Journal of Earth Science  2017, Vol. 28 Issue (6): 987-995   PDF    
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Space-Time Evolution and Controlling Factor of Fine-Grained Sediments in the Upper Fourth Member of Shahejie Formation, Dongying Depression
Guoqi Song1, Jing Wu2,3, Yong Wang1, Xuefeng Hao1    
1. Geological Science Research Institute, SINOPEC Shengli Oilfield Company, Dongying 257000, China;
2. Petroleum Exploration and Production Research Institute of SINOPEC, Beijing 100083, China;
3. SINOPEC Key Laboratory of Shale Gas/Oil Exploration & Production, Beijing 100083, China
Abstract: Fine-grained rocks (FGR) are the important source rocks and reservoirs of shale hydrocarbon which is the prospect hotspot at present. Widely distributed fine-grained sediments (FGS) of the upper fourth member of Shahejie Formation in Dongying depression are taken as an example to study the space-time evolution and controlling factor of FGS in this paper. Based on the analysis of well cores, thin sections, inorganic and organic geochemistry indicators, FGR are divided into 7 types of lithofacies. Through the study of 'point-line-plane', this study shows that FGS has the characteristics of rhythum, diversity and succession. The first stage is characterized by clayey FGS (massive claystone). The second stage is characterized by carbonate FGS (low-TOC laminated limestone) and dolomitic FGS (dolomitic-silty shale) formed by transgression. The third stage is characterized by organic-rich carbonate FGS (middle/high-TOC laminated limestone) distributed in cycle. The fourth stage is characterized by FGS mixed carbonate and siliciclastic sediments (calcareous-silty shale). A variety of space-time evolution of FGS are controlled by multiple factors including tectonism, climate and lake conditions.
Keywords: fine-grained sediments    lithofacies    space-time evolution    controlling factor    Dongying depression    the upper fourth member of Shahejie Formation    
0 INTRODUCTION

The fine-grained sediments (FGS) with abundant organic matter deposited in deep-water are important hydrocarbon source rocks and reservoir of shale hydrocarbon resources (Loucks et al., 2012; Stow and Mayall, 2000). With the development of exploration in shale hydrocarbon, deep-water FGS have received increasing attention (Liang et al, 2017a, b, 2016; Li et al., 2016; Wang et al., 2016; Jiang et al., 2014, 2013; Schieber et al., 2010; Algeo et al., 2004; Arthur and Sageman, 1994). FGS are clay-and silt-sized sediments that have the particle size less than 62.5 μm and mainly comprise clay, silt, carbonate and organic matter (Aplin and Macquaker, 2011; Schieber and Zimmerle, 1998). The studies of FGS mainly involve petrology, reservoir geology, sequence stratigraphy and reservoir geology etc. (Nie et al., 2016; Wang et al., 2016; Hu et al., 2015; Wu et al., 2015), which still have two problems. Firstly, most present researches are about marine FGS which develop in the shelf and deep sea environment (Zou et al., 2015; Stow and Mayall, 2000). Obviously, the differences between marine and lacustrine are huge, including sedimentary environment, structural setting and waterbody conditions. The lacustrine facies change fast with complex controlling factor that is required to study. Secondly, the current studies of space-time evolution of FGS focus on facies and subfacies, which are not adequate for researches and exploration of shale hydrocarbon (Wang et al., 2015). Space-time evolution and controlling factor of deepwater and shallow water FGS need to be further researched.

Dongying depression is a third-order tectonic unit of Bohai Bay Basin with abundant shale hydrocarbon resource (Zhang et al., 2012; Zhu et al., 2004). Paleogene formations distribute widely and thickly whose maximum depth can reach more than 7 000 m. Among them, Shahejie Formation developed thick oil source rocks (Zeng et al., 1998). The upper fourth member of Shahejie Formation (Es4s) is characterized by widely distributed lacustrine FGS with thickness between 150-300 m. For these characteristics, it becomes the main hydrocarbon source interval of Dongying depression. Core and cutting samples for mineral composition and geochemical test of the FGS from the Es4s were collected. This provides an advantageous data support for the research of this paper. The research tends to analyze sedimentary sequence of single well as FGS change slightly. Characteristics and relationship of deepwater and shallow water FGS are going to be researched used connecting-well section. At last, space-time evolution and genesis of FGS can be explored on the plane.

