Palynofacies and geochemical analysis of the Triassic Yanchang Formation, Ordos Basin: Implications for hydrocarbon generation potential and the paleoenvironment of continental source rocks

International Journal of Coal Geology 152 (2015) 159–176

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Palynofacies and geochemical analysis of the Triassic Yanchang Formation, Ordos Basin: Implications for hydrocarbon generation potential and the paleoenvironment of continental source rocks Mingzhen Zhang, Liming Ji ⁎, Yuandong Wu, Cong He Key Laboratory of Petroleum Resources, Gansu Province/Key Laboratory of Petroleum Resources Research, Institute of Geology and Geophysics, Chinese Academy of Sciences, Lanzhou 730000, China

a r t i c l e

i n f o

Article history: Received 15 May 2015 Received in revised form 7 November 2015 Accepted 10 November 2015 Available online 11 November 2015 Keywords: Palynofacies Organic geochemistry paleoenvironment Source rock potential Yanchang Formation Ordos Basin

a b s t r a c t The petroliferous basins that have formed in China since the Mesozoic are characterized by non-marine organic matter input, a characteristic that is significantly different from that of marine organic sources. The Triassic Yanchang Formation in the Ordos Basin is a suite of typical intra-continental lacustrine sediments that comprise the most important source rocks for the Triassic oil reservoirs. The Yanchang Formation is divided into ten subsections from top to bottom (Chan 1 to Chan 10). Three borehole successions in the southern Ordos Basin (located in the Huachi, Zhidan and Yichuan areas) cross the Chan 4 + 5 to Chan 10 subsections and have been studied using the palynofacies method combined with organic geochemistry data. Palynofacies analysis indicates that the sediments are rich in amorphous organic matter (AOM) and phytoclasts. The AOM content in all of the samples positively correlated with the hydrogen index (HI). In contrast, the transparent ligno-cellulosic fragments (TLF) + opaque particles (OP) content are negatively correlated with the HI values. Additionally, the gelified particles (GP) content has no linear correlation with the geochemistry data. Based on the quantitative composition of the particulate organic matter, three palynofacies types are identified, reflecting depositional settings in a distal dysoxic–anoxic deep basin, a shelf-to-basin transition zone and a proximal suboxic shelf. The palynofacies, total organic carbon (TOC), and Rock-Eval data together indicate that type I and II kerogen are abundant in the D48 and W22 wells in the Zhidan and Yichuan areas, respectively. These kerogen types are uncommon in the L94 well in the Huachi area, which suggests low hydrocarbon generation potential. In detail, the Chan 7 and Chan 9 subsections of the three wells contain abundant type I and II kerogen, indicating that these layers are likely the two primary source rocks in the southern Ordos Basin. © 2015 Elsevier B.V. All rights reserved.

1. Introduction The term palynofacies was defined by Powell et al. (1990) as a distinctive assemblage of specific HCl- and HF-insoluble organic matter (“palynoclasts”) whose composition reflects a specific sedimentary environment. This differs from the organic facies, defined as a body of sediment containing a distinctive assemblage of organic constituents, which can either be recognized by microscopy, or is associated with a characteristic bulk organic geochemical composition (Tyson, 1995). Thus, the use of the palynofacies method on the organic matter composition represents only a specific aspect of organic facies research. The advantage of palynofacies research is that it can quantify the various types of particulate organic matter within a source rock using transmitted light microscopy, which was described by Tyson (1995) and is widely applied, including in recent work (e.g. Roncaglia and Kuijpers, 2006; Ghasemi-Nejad et al., 2009; Graz et al., 2010; Garcia et al., 2011). This HCl- and HF-insoluble organic matter (spores, pollen grains, algae, ⁎ Corresponding author. E-mail addresses: [email protected] (M. Zhang), [email protected] (L. Ji).

http://dx.doi.org/10.1016/j.coal.2015.11.005 0166-5162/© 2015 Elsevier B.V. All rights reserved.

acritarchs, chitinozoa, foraminiferal linings, and fragments of different plant tissues) depends on primary productivity, depositional processes and biochemical degradation (Tyson, 1995; Ercegovac and Kostić, 2006). Thus, it can be used with sedimentological evidence to identify the depositional environment, such as proximal–distal changes, the time of maximal marine or terrestrial influx to a depositional area, the oxidation–reduction environment or variations in the water depth (Tyson, 1993; Tyson and Follows, 2000; Zobaa et al., 2011; Mueller et al., 2014). Consequently, the petroleum potential of a sedimentary succession can also be successfully identified from the palynofacies type (e.g. Schiøler et al., 2010; El Atfy et al., 2014). In fact, palynofacies studies have been widely used to interpret paleoenvironments and to evaluate source rocks in marine sediments (Carvalho et al., 2013; El Atfy et al., 2014 and others) but have been rarely used to examine performed on continental source rocks. The Ordos Basin is a large intracontinental sedimentary basin in China with an area of approximately 37 × 104 km2. The crude oil reserve in the Mesozoic reservoirs was estimated to be approximately 10 × 108 metric tons. The basin is considered to be a typical model of a nonmarine oil generating sedimentary basin because the crude oils were

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mainly derived from the non-marine source rocks of the Yanchang Formation (Duan, 2012; Wang et al., 1995). The molecular and isotopic organic geochemical data suggest that the oils were generated from a source with mixed terrigenous and algal–bacterial organic matter (Wang et al., 1995; Duan, 2012). The important discovery of leiosphaerid acritarchs (Leiosphaeridia) and oleaginous Botryococcus in the Yanchang Formation by Ji et al. (2008, 2010) first demonstrated the type and character of the algae sources. Although palynofacies are an important parameter for paleoenvironment reconstruction and the evaluation of hydrocarbon generation, few studies have focused on the palynofacies of the source rocks in the Yanchang Formation. In recent years, several major oilfields, such as the Xifeng, Huaqing and Jiyuan oilfields, have been discovered, which confirms significant hydrocarbon generation in the Yanchang Formation. The sedimentary sequence exhibits a cyclic pattern formed by alternating lake levels, which led to varying qualities of the different source rock layers in the Yanchang Formation. The different water depths, sources and depositional rates in the lake resulted from varying tectonic conditions during the same depositional interval, leading the deposition of different source rocks in the various regions of the Ordos Basin (Li et al., 2012). Most studies have focused on reservoir research with respect to production units in the oilfields and the difficulties in obtaining drill core materials. There are few studies that involve comparisons of the source rocks in the Yanchang Formation. In this study, palynofacies, Rock-Eval and total organic carbon (TOC) analyses are performed on source rock samples from the Yanchang Formation from three wells in different areas of the southern Ordos Basin. The objective of this research is to 1) interpret the composition and characteristics of the terrestrially derived organic matter, 2) interpret

the depositional environment of the sediments using the palynofacies, and 3) distinguish the hydrocarbon generation potential for various oil subsections in the three areas and determine the primary hydrocarbon source rocks. 2. Geological setting The Ordos Basin, which contains abundant petroleum resources, is a superimposed multicycle cratonic basin located on the stable Northern China Platform. This basin experienced two main developments, Paleozoic marine deposition and Mesozoic continental deposition. Deposition in the basin was controlled by paleotopography, which can be divided into the Yishan slope, where the studied wells are located; the Yimeng uplift zone; the Weibei uplift zone; the Jinxi flexural fold zone; the Xiyuan obduction zone; and the Tianhuan depression (Fig. 1A). The strata of the Yishan slope have a gentle western tilt with an angle of approximately 1°. This area is presently a major area of petroleum production in the Ordos Basin. The Ordos Basin was filled by Paleozoic to Cenozoic sediments. The source rocks were mainly formed in the late Paleozoic and early to mid-Mesozoic (Fig. 1B). The Late Paleozoic stratigraphy includes the Carboniferous Jingyuan and Yanghugou Formations and the Permian Taiyuan, Shanxi, Xiashihezi, Shangshihezi and Qianfengshan Formations. The Carboniferous deposits are composed of limestone, sandstone and dark mudstone with several coal beds. The Permian succession is mainly composed of sandstone, mudstone and limestone. Although the Late Paleozoic source rocks have a high TOC (2.0–3.0%), their high maturity means that they are primarily gas source rock. The main oil-bearing sequences in the Ordos Basin are the Upper Triassic Yanchang Formation and the Lower Jurassic Yanan

Fig. 1. A) The location of the research area and the tectonic units of the Ordos Basin; B) the generalized stratigraphic column of late Paleozoic and Mesozoic.

