Sedimentary Characteristics and Model of Estuary Dam-Type Shallow-Water Delta Front: A Case Study of the Qing 1 Member in the Daqingzijing Area, Songliao Basin, China

Abstract

The sedimentary characteristics and model of the shallow-water delta front are of great significance for the development of oil and gas reservoirs. At present, there are great differences in the understanding of the distribution patterns of estuary dams in the shallow-water delta front. Therefore, this paper reveals the distribution characteristics of estuary dams through the detailed dissection of the Qing 1 Member in the Daqingzijing area and establishes a completely new distribution pattern of estuary dams. By using geological data such as logging and core measurements, sedimentary microfacies at the shallow-water delta front are classified and logging facies identification charts for each sedimentary microfacies are developed. Based on the analysis of single-well and profile facies, the sedimentary evolution laws of the Qing 1 Member reservoirs are analyzed. On this basis, the sedimentary characteristics and model of the lacustrine shallow-water delta front are established. The results indicate that the Qing 1 Member in the Daqingzijing area exhibits a transitional sequence from a delta front to pro-delta facies and finally to deep lacustrine facies, with sediments continuously retrograding upward. Subaqueous distributary channels and estuary dams constitute the skeletal sand bodies of the retrogradational shallow-water delta. The estuary dam sand bodies are distributed on both sides of the subaqueous distributary channels, with sand body development gradually decreasing in scale from bottom to top. These bodies are intermittently distributed, overlapping, and laterally connected in plan view, challenging the conventional understanding that estuary dams only occur at the bifurcation points of underwater distributary channels. Establishing the sedimentary characteristics and model of the shallow-water delta front is of great significance for the exploration and development of reservoirs with similar sedimentary settings.

1. Introduction

Shallow-water deltas typically develop in either the platform and epicontinental seas with shallow water and relatively stable structures or depression basins with gentle terrain and overall slow subsidence [1,2,3,4,5]. Fisk first proposed the concept of a shallow delta when studying the Mississippi River Delta on the basis of water depth. Donaldson further demonstrated that river-controlled deltas typically exhibit the sedimentary characteristics of shallow-water deltas. Postma identified eight depositional units in low-energy basins based on the sedimentary process and the tectonic background. Numerous investigators have examined sedimentary characteristics and formation dynamics, and they have pointed out that the Gilbert three-layer structure is not developed in shallow deltas [6,7,8,9,10,11,12,13]. Exploration and development practices have demonstrated that shallow-water deltas host abundant oil and gas resources. In recent years, such deltas have become key targets for lithologic hydrocarbon reservoir exploration. The sedimentary characteristics and spatial distribution of sand bodies form the geological foundation for reservoir development planning and are critical factors influencing the success rate of hydrocarbon exploration and production.

The Daqingzijing area is located in the southern part of the Songliao Basin. Recent research shows that depression basins, due to their overall tectonic stability and gentle terrain, are sites for the development of large shallow-water deltas. The traditional view holds that the Songliao Basin follows the lake-controlled shallow-water delta sedimentary model. It has been summarized as a lobate model, characterized by thin individual delta bodies with widespread distribution, numerous distributary channels, extensive frontal sheet sands, and well-developed facies differentiation. Previous studies have generally concluded that estuary dam sands are poorly developed and are primarily located at the bifurcation points of underwater distributary channels [14,15], and they belong to the underwater distributary channel-type shallow-water delta front. However, with the encryption of the development well network and increased understanding of depositional factors, it is found that the delta front is not a simple depositional feature dominated by underwater distributary channels. Core observation and well logging curve analysis both indicate that there are extensive estuary dams, both of which are developed at the same time. This undoubtedly overturns the traditional geological understanding and urgently requires the exploration of a new sedimentary model.