1 GEOLOGICAL SETTING

Dongying depression represents a Mesozoic-Cenozoic basin, with Luxi uplift in the southern, Chenjiazhuang uplift in the northern, Qingtuozi uplift in the eastern, and Binxian and Qingcheng uplifts in the western. It forms a typical asymmetric half-graben depression characterized by north-fault, south-overlap, north-steep, and south-glacis. The depression has an approximately EW length of 90 km and SN width of 65 km, with an exploration area of approximately 5 760 km2. The Es4s shale was developed during the chasmic stage. In this period, rift was intensely accompanied by a large extent of subsidence and a fast basin expansion, forming vast semi deep-deep lake environments, and developing lacustrine FGS in the center of the depression and coarse-grained detrital sediments in the area of basin edge (Yang et al., 2011; Sun et al., 2010; Wang et al., 2005; Yan et al., 2005; Zhao, 2005). Studies show that the Es4s shale is mainly deposited in a semiarid-humid saline lake (Li and Xiao, 1988).

2 SAMPLES AND METHODS

A total of 300 samples are collected from the Es4s in 3 wells (NY1, LY1 and FY1, Fig. 1). The basic data include 835.94 m continuous core observation, well logs, 300 thin sections, XRD (whole rock X-ray diffraction) of all samples, major and trace element geochemistry, TOC (total organic carbon) of all samples.

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Figure 1. Map of structural units and stratigraphy in the Dongying depression.

The thin sections observation was taken using a Zeiss microscope Axio Scope A1. The relative content of each mineral composition of all samples were measured by using X'Pert-MPD diffraction instrument (Philips Corp.): pipe pressure 30 kV, conduit flow 40 mA, scanning speed 2° (2θ)/min. Computer analysis of the diffractograms enabled the identification and semi-quantitative analysis of the relative abundance (in weight percent) of the various mineral phases. A total of 65 samples were collected for element geochemistry analysis. All samples were ground to the grain of 200-mesh and subjected by inductively coupled plasma-atomic emission spectroscopy ICP-AES (JY38S) with a focus on the elements Ca, Al, Mg etc. TOC, which is the same with the free hydrocarbons (S1), mg HC/g rock, the hydrocarbons cracked from kerogen (S2), mg HC/g rock, was measured using rock-eval pyrolysis. TOC was determined using a LECO CS-200 carbon analyzer. The measurement technique is based on the combustion of the sample in an oxygen atmosphere to convert the total organic carbon to CO2. With the aid of combustion calculation, the total organic carbon content of the sample can be determined.

3 RESULTS 3.1 Lithofacies Classification

In the Es4s strata, the major minerals of FGS are calcite, clay, and quartz. Other minerals involve feldspar, dolomite and pyrite, etc. The average content of calcite is 35.50% with a maximum up to 80%. The average content of clay is 21.48% with few reach up to 50%. The quartz content is mainly of 20%-30%, with an average of 23.79%. The total organic carbon (TOC) content is of 0.11%-11.40% with an average of 2.49% (Table 1).

Table 1 Lithofacies types of fine-grained sedimentary rocks of the upper fourth member of Shahejie Formation in Dongying depression

Firstly, we classify the sediments into four types with major mineral compositions and sedimentary structures. Secondly, organic matter plays a highly important role in FGS. The presence of organic matter is conducive to calcite deposition (Michaelis et al., 2002). Mineral sequence and combination are influenced by organic matter (Aplin and Macquaker, 2011; Tucker, 2001). In the study area, organic matter mainly exists in laminated limestone. Based on these, we subdivide the lithofacies of limestone according to the contents of TOC (2% and 4%) (Jiang et al., 2013). Taking into consideration of the special mineral composition in laminated claystone and silty shale, seven lithofacies are identified.

3.2 Space-Time Evolution of Fine-Grained Sediments 3.2.1 Sedimentary sequence of single well

Based on the detailed analysis of the Well FY1, the vertical sedimentary sequence was constructed. The Es4s interval in the Well FY1 is characterized by various lithofacies and comprehensive data, which helps to analyze the FGS assemblage variation in vertical and well connection contrasts, avoiding overgeneralization caused by contingency of lithofacies development.