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Formation. The Yanchang is the most important due to its cumulative 25–35-m of dark gray mudstone, with a TOC of 2.0–15.0%. A thick, stratified, fine-grained sandstone is the main reservoir in the target zone of the Mesozoic oil province. Thus, the Yanchang Formation provides both the primary source rocks and important oil and gas reservoirs in the Ordos Basin. Based on lithology, the formation can be upwardly divided into five members (Y1 to Y5) and mainly consists of lacustrine, deltaic and fluvial deposits. Ten oil subsections (Chan 1 to Chan 10) are distinguished based on index beds (tuffs, dark mudstones, carbonaceous mudstones and coal seams), conductance data, and oil bearing characteristics from the top to bottom. Due to the controls of the regional geological structure, the thickness, organic matter content and type of each gas- or oil-bearing subsection differ between various oilfields. This variation is the main reason for the difficulty associated with finding the target oil horizon in hydrocarbon exploration. The southern Ordos Basin is the main hydrocarbon-bearing area (Fig. 1), and currently features many large oilfields, such as the Jiyuan, Yanan and Xifeng oilfields, among others. The burial depth of the source rocks is usually between 1000- and 2000-m, with a maximum burial phase in the late Mesozoic (Early Cretaceous) that led to the main stage of hydrocarbon expulsion (Ren et al., 2007). The source rocks of

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the Yanchang Formation have generally entered the oil threshold at a mature stage (near Ro = 1.0%) (Ren et al., 2007). The total thickness of the Yanchang Formation is approximately 500–1500 m, and individual shale layers reaches 30 m in thickness. 3. Materials and methods 3.1. Materials There were 134 samples that were studied from the L94, D48 and W22 wells, which crossed the Chan 4 + 5 to Chan 10 subsections of the Yanchang Formation (Fig. 2). All of the samples were examined for microscopic palynofacies, TOC and Rock-Eval pyrolysis analyses were performed on all of the samples. The three wells are located in the east, middle and west of the southern Ordos Basin, spanning a distance of 200 km. The L94 well was drilled in the Huachi area, which belongs to the Jiyuan oilfield and is situated in the southwestern Yishan slope. A total of 58 samples were collected from the Chan 4 + 5 to Chan 9 subsections of the drillcore (Fig. 2). The D48 well is located in the Zhidan area in the central southern Yishan slope. A total of 53 samples were collected from the Chan 6 to Chan 9

Fig. 2. Lithostratigraphic correlation of the Triassic Yanchang Formation in the studied wells in the Ordos Basin, China.

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subsections of the well (Fig. 2). The W22 well is located in the Yichuan area in the southeastern Yishan slope. A total of 22 samples were collected from the Chan 7, Chan 9 and Chan 10 subsections of the well (Fig. 2). 3.2. Method 3.2.1. Palynofacies These samples were processed for palynofacies analysis that followed the general procedures by Tyson (1995). Samples of approximately 10–30 g of sediments were treated with 10% HCl and 70% HF to remove the carbonates and silica. Then, the samples were cleaned to neutral by filtered water treatment. Palyno-residues were sieved using 10 μm mesh nylon sieves. Finally, the kerogen residues were mounted on slides using glycerin jelly. In this study, the procedures of oxidative and heavy liquid separation were not applied. More than 200 organic matter particulates were determined in each sample to obtain statistically significant organic matter content and diversity. The minimum sizes of the amorphous organic matter (AOM) and phytoclast particles were 4 μm. The phytoclast size parameter was obtained by counting the macroaxis size of at least 100 pieces of phytoclast debris in each sample. All of the samples were studied using a Zeiss AXO 40 microscope with a transmitted light mode at Key Laboratory of Petroleum Resources Research, Institute of Geology and Geophysics, China. 3.2.2. Organic geochemistry All of the palynofacies samples were processed for organic geochemistry at the Key Laboratory of Petroleum Resources Research in Lanzhou, China. The TOC and Rock-Eval pyrolysis measurements were prepared by crushing the samples to approximately 0.15 mm. Approximately 1–2 g of each sample was prepared for TOC analysis using a LECO CS344 apparatus. For all of the samples, the Rock-Eval pyrolysis analysis was conducted using a Rock-Eval 16 instrument. Several important parameters were provided by the Rock-Eval pyrolysis, including S1, S2, S3 and Tmax (°C). S1 represents the amount of free hydrocarbon (mg HC/g rock) volatilized out of the rock at 300 °C, S2 represents the amount of hydrocarbon (mg HC/g rock) under temperature-programmed pyrolysis (300–600 °C), and S3 represents the amount of released CO2 at temperatures of 300 to 390 °C. Tmax (°C) represents the temperature at the time of the S2 peak during the pyrolysis process. The hydrogen index (HI = (S2 / TOC) × 100, mg HC/g TOC) and oxygen index (OI = (S3 / TOC) × 100, mg CO2/g TOC) are both deduced parameters. The hydrogen index indicates the potential to generate petroleum, and the oxygen index is related to the amount of oxygen in the kerogen (Peters and Cassa, 1994). 4. Results 4.1. Characteristics of the dispersed organic matter In the present study, the particulate organic matter is all of continental origin, which is obviously different from marine deposits. The algae and phytoplankton-derived organic matter are relatively scarce. Additionally, some common marine algae are missing, which is a prominent mark of non-marine sediment. The palynofacies components in this study are broadly classified as AOM, phytoclasts and palynomorphs, for quantitative analysis. The detailed classified methods and criterions can be referred to previous research (e.g. Tyson, 1993; Ercegovac and Kostić, 2006; Ţabără et al., 2015). The AOM components are common and represent an important organic matter type in the Yanchang Formation in this study. Based on

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the shape, color and fluorescence, the AOM can be classified into granular or gelified forms. The granular form is mainly yellow to brown under natural light and exhibits irregular aggregated shapes formed by fibrous and ultramicro scopic organic particles (b3 μm) (Wang et al., 1993; Xiao et al., 1997) (Plate II, 2 and 3). Weak or no fluorescence under a blue light could be due to bacterial degradation (Ţabără et al., 2015). The aggregated forms sometimes contain framboidal pyrite and small-sized phytoclasts (Plate II, 3). These organic matter types are likely derived from phytoplankton, freshwater algae and bacteria that accumulated in environments of O2-depleted water (Pacton et al., 2011; Ţabără et al., 2015). The second common type is the gelified AOM, which is orange to brown and sometimes contains internal structures (Plate II, 1). Its fluorescence is relatively weak (Plate II, 1a) but is stronger than that of the first type. The gelified AOM appeared to form clumpy masses with angular sharp shapes. The fuzzy fibrous margins differ significantly from the central parts, which still retain the original texture of a light yellow smooth surface. This AOM type is usually associated with microbial reworking of terrestrial fragments (Pacton et al., 2011; Ţabără et al., 2015). Phytoclasts are plant-derived fragments, including cuticles, cortex tissues, woody tissues and charcoal (Ercegovac and Kostić, 2006). This group is the dominant composition for most of the source rock samples. Cuticles are generally translucent yellow to light yellow under transmitted natural light and show typical fluorescence (Plate I, 5 and 5a). The well-preserved parts of this component still retain the surface cellular structure (Plate I, 5a). Cortex tissues are rare and are usually difficult to identify because they do not have obvious characteristics and are very similar to woody tissues following thermal degradation and diagenesis. Charcoal has distinct morphological and optical features and is easily distinguished. To effectively distinguish the compositions of the phytoclast group, we decided to categorize them according to their morphological characteristics under the transmission microscope. In this study, the cortex tissues, woody tissues and charcoal components are separated into three categories: 1) transparent ligno-cellulosic fragments (TLF), 2) opaque particles (OP) and 3) gelified particles (GP). The TLF are characterized by typical cellular structures of wood (secondary xylem) and the gray, light-brown, or black color of the lignin. The lack of fluorescence, high translucency, and low lignification are important characteristics of the TLF. This category of grains is common in the palynodebris of the studied samples. The opaque particles (OP) have no visible structure and often appear to be mostly homogeneous, highly corroded opaque fragments with elongated shapes and sharp angular outlines. These components are included in the inertinitic maceral group (mostly fusinite and inertinite). The GP group has recently been proposed in several research studies (Sebag et al., 2006; Graz et al., 2010; Ţabără et al., 2015). These studies focused more attention on the environmental and climate in formation of the deposits, whereas few studies have focused on the organic origin of these common phytoclasts. In this study, a portion of the GP has weak-to-moderate fluorescence intensity under blue transmitted light (Plate II, 4 and 4a). To explore the plant sources of these constituents, we specifically study the leaf organs of coniferous plants, which represented the dominant vegetation in the Mesozoic. In addition, the palynoflora of the Yanchang Formation are also dominated by the bisaccate pollen produced by the conifers (Ji and Meng, 2006). Thus, we gathered many Lower Cretaceous conifer leaf fossils from the Jiuquan Basin. Their detailed shape features and plant nomenclature were studied by Du et al. (2013). The fossil debris was processed by the same experimental methods as in the palynofacies research. Based on the comparability of the surface gloss and color between the GP and the leaf cuticles, the shape and color of the leaf fossils are very

Plate I. (Scale bar: 40 μm) Representative photomicrographs of the palynological organic matter from the studied wells in the Ordos Basin. 1. Isolated bisaccate pollen (Protopinus sp.); 1a. highly fluorescent bisaccate pollen (Protopinus sp.); 2. trilete spore (Asseretospora gyrata); 2a. highly fluorescent Trilete spore (A. gyrata); 3. isolated Leiosphaerida; 3a. highly fluorescent Leiosphaerida; 4. isolated coenobium of Botryococcus; 4a. highly fluorescent coenobium of Botryococcus; 5. dispersed leaf cuticle phytoclast; 5a. a highly fluorescent cuticle; 6. transparent ligno-cellulosic fragments (TLF) with structured tracheid; 7. opaque particles.