During the deposition period of the Qingshankou Formation, the Songliao Basin underwent expansion under deep-water conditions, and the sediments were predominantly fine-grained. From the Qing 1 Member to the Qing 3 Member, as the lake basin contracted and the water depth decreased, the sediment grain size gradually became coarser. The Qingshankou Formation in the Daqingzijing area is the main oil-bearing layer of the Jilin Oilfield. In 2015, this layer achieved high-yield oil production, and it became the main development layer of the Qing 3 section. The Qing 1 Member and the Qing 2 Member are the prospective layers. The Daqingzijing area is a key development zone in the southern part of the Songliao Basin. A wide range of shallow-water delta front subfacies and pro-delta subfacies depositional systems occur in the Qing 1 Member. However, sustained exploitation has progressively degraded reservoir quality. The remaining untapped potential is mainly distributed in the frontal area, where the resource quality is poor, and it is difficult to select sweet spots [16,17]. Therefore, the sedimentary model and dynamic evolution laws of the shallow-water delta front and their control over the distribution of oil and gas are key issues for geological sweet-spot optimization and the exploration of lithological reservoirs in this area.

The Daqingzijing area has unique advantages for sedimentary research: Firstly, the study area is large in size and encompasses a wide variety of sedimentary types, including the complete sedimentary types of the delta front, pre-delta, and deep lake deposits, which is a natural testing ground for the study of the sedimentary characteristics of the delta front. Secondly, this area features a high-density development well network with a 100 m (minimum) well spacing, which has strong control over the distribution of sand bodies and is conducive to the dissection of the depositional characteristics and patterns of the reservoir. Furthermore, this area is one of the rare cases where underwater distributary channels and estuary dams coexist in the terrestrial lake–shallow-water delta front, providing important geological support for the study of a estuary dam-type shallow-water delta front. Under these favorable conditions, this study investigates the sedimentary characteristics and evolution in the Qing 1 Member, establishes a sedimentary model for estuary dam-type lacustrine shallow-water delta fronts, and proposes a new conceptual framework for their depositional architecture. This is of great significance for advancing understanding of shallow-water delta systems and the exploration and development of lithologic oil and gas reservoirs. It also provides important technical guidance for the hydrocarbon development of the Qing 1 Member and analogous settings elsewhere.

2. Geological Background

The Daqingzijing area is located in the central part of the Changling Depression in the southern Songliao Basin. It is a critical area for studying reservoir characteristics and hydrocarbon exploration potential (Figure 1). Daqingzijing is located in the saddle zone between the northern Qian’an sub-depression and the southern Heidimiao sub-depression. Its favorable tectonic setting has made this area a large-scale lithologic reservoir group of the Jilin Oilfield. It has reserves exceeding 100 million tons, and the exploration area is approximately 1500 km². The sedimentary cover structure is clear, consisting of the Mesozoic Upper Jurassic, Cretaceous, and Cainozoic Neogene and Quaternary from the bottom to the top. Since the Early Cretaceous, the Songliao Basin entered the depression stage, and a series of secondary depressions successively formed the central depression area. The sedimentary strata are mainly Cretaceous and exceed 5 km in thickness, covered in the interior of the basin. From the bottom to the top, the strata are the Shahezi Formation, Yingcheng Formation, Denglouku Formation, Quantou Formation, Qingshankou Formation, Yaojia Formation, Nenjiang Formation, Sifangtai Formation, and Mingshui Formation [18,19,20].

3. Materials and Methods

3.1. Lithologic Characteristics

Through detailed observation and description of core data from coring wells in the Daqingzijing area, the rock types are mainly sandstone and mudstone. The sandstone is predominantly composed of gray to gray-black siltstone, argillaceous siltstone, and a small amount of fine sandstone, mainly developed in Sand Groups II–IV (bottom and middle) of the Qing 1 Member. The particles are well-sorted and highly rounded, reflecting good hydrodynamic conditions, primarily formed in channel deposits. Meanwhile, the color of mudstones is one of the most intuitive and significant characteristics, as it can directly reflect the environmental conditions during their formation. For rocks in shallow water, the colors appear light gray, grayish yellow, purple-red, and in oxidized colors; for rocks in deep water, they appear gray, dark gray, gray-black, or black and in other dark shades [21,22,23,24,25,26]. The mudstones of the Qing 1 Member are mainly dark in color, being predominantly gray-black or black, indicating a underwater depositional environment (Figure 2).

More than 40 core wells were completed in the study area. Based on the direction of the sediment source [27], four long and continuous cores, Hei 62, Hei 68, Hei 57, and Qian144, were selected along a southwest-to-northeast transect. The total core length was 205.8 m. Lithological identification was performed by detailed visual description. The primary rock types at this location include fine sandstone, siltstone, muddy siltstone, silty mudstone, mudstone, and minor Ostracoda-bearing layers.

