Organic geochemistry and petrology of Lower Cretaceous black shales in the Qiangtang Basin, Tibet: Implications for hydrocarbon potential

Organic Geochemistry 86 (2015) 55–70

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Organic geochemistry and petrology of Lower Cretaceous black shales in the Qiangtang Basin, Tibet: Implications for hydrocarbon potential Ruofei Yang a, Jian Cao a,⇑, Guang Hu b,a, Xiugen Fu c a

State Key Laboratory for Mineral Deposits Research, Department of Earth Sciences, Nanjing University, Nanjing, Jiangsu 210023, China School of Geoscience and Technology, Southwest Petroleum University, Chengdu, Sichuan 610500, China c Chengdu Institute of Geology and Mineral Resources, Chengdu, Sichuan 610081, China b

a r t i c l e

i n f o

Article history: Received 4 January 2015 Received in revised form 17 May 2015 Accepted 12 June 2015 Available online 29 June 2015 Keywords: Shengli River black shales Hydrocarbon generation Lagoon Unconventional hydrocarbons

a b s t r a c t Lower Cretaceous black shales have recently been discovered in the Qiangtang Basin (especially in its northern basin), Tibet, implying a new interval with hydrocarbon resource potential in the region. This potential, however, has not been investigated to date and is addressed in this paper based on organic petrology and geochemistry data from the representative Shengli River outcrop. Organic petrology, including optical and scanning electron microscope observations, indicates that the organic matter (OM) within these shale sequences has precursors from abundant benthic algae, some bacteria and amorphous OM and a few land plants, suggesting that the black shales were deposited in a generally reducing platform–lagoonal environment favorable for OM preservation. The shales have high total organic carbon (TOC) contents (1.74–7.71 wt%), type II kerogen (tending to type III), and high thermal maturity (vitrinite reflectance equivalent of ca. 1.3 %Ro). Biomarkers in the shales suggest deposition under reducing brackish or saline water with aquatic benthic organisms, bacteria and amorphous OM as the dominant input. Thus, the results of organic petrology, organic geochemistry and biomarker geochemistry are generally consistent and imply that the Shengli River black shale samples have significant hydrocarbon resource potential and most likely produced gas due to relatively high maturity and gas-prone kerogen. The neighboring black shales within the same stratigraphic interval as the Shengli River shales, if having moderate OM maturity and oil-prone kerogen, can be expected to generate oil. The presence of well-developed micropores and fractures, and abundant brittle minerals within the black shales suggests that they may have unconventional hydrocarbon resource potential, especially in intervals with high TOC > 4.0 wt%. These results provide new data and understanding for regional hydrocarbon exploration. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction The Qiangtang Basin is located in Tibet (western China), and is predicted to be prospective for oil and gas exploration (Zhao et al., 2000; Li et al., 2005), because it is located in the eastern sector of the Tethyan Hydrocarbon Province, one of the most significant regions of hydrocarbon resources worldwide (Klemme and Ulmishek, 1991). However, the high altitude of the area has resulted in only limited oil and gas exploration and the hydrocarbon resource potential remains unclear (Qin, 2006a). Previous research based on outcrop and limited core samples has focused on source rock intervals including the Upper Triassic Xiaochaka Formation (T3x), the Middle Jurassic Buqu (J2b) and Xiali (J2x) formations, and the lower member of the Upper Jurassic Suowa

⇑ Corresponding author. Tel.: +86 25 83686719; fax: +86 25 83686016. E-mail address: [email protected] (J. Cao). http://dx.doi.org/10.1016/j.orggeochem.2015.06.006 0146-6380/Ó 2015 Elsevier Ltd. All rights reserved.

Formation (J3s1) (Qin, 2006b; Ding et al., 2011, 2013; Wang et al., 2013). Recently, a set of Lower Cretaceous black shales/oil shales were discovered within the basin and are particularly widespread in the north (Wang et al., 2007, 2009, 2010; Fu et al., 2009). Preliminary stratigraphic correlations across the area indicate that the shales are widely distributed, suggesting possibly significant hydrocarbon resource potential. However, most previous research on the shales has been focused on the age and depositional environment (Wang et al., 2007, 2009; Fu et al., 2009, 2010). Here, we present organic petrology and geochemical analyses to evaluate the hydrocarbon resource potential and provide new data and understanding for regional hydrocarbon exploration. As the shales were influenced by key events in geological history, such as oceanic anoxic events (OAEs) (Erbacher et al., 2001; Herrle et al., 2003; Pauly et al., 2013), the results from this study may also have implications for understanding these geological events.