The lithofacies is dominated by various limestone in the Well FY1 (Fig. 2, most of the calcite content is more than 50%). Low-TOC laminated limestone consist of light and dark laminae. Light laminae are dominated by micritic calcite and dark laminae are clay lamina. TOC content is less than 2% as algae provide a small amount of organic matter (Fig. 2, core and thin section pictures of 3 290.2 m). Middle-TOC laminated limestone consist of light and dark laminae. Calcites are grainy by a certain degree of recrystallization in the light laminae. Dark laminae mainly comprise organic matter and clay minerals. TOC content is 2%-4% as algae provide more organic matter (Fig. 2, core and thin section pictures of 3 414.6 m). Lamellation of high-TOC laminated limestone are developed with relatively pure light and thin dark laminae. The light lamina are comprised of recrystallization column calcite, and dark laminae are mainly organic-rich clay. Boundaries are very clear between laminae. Core are gray black as TOC content is greater than 4% (Fig. 2, core and thin section pictures of 3 397.5 m).

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Figure 2. Distribution characteristics of fine-grained sedimentation of the upper fourth member of Shahejie Formation in Well FY1, Dongying depression.

In the rest lithofacies, massive claystone distributes in the bottom of Es4s, associated with beach-bar siltstone (Fig. 2, the interval of 3 439.1-3 450.0 m, core picture of 3 444.4 m). Other wells have developed laminated gypsiferous claystone at the same interval (Fig. 3, Well NY1). In addition, calcareous-silty shale is the dominant lithofacies of the upper Es4s (Fig. 2, core and thin section pictures of 3 288.7 m). The dolomitic-silty shale is concentrated in the lower Es4s with abnormally high salinity (Fig. 2, the interval of 3 391.5-3 439.1 m with abnormally high Sr/Ba ratio). This phase corresponds to the development of cryptomeric dolomite (Fig. 2, core and thin section pictures of 3 414.9 m), the high salinity (Sr/Ba), and gradual TOC increase, likely affected by transgression. The presence of sediments (dolomite and glauconite), paleontologic evidence (e.g., Chinese Cladosiphon and calcareous algae), and geochemical index, together with the above findings indicate that transgression occurred in the study area during this interval (Wu et al., 2014).

Sedimentary sequence of FGS vary periodically in vertical. In the first stage, massive claystone is the dominant lithofacies. The second stage give priority to low-middle TOC laminated limestone. Dolomitic-silty shale appears by transgression. The lithofacies is dominated by limestone in the third stage. Middle-high TOC laminated limestone are developed. Calcareous-silty shale and low-middle TOC laminated limestone are the main lithofacies in the fourth stage.

3.2.2 Characteristics of connecting-well section

Fan delta gravel are developed in the north basin edge and beach-bar gravel are developed in the south basin edge, both of them are coarse-grained detrital sediments of terrigenous origin. Clayey FGS (massive claystone) is the main type in the area of basin edge influenced by terrestrial input as near provenance. For example, the thickness of claystone in Well L564 is about 65 m that accounts for 26.5% of overall thickness (Fig. 3). The thickness of claystone in Well T20 is about 35 m that accounts for 66% of overall thickness of FGS.

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Figure 3. Distribution characteristics of fine-grained sedimentation of the upper fourth member of Shahejie Formation in connecting-well section, Dongying depression.

Because the transport distance of detrital sediments increases, the water depth deepens, and coarse-grained detrital sediments reduce, carbonate FGS begin to develop. Low-TOC laminated limestone and massive claystone become the dominant lithofacies of FGS in shallow water area. For example, the thickness of massive claystone in Well L96 is about 77 m with 70 m of low-TOC laminated limestone.

Siltstone content drastically decrease in the center area with deep water where carbonate FGS are the dominant lithofacies. Low-middle-high TOC laminated limestone are widely distributed in stages 2-4. In addition, there are calcareous-silty shale in stages 4. Dolomitic sediments formed by transgression can be tracked in the transverse direction, such as dolomitic-silty shale of wells FY1 and NY1 in deep water area, dolomitic sandstone of wells L96 and Y93 in shallow water area. It is a further evidence from the perspective of regional comparison that the transgression has happened.

Massive claystone is the main type of FGS in the area of basin edge and shallow water. It is same with coarse-grained detrital sediments that are offered by provenance and migrated into basin through physical process. According to the sedimentary differentiation, FGS increase gradually from the basin edge to the center area. Carbonate FGS are the dominant lithofacies including low-middle-high TOC laminated limestone. Based on material basis provided by the provenance, they are mainly deposited through biochemical process in the basin. It is shown that carbonate FGS start to form when the content of coarse-grained detrital sediments and siltstone begin to decrease, which means terrestrial input decrease. No matter clayey FGS or carbonate FGS has an inverse association with coarse-grained detrital sediments and siltstone.