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similar to those of the GP under the microscope using transmitted white light (Plate II, 5). These leaf fossils also have strong fluorescence under blue light (Plate II, 5a). The fluorescent intensity of the leaf fossils is obviously higher than that of the GP, which probably results from the higher biodegradation and stronger weathering of small GP debris than the large leaf fossils that have been well preserved. However, the estimated leaf-derived sources also ensure that there is sufficient biomass material. For this reason, the GP are very common in palynofacies associations. Thus, these similar characteristics probably indicate that the GP are mainly derived from the leaf organs of plants. The cuticles on the surface are the main cause of the fluorescence excitation for these leaf fossils and the GP. Palynomorphs are observed in minor amounts and are dominated mainly by bisaccate pollen, Monocolpate gingko pollen and trilete spores (Plate I, 1, 1a and 2, 2a). Algae are very rare and include only Botryococcus and Leiosphaeridia, which have strong fluorescence (Plate I, 3, 3a and 4, 4a).

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4.3. TOC and Rock-Eval pyrolysis data The TOC represents insoluble kerogen and soluble bitumen, which are the common parameters used to express organic matter quantity. In general, the TOC values of the Chan 4 + 5 to Chan 10 subsections of the L94 well vary from 0.47 to 3.29 wt.% (excluding the outlier from Chan 9 at 1975.5 m) with a mean of 1.07 wt.% (Table 1). The Chan 6 to Chan 9 subsections in the D48 well have relatively higher TOC values that range from 0.29 to 8.21 wt.%, with an average of 2.58 wt.% (Table 1). In the W22 well samples, the Chan 7, Chan 9 and Chan 10 subsections feature much higher values between 0.21 and 15.6 wt.%, with an average of 2.6 wt.% (Table 1). The Rock-Eval pyrolysis values for S1, S2 and S3 and the derived parameters HI and OI are important criteria for determining petroleum generation potential. The detailed results are shown in Table 1. The S1, S2, HI and OI values are presented and discussed in detail with respect to the hydrocarbon generation potential and the associated palynofacies components in the sections below.

4.2. Palynofacies assemblages

5. Discussion

Based on the quantitative data on the particulate organic matter obtained from the analyzed samples, three palynofacies types have been defined. AOM dominates the palynofacies-I group (PF-I: AOM), which features a sub-dominance of phytoclast organic matter (Plate III, 2). The phytoclasts are mainly composed of TLF and OP. Cuticles, palynomorphs, and algae are very rare. The average size of the phytoclasts is approximately 15 μm larger than that of the other palynofacies types. The color of the organic matter appears to be brownish yellow and dark brown. This palynofacies type can also be divided into two types. The first type is characterized by a higher AOM content (N60%), which mainly occurs in the Chan 7 and Chan 9 subsections of the D48 and W22 wells. The second type has relatively low AOM (40–60%) compared with that of the first type. In comparison, the TLF and OP contents are relatively high. This type is scattered among the various layers of the studied succession. The palynofacies-II group (PF-II: gelified phytoclasts and AOM) is dominated by yellow AOM and GP (Plate III, 2). The obvious feature of this facies is the high GP content. The GP usually have a large size and feature a yellow color under natural light and moderate yellow fluorescence under blue light. The color of all of the organic matter for this type is yellow, which is usually lighter than that of the other palynofacies types. Cuticles and palynomorphs/algae are also common constituents and are markedly obviously more prevalent than in the other types. The mean phytoclast size is approximately 25 μm, which is obviously larger than that in PF-I. This palynofacies type is common in the L94 well samples and occurs occasionally in the Chan 4 + 5 subsections of the D48 well. The palynofacies-III group (PF-III: phytoclasts) is characterized by a high phytoclast content with good preservation (Plate III, 2). TLF and OP are the main types of phytoclasts. Cuticles are rare in the organic matter. The mean size of the phytoclasts has a large range, from approximately 15 to 25 μm. Palynomorphs are relatively common and usually have well-formed shapes under good preservation conditions. The color of this type has a large range, from brownish yellow to dark brown. This type dominates the large samples of the L94 well sequence, is moderately common in the Chan 4 to Chan 6 subsections of the D48 well, and are very rare in the Chan 7 and Chan 9 subsections of the D48 and W22 wells.

5.1. Kerogen characteristics 5.1.1. Thermal maturity The term maturation refers to the extent of the temperature- and time-dependent reactions involved in hydrocarbon generation (Peters and Cassa, 1994). These reactions significantly affect the color, shape and form of the organic matter. In this study, the degree of thermal maturation is mainly evaluated using the Rock-Eval Tmax data and the spore color index (SCI). The correlation between HI and Tmax is a useful tool for determining the organic matter maturation (Fig. 3). The Tmax readings of the samples from the three wells range between 429 and 476 °C and mostly cluster together at 460 °C. The HI values vary from 50 to 300 mg HC/g rock. These results indicate mature to over-mature stages (after Peters, 1986) for the majority of the Yanchang source rock. The Tmax values also show a slight increasing trend with depth in the three wells (Table 1, Fig. 4). The burial depth is likely a major factor in the thermal evolution of the organic matter of the Yanchang Formation. The SCI is an optical method for evaluating the maturation degree of the organic matter (e.g. Marshall, 1991). In the studied samples, the colors of mainly smooth-walled palynomorphs were correlated with previously established SCI standards (e.g. Pearson, 1984; Pross et al., 2007; Suárez-Ruiz et al., 2012). Most or nearly all of the examined palynomorphs have a yellowish brown color, which corresponds to an early mature to mature stage. Comparatively, the L94 samples have lower Tmax values and lighter palynomorph colors than the samples from the D48 and W22 wells, indicating lower extents of thermal evolution. The HI versus Tmax plots show that most of the samples in the estimated field have a Ro value of 1.0% (Fig. 3). In the D48 well, the Tmax values have a relatively narrow range, from 445 to 470 °C (excluding the outliers from the Chan 6 and Chan 8 subsections, 1233.8 m and 1424.5 m, respectively), which is mostly higher than the range calculated for the L94 well samples. The HI versus Tmax plots indicate a mainly mature stage for the source rock samples (Fig. 3). The W22 samples show a mature to highly mature stage, which is estimated by a high Tmax and SCI standard values. The Tmax values of all the samples range from 448 to 473 °C, which are higher than those of the L94 and D48 well samples. Most of the samples were close to the 1.35% Ro field in

Plate II. (Scale bar: 40 μm) 1. Under natural light, yellowish brown gelified AOM; 1a. idem previous image (blue-light fluorescence), weak fluorescence exhibited by the gelified AOM; 2. brown granular AOM with a diffuse edge; 3. brown granular AOM that contains framboidal pyrite (black arrow); 4. brown gelified phytoclast (GP) under natural light; 4a. idem previous image (blue-light fluorescence), GP exhibits weak fluorescence; 5. under natural light, yellowish brown leaf fossil debris from Fig. 3 in this plate; 5a. idem previous image (blue-light fluorescence), leaf fossil debris exhibits strong fluorescence.

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the HI versus Tmax plot. In summary, all the thermal maturity data indicate that the studied Yanchang Formation source rock samples have entered the mature to highly mature stage of hydrocarbon generation.

5.1.2. The hydrocarbon potential of the particulate organic matter The composition and characteristics of the particulate organic matter are those of a hydrocarbon-generating material that could form the basis of a source rock. The TOC and Rock-Eval parameters are well-accepted traditional evaluation methods for determining the hydrocarbon generation potential of a source rock. Thus, the hydrocarbon generation potential and original material sources can be derived by the correlation between the above two factors. In this study, four main organic matter groups (AOM, GP, TLF and OP) are chosen for correlation with the TOC and Rock-Eval parameters. The AOM is the main component, but it has a large range among the different samples from the three wells. Fig. 5A–C shows that the AOM content is linearly associated with the HI, indicating that the AOM has a high potential for hydrocarbon generation. The high AOM content reflects the high quality of the source rock, which is an essential factor for the hydrocarbon generation of kerogen. This trait can be estimated using the material sources and the preservation condition of the AOM. The granular AOM most likely originates from phytoplankton and freshwater algae. Another important component is the gelified AOM, which most likely originated from the liptinite content of high plants (Ercegovac and Kostić, 2006; Pacton et al., 2011; Ţabără et al., 2015). The AOM is initially an organo–mineral association that is interpreted to be an early flocculation at or near the sediment–water interface (Tyson, 1995; Ercegovac and Kostić, 2006). AOM is widely accepted to form in anoxic environments (e.g. Tyson, 1995; Ercegovac and Kostić, 2006). Anoxic environments are also favorable for the deposition of high-quality source rocks. The phytoclasts originated from the various tissues of terrestrial plants, and they have different hydrocarbon generation abilities. In general, the fluorescent cuticles have high hydrocarbon generation potential, and they are classified as oil-prone kerogen types. However, this component usually comprises a minor portion of the total particulate organic matter; thus, it cannot be the main source of the hydrocarbons. The GP, TLF and OP of phytoclasts are the main components of the total particulate organic matter. The xylogen-derived TLF and OP are both low hydrogen-containing components; therefore, they have low hydrocarbon generation potential. Thus, the two components are combined into a single group to study. The TLF + OP and HI show a good negative correlation (Fig. 5D–F), whereas the GP and HI show a relatively poor correlation compared with the TLF + OP compositions (Fig. 4G–I). A relatively good correlation of the first analysis result can be easily understood. Similar to the usual vitrinite component, TLF are classified according to the gas-prone kerogen types, which have low hydrocarbon generation capabilities. The OP theoretically has no hydrogen content. These components are often present in proximal delta zones with oxic environments, which are not good for the preservation of source rocks. Although few studies have addressed the hydrocarbon generation potential of GP compositions, the relatively higher fluorescence of these constituents indicate the existence of a certain amount of lipids with hydrocarbon generation potential. However, this composition is common in various palynofacies assemblages and deposited in various environments, from shallow to deep water. Thus, a poor correlation between GP and HI is observed.

