Lacustrine Basin Fills in an Early Cretaceous Half-Graben, Jiuquan Basin, NW China: Controlling Factors and Implications for Source Rock Depositional Processes and Heterogeneity

0. INTRODUCTION

Ancient lake deposits are of significant academic and economic interest, primarily because they (1) contain significant hydrocarbon resources (Katz, 2001); and (2) can be used to explore tectonic and paleoclimate issues (Carroll and Bohacs, 1999). Compared to marine basins, lacustrine rift basins are much more dynamic and sensitive to the changing geological processes (Bohacs et al., 2000; Sladen, 1994; Kelts, 1988). Consequently, sedimentary fills and stratigraphic and facies architectures of lacustrine rift basins are complicated and varied.

Many studies have documented the sedimentary fills in lacustrine rift basins and several models have been developed from well-known analogs (e.g., those from the Bohai Bay Basin in eastern China, Wang et al., 2011; Lin et al., 2004, and those from Lakes Malawi and Tanganyika in East Africa, Scholz and Rosendahl, 1990). These models, highlighting the distribution patterns of depositional systems, provide valuable insights into prediction of reservoir geometry and distribution (Wang H et al. 2017; Wang G H et al., 2016; Huang et al., 2012; Lin et al., 2001; Gawthorpe and Leeder, 2000). However, their applications in revealing source rock characteristics are very limited. A lack of understanding of the source rocks may result in errors in determining source-reservoir-cap assemblages as well as significant risks in hydrocarbon exploration. In fact, lacustrine source rocks are characterized by strong heterogeneity in organic matter type and richness under different basin filling conditions, which has been noted and attempted to be interpreted by many researchers. For example, Eugster and Kelts (1983) suggested the lake type (hydrologically open or closed) affects source rock properties. Bohacs et al. (2000) proposed that the balance of sediment and water supply (mostly a function of climate) with potential accommodation (mostly tectonic) mainly controls the development of three types of basin fills (termed overfilled, balanced fill, and underfilled) and their associated inorganic and organic components. Classification of ancient lakes into these three basin filling types is helpful for roughly assessing their source rock property and potential (Zhang et al., 2017; Carroll and Bohacs, 2001). Nevertheless, this approach may not be appropriate to explore the depositional processes and difference of source rocks within a specific filling type of basin, such as an overfilled basin or an underfilled basin. Therefore, there is still much work to be done, in order to deepen the understanding of the relationship between basin filling characteristics and source rock properties.

The Qingnan subsag, a half-graben rift unit of the Jiuquan Basin, is one of the small but most hydrocarbon-rich areas in China (Han et al., 2007; Yang et al., 2003). Its Lower Cretaceous Xiagou Formation is the most important interval of current hydrocarbon exploration. This formation is also the main source rock interval that displays strong heterogeneity in vertical succession (Ma et al., 2007). However, previous investigations of the basin fill of the Xiagou Formation were mainly based on the overall context of this stratigraphic unit (Pan et al., 2012; Wang et al., 2005), which makes it hard to elaborate on the basin filling characteristics and also the vertical difference of depositional environments of the source rock. Hence, based on a division of the Xiagou Formation into multiple lower-order stratigraphic units, this study proposes to illustrate the basin fill and its effect on the source rock properties of the Xiagou Formation in the Qingnan subsag. Its specific goals are to: (1) reveal the basin filling characteristics and controlling factors of the Xiagou Formation, (2) develop depositional models to explain the depositional process and vertical heterogeneity of the source rocks in terms of the evolution history of basin fills.

1. GEOLOGICAL SETTINGS

1.1 Jiuquan Basin

The Jiuquan Basin is an economically important oil and gas province in the Northwestern China. It is an Early Cretaceous extensional basin, covering an area of approximately 22 000 km² (Fig. 1a) (Pan et al., 2012; Li et al., 2006; Zhu et al., 2006; Shi et al., 2001). From west to east, the basin is composed of three first-order tectonic units: the Jiuxi subbasin in the west, the Jiayuguan uplift in the center, and the Jiudong subbasin in the east (Zhang et al., 2016; Chen et al., 2014; Vincent and Allen, 1999). Both of the two sub-basins are also composed of their tectonic sub-units (Li et al., 2006). Like other sedimentary basins in the Hexi Corridor region of the northwestern China, the Jiuquan Basin originated during the Late Jurassic to Early Cretaceous strike-slip and extensional deformation as a result of the Lhasa Block-Asia collision (Vincent and Allen, 1999; Hendrix et al., 1992). During the Early Cretaceous, the basin experienced three different phases of rifting, i.e., initial rifting, rifting extension, and post-rifting (Wang M F et al., 2008; Wang C X et al., 2005). These three tectonic phases correspond to three sets of lithostratigraphic units, namely, Chijinpu Formation, Xiagou Formation, and Zhonggou Formation from base to top (Figs. 1b, 1c). Evolution of the rift basin was terminated by subsequent tectonic inversion resulting from the Late Yanshanian and the Early Himalayan tectonic events (Huo, 1989). Since the Cenozoic Era, the basin then evolved into a compressional foreland basin caused by the northward movement of the northeast Tibetan Plateau, which is associated with the collision of Indian Plate and Eurasian Plate (Zhao et al., 2004).