3.2.3 Sedimentary facies distribution and space-time evolution of FGS

Nearshore subaqueous fans, fan delta, and glutenite of beach-bar are developed in the basin edge. FGS distribute in the center of basin. Beach-bar siltstone widely distribute in the basin. Clayey FGS is developed when the siltstone is lacking (Fig. 4, Well Y93). Low-TOC laminated limestone is developed in these area with low terrestrial input and distribute in isolation with limited scale in the first stage (wells G6 and B184). Only in the Lijin sag with the deepest water, carbonate FGS distribute much wider (Well L79). Laminated gypsiferous claystone affected by the drought climate is rather sporadic (Well NY1).

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Figure 4. Lithofacies map of the first stage of the upper fourth member of Shahejie Formation in the Dongying depression.

Transgression brought sea water into the lake, which lead to the lake level rise and FGS distribute much wider in the second stage. Beach-bar siltstone only distribute near provenance (Fig. 5, wells S25 and M19). The distribution variety of carbonate FGS increases and low-TOC laminated limestone has been connected. There are middle-TOC laminated limestones in Boxing sag and Lijin sag. High-TOC laminated limestone is still only developed in Lijin sag. Dolomitic sediments are formed by transgression from north to south. Dolomitic sandstone are developed in the north basin as sediments are corase-grained. Dolomitic-silty shale appears in the center of basin as sediments are fine-grained.

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Figure 5. Lithofacies map of the second stage of the upper fourth member of Shahejie Formation in the Dongying depression.

The distribution range of FGS continue to increase and carbonate FGS is the main type in the third stage after transgression (Fig. 6). Low-TOC laminated limestone has been connected in the area to the largest distribution. Middle-TOC laminated limestone is widely developed in the depression. There are large range of high-TOC laminated limestones in Niuzhuang and Lijin sags. These three types of laminated limestone are distributed in cycle. Low-middle-high-TOC laminated limestones are developed in turn from the shallow water to deepwater.

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Figure 6. Lithofacies map of the third stage of the upper fourth member of Shahejie Formation in the Dongying depression.

Distribution range of FGS begins to decrease in the fourth stage, while the lithofacies is still dominated by limestone. Beach-bar siltstone is developed again in the south basin (Fig. 7, Well B104). The range of connected low-TOC laminated limestone decrease. The range of middle-TOC laminated limestone decrease obviously and even disappear (Well W589). High-TOC laminated limestone is still distributed with limited range. There are mixed sediments of carbonate and siliceous clastic-calcareous-silty shale (wells NY1 and B104).

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Figure 7. Lithofacies map of the fourth stage of the upper fourth member of Shahejie Formation in the Dongying depression.

Plane distribution and evolutionary characteristics of FGS are various. The first stage is characterized by clayey FGS. The second stage is characterized by dolomitic sediments formed by transgression. The third stage is characterized by organic-rich carbonate FGS. The fourth stage is characterized by FGS mixed carbonate and siliciclastic sediments. Each stage develops on the basis of the previous stage and has the characteristics of succession.

4 DISCUSSION 4.1 Tectonism

Firstly, influenced by the tectonic activity of northwest, northeast and eastwest faults, Chenjiazhuang uplift, Luxi uplift and Qingtuozi uplift were formed respectively in the north, south and east basins, which provide material basis as provenance. Secondly, at the same time, the differences of tectonic activity lead to the difference of accommodation space in different areas. This difference affects the development of FGS. Strong tectonic activity formed small plane range of accommodation space with large vertical depth in the north basin that results in the small plane distribution range of FGS and quick change of lithofacies. Weak tectonic activity formed large plane range of accommodation space with small vertical depth in the south basin that results in large plane distribution range of FGS and slow change of lithofacies. Finally, in different period, the difference of tectonic activity lead to the evolution of FGS. Affected by the Himalayan movement, the study area experienced subsidence after a short-term rise and started the rift stage in the Es4s. Average value of quartz/feldspar in the first to third stage during Es4s is 7.1 (Fig. 2) which is inversely proportional with the strength of tectonic activity (Wang et al., 2006). It shows a fast expansion and wide subsidence in the first to third stage. Accordingly, the evolution of FGS experienced the first stage characterized by clayey FGS (massive claystone), the second stage changed from clayey FGS to carbonate FGS (low-middle-TOC laminated limestone), and the third stage characterized by organic-rich carbonate FGS (middle-high-TOC laminated limestone). Average value of quartz/feldspar in the fourth stage increase to 13.0, which suggests a weak expansion. This leads to strong weathering, which makes the increasing of the compositional maturity of sediments and content of quartz. Quartz and carbonate mixed and formed calcareous-silty shale.