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The palynomorph/algae have a high capability for hydrocarbon generation (e.g. Dutta et al., 2013) but contributed only a minor proportion to the total particulate organic matter. Thus, these components can not be the major contribution for the total hydrocarbon content of the source rock. 5.2. Paleoenvironmental reconstruction The palynofacies represent the primary composition of the organic matter in the sedimentary rock after the long process of sedimentation and organic diagenesis. The paleoenvironments control the organic matter composition. The quantitative assemblages of particulate organic matter exhibit an excellent capacity to record environmental evolution under different climatic and sedimentary settings (de Araujo Carvalho et al., 2006; Sebag et al., 2006). Thus, the palynofacies analysis was applied as one of the traditional parameters for paleoenvironmental interpretation (Buchardt and Nielsen, 1991). According to the principle that the relative abundances of different organic matter groups primarily reflect oxygenation conditions, it is similarly suitable for considering the lake level changes and estimated distances from the lakeshore (Sebag et al., 2006). The method for paleoenvironmental interpretation of the Yanchang Formation is not the same as for traditional marine deposits due to its continental depositional setting. The AOMPhytoclasts-Palynomorphs ternary plot after Tyson (1993, 1995) has been widely applied to infer the depositional conditions and the transportation pathways of the organic matter in a shallow sea depositional environment. In this study, we modify the classification of organic matter for the ternary plot by Tyson (1993, 1995) to the AOM–TLF + OP–GP + Cuticles + Palynomorphs (AOM–TO–GCP) ternary plot. The TLF and OP are derived from the stem xylogen of terrestrial plants, which have a large volume and are deposited in the proximal shallow or high-energy aquatic environment. GP and cuticles likely originate from leaf organs, which can be easily transported by the wind and water currents. This transport property is theoretically similar to that of the palynomorphs. Thus, in this study, the GP and cuticles are classified into the palynomorph field in the ternary plot. In addition, the phytoclast size parameter is a useful indicator of the proximity to land or debris flows in deep water settings (Tyson, 1995; Tyson and Follows, 2000), and this indicator is also employed in this study. 5.2.1. Palynofacies-I (PF-I: AOM): distal dysoxic–anoxic deep basin The high percentage of AOM and low to intermediate percentage of degraded phytoclasts characterize palynofacies-I (Fig. 6). AOM is generally deposited in environments with high preservation rates and low energy (Tyson, 1993). Dysoxic conditions are associated with the preservation of AOM, which correlates with but is not necessarily dependent upon high primary productivity (Tyson, 1993). In general, deep and low-energy aquatic environments result in anoxic deposits. Algal and aerobic microbe blooms increase the organic matter input, which is an important source of granular AOM. Furthermore, algal blooms can result in an anaerobic depositional environment (Liu and Wang, 2013). The remarkable laminated structures in most of the source rock samples are commonly associated with anoxic bottom conditions (Liu and Wang, 2013). The samples of this type plot close to the AOM field in the AOM–TO–GCP ternary diagram, which indicates a tendency toward

Plate III. (Scale bar: 40 μm) 1. PF-I: Abundant AOM (granular and gelified) with yellow color in association with palynomorphs and phytoclasts. The color and abundance of the AOM suggest a dysoxic–anoxic environment. This sample has high TOC and HI values, i.e., 2.21 wt.% and 182 mg HC/g rock, respectively, and the contained organic matter was identified as kerogen type I (oil prone) (Chan 6 subsection of the Yanchang Formation, 1302.1 m of the D48 well). 2. PF-II: AOM with a yellow-to-sunny-yellow color in association with gelified phytoclasts (GP). The depositional environment was mainly dysoxic. The sample has a high TOC content and hydrocarbon index, with values of 1.93 wt.% and 137 mg HC/g rock, respectively (Chan 7 subsection of the Yanchang Formation, 1837.6 m of the L94 well). 3. PF-III: Abundant TLF, opaque phytoclasts and GP in the particulate organic matter, which suggests a shallow water oxic environment. The kerogen is likely type III (gas prone). This sample has low values for TOC (0.87 wt.%) and HI (62 mg HC/g rock) (Chan 7 subsection of the Yanchang Formation, 1857.2 m of the L94 well).

168

Table 1 The palynofacies, TOC and Rock-Eval pyrolysis data of the studied samples. Well name

L3 L4 L5 L6 L7 L8 L9 L10 D3 D4 D5 L11 L12 L13 L14 D7 D8 D9 D10 D11 L15 L16 L17 L18 L19 L20 L21 L22 L23 L24 L25 L26 L27 L28 L29 L30 L31 L32 L33 L34 L35 L36 L37 L38 L39 L40 D12 D13 D14 D15 D16 D17 D18 L41

Depth (m)

Subsections

1630.4 1635.5 1649.3 1651.2 1707.6 1708.8 1720.5 1727.5 1233.8 1235.4 1249.8 1758 1762.6 1771.3 1775.1 1277.8 1278.7 1302.1 1317.8 1319 1777.9 1780.2 1783.8 1787.3 1793.3 1794.3 1797 1800 1802 1802.9 1804 1805 1811.8 1813.5 1820.7 1823.5 1825.9 1827.4 1829.7 1831.9 1837.6 1839.5 1841.6 1844.1 1845.8 1846.5 1373.3 1374.5 1375.2 1376.5 1377.3 1378.1 1379.1 1853.5

Chan 4 + 5

Chan 6

Chan 7

Lithology

Gray silty mudstone Gray silty mudstone Dark gray silty mudstone Gray silty mudstone Dark gray silty mudstone Dark gray mudstone Dark gray silty mudstone Dark gray mudstone Dark gray mudstone Black carbon mudstone Dark gray mudstone Gray mudstone Dark gray silty mudstone Dark gray mudstone Dark gray silty mudstone Dark gray mudstone Dark gray mudstone Black mudstone Black silty mudstone Dark gray silty mudstone Dark gray mudstone Dark gray mudstone Dark gray mudstone Dark gray mudstone Dark gray mudstone Dark gray mudstone Gray mudstone Gray silty mudstone Dark gray mudstone Dark gray silty mudstone Dark gray mudstone Gray silty mudstone Dark gray silty mudstone Dark gray silty mudstone Dark gray mudstone Dark gray mudstone Gray silty mudstone Gray silty mudstone Gray silty mudstone Dark gray silty mudstone Gray silty mudstone Gray silty mudstone Gray silty mudstone Gray silty mudstone Dark gray silty mudstone Dark gray muddy siltstone Black carbon mudstone Black shale Dark gray mudstone Black carbon mudstone Black carbon mudstone Dark gray silty mudstone Dark gray mudstone Gray silty mudstone

AOM (%) 8.5 41.0 5.0 1.0 29.8 75.2 15.0 81.8 27.3 35.7 19.8 46.4 4.5 4.1 51.2 9.4 34.4 50.1 42.5 3.6 40.7 6.1 45.2 2.9 2.0 26.0 30.6 6.1 14.2 63.3 2.4 1.0 1.0 16.6 65.2 2.0 13.0 3.0 1.0 75.0 17.4 6.0 49.0 65.0 24.7 73.1 72.3 78.4 84.0 87.0 76.4 38.6 17.2 6.5

Phytoclasts (%) GP

TLF

OP

Cuticle

35.9 15.2 24.1 60.7 17.9 2.0 38.6 1.5 14.5 29.8 1.9 19.1 30.0 25.8 14.2 13.7 3.9 16.9 24.6 20.1 19.1 30.6 31.4 31.7 30.4 8.9 11.4 35.9 10.5 11.8 29.1 32.9 38.3 38.9 4.5 35.1 28.1 40.7 40.7 6.4 35.2 25.7 10.9 8.1 13.2 2.6 1.4 0.5 0.0 0.3 3.1 0.6 30.7 28.7