Figure  1.  Location and structural map of the Jiuquan Basin. (a) Tectonic setting of the Jiuquan Basin during the Early Cretaceous (modified after Wang et al., 2005). The red box shows the location of the Qingxi sag. (b) Cross section (A–A') shows the half-graben and graben structural framework of the Qingxi sag and the Shida sag (modified after Pan et al., 2012). (c) Cross section (B–B') shows the half-graben and graben structural framework of the Yinger sag and the Maying sag (modified after Pan et al., 2012).

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1.2 Qingnan Subsag

The Qingnan subsag is a secondary unit of the Qingxi sag that is located in the southwest of the Jiuxi subbasin. It is bordered by the Qingxi low uplift to the northwest, the Yabei uplift to the northeast, the Southern uplift to the southeast, and the Qilian Mountain tectonic belt to the southwest, with an area of about 200 km² (Fig. 2). Controlled by the NE-trending boundary fault, the Qingnan subsag exhibits a half-graben structure with boundary fault margin in the southeast and flexure margin in the northwest (Fig. 3). The Lower Cretaceous succession (i.e., Xinminpu Group) in the Qingnan subsag comprises three secondary stratigraphic units: Chijinpu Formation (K₁c), Xiagou Formation (K₁g), and Zhonggou Formation (K₁z) (Chen et al., 2014; Wang et al., 2005; Wen et al., 2005). The Xiagou Formation (K₁g) reaches a maximum thickness of 2 000 m, which is divided into three members, i.e., K₁g₀, K₁g₁, and K₁g₂₊₃ (Fig. 3) (Sun et al., 2004). Each of these members encompasses ~2 Myr (Pan et al., 2012).

Figure  2.  Geological outline map of the Qingxi sag and location of Qingnan subsag. See red box in Fig. 1 for the location of the Qingxi sag. The Qingxi sag consists of three structural units: the Hongnan subsag in the north, the Qingxi low uplift in the center, and the Qingnan subsag in the south.

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Figure  3.  Stratigraphic framework of the Early Cretaceous Qingnan subsag. K₁c corresponds to the Chijinpu Formation; K₁g corresponds to the Xiagou Formation, which is divided into K₁g₀, K₁g₁, and K₁g₂₊₃; K₁z corresponds to the Zhonggou Formation. See dotted line C–C' in Fig. 2 for the cross-section location.

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1.3 Source Rock in the Qingnan Subsag

Most of the proven hydrocarbon reservoirs in the Qingnan subsag were developed in the Xiagou Formation, which also serves as the major source rock interval (Chen et al., 2001a). The accumulated thickness of this set of source rock exceeds 1 000 m in the central basin. It not only supplies hydrocarbon to reservoirs inside the Qingnan subsag but also those of surrounding oil fields, such as the Laojunmiao Oilfield and Shiyougou Oilfield (Chen et al., 2001b). Recent exploration also indicates that this thick source rock interval displays excellent potential of tight oil (Sun et al., 2015).

Vertical heterogeneity of this set of source rock mainly lies in the difference of organic matter types and abundance in the vertical succession of the Xiagou Formation. For example, the source rock of K₁g₀ in well K105 (Fig. 4) contains 0.77%– 2.08% TOC (average 1.49%), HI (hydrogen index) ranging from 65 to 340 mg/g with average 204 mg/g, and OI (oxygen index) ranging from 35 to 135 mg/g with average 80 mg/g; while the source rock of K₁g₁ and K₁g₂₊₃ contains 0.87%– 2.38% TOC (average 1.53%), HI ranging from 134 to 517 mg/g with average 366 mg/g, and OI ranging from 17 to 162 mg/g with average 48 mg/g (Chen, 2004). Therefore, the K₁g₀ source rock mainly contains gas-prone as well as oil and gas prone organic matter; while the K₁g₁ and K₁g₂₊₃ source rock contains abundant oil-prone organic matter. In addition, the organic matter in K₁g₀ is usually dispersed irregularly within the source rock (Figs. 5a, 5b), while the oil-prone organic matter especially in K₁g₂₊₃ usually occurs as laminar algae within the source rock (Figs. 5c, 5d) (Tu et al., 2012; Ma et al., 2007; Chen et al., 2005).

