Sedimentary Environment and Evolution of the Lower Cretaceous Jiufotang Formation in the Pijiagou and Tanjiagou Sections, Southern Fuxin Basin, NE China

Abstract

The Lower Cretaceous Jiufotang Formation in the Fuxin Basin contains a proven petroleum system. However, its southern part remains underexplored due to limited drilling and fragmentary sedimentary studies. To address this issue, we conducted detailed sedimentological logging of the two typical outcrop sections, Pijiagou and Tanjiagou. Field observations, petrographic data, and grain-size analysis were integrated to decipher hydrodynamic conditions, calibrate microfacies associations, and reconstruct the sedimentary evolution through facies stacking pattern analysis. The results show that the Jiufotang Formation predominantly consists of calcareous fine-grained clastic rocks, with poorly sorted sandstones indicative of low-energy conditions. Sediment transport mechanisms include both traction and turbidity currents, with suspension being predominant. The succession records a depositional transition from fan-delta to lacustrine environments. Two subfacies, fan-delta front and shore-shallow lacustrine, were identified and subdivided into seven microfacies: subaqueous distributary channels, interdistributary bays, subaqueous levees, mouth bars, muddy shoals, sandy shoals, and carbonate shoals. The sedimentary evolution reflects an initial lacustrine transgression followed by regression, interrupted by multiple lacustrine-level fluctuations. The alternating depositional pattern of lacustrine and deltaic facies has formed complete source-reservoir-seal assemblages in the Jiufotang Formation in the study area, making it a potential favorable target for hydrocarbon accumulation.

1. Introduction

The Fuxin Basin, located in western Liaoning Province, NE China, is a representative small-sized (<1 × 10⁴ km²) Late Mesozoic hydrocarbon-bearing basin. It is rich in diverse energy resources, including conventional oil and gas, coalbed methane, and geothermal energy [1,2]. The basin has a long history of petroleum exploration. To date, more than 300 shallow wells have encountered varying degrees of hydrocarbon shows, with oil and gas flows having been obtained from the Lower Cretaceous Shahai and Jiufotang formations [3]. The LFD2 well, deployed and implemented by Shenyang Center of China Geological Survey, produced an industrial oil flow of 15.3 m³/d from the Shahai Formation, representing the first major breakthrough in oil and gas exploration in the Fuxin Basin [4].

Previous researchers have extensively investigated the petroleum geological conditions in the Fuxin Basin. These works revealed that, in contrast to both the coal-measure and lacustrine source rocks in the Shahai Formation, the Jiufotang Formation is dominated by lacustrine source rock [5,6,7]. The Jiufotang Formation contains multiple thick layers of lacustrine dark mudstones rich in organic matter, characterized primarily by Type II kerogen and high organic matter maturity, which provide excellent conditions for hydrocarbon generation [8,9,10]. It represents the most important source rock layer in the Fuxin Basin. Meanwhile, the fan delta sand bodies within the Jiufotang Formation provide favorable reservoir space for hydrocarbons, and the thick mudstone and shale layers act as effective caprocks [11]. It has been inferred that the Jiufotang Formation has a favorable source-reservoir-seal configuration [12].

Reconstruction of sedimentary environments is critical for delineating source–reservoir–seal configurations, elucidating hydrocarbon migration and accumulation mechanisms, and pinpointing high-potential exploration fairways. Previous exploration efforts have been predominantly focused on the northern Fuxin Basin, whereas the southern region of this basin remains relatively underexplored. Fundamental geological research in the southern Fuxin Bassin is still in its early stages, and systematic research on the sedimentary environment of the Jiufotang Formation has not yet been conducted. Given the distinct sedimentary environments between the northern and southern parts of the basin, the hydrocarbon accumulation conditions are also likely to differ significantly. In recent years, significant intervals of organic-rich mudstone and shale have been identified within the Jiufotang Formation in the southern Fuxin Basin. These units exhibit high total organic carbon (TOC) content and are predominantly within the oil window, indicating favorable geological conditions for shale gas accumulation [13]. Nevertheless, systematic evaluation of the geological controls on hydrocarbon accumulation in this region remains lacking. Therefore, a systematic analysis of the sedimentary environment of the Lower Cretaceous Jiufotang Formation in the southern Fuxin Basin is essential to establish a foundation for understanding hydrocarbon accumulation mechanisms.

Sedimentary environment analysis enables the reconstruction of rock formation processes, sediment provenance, and transport-depositional mechanisms, forming a critical foundation for hydrocarbon resource prediction and holding significant scientific and practical value. The southern Fuxin Basin remains relatively underexplored, with a limited research foundation. The selection of appropriate sedimentological methods is therefore essential to enhance the reliability of the results.