4.2 Climate

Tectonic activity offered provenance that provided FGS with the material basis and weathered by climate. The rhythm climate leads to the rhythmical change of FGS. The climate in the first stage is still arid and cold as influenced by the arid climate of the Es4x (Wu et al., 2016). The high Th/U ratio means the shallow water (Davies and Elliott, 1996). The low pyrite content shows the water with weak reducibility (Zhang and Ren, 2003). Massive claystone was developed as abundant terrigenous detrital without organic matter. Influenced by the arid-cold climate, laminated gypsiferous claystone was developed in the area with little terrigenous detrital. In the second stage, the Na/Al ratio is low, which is inversely proportional with the degree of warm-damp of climate (Yang et al., 2004). It indicates relatively warm-damp climate. Transgression made the depth and reducibility of water increase. Therefore, low-TOC laminated limestone and dolomitic-silty shale replaced the development of massive claystone and laminated gypsiferous claystone. In the third stage, the ratio of Na/Al continues to decrease that reveals a warm-damp climate. Low Th/U ratio and high pyrite content show the waterbody deepen rapidly to the deepest with strong reducibility. Organic matter can be preserved. The lithofacies is dominated by middle-high-TOC laminated limestone. The climate is relatively warm-damp in the fourth stage. The water depth, reducibility and TOC content decrease with increased terrigenous detrital. calcareous-silty shale low-middle-TOC laminated limestone are the main lithofacies.

4.3 Lake Conditions

The study area suffered from transgression in the second stage which changed the condition of lake and influence the development of FGS. Its influence on FGS is embodied in the following three points: (1) Physicochemical conditions of water were influenced by the transgression which affected the development of FGS. Transgression resulted in the depth and reducibility of water increase and help to preserve the organic smatter which prompt the development of middle-TOC laminated limestone. (2) Transgression offered special mineral composition. The sea water carrying Mg2+ lead to the development of dolomitic-silty shale. (3) The rhythm of transgression leads to the rhythmical superposition of FGS. In the early transgression, dolomitic-silty shale and low-TOC laminated limestone are developed. As multiphase transgression happened, the lithofacies are dominated by dolomitic-silty shale and middle-TOC laminated limestone.

The space-time evolution of FGS are controlled by multiple factors including tectonism, paleoclimate and lake conditions. Tectonism offered provenance that influenced the material basis of FGS. It controls the distribution of FGS through the difference of accommodation space and affects the evolution of FGS through its evolution. The climate influences the waterbody conditions (depth and reducibility), thus leads to the rhythmical change of FGS. As the influence of transgression, the evolution of FGS is no longer a stable sedimentary cycle affected only by tectonism and climate, while has a sudden change on the basis of stable cycle. By influencing lake conditions of water, providing special material composition and rhythmical superposition, transgression controls the FGS. The main controlling factors of the above three aspects jointly produced a variety of FGS evolution.

5 CONCLUSIONS

The FGS of the Es4s in Dongying depression are taken as an example to study the space-time evolution and controlling factor of lacustrine FGS in this paper. Based on the sedimentary sequence analysis of single well, FGS with the characteristics of rhythm are discovered. The analysis of connecting-well section suggests that massive claystone is the main type of FGS in the area of basin edge, low-TOC laminated limestone and massive claystone become the dominant lithofacies in shallow water area. Genesis and sedimentary process of the massive claystone are the same with coarse-grained detrital sediments. They all offered by provenance and migrated into basin through physical process. Carbonate FGS are the dominant type in the basin center. Based on material basis provided by the provenance, they are mainly formed through biochemical process in the basin. By plane study found that FGS have the characteristics of diversity and succession as controlled by multiple factors including tectonism, climate and lake conditions. Raveling out the genesis and evolution rule of lacustrine FGS can helps to determine the most favorable lithofacies that makes the exploration of shale hydrocarbon more effective.

AKNOWLEDGMENTS


The research presented in this paper was supported by the National Science and Technology Special Grant of China (No. 2017zx05036-004). The final publication is available at Springer via https://doi.org/10.1007/s12583-016-0938-3.


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