15.9 7.6 18.2 9.1 8.6 5.6 13.9 1.5 21.3 9.3 32.7 15.2 17.0 25.8 11.3 34.1 21.1 15.7 20.2 29.6 11.1 20.1 2.9 15.8 21.8 16.4 14.9 9.8 21.0 4.9 18.1 26.7 9.2 9.3 8.1 18.3 10.1 16.3 27.2 3.7 11.7 36.3 17.3 8.8 32.4 7.6 4.3 1.6 1.5 1.2 8.7 13.7 20.2 43.4

15.4 24.3 32.6 16.1 34.5 15.7 27.1 13.9 24.2 8.4 37.5 17.5 19.6 19.2 19.6 24.6 24.3 3.4 2.3 32.1 28.1 22.6 6.8 36.8 36.9 37.3 36.7 25.1 44.6 17.8 31.8 26.2 24.4 24.4 12.6 23.5 38.1 18.1 13.4 12.4 21.5 20.0 13.4 14.4 21.6 4.4 16.4 14.0 12.4 10.4 7.7 45.0 19.4 13.3

23.4 12.0 18.5 13.0 9.2 1.5 5.4 1.2 1.0 11.1 0.3 0.7 27.0 24.3 2.3 1.8 2.1 0.7 0.2 1.4 1.0 17.9 12.8 10.4 8.9 10.0 6.4 21.4 9.7 1.3 15.4 11.8 25.1 10.8 8.7 21.1 10.7 20.4 16.7 2.5 13.5 9.1 8.3 3.1 7.0 10.7 1.5 0.3 0.0 0.0 0.9 1.0 0.3 6.2

Palynomorphs (%)

Phytoclast size (um)

1.0 0.0 1.5 0.0 0.0 0.0 0.0 0.0 11.7 5.7 7.8 1.0 2.0 0.9 1.4 16.4 14.1 13.1 10.2 13.2 0.0 2.7 1.0 2.4 0.0 1.4 0.0 1.6 0.0 0.9 3.2 1.4 2.0 0.0 0.8 0.0 0.0 1.5 1.0 0.0 0.8 2.9 1.1 0.6 1.1 1.5 4.2 5.1 2.1 1.2 3.2 1.0 12.2 1.9

23.6 18.4 24.8 24.8 18.4 13.6 20 12.4 13.6 20.4 20.8 21.6 22.4 22.4 18.4 16.4 20.4 14.4 16.8 18.4 17.6 25.2 21.6 27.2 19.6 16.4 20.4 20.8 18.8 18.4 23.2 22.8 24.4 24.8 20 26.8 22.8 26 27.2 20.4 22.8 24.8 20 19.6 22.4 25.2 15.6 16 17.2 15.6 16.8 12.4 12.8 26

TOC

0.49 0.54 0.48 0.61 0.47 0.67 0.63 0.82 0.64 1.9 0.39 0.65 2.78 0.64 2.13 0.96 0.59 2.21 1.58 0.36 0.6 0.59 1.71 1.03 0.5 0.45 0.59 0.49 0.61 3.29 1.18 0.63 0.85 0.89 1.92 1.05 0.48 0.63 1.18 0.55 1.93 1.13 1.46 1.77 1.17 2.76 5.18 3.01 3.83 4.86 3.42 0.48 0.31 1.17

Rock-Eval pyrolysis Tmax

S1

S2

S3

HI

OI

454 456 453 455 455 455 459 450 507 447 468 458 451 457 446 457 457 445 451 461 459 456 448 452 461 462 463 459 454 447 454 456 453 456 439 451 458 454 450 455 448 453 458 451 455 447 451 459 449 453 458 451 470 456

0.09 0.1 0.09 0.11 0.14 0.26 0.19 0.43 0.02 0.53 0.08 0.15 0.78 0.13 0.41 0.2 0.14 0.61 1.3 0.14 0.11 0.11 0.33 0.2 0.07 0.1 0.13 0.1 0.13 0.75 0.18 0.12 0.21 0.22 0.28 0.22 0.1 0.13 0.22 0.12 0.41 0.23 0.22 0.49 0.41 0.97 3.15 2.25 2.46 2.76 0.99 3.38 0.1 0.22

0.37 0.36 0.32 0.37 0.36 0.62 0.41 1.15 0.13 6 0.1 0.51 5.28 0.38 4.06 0.89 0.37 4.03 3.24 0.02 0.37 0.31 2.79 0.88 0.18 0.2 0.3 0.24 0.36 7.42 0.91 0.35 0.82 0.67 2.02 0.89 0.26 0.39 1.64 0.35 2.65 1.04 1.31 2.2 0.99 2.89 11.6 6.2 7.8 9.6 6.28 0.71 0.16 0.93

0.11 0.18 0.17 0.1 0.16 0.15 0.13 0.16 0.06 0.1 0.12 0.17 0.24 0.08 0.11 / 0.08 0.24 0.1 / 0.18 0.14 0.12 0.16 0.14 0.12 0.19 0.19 0.18 0.34 0.33 0.32 0.23 0.16 0.14 0.29 0.28 0.21 0.25 0.24 0.35 0.06 0.32 0.11 0.04 0.22 0.28 0.19 0.47 0.3 0.22 0.29 0.15 0.18

75 66 66 60 76 92 65 140 20 315 25 78 189 59 190 92 62 182 205 55 61 52 163 85 36 44 50 48 59 225 77 55 96 75 105 84 54 61 138 63 137 92 89 124 84 140 224 205 203 197 183 147 51 79

22 33 35 16 34 22 21 20 9 5 30 26 9 13 5 / 13 10 6 / 30 23 7 15 28 26 32 38 29 10 27 50 27 17 7 27 58 33 21 43 18 5 21 6 3 7 5 6 12 6 6 60 48 15

M. Zhang et al. / International Journal of Coal Geology 152 (2015) 159–176

L94 L94 L94 L94 L94 L94 L94 L94 D48 D48 D48 L94 L94 L94 L94 D48 D48 D48 D48 D48 L94 L94 L94 L94 L94 L94 L94 L94 L94 L94 L94 L94 L94 L94 L94 L94 L94 L94 L94 L94 L94 L94 L94 L94 L94 L94 D48 D48 D48 D48 D48 D48 D48 L94

Sample

L42 L43 L44 D19 D20 D21 W1 W2 W3 W4 W5 L46 L47 L48 D22 D23 D24 D25 D26 D27 D28 D29 D30 D31 L49 L50 D32 D33 D34 D35 D36 D37 D38 D39 D40 D41 L51 L52 L53 L54 L55 L56 L57 L58 L59 W6 W7 W8 L60 W10 W12 W13 W14 W15 W16 W17 W18 W19 W20

1855.5 1857.2 1860.1 1380.2 1380.8 1381.6 299.3 300.2 303 304 322.3 1901.6 1915 1916 1424.5 1425.4 1426.3 1427.2 1428.1 1429.2 1430.3 1431.2 1431.9 1432.8 1938.9 1940.2 1477.2 1478.3 1479.6 1480.4 1481.3 1481.8 1482.3 1483.6 1484.6 1485.6 1963.4 1964.7 1966.2 1974.4 1975.5 1976.3 1977.1 1977.8 1978.6 441.2 443.1 444 2058 452 454.2 455.2 460.5 461.2 462 462.9 463.5 464.6 466.8

Chan 8

Chan 9

Gray silty mudstone Gray silty mudstone Dark gray silty mudstone Dark gray mudstone Black mudstone Dark gray mudstone Dark gray shale Dark gray shale Dark gray shale Dark gray shale Dark gray mudstone Dark gray silty mudstone Dark gray silty mudstone Black silty mudstone Dark gray silty mudstone Dark gray silty mudstone Dark gray mudstone Dark gray mudstone Gray silty mudstone Gray silty mudstone Dark gray silty mudstone Dark gray silty mudstone Dark gray mudstone Dark gray silty mudstone Dark gray mudstone Dark gray silty mudstone Gray silty mudstone Dark gray silty mudstone Dark gray mudstone Dark gray mudstone Black carbon mudstone Black carbon mudstone Dark gray silty mudstone Black carbon mudstone Dark gray mudstone Dark gray silty mudstone Dark gray silty mudstone Dark gray silty mudstone Dark gray mudstone Dark gray mudstone Dark gray mudstone Dark gray silty mudstone Dark gray shale Dark gray carbon mudstone Gray silty mudstone Gray mudstone Dark gray mudstone Gray silty mudstone Dark gray shale Dark gray mudstone Dark gray mudstone Dark gray mudstone Dark gray shale Dark gray mudstone Dark gray shale Dark gray silty mudstone Dark gray silty mudstone Dark gray silty mudstone Gray silty mudstone