Based on core observations, lithological statistical analysis was conducted. In the Hei62 well, siltstone accounts for 49.03% and mudstone for 36.06%. In Hei 68, the siltstone content is 37.47% and mudstone is 40.82%. In Hei 57, siltstone makes up 17.61%, while mudstone constitutes 57.88%. In Qian-144, siltstone accounts for 16.17%, and mudstone accounts for 66.47% (Figure 3). These four coring wells are located in the study area, occurring sequentially from southwest to northeast. In comparison, it was found that the contents of fine sandstone, argillaceous siltstone, and silty mudstone gradually decline, while the content of mudstone gradually rises, indicating that the provenance lies to the southwest. As the distance from the provenance direction increases, the hydrodynamic conditions become poorer, and the sand and silt gradually decrease, forming argillaceous sediments dominated by mudstone.

Casting thin sections were prepared by vacuum impregnation using blue-dyed epoxy resin. For each core sample, three thin sections were systematically prepared to represent the upper, middle, and lower parts of the interval. Detrital mineral compositions were determined through petrographic analysis. A total of 63 representative casting thin sections were analyzed using point counting, with 350 points counted per sample under an optical polarizing microscope (Leica DFC500, Leica Microsystems, Wetzlar, Germany). This method yielded quantitative data on the mineralogical composition of the rocks. The detrital particle composition in the reservoir is feldspar, rock debris and quartz. The feldspar content is from 28.1% to 64.2%, with an average of 40%. The rock debris content is from 18.8% to 58.4%, with an average of 28%. The quartz content varies between 19.8% and 42.6%, with an average of 32%. The main component type of clastic rocks in the reservoir is lithic feldspar sandstone, with a matrix content of less than 10%. The cement is mainly calcite cement, with a content of less than 15%, as well as a small amount of secondary feldspar enlargement and authigenic quartz cement (Figure 4). In addition, mineralogical composition was analyzed using a D8 ADVANCE X-ray diffractometer (XRD). Prior to experiments, samples were ground into < 200 mesh, and aliquots of 1 g particles were placed into the ceramic tube. The diffraction angle of 2θ was set in the range of 5–80° with Cu Target—Kα radiation; the diffraction voltage and current were set as 40 kV and 30 mA, respectively. Finally, the mineralogical composition of samples (Figure 5) was obtained by comparison with the standard powder diffraction data, which was provided by the International Data Center of the Powder Diffraction Federation. Compared with thin-section petrography, XRD is more effective at quantifying fine-grained mineral components but is unable to detect detrital rock fragments. Despite methodological differences, the results from both approaches show a high degree of consistency in mineral composition.

3.2. Sedimentary Structural Characteristics

The primary sedimentary structure can directly reflect the strength of hydrodynamic conditions. This structural morphology remains stable under the influence of other factors, and it is a reliable basis for inferring sedimentary environments [28]. By observing sedimentary structures based on core data, the sedimentary characteristics of the target layer can be further judged.

3.2.1. Types of Bedding

There are seven types of bedding: horizontal wavy bedding, horizontal bedding, compound bedding, parallel bedding, ripple bedding, small-scale cross-bedding, and scour surfaces.

Horizontal wavy bedding is manifested as a series of nearly horizontally extended and mutually parallel wavy undulating layers formed under low-energy shallow-water conditions, commonly seen in the upper parts of subaqueous distributary channels. It is relatively commonly developed, with a development chance of about 90%. Horizontal bedding represents sediments that accumulate in a stable and continuous manner, being widely developed in mudstones and formed in a static water depositional environment. Development is extensive and at almost 100%. Compound bedding includes flaser bedding, wavy bedding, and lenticular bedding, manifested as periodically changing sand–mud bedding combinations, mainly affected by the intermittent low-energy supply environment, distributed in argillaceous siltstones. It is relatively commonly developed, with a development chance of about 90%. Parallel bedding represents thin laminae formed under strong hydrodynamic conditions, as seen in the lower parts of channel deposits. The likelihood of development is low, representing only about 10%. Ripple bedding is formed in an environment with higher energy than simple wavy bedding, with coarse grain sizes and laminae distributed in a wavy and interleaved manner, commonly found in channel deposits. It had a moderate development likelihood of around 30%. Small-scale cross-bedding is formed in a low-energy depositional environment in weaker hydrodynamic conditions, with finer laminae, as visible in the upper parts of subaqueous distributary channels. It has a moderate development likelihood of around 30%. Scour surfaces are formed at the bottom of channel deposits, resulting from the erosion of underlying weak hydrodynamic depositional environment sediments by sediments under strong hydrodynamic conditions (Figure 6a). It is relatively commonly developed, with a development chance of about 90%.