Various approaches are available for studying sedimentary environment in continental lacustrine basins. Commonly applied methods include lithological description, grain-size analysis, paleontological indicators, elemental ratio method, gamma-ray spectrometry, and seismic interpretation [14]. Among these, the integration of well-log and seismic data has been widely used in the characterization of sedimentary facies in petroliferous basins. The abundance of radioactive uranium (U) and the U/Th ratio have been employed to assess organic carbon content in reducing paleoenvironments [15]. Seismic reflection configurations and attribute analysis have proven effective for identifying sand-body geometry and depositional features [16,17]. However, in the study area, the absence of seismic surveys and well data precludes the application of gamma-ray spectrometry and seismic-based methods. Paleontological approaches, which infer ancient depositional conditions based on the distribution of fossil assemblages and ecological principles [18], are also limited here due to sparse and low-diversity fossil records. Moreover, water-depth interpretations based solely on isolated fossil evidence tend to exhibit high uncertainty and lack the precision needed for detailed paleoenvironmental reconstruction. Elemental ratio, particularly ratios of major and trace elements, are widely used to infer paleoclimate, paleosalinity, and paleowater depth [19,20,21]. Nevertheless, prolonged surface exposure and weathering of outcrop samples may alter elemental concentrations through leaching, introducing potential errors in such proxies. Lithological characteristics—such as color, sedimentary structures, texture, mineral composition, and vertical sequences—provide critical insights into depositional conditions, including hydrodynamic energy, transport distance, redox state, and sediment source direction [22,23]. Similarly, grain-size parameters (e.g., grain-size distribution, mean size, sorting, skewness, and kurtosis) are effective indicators of sedimentary environments [24]. In the early stages of basin exploration, lithological description and grain-size analysis represent particularly practical and informative methods.

Given the current level of exploration and data availability in the southern Fuxin Basin, this study employs a combined approach of lithological description and grain-size analysis. Representative sections of the Lower Cretaceous Jiufotang Formation were selected for sedimentary environment analysis, with the aim of providing a reliable basis for ongoing and future hydrocarbon exploration in the study area.

2. Geological Background

2.1. Regional Geological Features

The Fuxin Basin is located in western Liaoning Province, spanning from Fuxin City in the north to Jinzhou City in the south. Its tectonic position is at the intersection between the E-W trending Yanshan Tectonic Belt and the NE-SW trending Second Depression of the Neocathaysian Structural System. More precisely, the basin is bounded by two major fault systems: the approximately E-W trending Chifeng-Kaiyuan Fault and the NNE-SSW trending Tanlu Fault, which collectively define its structural framework. This configuration results in an asymmetrical, doubly faulted graben basin [25] (Figure 1a,b).

The Fuxin Basin was formed during the Late Jurassic–Early Cretaceous. Crustal movement in the Late Jurassic evolved from compressional and sinistral strike-slip folding to dominantly extensional rifting. During the Early Cretaceous, the basin underwent large-scale subsidence, resulting in the deposition of a set of Late Mesozoic sedimentary strata over the basement composed of the Changcheng System and Archean metamorphic rocks, which infilled the graben structure [26,27]. The sedimentary cover consists of, in ascending stratigraphic order, the Lower Cretaceous Yixian Formation (K₁y), Jiufotang Formation (K₁jf), Shahai Formation (K₁sh), Fuxin Formation (K₁f), and Sunjiawan Formation (K₁sj) [28] (Figure 1c).

The Early Cretaceous evolution of the Fuxin Basin can be divided into three distinct stages: initial rifting, rapid rifting, and basin contraction. During the initial rifting stage, frequent fault activity and intense volcanism resulted in the formation of the volcanic rocks within the K₁y. The rapid rifting stage, characterized by two cycles of extension and contraction, was divided into two substages: extensional-torsional and compressional-torsional. The K₁jf was formed during the extensional-torsional substage (Figure 1d), when the basin underwent rapid rifting and basement subsidence, leading to a generalized transgression. River, fan delta, and lacustrine sediments were deposited in this period [29]. Subsequently, the lacustrine basin contracted, accompanied by widespread development of fan-delta systems [30]. The second phase of Yanshan tectonic movement at the end of the K₁jf deposition caused the folding and deformation of the strata. The western part of the basin uplifted, and the subsidence center shifted eastward. During the sedimentation period of the K₁sh and K₁f, the basin experienced the second cycle of extension and contraction, forming extensive lacustrine deposits of the K₁sh and coal-bearing strata in the K₁f. By the time of the K₁sj sedimentation, the basin underwent contraction and tectonic inversion, accompanied by crustal uplift, erosion, and gradual shrinkage until its eventual disappearance [31].

The sedimentary thickness of the K₁jf in the Fuxin Basin is typically between 350 and 774 m, with a maximum thickness reaching up to 1800 m. The K₁jf is divided into three members in ascending stratigraphic order. The first member is only exposed in the Qinghemen area in the northern Fuxin Basin, where multiple fining-upward cycles composed of gravelly fine sandstone to argillaceous siltstone and silty mudstone are developed. In the southern Fuxin Basin, the second and third members of the K₁jf are predominantly exposed. These consist of interbedded gray to gray-white medium-fine sandstone and gray to gray-black mudstone and shale, intercalated with brown tuff and gray argillaceous siltstone, collectively forming complete transgressive-regressive cycles [32] (Figure 1d). The lacustrine basin expanded to its maximum area during the period of the second member of K₁jf, and the interior of the basin was in a reducing semi-deep to deep lacustrine environment. The mixing of aquatic organic matter and terrestrial higher plants supplied abundant organic matter, while the eutrophic water column supported prolific algal growth, leading to the formation of multiple sets of high-quality source rocks [33]. In the period of the third member, the fan delta around the lacustrine prograded basinward, exhibiting a shallowing-upward sedimentary pattern [34].

