcene. The formation and evolution of the Gaoyou Sag was controlled by the Zhenwu, Hanliu, and Wubao fault belts, which were distributed in the middle, west, and south of the depression, respectively. These faults made the depression manifest a half-graben-like fault structure with faults in the south and overlapping in the north [33]. The depression was created in the late stage of the Eocene–Oligocene, and the Zhen1 fault of the southern fault order was formed in the Taizhou Formation, while the Zhen2 fault was formed around the Sanstack stage. It can be seen from the profile of the tectonic evolution and the seismic depression (Figure 10) that the study area did not experience any large-scale tectonic events during the deposition period of the third member of the Funing Formation to the fourth member of the Funing Formation. The half-graben-like structure was relatively simple, and the base structure did not have any obvious large fluctuations. In particular, the northern gentle slope formed after the Yizheng Movement and is a regional southern-sloping slope with a gentle terrain and a large area. The Hanliu fault is not developed in the central and western parts of the depression and has no typical gradient break zone, but a small flexural hub zone with small relief was developed in the central Shanian area.

Overall, the northern slope formed in this period is a broad and gentle topological platform. In the late Funing Formation, after the depression created by the Wubao Tectonic Event, the uplift of the northern slope increased significantly. The northern slope has a low topography, a small slope angle, and a large platform-like area. These geological features increase the accommodation space for sediments carried by water and lay a foundation for extensive deposition in the depression. Paleo-structures with weak tectonic activities and a gentle terrain are good foundations for the formation of shallow-water delta deposition. The slope angle of the northern slope area of the Gaoyou Sag terrain was small before the Wubao Movement, which provided favorable structural conditions for the formation of shallow-water delta deposition in E₁f₃.

5.1.2. The Characteristic of Warm and Humid Climate

In the deposition period of the E₁f₃ member, the palynological assemblage consisted of Ulmaceae, Quercus lonicera, Proteaceae, Potamogeton, and a small amount of Anacardiaceae, and the characteristic indicator for these is the occurrence of more coniferous pollen in the palynological assemblage [15]. The tropical and subtropical molecules of broad-leaved tree species increased in this period, and the clay minerals were still dominated by montmorillonite and hydromica, with local scattered gypsum, indicating that the paleoclimate was relatively hot [15]. The paleoclimate is one of the key factors affecting the deposition of shallow-water deltas. Today, many scholars believe that warm and humid subtropical climates are important for the open flow of lake basins and, subsequently, are very conducive for the development of shallow delta sand bodies [2,10]. To sum up, the climate during the sedimentary formation period of the E₁f₃ member was humid, hot, rainy, and subtropical, and this warm and humid environment provided favorable conditions for the formation of shallow-water delta deposition.

5.1.3. Shallow Paleo-Water Depth

The main paleontological species from the sedimentary formation period of the E₁f₃ member were limnobios, indicating that the organisms in this period had poor salt tolerance. According to geochemical analysis, the salinity of the water in this period was brackish-fresh water, which indicates that the water in the lake basin was shallow [35]. The primary color of sedimentary rocks is one of the most important features for judging the climate and oxidation–reduction conditions of water during the deposition of a rock stratum [36,37]. The colors of various cores from different wells are shown in Figure 11, and it can be seen that the sedimentary rocks in the study area during this sedimentary period are mainly light-brown, gray-green, brown-yellow, light-gray, dark-brown, and other weak oxidizing–weak reducing colors, which alternate with weak reducing colors, including gray and white. At the same time, brown-yellow, light-brown, and dark-brown mudstone developed alternately with gray and gray-green mudstone. Generally, the overall water depth in the sedimentary period was relatively shallow, and the paleo-water depth fluctuated frequently (Figure 11). As stated above, the rock colors of the E₁f₃ member indicate a weakly oxidizing–weakly reducing environment in the study area during this period, suggesting that the overall water depth was relatively shallow, and the paleo-water depth fluctuated frequently in the sedimentary period. Generally, during the deposition period of E₁f₃ in the study area, the shallow water depth and frequent changes in the lake level provided favorable water conditions for the formation of a shallow-water delta deposition.