15.0 1.0 70.4 28.6 54.2 7.4 76.1 76.7 74.3 81.3 69.2 5.6 9.0 4.9 73.3 40.2 64.9 62.1 7.6 57.6 71.1 55.1 39.0 45.1 79.8 71.7 13.2 54.4 54.4 54.8 57.8 35.4 41.6 80.2 16.2 50.8 2.0 3.8 1.0 3.6 57.4 53.0 3.0 3.2 17.5 21.1 54.6 12.4 60.2 5.5 17.9 28.5 67.2 59.0 78.4 76.7 8.2 53.4 52.9

25.4 28.5 3.5 4.9 5.2 51.7 1.0 0.5 0.0 0.0 4.0 20.9 16.7 25.1 0.2 8.0 3.2 3.2 9.1 9.8 3.6 5.4 12.7 15.5 11.6 16.4 12.0 7.0 4.9 2.3 11.7 22.0 15.3 10.8 11.4 5.7 23.3 24.5 16.3 26.1 28.7 21.7 9.2 40.0 27.4 12.7 8.2 10.2 18.0 7.1 2.1 3.4 0.0 0.0 0.0 1.1 11.3 11.7 16.3

36.4 34.9 12.9 9.1 20.9 27.4 3.0 3.6 2.6 1.9 7.9 19.0 13.8 11.6 6.0 15.0 13.0 8.2 29.8 12.6 8.6 11.2 16.3 21.7 1.3 1.0 43.2 24.6 11.6 13.5 16.2 28.2 19.7 6.1 31.8 20.5 25.8 23.2 41.2 22.2 8.9 10.9 27.5 27.4 13.4 28.4 17.8 37.0 7.2 39.3 27.3 23.6 1.9 4.1 2.0 7.2 56.2 11.6 10.0

9.0 12.4 8.8 40.1 14.6 6.3 16.0 16.4 18.0 15.5 6.3 36.8 50.3 52.7 17.2 22.2 9.0 17.1 38.1 11.2 10.4 12.0 15.4 11.6 6.1 8.6 14.3 10.6 21.0 26.1 7.9 12.3 16.8 1.6 23.5 11.0 34.7 31.8 33.7 36.2 2.8 9.3 47.5 7.3 23.3 23.1 12.0 31.6 12.5 34.3 46.3 33.8 29.6 30.8 17.3 9.8 11.3 11.6 7.4

11.9 23.2 4.2 1.0 0.3 1.1 3.4 2.4 5.1 1.3 10.0 16.0 10.2 4.8 0.0 0.5 1.1 0.0 0.3 0.2 0.7 1.0 0.3 0.0 1.2 2.3 0.0 0.3 0.0 0.0 0.3 0.0 0.4 0.0 0.9 0.0 14.2 14.9 7.8 10.8 2.1 5.1 10.9 22.1 17.4 14.7 7.0 8.8 2.1 13.5 6.4 9.7 1.3 6.1 2.3 4.6 12.5 10.1 13.2

2.3 0.0 0.0 16.3 4.6 6.2 0.5 0.5 0.0 0.0 2.7 1.8 0.0 0.9 3.2 14.1 8.8 9.4 15.1 8.6 5.6 15.4 16.3 6.1 0.0 0.0 17.3 3.1 8.1 3.2 6.2 2.1 6.2 1.2 16.2 12.1 0.0 1.7 0.0 1.1 0.0 0.0 2.0 0.0 1.0 0.0 0.3 0.0 0.0 0.3 0.0 0.9 0.0 0.0 0.0 0.6 0.4 1.6 0.3

24.8 27.2 20.8 14 13.2 13.6 14 12.8 11.2 10.4 19.2 24.4 23.2 22.8 14.4 12.4 15.2 16.8 18.4 19.2 16.8 14 19.6 19.6 20 21.2 20.4 12.4 12.8 12.6 19.2 20 16.4 18.4 22.4 17.2 22.8 22.8 22.4 23.2 19.2 18.4 21.2 20.8 22.4 16.8 20.4 24.4 20.4 23.2 16.4 21.6 12.4 16.8 14 15.2 21.2 21.6 18

1.42 0.87 0.82 2.04 1.88 0.29 2.58 3.3 3.32 3.54 1.71 0.71 0.79 0.69 0.83 2.73 2.55 1.47 0.31 1.13 2.78 0.59 1.07 0.72 1 2.95 0.34 0.76 3.89 0.49 1.79 0.49 0.4 5.18 0.65 0.97 0.74 0.69 0.52 0.98 12.7 3.02 0.99 0.49 0.77 0.21 0.85 0.36 1.05 0.27 0.36 0.48 3.41 3.27 3.77 3.39 0.57 1.36 1.12

455 458 467 460 456 465 460 457 457 457 462 469 469 473 437 460 461 453 450 456 458 457 458 460 469 455 469 465 460 456 462 468 469 461 466 469 470 476 476 466 456 455 464 429 471 470 463 469 465 472 473 473 455 454 462 460 470 463 454

0.23 0.19 0.24 0.76 0.97 0.08 0.88 1.52 1.79 1.68 0.42 0.12 0.11 0.11 0.94 0.94 1.03 0.58 0.45 0.51 0.88 0.31 0.42 0.31 0.17 0.6 0.12 0.28 1.55 0.21 0.5 0.14 0.12 1.52 0.18 0.28 0.11 0.1 0.09 0.16 2.78 0.51 0.14 0.3 0.11 0.04 0.18 0.11 0.16 0.05 0.06 0.09 1.36 1.4 0.88 1.11 0.2 0.29 0.76

1.18 0.54 0.4 3.52 2.71 0.09 4.44 5.92 5.76 6.12 2.2 0.29 0.3 0.22 1.38 4.09 4.07 2.35 0.26 1.25 4.6 0.59 1.21 0.58 0.45 3.67 0.15 0.55 5.69 0.39 2.3 0.2 0.16 9.16 0.38 0.53 0.33 0.23 0.15 0.57 25.1 4.42 0.68 0.2 0.32 0.04 0.51 0.19 0.7 0.05 0.1 0.21 6.45 7.2 5.26 5.5 0.44 1.89 1.76

0.17 0.2 0.8 0.1 0.08 / 0.19 0.22 0.47 0.39 0.27 0.3 0.14 0.1 0.05 0.02 0.03 0.03 0.02 / 0.21 0.08 0.18 0.11 0.2 0.21 / 0.04 0.19 0.25 0.06 0.11 0.03 0.21 0.16 0.11 0.08 0.03 0.13 0.06 0.97 0.42 0.21 0.33 0.08 0.03 0.05 0.02 0.1 0.21 0.01 / 0.21 0.23 0.27 0.21 0.09 0.05 0.1

83 62 48 172 144 31 172 179 173 172 128 40 37 31 166 149 159 159 83 110 165 100 113 80 45 124 44 72 146 79 128 40 40 176 58 54 44 33 28 58 197 146 68 40 41 19 60 52 66 18 27 43 189 220 139 162 77 138 157

11 22 97 4 4 / 7 6 14 11 15 42 17 14 6 1 1 2 6 / 7 13 16 15 20 7 / 5 4 51 3 22 7 4 24 11 10 4 25 6 7 13 21 67 10 14 5 5 9 77 2 / 6 7 7 6 15 3 8

169

(continued on next page)

M. Zhang et al. / International Journal of Coal Geology 152 (2015) 159–176

L94 L94 L94 D48 D48 D48 W22 W22 W22 W22 W22 L94 L94 L94 D48 D48 D48 D48 D48 D48 D48 D48 D48 D48 L94 L94 D48 D48 D48 D48 D48 D48 D48 D48 D48 D48 L94 L94 L94 L94 L94 L94 L94 L94 L94 W22 W22 W22 L94 W22 W22 W22 W22 W22 W22 W22 W22 W22 W22

170

Table 1 (continued) Well name

W21 W22 D42 D43 D44 D45 D46 D47 D48 D49 D54 D55 D56 D57 D58 D59 D60 L61 W23 W24 W25

Depth (m) 467.7 468.5 1507.4 1508.6 1509.3 1510.2 1510.8 1511.1 1512.2 1513.1 1527.5 1528.5 1529.4 1530.5 1531.6 1532.5 1533.4 2079.4 477.9 478.3 480.3

Subsections

Chan 10

AOM = amorphous organic matter. TOC = total organic carbon, wt.%. GP = gelified particle. OP = opaque particle. TLF = transparent ligno-cellulosic fragments. S1: Volatile hydrocarbon (HC) content, mg HC/g rock. S2: Remaining HC generative potential, mg HC/g rock. S3: Carbon dioxide yield, mg CO2/g rock. Tmax: Temperature at maximum of S2 peak, °C. HI: Hydrogen Index = S2 × 100 / TOC, mg HC/g TOC. OI: Oxygen Index = S3 × 100 / TOC, mg CO2/g TOC.