2.2. Stratigraphic Log of Typical Sections

Although the K₁jf is widely exposed in the southern Fuxin Basin, most of the outcrops are discontinuous. Large-scale and relatively continuous complete sections are only observed in Pijiagou and Tanjiagou (Figure 1c).

The Pijiagou Section (121°10′31.42″ E; 41°31′15.99″ N): The exposed strata comprise the K₁jf and K₁sh (Figure 2a,b). The lower part of the K₁jf consists of two fining-upward cycles, represented by gray-white calcareous siltstone to green-gray silty mudstone (Layers 1–4), followed by yellow-gray silty fine sandstone to green-gray silty mudstone (Layers 5–6). The middle part is characterized by fine-grained deposits of gray-black mudstone interbedded with gray-white calcareous siltstone (Layers 7–10). The upper part includes two distinct cycles: a fining-upward sequence of yellow-gray silty fine sandstone to green-gray silty mudstone (Layers 11–13), and a coarsening-upward sequence of green-gray argillaceous siltstone to gray-white calcareous siltstone (Layers 14–15). The uppermost part consists of green-gray silty mudstone interbedded with gray-white calcareous siltstone and gray-white micrite (Layers 16–18). The K₁sh, distinguished by heterogeneous conglomerate, overlies the K₁jf with an angular unconformity (Layer 19) (Figure 2a,b).

The Tanjiagou Section (121°14′40.06″ E; 41°22′23.87″ N): The exposed strata comprise the K₁jf overlain by the Upper Cretaceous Daxingzhuang Formation (K₂dx) (Figure 2c,d). The K₁jf comprises four complete coarsening-upward cycles, each progressing from white shale and gray-black mudstone at the base, through light-gray calcareous argillaceous siltstone, to light-gray and yellow-gray fine sandy siltstone at the top (Layers 1–13). The K₂dx, consists of gray-black dacite, overlies the K₁jf with an angular unconformity (Layer 14) (Figure 2c,d).

3. Sample and Methods

All samples were collected from the K₁jf in the Pijiagou and Tanjiagou sections, located in the southern Fuxin Basin. Each sample was collected from a distinct sandstone layer. During fieldwork, care was taken to avoid contamination from adjacent layers and foreign substances. In the laboratory, fresh to slightly weathered samples were selected for thin section preparation and grain size analysis. A total of ten samples were collected, comprising six from the Pijiagou Section and four from the Tanjiagou Section. The specific sampling locations are shown in Figure 2a,c.

The thin section identification was carried out at the Experimental and Testing Center of Shenyang Center of China Geological Survey. Relatively intact hand specimens should be selected for sectioning and grinding to produce thin sections, each with an area greater than 18 mm × 18 mm. Microscopic observations were conducted using a polarizing microscope, with photomicrographs taken for documentation. The content of quartz (Q), feldspar (F) and lithic fragments (R) was statistically analyzed by the point-count method [35]. The number of points counted for each sample is over 300. The rock classification and naming standards followed the Chinese National Standard GB/T 17412.2-1998, and the Q-F-R ternary plots was drawn accordingly.

Grain size analysis was performed at the Key Laboratory of Unconventional Oil and Gas Reservoirs and Development, Northeast Petroleum University. Given the fine-grained nature of all collected clastic rock samples, laser diffraction analysis was employed due to its broad measurement range (0.02–2000 μm) and high precision. Measurements were conducted using a Malvern MS2000 laser particle size analyzer in accordance with the Chinese National Standard (GB/T 19077-2016). Sample preparation was carried out by wet dispersion, where samples were initially subjected to mechanical homogenization at 2500 rpm, followed by ultrasonic treatment for 120 s to ensure complete particle disaggregation. Optical parameters were optimized with a refractive index of 1.80 and absorption coefficient of 0.10 for accurate mineralogical characterization. Quality control metrics demonstrated excellent analytical precision, with repeatability tests yielding relative standard deviations (RSD) ≤ 0.5%. The expanded measurement uncertainty ranged from ±1.5% to ±2.5% at 95% confidence level. All analyses were performed under controlled laboratory conditions (18 ± 1 °C, 37 ± 2% relative humidity) to ensure data reproducibility.

This paper integrates grain-size analysis with field observations and petrographic data to reconstruct the sedimentary environment of the K₁jf in the southern Fuxin Basin. The methodology proceeded as follows: After field description and sampling, the petrological characteristics of the samples were analyzed examined via thin section microscopy, leading to their classification based on lithology. Then, representative samples from each category were subjected to grain-size analysis. Grain sizes were classified according to the Udden-Wentworth-φ standard [36,37]. Next, the grain size parameters such as maximum grain size, median, mean, deviation, skewness, and kurtosis, were calculated using the formulas proposed by Folk [38]. Subsequently, grain size distribution diagrams, probability cumulative curves, C-M diagrams, and other parameter plots were generated. These diagrams, in conjunction with the Sahu discriminant function [39,40,41], were used to interpret the sedimentary environment. Finally, a detailed sedimentary facies classification and an analysis of the sedimentary evolution were conducted by synthesizing the sedimentary characteristics observed in the sections with the grain-size data.

4. Results

4.1. Petrological Characteristics

All the samples can be classified into three distinct lithotypes: Type I (Feldspar lithic fine sandstone), Type II (Feldspar lithic siltstone), and Type III (Argillaceous siltstone). All three types are observed in the six samples from the Pijiagou Section, while the four samples from the Tanjiagou Section exclusively belong to Type II (

Table 1. Petrological characteristics of samples in the Pijiagou and Tanjiagou sections.