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2. Geological setting The Qiangtang Basin is located between 32°–35°N and 83°– 93°E, and covers an area of approximately 180  103 km2 (Fig. 1a). It is tectonically bounded by the Hoh Xil–Jinsha River Suture to the north and the Pangong Lake–Nu River Suture to the south, and consists of three second-order structural units: the northern Qiangtang depression, the southern Qiangtang depression, and a central uplifted area (Fig. 1a). The Qiangtang Basin is a typical multi-stage basin which developed on a pre-Devonian basement. The basin has experienced a range of tectonic settings, including a Devonian–Triassic rift basin, a Triassic foreland basin, and a Jurassic passive continental margin that continues to develop to the present day (Nan et al., 2013). Most of the basin was covered by seawater between the Triassic and Late Jurassic, leading to widespread deposition of carbonates with some interbedded clastic sediments, forming a sedimentary package of several kilometers in thickness (Zhao et al., 2000). The Yanshanian orogeny at the end of the Jurassic caused the basin to shrink and transform into a residual continental basin by the end of the Late Cretaceous (Zhao et al., 2000). Thus, the Lower Cretaceous black shales in the study area were most likely deposited during the transition from marine to residual continental settings, resulting in sediments that include both carbonates and clastic rocks (Zeng et al., 2012). These sediments are divided into the following two lithofacies:

(1) Delta facies, present locally in the east of the Quemocuo area that contain mudstone, siltstone, and sandstone, all of which show a transition into continental sediments of the Xueshan Formation to the north and the Zhaworong Formation to the east; (2) tidal flat–lagoon facies, dominating the central part of the northern Qiangtang depression, which include a lower sequence of sandstone, micritic limestone, marl, limestone and shale, and an upper sequence dominated by gypsum. The age assignment of the Lower Cretaceous has been characterized mainly from isotopic dating and palynological data (Wang et al., 2007; Fu et al., 2009). This is believed to be more precise than the stratigraphic constraints, which are still debated and cannot be precisely conducted due to a lack of continuously distributed outcrops. Stratigraphically, the Lower Cretaceous in this study roughly represents the upper part of the Upper Jurassic Suowa Formation (J3s) in previous studies (Wang et al., 2013), while the lower part of the Suowa Formation belongs to the Upper Jurassic.

3. Samples and methods A total of 28 samples were analyzed in this study. They were collected from an outcrop in the central part of the black shales in the Shengli River area which provided good sampling conditions (Fig. 1b), where the shales are intercalated with marls and micritic limestones (Fig. 2). The surfaces of the samples were removed

Fig. 1. Geological map of the study area: (a) structural units of the Qiangtang Basin; (b) location of the Shengli River black shale sequences.

R. Yang et al. / Organic Geochemistry 86 (2015) 55–70

before organic petrological and geochemical analyses to minimize the effects of surface weathering and possible modern contamination. The organic petrological analysis included conventional optical microscopy based on rock thin sections and scanning electron microscopy (SEM) for all of the studied samples. The samples were sectioned perpendicular to bedding before being embedded in a homogeneous mixture of Buehler’s epoxy resin and hardener (ratio 5:1), dried and then polished according to the procedures described by Taylor et al. (1998) and Amijaya and Littke (2006). The rock thin sections were examined at different magnifications to characterize organic matter (OM) features in incident white light and blue light excitation using a Nikon LHS2H100C21 microscope. The bedding planes usually visible in sections perpendicular to the bedding were not easily observed due to relatively poor preservation conditions and high maturity, and thus a quantitative evaluation of maceral compositions could not be precisely obtained. The SEM observations used air dried samples that were mounted on stubs using double sided tape and coated with Pt–Pd in a Polaron E5000 sputter coater at 1.2 kV for 2  2 min. The SEM analysis used a JSM-6490 instrument operated at an accelerating voltage of 15.0 kV with a beam current of 1.0–2.0  10 9 A. The 28 samples were collected and crushed to powder that was used for total organic carbon (TOC) analysis. This analysis used sample splits (200 mg) that were treated with HCl at 60 °C to remove carbonate before being washed with distilled water to remove the HCl. The samples were then dried overnight at 50 °C before analysis using a LECO CS-200 analyzer. Rock–Eval pyrolysis was performed on the 28 crushed rock samples using 100 mg of sample per analysis and a Rock–Eval VI instrument. These samples were heated to 600 °C in a He