Figure  4.  Total organic carbon (TOC), hydrogen index (HI), oxygen index (OI) from deep lacustrine mudstones in well K105 (modified from Chen, 2004). For well location, see Fig. 2.

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Figure  5.  Thin section micrographs of mudstones. (a) (b) Dispersed organic matter, well L9, K₁g₀, 5 127 m (modified from Li et al., 2015). (c) (d) Laminar organic matter, well K1, K₁g₂₊₃, 4 091 m (modified from Yang et al., 2003). (a) and (c), polarized micrographs; (b) and (d), fluorescence micrographs. For well locations, see Fig. 2.

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2. DATA AND METHODS

The database of this study includes 280 km² of 3D seismic reflection data, log data from 110 wells, and approximately 800 m of core data from 30 wells. All of the data came from the Qingnan subsag and were provided by the Petroleum Exploration and Development Institute of Yumen Oilfield, PetroChina.

Drill cores from representative wells (K7, Q24, K115, YX111, X1, and QK1) were selected to investigate the sedimentological characteristics of the depositional systems developed in the study area. The representative cross-sections of inter-well correlation (using lithology data, SP, GR, and RD log curves) and log-constrained acoustic impedance inversion were combined to determine the extent and vertical stacking patterns of the depositional systems. Locations of the wells and cross- sections in the study area are shown in Fig. 2. Planar distribution of depositional systems was defined by the integration of cores, well logs, lithological statistics, planar seismic facies, and acoustic impedance inversion interpretations. Statistics of lithology percentage (conglomerate, coarse- and fine-grained sandstone, and mudstone) from about 40 wells were used in delimiting the depositional boundaries and identifying the lateral changes of depositional facies. Specific seismic facies such as the foreset reflection configuration were identified and used to determine dispersal directions of coarse-grained depositional systems.

Two representative seismic survey lines were selected to restore the subsidence history of the basement during the deposition of the Xiagou Formation using the backstripping (equilibrium profile) method (Watts and Ryan, 1976). The seismic profiles were converted from time domain to depth domain according to the synthetic seismogram and seismic wave velocity before the backstripping. Basin subsidence rates across these two survey lines were also calculated based on the restored strata thickness and the corresponding deposition time period.

3. BASIN FILLING CHARACTERISTICS

3.1 Depositional Systems

To document the sedimentary characteristics of depositional systems in the study interval of interest, drill cores were used as first-hand evidence and were described in detail. The depositional systems observed in the Qingnan subsag include flexure margin fan deltas, boundary-fault margin fan deltas, and shallow- and deep-lacustrine systems. Similar depositional systems have also been reported in other areas of the Jiuquan Basin, such as Shida sag (Cao et al., 2009; Wang et al., 2009) and Yinger sag (Li et al., 1997). Investigation of the sedimentary characteristics of these systems helps to understand the depositional environment, especially the hydrodynamic conditions.

3.1.1   Flexure margin fan deltas

Fan delta systems with coarse-grained deposits are observed in the basin flexure margin, where sediments are derived from the adjacent Qingxi low uplift (Wang et al., 2008). The flexure margin fan deltas can be divided into subaerial fan delta facies (fan delta plain) and subaqueous fan delta facies (fan delta front and prodelta) (cf., McPherson et al., 1987). The sedimentary characteristics of deposits in these two facies are provided in Table 1 and discussed in the following sections.

Table  1.  Summary of the sedimentary characteristics of deposits in the flexure margin fan delta system of the Qingnan subsag