Lithotypes	Type I	Type II	Type III
Lithology	Feldspar Lithic Fine Sandstone	Feldspar Lithic Siltstone	Argillaceous Siltstone
Representative sample	PJG-05, PJG-11, PJG-12	PJG-03, PJG-13, TJG-03, TJG-06, TJG-09, TJG-11	PJG-18
Texture	Fine sandy texture	Silty texture	Muddy silty texture
Sorting	Poor	Poor	Poor
Rounding	Subangular to subrounded	Subangular	Unobservable
Clastic composition	Quartz (15–20%); Feldspar (20–26.3%); Lithic (45–46.3%)	Quartz (5–15%); Feldspar (16.3–22.5%); Lithic (37.5–45%)	Abundant clay and minor feldspar
Interstitial material	Clay (6.3–10%)	Clay (17.5–38.8%)	Unobservable
Supporting style	Grain supported	Grain supported	Unobservable
Cementation style	Porous cementation	Porous cementation	Unobservable

).

Type I exhibits a fine sandy texture with inclusions of silty particles. It is poorly sorted, and grains are subangular to subrounded. The clastic components are predominantly lithic fragments, with subordinate quartz and feldspar. Quartz grains are granular or broadly tabular, with euhedral to subhedral shapes, and some display undulatory extinction. Feldspar, composing both orthoclase and plagioclase, occurs as broadly tabular, subhedral to euhedral crystals, showing Carlsbad and polysynthetic twinnings. Lithic fragments are predominantly sandstone and mudstone clasts with irregular shapes. The interstitial material is primarily clay, with a grain-supported fabric and pore-filling cementation (Figure 3a,b and Figure 4). Type II displays a silty texture with minor fine sandy particles. It is poorly sorted with subangular grains. The composition of the clastis grains and interstitial material are broadly similar to Type I, with a matrix-supported cementation fabric (Figure 3c,d and Figure 4). Type III has a muddy silty texture, containing abundant clay and minor feldspar. It is poorly sorted, and clay minerals show complete extinction under cross-polarized light (Figure 3e,f and Figure 4).

4.2. Grain Size Characteristics

4.2.1. Grain Size Grades and Parameters

The sandstones from the Pijiagou Section have mean grain sizes ranging from 3.486 to 7.070 φ [φ = −log₂(d/mm)], which predominantly fall within the fine sand to fine silt grades (

Table 2. Distribution of sandstone grain size parameters in the Pijiagou Section.

Sample		PJG-03	PJG-05	PJG-11	PJG-12	PJG-13	PJG-18
Lithology		Mud-Bearing Fine Sandy Siltstone	Silt-Bearing Fine Sandstone	Silty Fine Sandstone	Silt-Bearing Fine Sandstone	Mud- and Fine Sand-Bearing Siltstone	Argillaceous Siltstone
Coarse sands	0 < φ ≤ 1	0	0	0	0	0	0
Medium sands	1 < φ ≤ 2	0	6.299	4.577	6.818	0	0.197
Fine sands	2 < φ ≤ 3	5.157	42.274	36.119	39.985	2.296	1.930
Extremely fine sands	3 < φ ≤ 4	24.458	27.919	29.328	26.267	19.913	4.931
Coarse silts	4 < φ ≤ 5	24.855	8.435	11.125	10.239	28.879	9.760
Fine silts	5 < φ ≤ 8	32.813	10.767	14.145	12.622	38.608	52.232
Clay	φ > 8	12.717	4.304	4.706	4.069	10.304	30.950
C (mm)		0.169	0.307	0.298	0.314	0.142	0.171
M (mm)		0.036	0.122	0.106	0.118	0.032	0.008
Mz (φ)		5.282	3.398	3.670	3.486	5.327	7.070
σ (φ)		1.967	1.515	1.631	1.562	1.749	2.057
SK		0.406	0.524	0.512	0.513	0.361	0.010
KG		0.970	1.704	1.393	1.419	1.048	0.962

φ = −log₂(d/mm); C: maximum grain size; M: median grain size; M_z: mean grain size; σ: deviation; S_K: skewness; K_G: kurtosis.

). Corresponding to the three petrologically defined types, the sandstone samples are classified into three categories (Type I, Type II, and Type III) based on their grain size distribution curves and Folk’s classification criteria [38].

Type I sediments consist of a mixture of fine sand and silt. The grain size parameters are as follows. The maximum grain size (C) ranges from 0.298 to 0.314 mm, with an average of 0.306 mm. The median grain size (M) ranges from 0.106 to 0.122 mm, with an average of 0.115 mm. The mean grain size (M_z) ranges from 3.398 to 3.670 φ, with an average of 3.518 φ. The deviation (σ) ranges from 1.515 to 1.631 φ, with an average of 1.569 φ. The skewness (S_K) ranges from 0.512 to 0.524, with an average of 0.516, indicating a very positively skewed distribution. The kurtosis (K_G) ranges from 1.393 to 1.704, with an average of 1.505, indicating a leptokurtic distribution.