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atmosphere to measure the Rock–Eval parameters S1, S2, S3 and Tmax, where S1 is the amount of free hydrocarbon that can be volatilized from the rock sample (mg HC/g rock), S2 is the amount of hydrocarbon produced by the cracking of OM (mg HC/g rock), S3 is the amount of CO2 produced during the analysis (mg CO2/g rock), and Tmax (°C) is the temperature at which the maximum S2 yield is reached, giving a rough estimate of the thermal maturity of the sediment. Carbon isotope (d13C) values of kerogen were determined using powdered samples that had been reacted with concentrated HCl and heated in HCl–HF to remove carbonate and silicate minerals. The residues then underwent heavy liquid separation and subsequent washing, before being extracted with CHCl3 to eliminate soluble OM, leaving kerogen. d13C values were determined using a MAT 253 instrument with a precision better than 0.1‰. Isotopic ratios are reported in standard d-notation relative to the Vienna Peedee Belemnite (VPDB) standard. The OM in the black shales of the study area is generally of high maturity (Qin, 2006c); consequently, it was analyzed using laser Raman (LRM) spectroscopy as a complement to vitrinite reflectance (VRo) and bitumen reflectance (BRo) measurements that are more conventionally used for maturity analysis (Liu et al., 2013; Zhou et al., 2014). Eleven polished block samples were analyzed for reflectance values using a Zeiss Axiokop 40 Pol incident light microscope with a wavelength (k) of 546 nm and a 50  0.85 oil immersion objective. An yttrium aluminum garnet standard (GWB13401) with a reflectance of 0.588% was used for calibration, and at least 50 measurements were performed on each sample. The LRM analysis used a Renishaw RM2000 micro-laser Raman spectroscope with a solid laser device setup at 532 nm and 30–50 mW, D1–D2 laser energy attenuation, 1800 raster lines, a 100–300 lm confocal pinhole, a 100 lm raster slit, observation objectives between 50 and 100, an exposure time of 10–40 s, a scanning wave number range of 100–4000 cm–1, and a silicon wafer for wave number calibration of the Raman spectroscope. Raman spectral parameters were calculated using the spectral analytical software supplied with the spectroscope. Saturated hydrocarbon analysis was undertaken on 11 selected samples extracted using a Soxhlet apparatus with CHCl3 for 72 h. The resulting extracts were fractionated using open silica gel column chromatography with n-hexane to yield saturated hydrocarbons that were analyzed by gas chromatography (GC) and gas chromatography–mass spectrometry (GC–MS). The GC analysis used a HP6890 gas chromatograph fitted with a 30 m  0.32 mm i.d. HP-5 column with a film thickness of 0.25 lm, using N2 as a carrier gas. The GC oven temperature was initially held at 80 °C for 5 min before being ramped from 80 °C to 290 °C at 4 °C/min, and then held there for 30 min. The GC–MS analysis was conducted using an Agilent 5973I mass spectrometer interfaced with a HP6890 gas chromatograph fitted with the same type of column as that used during GC analysis, employing He as a carrier gas. The GC oven temperature during the GC–MS analysis was initially held at 60 °C for 5 min before being ramped to 120 °C at 8 °C/min, from 120 °C to 290 °C at 2 °C/min, and then held at 290 °C for 30 min.

4. Results and discussion 4.1. Organic petrology

Fig. 2. Generalized lithology of the outcropping Shengli River black shale sequences, showing the location of the samples in this study.