3.2.2. Contemporaneous Deformation Structures

Convolute bedding is formed during the processing of sediments with high deposition rates and under complex hydrodynamic conditions after being disturbed and deformed, usually related to flows or slides shortly after deposition. The bedding no longer maintains a straight morphology, showing a complex bending, curling, and even winding state, as seen in channel deposits, with a small likelihood of development of around 20%. Water escape structures are formed in rapidly accumulated loose sediments with rapidly changing hydrodynamic conditions. Due to the rapid drainage of pore water during deposition, contemporaneous deformation occurs within the sediments. They are found in channel deposits. A large number of contemporaneous deformation structures, such as water escape structures and convolute bedding, have been found in the transitional lithology and the interval of interbedded siltstone and mudstone. These structural types reflect the sedimentary environment of the delta front and are highly indicative (Figure 6b). They have a moderate development likelihood of around 40%.

3.2.3. Sediment Gravity Flow Bedding

Gravity flow is the movement of high-density fluids rich in large amounts of sediments under the action of gravity. It has multiple types manifested mainly as thin, repetitive laminae. Internal visible slump tears and slump deformations are present. At the same time, the bedding is rich, including graded, superimposed inverse graded, horizontal wavy, and vortex bedding, as found in the lower parts of channel deposits (Figure 6c). This gravity flow is ephemeral, small in scale, and thin in thickness. It does not develop a complete gravity flow depositional sequence and cannot be considered an independent gravity flow layer. Only occasional traces of its presence are observed, and it does not constitute a major component. Instead, it typically coexists with other microfacies types within the delta front environment.

4. Results

4.1. Characteristics of Delta Sedimentary Facies

The delta facies in the study area can be further classified into delta front subfacies and pro-delta subfacies.

4.1.1. Delta Front Subfacies

The delta front subfacies represents the seaward extension of the delta plain, located in the lower region of the wave base. Depositional activities are frequent here. It is the frontal zone where river-borne sediments vigorously interact with the marine environment, forming the main area of sandy deposits in deltas. The notable features of this subfacies include mudstones with a grayish or grayish-green hue, reduced oxidation traces, and abundant pyrite and other deep-water authigenic reducing minerals. Compared to the inland distributary plain subfacies, it exhibits more developed and distinct bedding structures and more prominent lacustrine bedding; the lithology is mainly siltstone, dominated by an interlayered structure of very thin sandstone and mudstone. There is a variety and large number of fossils, particularly freshwater biogenic traces such as mussel shells, ostracod fossils, and abundant plant fragments. Subaqueous distributary channels are thin and narrow, with relatively weak scouring capacity, sandwiched between the delta distributary plain and the pro-delta [29,30]. This subfacies can be subdivided into five sedimentary microfacies: subaqueous distributary channels, estuary dams, estuary dam margins, sheet sands, and interdistributary bays (

Table 1. The logging interpretation criteria for the sedimentary microphases of the delta front.