5.1.4. Adequate Supply of Sufficient Provenance Recharge

According to the relative data on the heavy mineral content of 14 wells in the E₁f₃ formation, the heavy mineral assemblages in the study area are mainly garnet, white titanite, zircon, and magnetite, the total content of which accounts for 28.4%–98% of the total heavy mineral content, with an average of 76.17%. Unstable minerals mainly include pyrite and barite and occasionally mica, chlorite, and glauconite, which account for 2%–71.6% of the total mineral content, with an average of 24.08% (Figure 12a). Among the heavy minerals, zircon, rutile, and tourmaline have the most stable chemical compositions, so the sum of the ratios of these three heavy minerals (ZTR index) is often used to reflect the composition maturity of sedimentary rocks and is an important means of evaluating the direction of provenance [38]. In general, rocks with the same provenance, same sedimentary period, and same water system have the same or similar heavy mineral compositions and ZTR index. With the increase in transport distance, the proportion of unstable minerals was lost, but the ZTR index increased.

The ZTR index of the northern slope zone ranges from 0.15 to 0.56 in the E₁f₃ member (Figure 12b), increasing from north to south, which indicates that the provenance direction of the sedimentary area derives from the east and west of the Jianhu Uplift in the north. It is worth noting that the ZTR index and compositional maturity index of samples from individual wells are higher than those of the surrounding sampled wells. The reason for this phenomenon is due to the differences in the sedimentary period, migration path, and parent rock between different samples.

As shown in Figure 12c,d, the index of debris composition maturity increases from north to south, indicating that the provenance direction is derived from the east and west directions of the northern Jianhu Uplift, which is consistent with the results of the ZTR index analysis. So, based on the cathode luminescence characteristics, heavy mineral assemblages, ZTR characteristics, and the composition maturity law of clastic rocks, we observed that there were several different provenance systems in the east and west directions created by the Jianhu Uplift in the north in the form of a line plane.

As discussed above, both the ZTR index and compositional maturity suggest that the study area’s provenance was primarily derived from the east and west directions of the southern slope of the Jianhu Uplift during this period. Based on the cathodoluminescence characteristics, heavy mineral assemblages, ZTR characteristics, and the composition maturity law of clastic rocks in the study area, we analyzed the deposition provenance system of the E₁f₃ member comprehensively; it was found that there were several different provenance systems in the east and west directions created by the Jianhu Uplift in the north in the form of a line plane (Figure 12). Through the south–north equilibrium section (Figure 10), it can be seen that the paleo-geomorphology of the northern slope area during the deposition of the E₁f₃ member is very gentle and is part of the windward area [39], which results in the long-distance transport of sediment. Paleo-provenance is an important factor in controlling the deposition of shallow-water deltas, and sufficient material supply and rapid sediment recharge are prerequisites for the formation of shallow-water deltas [40]. Therefore, as a gentle slope with an abundant material supply and a large surface runoff area, the north slope provides favorable conditions for the formation of a shallow-water delta in the study area during the deposition of the E₁f₃ member.

5.2. Depositional Evolution

The sedimentary deposition period of the third member of the Funing Formation in the Gaoyou Sag is characterized by a gentle terrain profile, warm and humid climate, shallow water body, frequent changes in lake water level, and sufficient provenance supply, which provides an appropriate basis for the formation of shallow-water deltas. The continental lacustrine basin indicates a multi-factor sand control, the effective allocation of multi-factors in time and space, and dynamic factors regulating static factors, which further accurately reflect the plane distribution characteristics of sedimentary facies. There are eight different sedimentary effects of the sand control mechanism coupled with three paleo-geomorphic elements, namely the source system (ridge), the ditch system (ditch), and the slope-break system (slope) [41]. Of these, a ‘ridge’ refers to the source area where the basin or depression periphery provides an effective source for the lake basin; a ‘ditch’ refers to the incised valley formed by long-term erosion, that is, the distributary channel of the study area; and a ‘slope’ refers to various types of syndepositional slope break zones. In this study, the northern slope of the Gaoyou Sag is considered a gentle slope.

The front edge of the shallow-water delta is characterized by shallow water bodies and a gentle terrain. Underwater distributary channels frequently migrate and change course in low accommodating space conditions, forming a sedimentary pattern of multi-stage thin sand bodies, vertically stacked and horizontally connected. In the seismic profile (Figure 13), the sand bodies in the study area exhibit significant medium-to-strong amplitude reflection characteristics, manifested as parallel to sub-parallel reflection phase axes with good long extension distance continuity and small lateral variations in amplitude intensity. These reflection axes often exhibit thin, layered stacking structures in the vertical direction, forming clear wave impedance interfaces with the surrounding rock (mudstone or silty mudstone), further verifying the presence of shallow-water sedimentary environments.