Depositional environment	Drill core characteristics
	Bed types	Description	Interpretation	Typical sketch map
Subaerial fan delta	A1: Muddy-matrix supported conglomerate	Thick-bedded (> 0.5 m), massive pebble-cobble conglomerate (no specific grain alignments) with muddy matrix	Subaerial cohesive debris flow
	A2: Normal grading conglomerate, and pebbly sandstone or mudstone	Decimeter to meter-thick, fining- upward sequence from conglomerate to pebbly sandstone or mudstone with basal erosional surface	High-concentrated suspension in a decelerating debris flow
Subaqueous fan delta	B1: Clast-supported conglomerate	Thick-bedded (> 0.5 m), massive bedding or normal grading, clast- supported conglomerate with fine-grained matrix	Proximal deposits of subaquatic debris flow
	B2: Fine-grained siltstone or sandy mudstone	Decimeter to meter-thick bed made up of horizontal and/or faint laminated siltstone or dark sandy mudstone	Suspension load fall-out of fine-grained suspended sediments
	B3: Upward-fining sandstone and mudstone	Thin to medium bedded (< 0.5 m) sandstone displaying normal grading and parallel bedding. Individual sandstone beds are separated by interbedded dark grey mudstone	Turbidity current deposits
	B4: Deformed sandstone and mudstone	Fine-grained sandstone and dark mudstone with soft-sediment deformation structures	Slump-generated gravity flow deposits

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3.1.1.1   Subaerial fan delta facies

This facies primarily consists of variegated, thick-bedded (> 0.5 m) conglomerate, pebbly to coarse-grained sandstone, and pebbly mudstone (Fig. 6). Two major bed types of these deposits are recognized in cores according to grain size distribution and sedimentary structures. Bed type A1 is composed of massive bedding and muddy-matrix supported conglomerate (Fig. 6a). Most of the pebbles in the massive beds range between 0.5 and 5 cm (long axis), and are characterized by irregular shapes and disordered fabric. Generally, erosional surfaces are difficult to recognize between amalgamated beds. Bed type A2 is marked by normal grading, which is highlighted by distinct vertical variations in grain size and pebble content (Figs. 6b and 6c). They are usually observed as a fining-upward sequence from conglomerates to pebbly sandstones or mudstones, and range in thickness from a few decimeters to a few meters. Erosional surfaces are observed at the bottom of the beds and usually exhibit scours, truncating the underlying stratum (Fig. 6b). Statistics based on core observation shows that these two bed types make up ~95% of the drill cores in proximal areas of the depositional system. Widespread development of these two bed types in vertical core records in the proximal area indicates that the subaerial fan delta facies is dominated by gravity flows, especially the debris flows (cf., Park et al., 2013; Sohn, 2000a; McPherson et al., 1987; Steel and Thompson, 1983).

Figure  6.  Core photographs of subaerial fan delta facies in flexure margin. (a) Variegated, thick-bedded conglomerate (bed type A1), well K7 at 4 203.50– 4 205.41 m. (b) Normal graded conglomerate (bed type A2), well K7 at 4 213.32–4 214.20 m. (c) Normal graded conglomerate (bed type 2A), well K7 at 4 208–4 208.50 m. Gm. massive conglomerate; ng. normal grading. For well locations, see Fig. 2.

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3.1.1.2   Subaqueous fan delta facies

This facies refers to the subaqueous part of the fan delta systems including fan delta front and prodelta facies. Four major bed types are recognized. Bed type B1 is composed of gravel-supported conglomerates with massive bedding or normal grading, and is usually observed immediately above sharp erosional surfaces (Figs. 7a, 7b, 7c). It ranges in thickness from 0.2 to 5 m. This bed type is very common, accounting for ~75% of drill cores in this facies. These deposits are interpreted to be the product of subaquatic debris flows (cf., Liu et al., 2012; McConnico and Bassett, 2007; Martins-Neto, 1996). Bed type B2 consists of light gray siltstones or dark sandy mudstones (Figs. 7a, 7b, 7c). These deposits have horizontal and/or faint laminations, and are usually intercalated with conglomerates of bed type B1. They are interpreted to be deposited by fine-grained suspended-sediment clouds at the final stage of a debris flow event or time interval between debris flow events (Sohn, 2000b). Bed type B3 consists of fine-grained sandstones and interbedded dark grey mudstones (Fig. 7d), and is characterized by fining-upward sequences. The sandstones mainly display normal grading and parallel bedding, and range in thickness from 1 to 30 cm. In some cases, mudstone intraclasts are observed at the base of the beds. Bed type B3 is interpreted to have formed by turbidity currents flowing down the subaqueous slope of the distal fan margin (cf., Henstra et al., 2016; Zou et al., 2012; Martins-Neto, 1996). Bed type B4 consists of fine-grained sandstones and mudstones with deformed structures (Fig. 7e). This bed type is usually observed adjacent to bed type B3. This bed type is interpreted as a result of slump-generated gravity flows on the subaqueous slope (cf. Rohais et al., 2008). Bed types B3 and B4 comprise only a small part of the cores (~10%). As a whole, the subaqueous fan delta facies is also dominated by gravity flows, including debris flows, turbidity current flows, and slumps.