Sedimentary Microfacies	Subaqueous Distributary Channels	Estuary Dams	Estuary Dam Margins	Sheet Sands	Interdistributary Bays
GR (API)	50–100	50–100	80–135	70–130	120–145
RLLD/RLLS (Ω·m)	20–60	20–70	12–30	12–30	7–11
AC (μs/m)	200–230	200–230	220–240	220–240	230–260

, Figure 7a–e). The following list details the features of different subfacies:

(1)

The subaqueous distributary channel microfacies is the continuation of the river system underwater, inheriting characteristics such as positive rhythm deposition and bottom scour surfaces from inland distributary channels. However, due to the influence of multiple factors such as water flow, waves and tides in the depositional environment, it exhibits distinct features: the sediments are generally finer and of higher purity, dominated by siltstones with scattered fine sandstones and very few mudstones, while depositional mutations are mostly located at the top. Additionally, the electrical logging curves show high similarity. The natural potential and natural gamma present either medium-to-high-amplitude bell-shaped or box-shaped patterns or bell-shaped or box-shaped patterns with tooth-like features, and the thickness is reduced. GR, 50–100 API; RLLD/RLLS, 20–60 Ω·m; and AC, 200–230 μs/m for the subaqueous distributary channel. Core analysis of the Qing 1 Member in Well Hei 60 reveals that the bottom rock strata consist of alternating fine siltstones rich in light brown oil patches and gray siltstones. A typical scour mutation surface exists at the bottom of this interval, with prominent interface characteristics. It is an indicator of the scour surface formed during the deposition process of subaqueous distributary channels.

(2)

The estuary dam microfacies serves as a distinctive sedimentary body on both sides of subaqueous distributary channels. And it is the zone of the most intense interaction between rivers and marine waters, with the fastest deposition rates. Its sedimentary characteristics are prominent, mainly consisting of well-sorted, sand-pure silt and fine sand, exhibiting a reverse cyclic sequence structure and widely developed cross-bedding. In logging responses, natural gamma and natural potential show medium-to-high-amplitude funnel shapes, which are typical features of estuary dam deposits. GR, 50–100 API; RLLD/RLLS, 20–70 Ω·m; and AC, 200–230 μs/m for the estuary dam microfacies. Core analysis of the Qing 1 Member in Well Hei 60 indicates that depositional mutations are mostly at the top, with the lithology dominated by siltstone interspersed with sandy mudstone and calcareous argillaceous siltstone.

(3)

The estuary dam margin microfacies typically exhibits a gradual transition from sandy to muddy sediments. The depositional characteristics are similar to those of the estuary dam, but the thickness is thinner and the lithology is finer than that of the estuary dam. Sand–mud interbedding may occur or muddy sediments may dominate, mainly composed of siltstones, muddy siltstones, and mudstones. Compared with the estuary dam, the anomaly amplitudes of natural gamma and natural potential amplitudes decrease, with gentler shapes. GR, 80–135 API; RLLD/RLLS, 12–30 Ω·m; and AC, 220–240 μs/m for the estuary dam margin microfacies.

(4)

The sheet sand microfacies represents the lateral migration of sandy sediments from estuary dams and distal bars under continuous wave and shore current erosion. These sandy materials are gradually arranged into ordered and widely horizontally extended sheet-like or band-like forms and eventually deposited and distributed in the outer margin of the delta front. Sheet sands are pure and well-sorted, mainly made of siltstone and argillaceous siltstone, and the sand body thicknesses typically do not exceed 2 m, reflecting a strong current depositional environment. In terms of depositional structures, sheet sands share significant similarities with estuary dams, with both developing wavy bedding, cross-bedding, horizontal wavy bedding, and lenticular bedding, and there are relatively fewer biogenic fossils in sheet sands. Natural gamma and natural potential curves show medium-to-high-amplitude finger shapes. GR, 70–130 API; RLLD/RLLS; and 12–30 Ω·m, AC, 220–240 μs/m for the sheet sand microfacies.

(5)

The interdistributary bay microfacies can effectively distinguish between subaqueous and terrestrial environments. It is located in low-lying areas between subaqueous distributary channels, away from the direct impact of the main channels. The depositional environment is relatively mild, mainly depositing fine-grained suspended materials dominated by mudstone, with gray and grayish-black colors and developed horizontal bedding. Core analysis of the Qing 1 Member in Hei 60 reveals predominantly dark gray and black-gray horizontal bedding mudstone. GR, 120–145 API; RLLD/RLLS, 7–11 Ω·m; and AC, 230–260 μs/m for the interdistributary bay microfacies.

Natural gamma-ray logs were acquired by using the NaI scintillation tool of the Wiseye1000 system of Daqing Logging Company, calibrated using a calibration well cluster owned by the company, and verified on-site using a Cs137 field source. In this system, the natural gamma-ray log and dual laterlog has an effective vertical resolution of about 1 m; the acoustic log is 0.6 m.