The combined optical microscopy and SEM observations indicate that the Shengli River black shales contain diverse OM precursors that can be divided into four types based on morphology (e.g., shape and size) and fluorescence. These include, from highest to lowest abundance, marine benthic algae, bacteria, amorphous

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OM and terrigenous higher plants. This suggests that the black shales were deposited in a transitional tidal flat–lagoon environment and contain type II–III kerogen. The kerogen type is generally consistent with those kerogens composed dominantly of macroalgae in the study of Xie et al. (2014) and the geological setting of the study area. In addition, benthic organisms are abundant in this environment, including bivalves, gastropods, echinoderms, foraminifera and bryozoans. They are associated with OM enrichment and imply that the environment could not have been anoxic. More details on these precursors are provided below. 4.1.1. Benthic algae Benthic algae are the major bio-precursors within the Shengli River black shale sequence and were observed in all the samples, being especially common in the shale samples. They can be mainly subdivided into two types: the first type was identified in samples SLR-03 to SLR-28 with a rod-like shape or in a fragmented structure with tiny irregular holes (Fig. 3a). They commonly appear as reddish brown to black under plane polarized light with only minor fluorescence, suggesting a high maturation consistent with the results of OM maturity (see Section 4.2.3). The second type was observed in all samples but was not abundant. These macroalgae are present as grid-cell structures that contain single cells approximately 5–10 lm in size that are linked by filaments along the edges (Fig. 3b). They have pale brown and focused fluorescence which is readily discernable from the generally dark and fuzzy fluorescence of the mineral matrix. 4.1.2. Bacteria Bacteria are another important type of bio-precursor for the OM within the Shengli River black shales. These bacteria are generally disseminated throughout the sediments, as observed during optical microscopy, and they fluoresce as extremely fine green spots different from the dark fuzzy fluorescence of the mineral matrix (Fig. 3c). SEM imaging of these bacteria indicates that they are generally spherical with a diameter of 1–2 lm and are present as aggregations on the surface of minerals such as calcite and collophane (Fig. 3d). The presence of bacteria in the samples can also be demonstrated by the biomarker studies (see Section 4.3). 4.1.3. Amorphous OM Amorphous OM of algal-bacterial origin is less abundant than benthic algae, but also an important type of bio-precursor for the OM within the Shengli River black shales. They are generally disseminated throughout the sediments and fluoresce as extremely pale brown fine spots, discernable from the dark and fuzzy fluorescence of micro-calcite grains (Fig. 3c). 4.1.4. Higher plants Higher plants form only a minor part of the precursors of the OM within the Shengli River black shales. These higher plants are generally present as sporopollens and little woody debris was observed (Fig. 3e and f). The sporopollens commonly have a diameter of 10–15 lm and fluoresce with a golden appearance under optical observation (Fig. 3e) and can be further characterized by SEM observation (Fig. 3f). 4.2. Basic organic geochemistry 4.2.1. Abundance of OM High contents of OM are a requisite for a good source rock (Tissot and Welte, 1984; Katz, 2005). The indicators commonly used to evaluate OM abundance include total organic carbon content (TOC), Rock–Eval parameters, such as petroleum generation (PG) potential (PG = S1 + S2), and chloroform-extractable bitumen content (extractable OM).