Figure  7.  Core photographs of subaqueous fan delta facies in flexure margin. (a) Gravel-supported conglomerate (Bed type B1), well Q24 at 4 225.12–4 225.64 m. (b) Massive conglomerate (bed type B1) interbedded with siltstone and sandy mudstone (bed type B2), well Q24 at 4 230.91–4 231.56 m. (c) Massive conglomerate (bed type B1) interbedded with siltstone and sandy mudstone (bed type B2), well Q24 at 4 183.03–4 183.82 m. (d) Fining-upward sequence with sandstone and interbedded mudstone (bed type B3), well Q24 at 4 184.36–4 185.16 m. Black arrow indicates mudstone intraclasts. (e) Fine-grained sandstone and mudstone with deformed structures (bed type B4), well K115 at 4 631.99–4 632.78 m. Gm. massive conglomerate; Gng. normal graded conglomerate; ng. normal grading; ssds. soft sediment deformation structure. For well locations, see Fig. 2.

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3.1.2   Boundary fault margin fan deltas

These fan deltas along boundary fault are usually fed from basin margin uplifts and directly accumulate in a deep sublacustrine environment. Similar systems have also been observed from both modern rift lakes and their ancient counterparts (e.g., Zhu et al., 2008; Lin et al., 2001; Scholz and Rosendahl, 1990). Drill cores show that these fan delta deposits are mainly composed of massive bedded, clast- or matrix-supported conglomerates (Fig. 8). They directly contact with dark, deep- lacustrine mudstones, and their amalgamated beds can reach as thick as 30 m. Compared to the flexure margin fan deltas, these systems are richer in coarse clasts but have less bed types. Grain size of the gravels varies in a wide range, from pebbles to cobbles. Poor sorting and disordered fabric indicate that they were rapidly accumulated with a short transportation distance, and deposited by high-concentrated debris flows. Coarse- grained clasts that float within the underlying dark mudstone bed are also observed (Fig. 8b). They are interpreted to be the debris-fall deposits detached from the body of debris flows (cf., Sohn, 2000b).

Figure  8.  Core photographs of fan delta facies in boundary fault margin. (a) Thin-bedded, massive conglomerate, well YX111 at 4 374.63–4 377.30 m. (b) Massive conglomerate and floating gravels, well YX111 at 4 378.18–4 379.96 m. For well locations, see Fig. 2.

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3.1.3   Shallow- and deep-lacustrine systems

Shallow-lacustrine facies mainly developed in the flexure margin. Deposits of this facies include (1) thick-bedded (> 0.5 m) gray to greenish mudstone and calcareous mudstone (Fig. 9a); (2) thinly laminated siltstone and fine-grained sandstone that have burrows and bioturbation structures (Fig. 9b); (3) gray or greenish siltstone and sandstone with faint lamination or parallel bedding (Fig. 9c). Generally, shallow-lacustrine facies are relatively thin and restricted in the basin.

Figure  9.  Core photographs of shallow- and deep-lacustrine facies. (a) Greenish mudstone, well X1 at 4 507.13 m. (b) Burrows and bioturbation, well QK1 at 4 211.67 m. (c) Greenish siltstone, well X1 at 4 242.50 m. (d) Horizontal bedded shale, well K105 at 4 462.19–4 463.80 m. (e) Massive mudstone, well K105 at 4 795.31– 4 796.11 m. Fm. massive mudstone; Fmh. horizontal mudstone; Sm. massive siltstone; ssds. soft sediment deformation structure. For well locations, see Fig. 2.

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Deep-lacustrine deposits in the Xiagou Formation consist of dark gray to black mudstone, dolomitic mudstone, and thinly interbedded siltstone and sandstone. Two types of mudstone are recognized, i.e., horizontal bedded shale (Fig. 9d) and massive mudstone (Fig. 9e). The horizontal bedded shale usually exhibits well-developed laminated texture, which comprises alternating dark laminae of organic matter (planktonic algae) and gray laminae of dolomite or clay minerals (Li et al., 2015; Gao et al., 2007). This type of mudstone is commonly developed in the upper part of the Xiagou Formation, especially in K₁g₂₊₃. The massive mudstone beds are mostly structureless or show a disordered structure highlighted by soft-sediment deformation. In vertical succession, the thin-bedded (< 0.2 m) siltstone and sandstone are usually interbedded with the massive mudstone. Ratios of the coarse-grained deposit (sandstone and siltstone) and mudstone are below 10%. The massive mudstone beds were observed in the whole unit of the Xiagou Formation but mostly in K₁g₀.