Combined with the formation thickness map, sand thickness contour map, sand content percentage contour map, logging data, and core characteristics, a plane distribution map of sedimentary facies of five major sand formations in the deposition period of the E₁f₃ member was compiled by using the sand formation as a mapping unit. According to the plane distribution characteristics of sedimentary facies, the shallow-water delta front developed in a large area in the north slope area during the deposition of the E₁f₃ member. The underwater distributary channel was distributed along a strip, and the subaqueous distributary channel lateral margin front was distributed along a narrow strip or a wide strip close to the main underwater distributary channel. In addition, the distributary bays are isolated and dotted in underwater distributary channels.

In the sedimentary deposition period of the E₁f₃³ sub-member, large-scale lake encroachment occurred, and the whole depression experienced water regression. The major sand formation that occurred in this period (Figure 14) was the first stage of the delta deposition of the E₁f₃ member. The sediments were deposited near the center of the depression, and the vertical lithology was characterized by reverse cycle deposition with a thin bottom and a thick top. In this period, the gully system was mainly U-shaped, V-shaped, and occasionally W-shaped, which indicates the following coupling type: “ridge” good + “ditch” good + “slope” poor. The sedimentation was transported far and distributed widely, and the sand body was caused by traction flow. The sand body in the shallow-water delta front with a thickness of more than 8 m was mainly deposited by underwater distributary channels and distributed in a wide band to the east and west of the depression. The maximum sandstone content in the deposition period is 77%.

During the deposition period of the E₁f₃² sub-member, due to the gentle topography of the northern slope area of the depression, the frequent rise and fall of the lake level had a significant impact on the front deposition. When the lake plane fell, the shallow-water delta exhibited a progradation feature near the center of the lake, and when the lake plane rose, a small, irregular, potato-shaped beach bar deposition was formed in the area where wave action dominated. The sedimentary plane distribution characteristics of the major sand formations b (Figure 15) and c (Figure 16) during this period show that the front lobes of the shallow-water delta continue to deposit material in the center of the lake basin. The gully system of the major sand formation b and the major sand formation c in the sedimentary period was mainly W- and V-shaped, as well as occasionally U-shaped, and possessed the following coupling type: “ridge” good + “ditch” good + “slope” poor. During the sedimentary deposition period of the major sand formation c, the underwater distributional channel sand body reached up to 19m, and the percentage of sandstone could reach 74%. The thickest underwater distributary channel sand body of the major sandstone formation c could reach 22.5 m, and the sandstone content could reach 95%.

In the early stage of the E₁f₃¹ sub-member deposition, the shallow-water delta still showed progradation characteristics. In the later stage, the water body deepened continuously until E1f4 reached the maximum lacustrine flood surface in the sedimentary period of the Funing Formation. During the deposition of the major sand formation d (Figure 17), small, irregular, potato-shaped beach bar deposits were formed in the end area, dominated by the wave action during the rise in lake level. The gully system was mainly U-shaped and V-shaped, with a small W-shaped portion, demonstrating “ridge” good + “ditch” good + “slope” poor coupling. The thickest underwater distributary channel sand body reached 23.5 m, and the percentage of sandstone reached up to 73%. During the deposition period of the major sand formation e (Figure 18), the gully system was W-, V-, and U-shaped with the following coupling type: “ridge” good + “ditch” good + “slope” poor. The widest underwater distributary channel sand body was 16m, and the sandstone content reached 53%.

The development of the shallow-water delta during the deposition of the E₁f₃ member in the Gaoyou Sag was controlled by the Jianhu Uplift water system in the northern part of the depression, and two delta lobes formed by underwater distributary channels were developed in the east and west. Due to the frequent diversion of underwater distributary channels, a large network of interwoven sand bodies was distributed in the plane, and the two lobes at the lateral margin front were characterized by more mud and less sand. The channel sand bodies on both sides showed a thinning trend in the lateral margin front until the sand bodies peaked into argillaceous deposits.