Lithology

Gray silty mudstone Gray muddy siltstone Black carbon mudstone Dark gray silty mudstone Black carbon mudstone Black carbon mudstone Dark gray shale Dark gray shale Black carbon mudstone Dark gray silty mudstone Dark gray shale Dark gray shale Dark gray shale Dark gray shale Dark gray shale Dark gray shale Gray silty mudstone Black carbon mudstone Dark gray silty mudstone Dark gray silty mudstone Gray silty mudstone

AOM (%) 69.7 54.3 75.9 65.5 67.1 76.7 81.8 79.6 92.6 81.2 82.0 78.9 83.7 82.8 84.3 23.1 25.3 67.5 76.5 73.1 30.8

Phytoclasts (%) GP

TLF

OP

Cuticle

4.7 6.5 1.0 1.1 2.6 1.0 1.9 1.0 0.5 2.7 1.1 1.0 0.3 0.3 0.3 13.8 24.6 14.8 0.0 0.6 8.0

5.0 20.6 5.0 8.6 7.3 5.0 2.8 3.2 0.8 2.5 3.2 3.0 1.6 1.6 1.6 16.1 11.0 3.3 3.6 3.4 30.8

16.4 10.0 16.0 21.6 21.8 16.0 9.9 11.2 2.9 11.1 12.2 15.3 11.2 11.2 11.2 42.0 32.2 10.2 18.6 19.2 14.3

4.1 6.5 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 3.9 1.2 3.1 14.1

Palynomorphs (%) 0.0 2.2 2.1 3.2 1.3 1.3 3.6 5.1 3.2 2.4 1.6 1.8 3.2 4.1 2.6 5.1 6.9 0.3 0.0 0.6 2.0

Phytoclast size (um) 19.2 20.4 10 12.8 13.2 12.4 18 13.6 12.4 12.8 13.2 11.6 12.4 12.8 11.2 12.8 13.2 20.8 12.8 12.4 24.4

TOC

1.63 1.79 4.57 4.24 4.71 4.24 5.07 7.77 6.71 3.84 5.13 4.65 6.61 5.06 8.21 0.72 2.02 1.4 1.88 1.8 7.87

Rock-Eval pyrolysis Tmax

S1

S2

S3

HI

OI

457 459 456 455 457 456 457 462 457 453 462 459 458 461 461 462 464 461 464 461 459

0.74 0.53 3.08 3.44 3.4 3.42 3.91 4.09 4.5 3.62 2.57 2.28 3.78 2.24 3.45 0.47 1.19 0.21 0.49 0.55 1.92

2.73 2.42 7.54 8.06 9.24 9.9 10.3 14 12 7.26 8.18 7.38 12 9.2 13.1 0.61 3.3 1.44 2.33 2.21 16.2

0.17 0.04 0.22 0.32 0.27 0.34 0.34 0.29 0.3 0.23 0.41 0.34 0.33 0.3 0.31 0.18 0.15 0.08 0.11 0.1 0.57

167 135 164 190 196 233 203 180 179 189 159 158 182 181 159 84 163 102 123 122 205

10 2 4 7 5 8 6 3 4 5 7 7 4 5 3 25 7 5 5 5 7

M. Zhang et al. / International Journal of Coal Geology 152 (2015) 159–176

W22 W22 D48 D48 D48 D48 D48 D48 D48 D48 D48 D48 D48 D48 D48 D48 D48 L94 W22 W22 W22

Sample

M. Zhang et al. / International Journal of Coal Geology 152 (2015) 159–176

171

Fig. 3. Plot of HI versus Tmax for the analyzed source rock samples from the L94, D48 and W22 wells, delineating present-day thermal maturation and kerogen types.

anoxic depositional environments. The phytoclast sizes are usually small and indicate a distal deposition (Tyson and Follows, 2000). Thus, this palynofacies assemblage is considered to indicate a distal

dysoxic–anoxic deep basin environment. This facies mainly occurs in the Chan 7 and Chan 9 subsections of the studied sequences, especially in the D48 and W22 wells (Fig. 6).

Fig. 4. Relationship between depth and Tmax for the L94, D48 and W22 wells.

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5.2.2. Palynofacies-II (PF-II: GP and AOM): shelf-to-basin transition AOM compositions with high GP content and common palynomorphs characterize palynofacies-II (Fig. 6). Generally, GP, cuticles and palynomorphs are easier to transport than the TLF, which are most likely sorted in the transportation process. In addition, moderate percentages of AOM indicate a relatively dysoxic depositional environment. Most samples plot in or near the GCP field in the AOM–TO–GCP ternary diagram (Fig. 6). This field is interpreted to represent the heterolithic oxic shelf (proximal shelf) (Tyson, 1995). The phytoclast sizes are larger than those in PF-I, which reflects the fact that the proximal–distal trend requires a certain level of water dynamics. This palynofacies also has another characteristic, its high TOC values, which are an indication of low sedimentation rates and/or high organic input. This facies is mostly interspersed with the PF-III horizons in the L94 well sequences. In the anoxic depositional interval in Chan 7 and Chan 9 in the D48 and W22 well sequences, this palynofacies type is rare. Thus, the transportation distance, proximity, and oxidation reflected by this palynofacies, all between the PF-I and PF-III values, are indicative of a suboxic to dysoxic shelf-to-basin environment.

5.2.3. Palynofacies-III (PF-III: phytoclasts): proximal suboxic shelf Palynofacies-III is characterized by the high percentage of TLF and OP and the low to intermediate percentage GP (Fig. 6). The percentage and size of the phytoclasts group is generally inferred to be associated with the distance from the terrestrial organic matter flux (Tyson, 1995; Tyson and Follows, 2000). TLF mainly originated from terrestrial plant xylogen, which has a relatively large volume and tends to be deposited close to the source. Thus, the high value of TLF may indicate a shorter transport distance. Additionally, the low AOM content indicates a relatively oxic sedimentary environment. The TOC values are usually less than 1%, which reflects a low organic matter preservation ratio or high sediment accumulation rates. Most of the samples plot in the TO field of the AOM–TO–GCP ternary diagram (Fig. 6), which can be interpreted as a highly proximal shelf or basin environment. The phytoclast sizes are larger and are mainly in the range of 15–25 μm, which also indicates relatively proximal deposits. The sampled horizons are dominated by coarser-grained rocks, such as sandy or silty mudstones, which generally occur in shallow lake or delta depositional environments. Therefore, palynofacies-III most likely accumulated in a shallow near shore

Fig. 5. Relationships between of TOC, HI and organic components (AOM, GP and TLF).

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Fig. 6. Ternary AOM–TO–GCP kerogen plot of the sample data from the three studied wells showing the palynofacies types and depositional sedimentary environment (ternary diagram modified by Tyson, 1993, 1995). AOM = amorphous organic matter; TO = transparent ligno-cellulosic fragments + opaque particles; GCP = gelified particles + cuticles + palynomorphs.

environment. This palynofacies type is mainly observed in the samples from the L94 well and in a few from the D48 and W22 wells. 5.3. Source rock potential The dispersed original organic matter was preserved in the sediments and experienced favorable temperature and pressure conditions. These conditions formed kerogen as a diagenetic product, and this kerogen was converted to bitumen, oil and gas, depending on the type of organic matter and the diagenetic conditions. The original organic matter information can be estimated using the composition of the source rock's particulate organic matter (kerogen) that is resistant to the inorganic acids HCl and HF (Tyson, 1995). Thus, palynofacies analysis was successfully used to evaluate the hydrocarbon source rock potential, which can be compared to the results of traditional geochemical methods, such as TOC, vitrinite reflectance (Ro%) and Rock-Eval pyrolysis (Zobaa et al., 2011; El Beialy et al., 2010; El Atfy et al., 2014). In the present study, we combined palynofacies analysis and instrumental geochemical analysis (TOC and Rock-Eval Pyrolysis) results from the L94, D48 and W22 wells to determine the hydrocarbon source rock potential of the Yanchang Formation in the southern Ordos Basin. 5.3.1. Comparison among the different areas Unlike gentle marine petroliferous basins, continental basins feature larger basement relief and are controlled by the regional structure. Each depression in the Ordos Basin has a different hydrocarbon generation potential for each subsection of the Yanchang Formation (Zhang et al., 2008; Li et al., 2012). The average AOM value from the D48 well in the

Zhidan area is 56.7%, which is equivalent to the samples from the W22 well in the Yichuan area, which have an average of 55.2%. These values are far higher than those of the L94 well, which have an average of 26.8%. Therefore, most oil- and gas-prone materials that have the characteristics of type I and II kerogen are contained in the D48 and W22 samples from the Zhidan area. However, type III kerogen samples are abundant in the Huachi area, as demonstrated by the L94 samples. The plot of HI versus Tmax (Fig. 3) also corresponds to the palynofacies results and indicates better kerogen quality in the source rocks from the Zhidan and Yichuan areas. The AOM-rich source rocks usually have corresponding high TOC and pyrolytic hydrocarbon values, as shown in Fig. 5. The average TOC values of the source rocks in the D48 and W22 wells are 2.8 wt.% and 2.7 wt.%, respectively. The TOC values of the L94 well samples are relatively lower, with an average of 1.1 wt.%. Organic geochemists have suggested that effective source rocks composed of non-marine shale contain at least 1.0 wt.% TOC (Xia and Dai, 2000). Such rocks contain sufficient organic matter for significant generation and expulsion. The Zhidan and Yichuan areas are superior to the Huachi area. The HI values inferred by Rock-Eval pyrolysis are commonly used as a proxy for the hydrogen content of the OM, which is the most important factor controlling the generation of hydrocarbons (Hunt, 1996; El Atfy et al., 2014). The value of 100 mg HC/g TOC is a widely accepted hydrocarbon-generating minimum criterion for non-marine shales in China (Xia and Dai, 2000). The HI values of the D48 and W22 wells samples are similar, and they vary from 20 to 315 mg HC/g TOC. Approximately 70% of the samples are over 100 mg HC/g TOC. Although the HI values of the L94 well samples vary from 31 to 225 mg HC/g TOC, only

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Fig. 7. Plot of S2 versus the TOC of the sample data from three wells, showing the hydrocarbon potential and source rock efficiency.