3.2 Depositional System Distribution and Evolution

3.2.1   Cross-sectional distribution characteristics

A representative cross-section across the half graben is selected to illustrate the dip-oriented distribution characteristics of the depositional systems within the Xiagou Formation based on a combination of inter-well correlation and log-constrained acoustic impedance inversion data (Fig. 10). The inter-well correlation section, which intersects wells K108, K7, K5, K115, Q14, and K106, exhibits an overall retrogradational filling pattern with a decreasing extension of fan delta systems in both flexure and boundary fault margins. Wells K108, K7, K5, K115, and K106 are characterized by a fining-upward succession with a coarse-grained fan-delta dominated lower part and a fine-grained lacustrine system dominated upper part (Fig. 10a). The well log-constrained acoustic impedance inversion section shows distribution characteristics similar to what is observed in the inter-well correlation section (Fig. 10b). Acoustic impedance is a physical rock property, which depends on the rock density and compressional (P-wave) velocity. As indicated by sonic logging data from wells in the study area, the interval velocity of the sandstone and conglomerate is commonly greater than that of the mudstone. Accordingly, the high impedance (12 200 to 14 700 g/cm³×m/s) with red and yellow colors is more likely to represent the fan delta dominated deposits and the low impedance (9 500 to 12 200 g/cm³×m/s) with green and blue colors is more likely to represent deep lacustrine dominated deposits, although there are uncertainties in some local places. As a whole, the distribution of the high-impedance deposits corresponding to K₁g₀, K₁g₁, and K₁g₂₊₃ shows a vertical retrogradation.

Figure  10.  Dip-oriented cross-sections showing basin fills of the Qingnan subsag. (a) Well correlation section. (b) Log-constrained acoustic inversion section. For cross-section location, see dotted line D-D' in Fig. 2.

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3.2.2   Planar distribution and evolution

Based upon core observation, log curve analysis, lithological statistics, and planar seismic facies interpretation, planar distributions of the depositional systems were characterized within the context of each member of the Xiagou Formation (Fig. 11). It is noteworthy that seismic attributes, including maximum amplitude, root mean square (RMS) amplitude, and RMS instantaneous frequency, were extracted from the 3D seismic cube to constrain the depositional facies distribution. However, the results of the seismic attributes are, in some places, not compatible with information from boreholes.

Figure  11.  Planar distribution of depositional systems of the Qingnan subsag. (a) Depositional system distribution of K₁g₀. (b) Depositional system distribution of K₁g₁. (c) Depositional system distribution of K₁g₂₊₃.

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Sediments in the Qingnan subsag originated from surrounding sources (the red arrows), where the bedrocks revealed by deep boreholes are consistent with the gravels in the fan delta conglomerate. The content of coarse-grained deposits (conglomerate and sandstone) generally decreases from 100% proximally to less than 30% distally. It is evident from the planar depositional facies maps that areas of the fan delta systems along the basin margin decrease from bottom to top of the Xiagou Formation (Fig. 11). Such a tendency of fan delta variation also matches the current exploration result in the Qingnan subsag that the largest oil and gas finds are mainly focused on these fan deltas in K₁g₀. The decreasing areal extent of fan deltas also suggests that basin fill of the Xiagou Formation displays an overall retrogradational pattern and a tendency of being underfilled.

4. DISCUSSION

4.1 Controls on Basin Fills

Tectonics and climate are usually considered as the primary causes, which determine basin filling patterns and also facies belt migration (prograde basinward or retrograde landward) through controlling the balance between accommodation space (A) and sediment supply (S) (Lin et al., 2001; McCallum and Robertson, 1995; Scholz, 1995; Gawthorpe et al., 1994; Smith, 1994; Blair and Bilodeau, 1988). Therefore, investigation of these two factors is critical to evaluate their roles in the basin evolution.

4.1.1   Tectonics

The subsidence history of the Qingnan subsag is illustrated on the equilibrium profiles across the half graben of the Qingnan subsag presented in Fig. 12. Stratal thickening toward the boundary fault suggests that deposition of the Xiagou Formation was accompanied by boundary fault-driven differential subsidence during the early stages (i.e., K₁g₀). Reduced amounts of stratal thickening toward the boundary fault and a relatively smaller stratal thickness especially in K₁g₂₊₃ suggest uniform subsidence during the late stage. These interpretations are supported by the analysis of subsidence rates across the basin during each depositional stage (Figs. 12f and 12l). The change from the early differential subsidence to the late nearly uniform subsidence of the Qingnan subsag suggests that the activity rate of the main boundary fault gradually decreased through time. This change in basin subsidence also reflects the transition in basin evolution from the synrift phase to the postrift phase (Wang M F et al., 2008; Wang C X et al., 2005).