Type II sediments are predominantly composed of silt with subordinate amounts of fine sand and clay. The grain size parameters are as follows. The C ranges from 0.142 to 0.169 mm, with an average of 0.156 mm. The M ranges from 0.032 to 0.036 mm, with an average of 0.034 mm. The M_z ranges from 5.282 to 5.327 φ, with an average of 5.305 φ. The σ ranges from 1.749 to 1.967 φ, with an average of 1.858 φ. The S_K ranges from 0.361 to 0.406, with an average of 0.384, indicating a positively skewed distribution. The K_G ranges from 0.970 to 1.048, with an average of 1.009, which is mesokurtic and approaches a normal distribution.

Type III sediments are predominantly composed of silt. The grain size parameters are as follows. The C is 0.171 mm. The M is 0.008 mm. The M_z is 7.070 φ. The σ is 2.057 φ. The S_K is 0.010, indicating a near-symmetrical distribution. The K_G is 0.962, which is mesokurtic and approaches a normal distribution.

The sandstones from the Tanjiagou Section exhibit dominant fraction fall within the coarse silt to fine silt grade. The sediments are primarily composed of silt, with subordinate fine sand and clay. The grain size and parameters are similar to Type II (

Table 3. Distribution of sandstone grain size parameters in the Tanjiagou Section.

Sample		TJG-03	TJG-06	TJG-09	TJG-11
Lithology		Mud- and Fine Sand-Bearing Siltstone	Mud-Bearing Fine Sandy Siltstone	Mud-Bearing Fine Sandy Siltstone	Mud- and Fine Sand-Bearing Siltstone
Coarse sands	0 < φ ≤ 1	0	0	0	0
Medium sands	1 < φ ≤ 2	0.058	0.586	0.001	0.048
Fine sands	2 < φ ≤ 3	3.002	11.326	7.474	2.812
Extremely fine sands	3 < φ ≤ 4	12.001	22.709	22.923	11.325
Coarse silts	4 < φ ≤ 5	19.406	18.989	22.325	17.696
Fine silts	5 < φ ≤ 8	48.646	31.457	32.409	44.652
Clay	φ > 8	16.887	14.933	14.868	23.467
C (mm)		0.169	0.233	0.191	0.165
M (mm)		0.018	0.037	0.035	0.015
Mz (φ)		5.977	5.284	5.380	6.293
σ (φ)		1.954	2.223	2.120	2.136
SK		0.162	0.346	0.375	0.139
KG		0.915	0.850	0.902	0.827

). The C ranges from 0.165 to 0.233 mm, with an average of 0.190 mm. The M ranges from 0.015 to 0.037 mm, with an average of 0.026 mm. The M_z ranges from 5.284 to 6.293 φ, with an average of 5.734 φ. The σ ranges from 1.954 to 2.223 φ, with an average of 2.108 φ. The S_K ranges from 0.139 to 0.375, with an average of 0.256, indicating a positively skewed distribution. The K_G ranges from 0.827 to 0.915, with an average of 0.874, indicating a platykurtic distribution.

4.2.2. Grain Size Curve Characteristics

Three distinct types of sandstone samples were identified based on an analysis of grain-size distribution histograms and probability cumulative curves. The classification incorporated the modality of histogram peaks, the slopes of cumulative curves, and the line segments and their inflection points on probability cumulative curves. These differing curve morphologies reflect distinct sedimentary hydrodynamic conditions [39].

Type I is characterized by a nearly bimodal grain-size distribution with a narrow range. These samples display moderately sloping, S-shaped cumulative curves (Figure 5b–d) and three-segment probability cumulative curves (Figure 6b–d). Sediment composition is dominated by saltation components (45–55%), with subordinate suspended components (40–55%) and a negligible rolling population (<10%). The intersection of the suspended and saltation segments occurs at 3–4 φ, and that between the saltation and rolling segments at 1–2 φ (Figure 6b–d).

Type II is characterized by a unimodal, moderately to poorly sorted grain-size distribution. These samples display gently sloping, S-shaped cumulative curves (Figure 5a,e,g–j) and two-segment probability cumulative curves (Figure 6a,e,g–j). Sediment composition is dominated by suspended components (>90%), with minor saltation (<10%) and no rolling components. The intersection of the suspension and saltation segments occurs at 2–3 φ, except in sample PJG-13 (3–4 φ) (Figure 6a,e,g–j).

Type III is characterized by a unimodal, moderately sorted grain-size distribution. The sample displays a gently sloping, S-shaped cumulative curve (Figure 5f) and a two-segment probability cumulative curve (Figure 6f). Sediment composition is composed almost exclusively of suspended components (>95%), with trace saltation (<5%) and no rolling population. The intersection of the suspension and saltation segments occurs at 2–3 φ (Figure 6f).

5. Discussion

5.1. Sedimentary Environment

5.1.1. Sedimentary Environment Analysis

Field descriptions and microscopic observations show that the K₁jf in the southern Fuxin Basin consists primarily of fine-grained clastic sediments with minor chemical precipitates. The sediments are compositionally and texturally immature, showing poor sorting and rounding, suggesting deposition in a short-transport depositional system. Grain-size analysis reveals generally fine average diameters, consistent with a low-energy setting. Types I, II, and III sandstones correspond to high, low, and very low energy conditions, respectively. The σ range from 1.515 to 2.223 φ, indicating poor sorting. The S_K is generally positive to strongly positive, denoting a coarse-grained skew (Table 2 and Table 3). The K_G shows a bimodal distribution for Type I, likely resulting from scour-and-fill processes, whereas Types II and III are unimodal with moderate to platykurtic kurtosis, further reflecting poor sorting (Figure 5). Probability cumulative curves demonstrate that suspended load dominates the transport mechanism, with only Type I containing a significant saltation components (Figure 6).