4.1.2. Pro-Delta Subfacies

The pro-delta subfacies is located at the foremost seaward extension of the delta depositional system, and it is the area with the thickest sediment accumulation in river-dominated deltas. Sediments are generally deposited below the wave base, the deposition process is more significantly influenced by tidal currents and ocean currents, and the sediment composition is dominated by silty clay and dark colored clay [30,31,32]. Dark colored pro-delta mud is rich in organic matter, exhibiting good oil-generating potential. The depositional structure is relatively uniform, mainly characterized by horizontal bedding or fine wavy bedding, with visible bioturbation and lebensspur structures, locally visible slump deformation structures (Figure 7f), and logging curve values similar to the interdistributary bay microfacies.

4.2. Characteristics of Lacustrine Deposition

The deep lacustrine subfacies depositional zone lies below the wave base and is far from the lakeshore influence, characterized by weak hydrodynamic conditions and a typical static water environment. This environment is beneficial to organic matter preservation and hydrocarbon generation. It is dominated by gray-black mudstones, mud shales, and oil shales rich in organic matter, locally containing siltstone lenticles. Horizontal bedding and lamination are developed, and ostracods and pyrite can be observed (Figure 7g), along with logging curve values similar to the interdistributary bay microfacies.

4.3. Vertical Distribution Patterns

A total of 3085 wells have been drilled in the study area, resulting in a high well density. In particular, within the intensive development zones, well spacing ranges from approximately 100 to 300 m, providing a solid foundation for delineating the distribution characteristics. Based on sedimentary facies analysis, detailed interpretations of single-well and multi-well sedimentary facies profiles (Figure 8 and Figure 9) were conducted to summarize the formation characteristics and spatial distribution patterns of sand bodies in the study area (note: for clarity, a subset of wells is shown due to space limitations). The findings offer important guidance for accurately understanding the areal distribution of delta front sand bodies and lay the foundations for predicting their spatial extension [33,34].

Through detailed stratigraphic correlation of the Qing 1 Member in the Daqingzijing area, it was found that the drilling geological data are relatively detailed, and the sequences of each sand group have high integrity. Sand Group IV and even Sand Group III are absent in some wells. A multi-well profile from Hei 184 to Qianshen-11 was plotted to analyze the vertical development characteristics of sand bodies. The following features are summarized: (1) The Qing 1 Member strata are thicker in the southwest and thinner in the northeast, and the lake basin develops gradually. (2) In the southwest part of the study area, subaqueous distributary channel microfacies and estuary dam microfacies are mainly developed, with relatively thick layers, constituting the main sands of the delta front zone. In the central part, estuary dam marginal microfacies and sheet sand microfacies are developed, with relatively thin layers. In the northeast part, pro-delta microfacies and deep lacustrine facies are developed. (3) Sand bodies are distributed in a strip-like pattern, with thicknesses ranging from 2 to 8 m. (4) Multi-stage subaqueous distributary channel sand bodies are superimposed vertically and have good connectivity horizontally, developing in a contiguous manner. The multi-well profile shows that channel sand bodies are mostly developed in the shallow-water delta front and deteriorate from the bottom to the top of the profile.

4.4. Plane Distribution Characteristics

The Qing 1 Member in the Daqingzijing area mainly consists of delta front subfacies, pro-delta subfacies, and deep lacustrine facies. The deep lacustrine facies are distributed in the northeast, southeast, and north of the study area. The delta front is distributed in the southwest of the study area, located between the delta front and lacustrine facies, and it is the thickest area of the delta. This sedimentary pattern of delta front–pro-delta–deep lacustrine reflects the gradual reduction in provenance supply from the southwest to the northeast during this period and the expansion of the lake (Figure 10).