As a whole, in the deposition period of the five major sand formations in the third member of the Funing Formation of the Gaoyou Sag, two shallow-water delta front lobes, consisting of underwater distributary channels as skeletal sand bodies, were developed in the east and west. Due to the frequent diversion of underwater distributary channels, nearly planar sand bodies are distributed across the plane, which is a major difference between general delta sedimentation and shallow-water delta sedimentation. The lateral–frontal lobe is characterized by the presence of more mud than sand. The channel sand bodies on both sides show a thinning trend near the lateral–front side until the sand bodies peak into argillaceous deposits. At the end area of the underwater distributary channel, where wave action is dominant, scattered beach bar deposits appear.

5.3. Sedimentary Model of Shallow-Water Delta

By comprehensively studying the geological background, sedimentary characteristics, distribution, and sedimentary evolution, we established the sedimentary facies model of the E₁f₃ member in the Gaoyou Sag (Figure 19). The gentle sedimentary slope environment, sufficient material supply, and rapid sediment recharge are prerequisites for the formation of shallow-water deltas. Provenance is an important controlling factor for the formation, development, and distribution of shallow-water deltas. In the sedimentary area close to the material provenance location, a high-velocity river carrying a large amount of terrigenous detrital materials maintains strong river energy after entering the sedimentary basin. So, the underwater distributary channels that extend to the center of the basin are formed, which carry the sediments to the basin over a long distance. Because of the presence of a river with a fast-moving advance speed, the underwater distributary channel is constantly forked and rerouted, forming a thin and widely spread-out delta lobe with the sedimentation of the underwater distributary channel as the main body.

The front end of the underwater distributary channel continues to extend forward; the deposition of the bay between the distributary channels increases, and the sand bodies are distributed in broad bands or strips. The sand bodies of the underwater distributary channel gradually taper out in the front depositions or form small areas of irregular, potato-shaped beach bar deposits in the end areas where wave action dominates. In the E₁f₃ sedimentation deposition period of the Gaoyou Sag, the northern slope zone was low-lying. Meanwhile, the warm and humid climate, the sufficient material supply through the northern lake uplift, the strong hydrodynamic conditions, and the frequent rising and falling of the lake water level resulted in shallow-water delta deposition during the E₁f₃ deposition period in the Gaoyou Depression.

Compared with the general sedimentary model of the delta, the shallow-water delta has a particularity of the underwater distributary channel being a skeleton sand body. Laterally variable distributary channel and underwater distributary channel sand bodies were developed in the shallow-water delta of the third member of the Funing Formation in the Gaoyou Sag, which provides favorable reservoir space for the formation of large-scale lithologic reservoirs. The distribution channel sand body has a wide distribution range, and the sand bodies exhibit a ribbon-like distribution, with a general width of 2000–4300 m and an extension distance of more than 25 km. The size of a single sand body is the single lenticular layer of the distributor channel. The average thickness of the single layer is 7.05 m, and the average value of the sand-to-land ratio is 18%. Multiple sand bodies are stacked longitudinally and connected horizontally. Due to the wide distribution and large extension range of shallow-water delta sand bodies, they have become important targets for identifying hidden oil and gas reservoirs.

6. Conclusions

• The sedimentary conditions for the formation of shallow-water delta are a slow tectonic terrain, a warm and humid climate, a shallow water body, the frequent rise and fall of the lake water level, and abundant material supply. The sedimentary basis of the third member of the Funing Formation in the Gaoyou Depression was identified.
• The shallow-water delta developed in the third member of the Funing Formation in the Gaoyou Sag fluctuates frequently and exists in a weak oxidation–weak reduction sedimentary environment. The C-M diagram and the grain size probability curve show that the main sedimentary characteristic is traction flow. In the process of sandstone deposition, many scour surfaces and overlying scour surfaces are created, reflecting strong hydrodynamic characteristics.
• Provenance is an important controlling factor for the formation, development, and distribution of a shallow-water delta. In the sedimentary area near the source area, the high-speed river carries large amounts of terrigenous detrital material and maintains a strong, high-energy stream after entering the sedimentary basin and forming the underwater distributary channel, which extends to the center of the basin with the shallow delta front as the main body.
• The shallow-water delta has a skeletal sand body as its underwater distributary channel, and most planar sand bodies are widely distributed, which is the difference between general delta sedimentation and shallow-water delta sedimentation.