The C (1% threshold) and M (50% threshold) values, derived from grain-size cumulative frequency curves, serve as proxies for flow competence (maximum and mean). Passega established the C-M diagram using these parameters to interpret the transport mechanisms and hydrodynamic conditions, thereby facilitating sedimentary environment identification [40]. Analysis of ten sandstone samples from the Pijiagou and Tanjiagou sections reveals that nine samples plot within the traction-current field (one marginal), while seven plot in the turbidity-current (gravity-current) field (two marginal). This finding indicates combined traction and gravity flow processes in the K₁jf (Figure 7).

The Pijiagou Section samples display distinct patterns: Type I occupies the QR segment (gradational suspension deposition). While Types II and III plot in the RS segment (uniform suspension deposition). All samples from the Tanjiagou Section cluster in the RS segment. Turbidity-current field samples show C/M ratios of Type I range from 2.52 to 2.81 and those of Type II range from 4.44 to 11.00, suggesting that the sorting of Type I is slightly better than that of Type II (Figure 7). Section-wide comparisons reveal that the Pijiagou Section (C/M = 2.52–4.69) exhibits superior to sorting to the Tanjiagou Section (C/M = 5.46–11.00) (Table 2 and Table 3, Figure 7).

Integrated analysis reveals that the two sections were deposited under predominantly weak hydrodynamic regimes, where the transporting system exhibited hybrid characteristics combining traction and turbidity current processes, and sediment transport was primarily transported through suspension mechanisms (Figure 7).

The discriminant functions developed by Sahu, which utilize grain size parameters (M_z, σ, S_K, and K_G) to differentiate sedimentary environments [41] (

Table 4. Sahu formula for identifying sedimentary environment [41].

Sedimentary Environment	Formula	Discriminantion Value	Average Value
Aeolian process or beach	(1) Yaeol: beach = −3.5688Mz + 3.7016σ2 − 2.0766SK + 3.1135KG	Yaeol < −2.7411 Ybeach > −2.7411	Yaeol = −3.0973 Ybeach = −1.7824
Beach or shallow agitated marine	(2) Ybeach: mar = 15.6634Mz + 65.7091σ2 − 18.1071SK + 18.5043KG	Ybeach < 65.3650 Ymar > 65.3650	Ybeach = 51.9536 Ysh.mar = 104.7536
Shallow lacustrine or deltaic	(3) Ylacu: delta = 0.2852Mz + 8.7604σ2 − 4.8932SK + 0.0482KG	Ylacu > −7.4190 Ydelta < −7.4190	Ylacu = −5.3167 Ydelta = −10.4418
Deltaic or turbidity current	(4) Ydelta: turb = 0.7215Mz + 0.4030σ2 − 6.7322SK + 5.2927KG	Ydelta > 9.8433 Yturb < 9.8433	Ydelta = 10.7115 Yturb = 7.9791

), were applied to the sandstone samples from the Pijiagou and Tanjiagou sections. Subsequent application of these functions provided the following insights. The Y₃ values derived from Formula (3), which are used to distinguish shallow lacustrine from deltaic deposits, range from −35.05363 to −21.61988 for the Pijiagou Section and from −43.43660 to −32.49218 for the Tanjiagou Section, confirming a deltaic environment for both sections. The Y₄ values from Formula (4), which discriminates between deltaic and turbidity current deposits, fall within 8.55471–14.07311 for the Pijiagou Section and 8.01455–9.36902 for the Tanjiagou Section. The Y₄ values of most samples lie between the average function values for deltaic and turbidity current environments, indicating a depositional system influenced by both traction and gravity flows. A comparative analysis shows that the Pijiagou Section was primarily shaped by traction currents, whereas the Tanjiagou Section was dominated by turbidity currents (

Table 5. Discriminates results by Sahu formula for the rock samples in the two sections.

Section	Sample	Discrimination Value of Y3	Discrimination Result of Y3	Discrimination Value of Y4	Discrimination Result of Y4
Pijiagou	PJG-03	−34.32823	Deltaic	10.11891	Deltaic
	PJG-05	−21.61988	Deltaic	14.07311	Deltaic
	PJG-11	−24.69557	Deltaic	12.39548	Deltaic
	PJG-12	−22.82162	Deltaic	12.49585	Deltaic
	PJG-13	−26.99474	Deltaic	10.58773	Deltaic
	PJG-18	−35.05363	Deltaic	8.55471	Turbidity current
Tanjiagou	TJG-03	−32.49218	Deltaic	8.70714	Turbidity current
	TJG-06	−43.43660	Deltaic	8.64903	Turbidity current
	TJG-09	−39.62984	Deltaic	9.36902	Turbidity current
	TJG-11	−38.81482	Deltaic	8.01455	Turbidity current

).