The development area of the delta front is mainly composed of gray fine sandstone, siltstone, and dark gray to gray-green mudstone, exhibiting extremely thin interlayered structures alternating between sandstone and mudstone. It contains mussel shells, ostracod fossils, and abundant plant fragments. The thickness of sandstone generally ranges from 0 to 4 m, with more in the range of 4 to 6 m, less in the range of 6 to 8 m, and a maximum thickness exceeding 10 m. The delta front is characterized by the development of subaqueous distributary channels and broad-band thick estuary dams. The sand bodies are oriented from southwest to northeast, gradually thinning from the thickest part in the middle to both sides, and they are deposited extensively, with multiple layers of sand bodies overlapping with and connecting to each other, providing conditions for the formation of lithologic reservoirs. The pro-delta development area is located in the seaward direction of the delta front, and the sediments are muddy materials rich in organic matter, appearing dark in color with fine textures. It is a good source bed, with significant bioturbation and abundant lebensspur structure development. Some areas contain slump deformation structures, with almost no sandstone development. The facies belt is thick and widely distributed. The deep lacustrine facies are distributed in front of the pro-delta facies and mainly developed in the northeast and east of the study area. The lithology is dominated by gray-black mud shales and oil shales rich in organic matter, with the development of horizontal bedding and lamination noted. At the end of the Qing 1 Member, the lake reaches its maximum extent and depth, representing an important hydrocarbon source rock system in the Songliao Basin [35].

5. Discussion

5.1. Sedimentary Evolution Characteristics of the Qing 1 Member

The sedimentary evolution process plays a crucial role in controlling the location, characteristics, development degree, and distribution pattern of hydrocarbon-generating formations and reservoirs, as well as the sedimentary processes and accumulation mechanisms in the study area [36,37,38]. Based on the analysis of single-well and profile facies, the planar distribution patterns of sand bodies at different stratigraphic levels were established. These patterns were then used to infer the sedimentary evolution of the Qing 1 Member within the study section.

The fastest transgression occurred at the Qing 1 Member, leading to the development of relatively developed source rocks. The maximum transgression sand bodies were formed at the end of the Qing 1 Member, resulting in the development of a large set of organic-rich source rocks in Sand Group I. Secondary transgressions occurred in Sand Group II, where a large set of mudstones was deposited nearby (Figure 11). This transgressive pattern broadly matched the sequence-stratigraphic framework proposed by Feng and Zhang (2020) for the Qingshankou Formation of the Songliao Basin, lending support to our interpretation [39,40].

The overall sedimentary process of the Qing 1 Member was mainly influenced by transgression. At the beginning of deposition, the lake basin underwent rapid transgression with the fastest water body expansion, followed by a sustained and stable depositional stage. During this process, good-quality hydrocarbon source rocks were formed at the bottom of the Qing 1 Member. Provenance analysis indicated that the sediments in this area mainly originated from the southwest. In the Daqingzijing area, as the lake basin advanced, delta front and pro-delta depositional facies belts were formed sequentially. The southwestern and central regions close to the provenance were dominated by delta front deposits, while the northeastern region far from the provenance mainly exhibited pro-delta depositional characteristics. In the sedimentary sequence from Sand Group IV to Sand Group I, the basin underwent significant changes. During the Sand Group IV period, the lake basin subsided rapidly, and the transgression scope of the study area expanded significantly. There was a short-lived fluctuation in the water body, which generally promoted significant changes in the scale of sand bodies. From Sand Group IV to Sand Group III, the thickness of sand bodies gradually increased; in Sand Group II, the maximum thickness decreased slightly, but the distribution range of sand bodies expanded. In Sand Group I, both the thickness and distribution range of sand bodies significantly decreased, reflecting the peak stage of lake basin development. During the Sand Group III period, the lake energy was stable, and the ability of the water body to carry sedimentary clastics increased. Estuary dams developed, and sand bodies were widely distributed with large thicknesses. Among them, Layer 12 was the most developed, being the thickest sedimentary layer in Sand Group III. During the Sand Group II period, rivers developed significantly, and the provenance supply from the southwest increased. Sand bodies gradually expanded northeastward, promoting the development of subaqueous distributary channels and estuary dam sand bodies. During the Sand Group I period, the lake basin expanded rapidly, and the development of subaqueous distributary channels worsened. The proportion of sand body area was the smallest across the entire region, resulting in poor sand body development at this stage. At this time, the lake basin experienced maximum transgression and dark mudstones developed extensively, generating a large amount of high-quality hydrocarbon source rocks at the top of the Qing 1 Member. In summary, the sedimentary process represents a complete cycle of the lake basin from rapid transgression to peak development, accompanied by complex evolution processes of sand bodies, mudstones, and hydrocarbon source rocks.