Figure  12.  Subsidence history recovery profiles of the Xiagou Formation in the Qingnan subsag (for profile locations, see dotted lines C–C' and EE' in Fig. 2). Distances in the subsidence rate profiles (f and l) represent the distance away from the boundary fault. Age data after Jin (2016).

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The tectonic effects on the retrogradational fan delta deposition in the flexure margin are considered to be reflected in subsidence of the Qingxi low uplift primarily due to downward flexure of the hanging wall during displacements along the boundary fault. The Qingxi low uplift occurred as a boundary-fault hanging-wall ramp high in the margin of the Qingnan subsag (Chen et al., 2001b), which was progressively buried during the development of the Xiagou Formation (Fig. 12). Reduction of the relief and the exposed area of the Qingxi low uplift would be expected to decrease the rate of sediment supply. Deposition of fan deltas along main boundary faults in rift basins is usually associated with uplift of the footwall at the basin margin due to the isostatic effects during normal faulting (Gawthorpe and Leeder, 2000). Decreasing of the boundary fault activity could reduce the isostatic uplifting of adjacent source areas, and therefore contributed to the retrogradation of the fan delta systems in the boundary fault margin.

4.1.2   Climate conditions

During the development of Xiagou Formation, the regional climate was characterized by alternating periods of humidity and aridity (Wang et al., 2005). The humid condition prevailed at the early stage of the Xiagou Formation (likely corresponding to K₁g₀) where subtropic humid pollen assemblages were very common (Liu, 2000). This humid climate probably initiated during the depositional period of the underlying Upper Chijinpu Formation, in which humid plant fossils (e.g., Athrotaxites and Solenites) and local coal seams were observed (Deng and Lu, 2008). The Qingnan subsag was subjected to the semi-arid climate at the middle and late stage of the Xiagou Formation (like corresponding to K₁g₁ and K₁g₂₊₃), as indicated by the carbon and oxygen isotopes records described by Li et al. (2013). This interval of semi-arid climate probably continued to the late stage of the Zhonggou Formation, in which the semi-arid Pseudofrenelopsis-Brachyphyllum assemblages were reported (Deng and Lu, 2008).

Climate change plays an important role in altering the efficiency of weathering, erosion, and sediment transport and supply (Yang and Ma, 2017; Zhu et al., 2017; Molnar, 2004; Leeder et al., 1998). Precipitation associated with regional climate influences the rates of weathering, erosion of the source area and the amount of terrigenous material input. This factor is particularly critical for the development of fan delta systems alongside the basin margin, where the sediments are directly derived from the adjacent uplifts. The effect of the climate change from the early humid to the middle and late semi-arid conditions is reflected in the reduction of water and sediment supply and thus the decrease in fan delta dimensions. This interpretation is supported by the geochemical character of organic matter in the source rock, quantified via HI and OI values of drill cores from the central basin (Fig. 4). To be specific, the relatively low HI values (average 204 mg/g) and high OI values (average 80 mg/g) in K₁g₀ reveal that the source rock included more terrigenous organic matter (Katz, 1988). This feature suggests high water inflow and high terrigenous inputs (both organic and inorganic) during the stage of K₁g₀, which corresponds to the humid climatic conditions as supported by humid pollen fossil assemblages reported by Liu (2000). In contrast, the relatively higher HI values (average 336 mg/g) and lower OI values (average 48 mg/g) in K₁g₁ and K₁g₂₊₃ reveal less terrigenous organic matter included (Katz, 1988). This feature suggests lower water inflow and lower terrigenous inputs during the stages of K₁g₁ and K₁g₂₊₃, which correspond to the semi-arid climatic conditions based on the carbon and oxygen isotopes by Li et al. (2013). In many geological records in other places, such as the Bengal Fan (France-Lanord et al., 1990), the Indus Fan (Clift et al., 2002a), and the South China Sea (Clift et al., 2002b) also suggest slower erosion, less sediment accumulation, and less precipitation during periods of regional aridity.

4.2 Implications for Source Rock Depositional Process and Heterogeneity

The characteristics of basin fills and control factors suggest that the depositional environment of the Qingnan subsag is different during the depositional history of the Xiagou Formation. Depositional environment of lacustrine basins strongly affects production and accumulation of organic matter that forms source rocks (Bohacs et al., 2000). Based on the basin filling history discussed above, two conceptual models have been established in order to better understand the difference of depositional environment and source rock depositional processes (Fig. 13).