25.4% of the samples have over 100 mg HC/g TOC. The low TOC contents and pyrolysis S2 production values in most of the L94 samples indicate poor to fair hydrocarbon generation potential (Fig. 7). The D48 and W22 well samples have relatively high TOC contents and pyrolysis S2 production values, which mostly indicate good to very good hydrocarbon generation potential (Fig. 7). Therefore, the correlation of the kerogen characteristics among the three well samples shows that the source rock qualities of the Zhidan and Yichuan areas are obviously better than those of Huachi area. 5.3.2. Comparison among the various oil subsections The ten oil subsections of the Yanchang Formation also have different hydrocarbon generation potential. The TOC, palynofacies and Rock-Eval pyrolysis data are used to evaluate the hydrocarbon generation potential of different oil subsections in the studied wells. In the L94 well, the Chan 7 layer has the thickest shale deposits (Fig. 2) with the highest TOC values, followed by the Chan 8 and Chan 9 layers. The highest AOM content and HI value also occurred in the Chan 7 layer, which features abundant oil to gas-prone type I and II kerogen. A good hydrocarbon generation potential is also indicated by the high TOC content and the pyrolysis S2 yields (Fig. 7). These analysis results indicate that Chan 7 maybe the primary source rocks in the Huachi area in the southern Ordos Basin. This finding is also supported by previous exploration data (Zhang et al., 2006). Similarly, we examined the D48 and W22 wells in the Zhidan and Yichuan areas, respectively. The Chan 9 and Chan 7 layers have abundant oil-shale layers that are greater than 10 m thick (Fig. 2). The TOC value in the Chan 9 layer of the D48 well ranged from 2.02 wt.% to 6.71 wt.%, with an average of 5.2 wt.% (excluding an outlier in the D48 well, 1532.5 m). The Chan 7 has relatively low TOC values, which range from 0.29 to 5.18 wt.%, with an average of

2.53 wt.%. The plot of S2 versus TOC shows that the Chan 9 samples have higher hydrocarbon potential than other subsections (Fig. 7). The high-quality source rock of Chan 9 in the Zhidan area is also demonstrated by several geochemical parameters, sedimentary characteristics and drilling data (e.g. Zhang et al., 2008). Based on the W22 samples from the Yichuan area, Chan 7 and Chan 9 both have high hydrocarbon generation potential, which is indicated by the TOC, palynofacies and pyrolysis results (Table 1; Figs. 3 and 7). The hydrocarbon generation potential of Chan 7 is slightly higher than that of Chan 9. In conclusion, the Chan 7 and Chan 9 oil layers are the primary source rocks. In the Huachi and Yichuan areas, Chan 7 likely has the highest hydrocarbon generation potential among the oil layers. In contrast, Chan 9 exhibits a relatively high hydrocarbon generation potential in the Zhidan area. 6. Conclusions A palynofacies and organic geochemical study was conducted using 134 samples of the Yanchang Formation from the L94, D48 and W22 wells in the southern Ordos Basin, China. This study is the first to combine optical palynofacies and organic geochemistry for source rock evaluation of Triassic continental sediments in the Ordos Basin. This study's observations can be summarized as follows: 1) The particulate organic matter is mainly composed of terrestrially derived AOM, phytoclasts (GP, TLF, OP and cuticles), and palynomorphs. Three distinct palynofacies assemblages are recognized in the Yanchang Formation on the basis of the quantitative content of the particulate organic matter. PF-I is dominated by the AOM components. PF-II is characterized by the dominance of gelified

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particles and AOM. PF-III has high contents of TLF and OP constituents. 2) The PF-I samples are mainly from the D48 and W22 wells in the Zhidan and Yichuan areas, respectively, and from the Chan 7 subsection of the L94 in the Huachi area of southern Ordos. Based on their palynofacies characteristics, these sampled sequences were deposited in a distal dysoxic–anoxic deep basin setting with a large contribution of degraded aquatic organisms. The PF-II samples are common in the L94 well and represent a proximal suboxic shelf setting with a moderate production of degraded aquatic organisms and an influx of terrestrial plant matter. The PF-III samples are scattered throughout the L94 well and in a few sections of the D48 and W22 wells. These samples represent a shallow shelf to basin transition setting that received a large influx of terrestrial plant matter. 3) The palynofacies, TOC and Rock-Eval data indicate that type I and II kerogen are abundant in the Yanchang Formation in the southern Ordos Basin. These parameters also indicate a mature to highly mature stage for the examined organic matter and present an increasing maturity trend with burial depth. In detail, the Yanchang source rocks in the D48 and W22 wells have higher hydrocarbon generation potentials than the L94 well in the Huachi area. Additionally, these parameters from the D48 well indicate that the Chan 9 subsection is the primary source rock stratum in the Zhidan area. In contrast, the Chan 7 source rocks exhibited the highest hydrocarbon generation potential in the L94 and W22 wells in the Huachi and Yichuan areas, respectively, of the southern Ordos Basin. Acknowledgments This work was financially supported by grants from the National Natural Science Foundation of China (No. 41172131), the “Strategic Priority Research Program” of the Chinese Academy of Sciences (No. XDB10010103), and the Key Laboratory Project of Gansu Province (No. KFJJ2015-06 and No. 1309RTSA041). We are grateful to Xu Jinli, the Senior Engineer of the Geology Science Academy of the Shengli Oilfield for his assistance with sample handling and fossil identification. References Buchardt, B., Nielsen, M.V., 1991. Comparison of organic geochemical and palynofacies methods: example from the Upper Triassic Gassum Formation in Denmark. Bull. Geol. Soc. Den. 38, 267–277. Carvalho, M.D.A., Ramos, R.R.C., Crud, M.B., Witovisk, L., Kellner, A.W., Silva, H.D.P., Grillo, O.N., Riff, D., Romano, P.S., 2013. Palynofacies as indicators of paleoenvironmental changes in a Cretaceous succession from the Larsen Basin, James Ross Island, Antarctica. Sediment. Geol. 295, 53–66. De Araujo Carvalho, M., Mendonça Filho, J.G., Menezes, T.R., 2006. Paleoenvironmental reconstruction based on palynofacies analysis of the Aptian–Albian succession of the Sergipe Basin, Northeastern Brazil. Mar. Micropaleontol. 59, 56–81. Du, B.X., Sun, B.N., Ferguson, D.K., Yan, D.F., Dong, C., Jin, P.H., 2013. Two Brachyphyllum species from the Lower Cretaceous Jiuquan Basin, Gansu Province, NW China and their affinities and palaeoenvironmental implications. Cretac. Res. 41, 242–255. Duan, Y., 2012. Geochemical characteristics of crude oil in fluvial deposits from Maling oilfield of Ordos Basin, China. Org. Geochem. 52, 35–43. Dutta, S., Hartkopf-Fröder, C., Witte, K., Brocke, R., Mann, U., 2013. Molecular characterization of fossil palynomorphs by transmission micro-FTIR spectroscopy: implications for hydrocarbon source evaluation. Int. J. Coal Geol. 115, 13–23. El Atfy, H., Brocke, R., Uhl, D., Ghassal, B., Stock, A.T., Littke, R., 2014. Source rock potential and paleoenvironment of the Miocene Rudeis and Kareem formations, Gulf of Suez, Egypt: an integrated palynofacies and organic geochemical approach. Int. J. Coal Geol. 131, 326–343. El Beialy, S.Y., El Atfy, H.S., Zavada, M.S., El Khoriby, E.M., Abu-Zied, R.H., 2010. Palynological, palynofacies, paleoenvironmental and organic geochemical studies on the Upper Cretaceous succession of the GPTSW-7 well, North Western Desert, Egypt. Mar. Pet. Geol. 27, 370–385. Ercegovac, M., Kostić, A., 2006. Organic facies and palynofacies: nomenclature, classification and applicability for petroleum source rock evaluation. Int. J. Coal Geol. 68, 70–78. Garcia, Y.C., Martinez, J.I., Velez, M.I., Yokoyama, Y., Battarbee, R.W., Suter, F.D., 2011. Palynofacies analysis of the late Holocene San Nicolás terrace of the Cauca paleolake and paleohydrology of northern South America. Palaeogeogr. Palaeoclimatol. Palaeoecol. 299, 298–308.

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