The C-M diagram and the discrimination results from Sahu’s formulas suggest that the sandstone intervals in the sections were deposited under relatively weak hydrodynamic conditions. The transporting flows exhibited dual characteristics of both traction and turbidity currents, with suspension serving as the dominant mode of sediment transport. Petrologically, the sandstones are characterized by low quartz and feldspar content, indicating low compositional maturity and supporting a proximal source. Additionally, the occurrence of breccia-bearing sandstones in the K₁jf at other outcrops in the region is consistent with fan delta sedimentation [33]. Dark mudstones and shales in the sections commonly display well-developed horizontal bedding, with Eosestheria fossils preserved on bedding planes and distinct evidence of chemical sedimentation, all of which are diagnostic features of lacustrine environment. Based on comprehensive analysis, the K₁jf in this area is interpreted as having been deposited in a fan delta to lacustrine environment.

5.1.2. Sedimentary Facies Division

Based on an integrated analysis of lithology, sedimentary structures, petrography, and grain size characteristics from the Pijiagou and Tanjiagou sections, the criteria for identifying sedimentary microfacies were established. The K₁jf in the southern Fuxin Basin is delineated into two subfacies (the fan delta front and the shore-shallow lacustrine) and seven microfacies (subaqueous distributary channels, interdistributary bays, subaqueous levees, mouth bars, muddy shoals, sandy shoals, and carbonate shoals) (

Table 6. Sedimentary facies types and characteristics of the K₁jf in the southern Fuxin Basin.

Facies	Subfacies	Microfacies	Sedimentary Characteristics
Fan delta	Fan delta front	Subaqueous distributary channel	The lithology is consists mainly of fine-grained sandstone (Type I), intercalated with thin silty mudstone layers. These sandstones typically exhibit parallel bedding and rest on an erosional base, coarsening upward to form a distinct positive grading. They are texturally immature, with grain size distributions often displaying a bimodal pattern and strong positive skewness due to scour-and-fill processes. Probability cumulative curves are predominantly three-segmented, indicating a dominance of suspended and saltation components.
		Interdistributary bay	The lithology is consists mainly of gray-green or green-gray silty mudstone or argillaceous siltstone, with minor interbeds of fine silty sandstone. Localized horizontal bedding is observed.
		Subaqueous levee	The lithology consists predominantly of siltstone (Type II) with intercalations of silty mudstone. An erosional base is present, while the succession contains multiple regular rhythmic beds, with lenticular bedding observed locally. The siltstone is poorly sorted, and its grain size distributions are mostly unimodal and positively skewed. The probability cumulative curves are predominantly two-segmented, reflecting a predominance of suspended sediments over a minor saltation population.
		Mouth bar	The lithology is composed of silty mudstone, argillaceous siltstone, and fine-grained silty sandstone (Type II), forming a coarsening-upward sequence. These texturally immature sandstones exhibit unimodal grain size distributions characterized by a flat peak and positive skewness. Probability cumulative curves are predominantly two-segmented, indicating a dominant suspended load with a subordinate saltation component.
Lacustrine	Shore-shallow lacustrine	Muddy shoal	The lithology consists chiefly of dark mudstone and shale, characterized by well-developed horizontal stratification and well-preserved fossils.
		Sandy shoal	The lithology is primarily composed of siltstone and argillaceous siltstone (Type III), interpreted as gravity flow deposits with a significant argillaceous matrix. These deposits exhibit a unimodal grain-size distribution, characterized by a high C/M ratio, poor sorting, and near-symmetrical skewness.
		Carbonate shoal	The lithology is predominantly micrite, indicating a depositional environment dominated by chemical precipitation.

).

5.2. Sedimentary Evolution

Following the Yanshanian tectonic movement, the Early Cretaceous paleoenvironment in western Liaoning Province underwent a significant transformation. The regional climate shifted from hot and arid to predominantly warm and humid conditions [42]. This warm and humid regime during the deposition of the K₁jf was punctuated by fluctuations to cooler climates and seasonal droughts [43]. These climatic shifts directly controlled the alternating development of lacustrine and fan-delta sedimentary systems [44]. Concurrently, seasonal droughts induced variations in lacustrine-water salinity [45], which facilitated the localized precipitation of carbonate deposits on a considerable scale.

The studied interval in the Pijiagou Section corresponds to the second member to the bottom of third member of the K₁jf. It records a complete lacustrine basin cycle characterized by initial expansion followed by contraction, punctuated by two minor lacustrine transgressions and one regression event. The sedimentary evolution comprises four stages. The first stage (Lower part; layers 1–6) was characterized by a relatively restricted lacustrine basin. Deposition was dominated by fan-delta facies, including subaqueous levees, interdistributary bays, and subaqueous distributary channels. The sedimentary characteristics indicate high-energy hydrodynamic conditions and significant erosional forces. The second stage (Middle part; layers 7–10) marks a major transgressive phase, during which the lacustrine basin expanded to its maximum depth within the section. This period of stable, low-energy conditions led to the deposition of a thick succession of dark-colored, organic-rich mudstone and shale in shore-shallow lacustrine environments. The presence of abundant fossils and high organic matter input identifies this interval as the primary source rock unit of the formation. The third stage (Upper part; layers 11–15) shows a process of lacustrine basin contraction. A progradational sedimentary sequence developed, exhibiting a vertical succession from subaqueous distributary channels to interdistributary bays, then to subaqueous levees, and finally to mouth bars, reflecting the basinward advance of the fan-delta system. The fourth stage (Uppermost part; layers 16–18) records a minor transgressive pulse that occurred within the overall regressive trend, causing a renewed rise in lacustrine level, though water depths remained shallower than during Stage II. Sedimentation was dominated by shore-shallow lacustrine deposits under very low-energy conditions. A gradual increase in chemical precipitation resulted in the formation of a substantial thickness of interbedded fine-grained clastic rocks and carbonate rocks (Figure 8a).