Figure  13.  Depositional models showing depositional environment and source rock deposition. (a) Depositional model of K₁g₀; (b) depositional model of K₁g₂₊₃.

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During the deposition period of K₁g₀, the Qingnan subsag was characterized by a distinct half-graben structure with strong boundary fault activities. Regional humid climate increased the intensity and frequency of rainfall, which triggered frequent flash floods and landslides in the surrounding uplifts and delivered large quantities of sediment and water into the lake with strong hydrodynamic conditions (Fig. 13a) (Zhang et al., 2014). These gravity flow events could result in lake water mixing, particularly for small lakes like the Qingnan subsag with the maximum width less than 12 km. Coarse-grained clasts linked to the gravity flows were deposited along the basin margin, forming fan deltas. Massive mudrock in the central basin was deposited after the deposition of coarse-grained gravity flows, with the massive structure formed from the rapid settling of suspended fine-grained particles. The gravity flows also acted as important transportation mechanisms of organic material from terrigenous higher plants, which made the source rock containing multisource-mixed organic matter with a large proportion of terrigenous component indicated by the relatively low HI values and high OI values of the source rock in this interval from the central basin (Fig. 4). On the other hand, mixing of the lake water could increase the availability of oxygen for various scavengers near the lake bottom, which could consume the preserved organic matter (Bohacs et al., 2000). In addition, rapid deposition rate during the process could cause dilution of organic matter by mineral sediments, which could decrease the ratio of organic matter relative to inorganic matrix. These two effects (destruction and dilution) jointly limited the richness of organic matter in the source rock.

During the late stage of Xiagou Formation, especially in K₁g₂₊₃, the Qingnan subsag presented a nearly-uniform subsidence with decreased boundary fault activities (Fig. 13b). The subsidence caused a larger areal extent of the lake basin and a smaller exposed area of the hanging-wall ramp high (Qingxi low uplift). Regional semi-arid condition decreased the amount of rainfall, which directly reduced the overall efficiency of sediment and water transport and supply. Gravity flow related deposition likewise occurred along the basin margin forming the fan deltas, but the frequency of the gravity flow events and the scale of the fan deltas were obviously decreased. In this scenario, agitation of the gravity flows on the water column of the lake was restricted, particularly in the central basin. Under the semi-arid climate setting, seasonal stratification and turnover of the lake occurred alternately, which produced the horizontal bedded shale. Seasonal turnover of the lake strengthened the convention of bottom water and surface water, which brought abundant nutrients from the lake bottom and led to surface algal bloom (Cohen, 2003). During the lake turnover, settling of the fine-grained particles and minerals formed the light color inorganic laminar of the shale. When the lake changed to be stratified, circulation of water among different layers was blocked, resulting in a shortage of nutrients and mass death of algae. Settling of the dead algae formed the dark organic laminar of the shale. During the process, terrigenous organic matter could be mixed into the source rock, but only a small proportion. Besides, lake stratification could also generate an anoxic environment near the lake bottom, favoring preservation of the organic matter.

Source rock deposition during basin filling processes is often underexplored, compared with the deposition of reservoirs. However, source rock characters have a great influence on hydrocarbon-generation potential and total hydrocarbon resources in sedimentary basins. It is suggested that source rock heterogeneity linked to basin fills in lacustrine basins should be given consideration in hydrocarbon exploration.

5. CONCLUSIONS

(1) Based on core observation, fan delta systems and shallow- and deep-lacustrine deposits are identified in the Xiagou Formation of the Qingnan subsag. Sedimentological characteristics of the fan delta along both the flexure margin and boundary fault margin reveal that deposition took place mainly through gravity flows.

(2) Fan deltas along the basin margin were developed from K₁g₀ to K₁g₂₊₃ with a decreasing areal extent, indicating basin fills of the Xiagou Formation characterized by an overall retrogradational pattern. Tectonics and climate conditions are interpreted to be the main controls that were responsible for the retrogradational basin fill. The boundary fault activities decreased through time and the basin presented a change from the early differential subsidence to the late nearly uniform subsidence. The regional climate changed from the early humid condition to the middle and late semi-arid condition, contributed to the reduction of water and sediment supply.

(3) Source rock depositional processes and heterogeneity during basin filling processes are often underexplored. This study illustrates the basin filling characters and their implications for the depositional processes and difference of source rock in a rift basin, and the proposed models may help to evaluate the source rock in other similar basins.