5.2. Sedimentary Model of the Qing 1 Member

Analyses of the sand body distribution characteristics and sedimentary evolution processes indicate that the Qing 1 Member of the Daqingzijing area in the Songliao Basin is a typical retrogradational shallow-water delta deposition, with strong control by the lake basin over sand body development and deposition [41,42,43,44,45,46,47,48]. Based on the above research results, a retrogradational shallow-water delta deposition model for this area was constructed using Gptmap (2017.1_v4.5.18.38745) software (Figure 12). Our retrogradational interpretation was broadly consistent with the ‘shoaling-upward shallow-lake delta pattern’ documented by Cai et al. (2022) for Sanzhao depression, thereby suggesting that similar accommodation–sediment supply dynamics can operate in intracratonic lake settings [49].

During the deposition of the Qing 1 Member, the lake basin base level rose overall, with well-developed hydrocarbon source rocks. At the same time, the flat terrain and abundant provenance provided a solid material basis for the extensive development of delta front sand bodies, forming a lake–shallow-water delta depositional system. Initially, the base level was in a rising stage. Influenced by lake basin expansion, the lake surface showed an upward trend, the water body became deeper, and the depositional system gradually contracted towards the southwestern provenance area. During the middle stage, the lake basin contracted and remained stable for a period. Influenced by provenance expansion, there was strong river sand transport, with numerous river channel branches and the broad-band development of estuary dams, collectively constituting the skeletal sand bodies. In the late stage, the lake basin continued to develop and extended towards the provenance direction. Subaqueous distributary channels and estuary dams were sporadically distributed near the provenance. At the same time, affected by lake water, the sand bodies were widely distributed on the near-lake side of the delta front, accompanied by a large number of interdistributary bays.

Due to the influence of retrogradation, the shallow-water delta front continuously contracted in the provenance direction. Lake basin deposition became increasingly developed. The color of the sediments transitioned from light to dark from the bottom to the top. The development scale of sand bodies gradually decreased, dark mudstones developed, and a retrogradational depositional sequence with extensive development of hydrocarbon source rocks was formed.

6. Conclusions and Suggestions

(1)

Based on geological data such as well logs and core observations, the sedimentary microfacies types of the shallow-water delta front were identified. These can be subdivided into five microfacies: subaqueous distributary channels, estuary dams, estuary dam margins, sheet sands, and interdistributary bays. Using the electrical response characteristics of the rocks, logging facies identification templates and corresponding interpretation criteria were established for each microfacies.

(2)

Based on single-well and profile facies analyses, the planar distribution characteristics of the shallow-water delta front and the sedimentary evolution of the Daqingzijing area were systematically delineated. The Qing 1 Member exhibits a transitional characteristic from the delta front to the pro-delta to the deep lacustrine. The spatio-temporal distribution characteristics in the study area are as follows: the provenance from the southwest is mainly the delta front, and as it migrates northeastward, vertically, it begins to transition into the pro-delta and deep lacustrine facies with the advance of a large lake area. With the continuous rise in the lake level, the development scale of the sand body decreases. And it shows a pattern of reduced channel development vertically, thinner sandstone layers, the gradual contraction of delta front sand bodies planarly, and the establishment of a sedimentary model of shallow-water delta.

(3)

The shallow-water delta retrogradational reservoirs are dominated by channel sand bodies and estuary dam sand bodies. The estuary dam sand bodies are relatively well-developed and distributed on both sides of the channels, and they exhibit notable characteristics of vertical stacking, lateral connectivity, and continuous development in the sedimentary sequence. This finding challenges the traditional understanding that river mouth sand bodies develop only at the bifurcation points of distributary channels. It lays a foundation for future reservoir development in the Daqingzijing area and offers new insights into the exploitation of similar sedimentary features in shallow-water delta systems.

(4)

Several factors were not fully considered in this study, including sample chronology, interpretational uncertainty, and potential alternative facies models. It is recommended that future research incorporates additional constraints, such as the integration of 3D seismic data, expansion of the coring grid, and application of provenance geochemistry.