The studied interval in the Tijiagou Section corresponds to the third member of the K₁jf, capturing an alternation of lacustrine and fan-delta facies. The sedimentary sequence exhibits an overall progradational motif, indicating a gradual shrinkage of the lacustrine basin and concomitant shallowing of water depth. This trend was punctuated by five minor lacustrine-level fluctuations. Transgressive phases favored the deposition of thick, dark lacustrine mudstones in shore-shallow environments, whereas regressive phases led to the progradation of fan-delta fronts, forming successions of mouth-bar deposits with inverse-grading rhythms (Figure 8b).

The sedimentary evolution recorded in the sections exhibits strong consistency with the paleoclimatic patterns documented in western Liaoning Province. Based on these correlations, we infer that the southern Fuxin Basin likely experienced comparable paleoenvironmental conditions, suggesting its sedimentary succession may demonstrate analogous evolutionary trends to those observed in the studied sections. In summary, the K₁jf in the southern Fuxin Basin documents an initial expansion followed by contraction of the lacustrine basin. This evolution was punctuated by multiple high-frequency lacustrine-level fluctuations during both transgressive and regressive phases. The interval associated with the maximum flooding event is identified as the primary source rock unit, although its hydrocarbon generation potential requires further evaluation.

5.3. Suggestions for Oil and Gas Exploration

The alternating depositional pattern of lacustrine and deltaic facies typically forms favorable zones for hydrocarbon accumulation. This classic model has been widely validated in exploration practices across numerous continental lacustrine basins, including the Ordos Basin (characterized by its extensive Triassic Yanchang Formation lacustrine-deltaic systems) (Figure 9a) [46] and the Bohai Bay Basin (notably the Paleogene Shahejie Formation, where lacustrine-delta transitions control high-quality source-reservoir-seal assemblages) (Figure 9b) [47]. Similarly, the K₁jf sedimentary sequence in the southern Fuxin Basin exhibits an alternation between lacustrine and fan-delta facies (Figure 8), closely resembling the depositional architectures of the aforementioned basins. This similarity suggests the potential for the development of high-quality source-reservoir-seal assemblages in this region.

The thick, dark mudstones and shales deposited during the maximum lacustrine flooding period in the second member of the K₁jf represent the primary source rock in this area. This unit is rich in fossils and has high organic matter content. Given that the studied sections are located at the basin margin, we suggest that the source rock interval is likely to be thicker and have attained higher content of organic matter maturity toward the depocenter. The fan-delta sand bodies of the third member serve as possible reservoirs, overlain by potential caprocks composed of shore-shallow lacustrine dark mudstones, shales, and micrites (Figure 8). Consequently, the K₁jf in the southern Fuxin Basin can possess a favorable geological foundation for hydrocarbon accumulation with significant potential for further industrial exploration.

5.4. Uncertainties

This paper provides a preliminary analysis of the sedimentary environments and evolution of the K₁jf in the southern Fuxin Basin, with implications for hydrocarbon exploration. However, the interpretations are subject to uncertainty due to data constraints. Based on our analysis of two typical sections and analogies with similar continental lacustrine basins, we hypothesize that the depocenter of the study area may hold hydrocarbon potential. Nevertheless, the absence of seismic data and wells in this area precludes a definitive assessment of fault distribution, the lateral continuity of the favorable intervals identified at the margins into the depocenter, and the evaluation of source rock quality and maturity. Therefore, further drilling and research in the southern Fuxin Basin are required for validation.

6. Conclusions

• The K₁jf in the southern Fuxin Basin consists of fine-grained clastics and local chemical sediments, characterized by low compositional and textural maturity. These features collectively suggest deposition from low-energy fluids systems with hybrid traction-turbidity flow properties, in which suspension was the dominant sediment transport mechanism.
• The K₁jf in the southern Fuxin Basin exhibits a vertical succession of fan-delta to lacustrine deposits, encompassing two subfacies (fan-delta front and shore-shallow lacustrine) and seven microfacies (subaqueous distributary channels, interdistributary bays, subaqueous levees, mouth bars, muddy shoals, sandy shoals, and carbonate shoals).
• The sedimentary record of the K₁jf suggests that the lacustrine basin in the study area underwent an overall transgressive-regressive cycle, superimposed by multiple high-frequency lacustrine-level oscillations.
• The K₁jf is considered viable for hydrocarbon accumulation, as evidenced by complete source-reservoir-seal combinations. This includes the source rocks from the dark mudstones and shales developed in the maximum lacustrine flooding period (corresponding to the second member of the K₁jf), the reservoir potential of the fan-delta sandstones in the third member of the K₁jf, and the sealing capacity provided by the interbedded shore-shallow lacustrine mudstones and micrites in the third member of the K₁jf.
• Overall, the K₁jf in the study area may have favorable oil and gas exploration prospects. However, further acquisition of seismic data and implementation of wells are needed to determine the planar distribution characteristics of sedimentary facies.