The 28 samples have TOC values from 1.74–7.71 wt%, with an average of 5.16 wt% (Table 1; Fig. 4), indicating that most of these samples can be classified as having good quantities of OM (> 2 wt%). The shale TOC values (average 5.51 wt%) are similar to the micritic limestones in the study area (average 6.04 wt%), but have higher values than the marls (average 2.80 wt%). The samples have PG values of 2.9–16.1 mg/g with an average of 10.7 mg/g, indicating that they are good quality source rocks (> 6 mg/g). These values vary between different lithologies in a similar fashion as the TOC values. The extractable OM concentrations of the samples range from 816–2241 ppm, with 25 samples having concentrations > 1000 ppm (moderate quality source rocks) and 12 samples have concentrations > 1500 ppm (good quality source rocks). These concentrations vary in a similar fashion to the TOC and PG values. In summary, all of the black shales from the Shengli River area are good quality source rocks. The marls are of slightly poorer quality, but are still classified as moderate to good quality source rocks, whereas both the micritic limestone and shale samples are high quality. 4.2.2. Type and origin of OM The type of OM is related to its origins and allows one to infer the composition and scale of hydrocarbon generation during maturation (Tissot and Welte, 1984). Here, we utilize the relationships between the hydrogen index (HI) and the oxygen index (OI) and Tmax as well as the carbon isotopes of kerogen to discern the origin of the OM (Katz, 1983; Huang et al., 1984). The organic petrology presented above (see Section 4.1) and biomarkers discussed later (see Section 4.3) also provide additional clues to the origin of the OM. The HI and OI values of the 28 Shengli River black shale samples vary from 152–252 mg HC/g TOC and from 31–70 mg CO2/g TOC, respectively, yielding averages of 200 and 46.5 mg/g (Table 1; Fig. 4). These values are indicative of type II kerogens tending towards type III (Fig. 5), suggesting that the black shales are mainly gas-prone. The dominant organic macerals of macroalgae are similar to those of higher plants in chemical structure and composition and thus indicative of type III kerogen (Xie et al., 2014), even though higher plants are not dominant in the macerals. Combined with the bacteria and amorphous OM of algal-bacterial origins which have more oil-prone kerogen types (Xie et al., 2014), the kerogen of the Shengli River black shales is characterized as type II–III. The carbon isotopic compositions of kerogen within the Shengli River black shales range between 22.3‰ and 20.3‰, with an average of 21.1‰ (Table 1). These values, when compared with the Kimmeridgian–Valanginian interval (close to the Early Cretaceous age of this study), seem to be consistent with the values of terrestrial material ( 26‰ to 19‰) rather than marine origin ( 30‰ to 24‰) (Lini et al., 1992; Morgans-Bell et al., 2001; Robinson and Hesselbo, 2004; Gröcke et al., 2005; Nunn et al., 2009). Therefore, it appears that the isotopic values of this formation of the Qiangtang Basin are closer to terrestrial OM than to marine OM. This, however, is not supported by the petrographic and biomarker data (see Sections 4.1 and 4.3). Since maturation does not explain these high isotopic values (Lewan, 1983), another possible explanation is needed. Interestingly, the d13C values of present-day benthic algae have a large variation from 33‰ to 7‰; in particular for red algae, which is likely the main organic maceral of the black shales in this study, the d13C value ranges between 8‰ and 32‰ and increases along with warmer climate (Mercado et al., 2009). According to Fu et al. (2009), the climate was warm during the deposition of the black shales in this study. Thus, the carbon isotopic values can indicate the presence of some special types of

R. Yang et al. / Organic Geochemistry 86 (2015) 55–70

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Fig. 3. Photomicrographs showing typical examples of organic matter within the Shengli River black shale sequences: (a) benthic algae with rod-like shapes (plane-polarized light, sample SLR-17); (b) benthic algae with grid-cell structures (fluorescent light, sample SLR-06); (c) bacteria and amorphous organic matter of algal-bacterial origin with bitumen in micropores and fractures (fluorescent light, sample SLR-26); (d) bacteria on the surface of crystallized minerals (SEM image, sample SLR-18); (e) spores of higher plants (fluorescent light, sample SLR-02); (f) spores of higher plants (SEM image, sample SLR-02). See Fig. 2 for sample locations.

red algae having relatively heavy carbon isotope values living in a warm climate. This preliminary deduction needs more detailed investigation.

4.2.3. Maturity of OM Source rocks can only produce hydrocarbons given a favorable thermal regime, indicating that the maturity of OM within sediments is another important parameter that needs to be assessed in terms of source rock potential (Tissot and Welte, 1984). Such parameters commonly include vitrinite reflectance (VRo) and pyrolysis peak temperature (Tmax). Here, we use bitumen reflectance (BRo) because few homogeneous vitrinite macerals were observed during the petrographic analysis (see Section 4.1). This may be caused by the lack of higher plants present in the samples, while high maturity and relatively poor preservation conditions can also add to this. In addition, we also undertook LRM analysis to compensate for the lack of VRo values as this technique is effective in evaluating highly mature to over-mature OM (Kelemen and

Fang, 2001; Liu et al., 2013; Zhou et al., 2014), which is the case in this study (Qin, 2006c). These two indirect indictors (i.e., BRo and Rmc values) are converted to equivalent vitrinite reflectance values (VReq) based on the formula suggested by Jacob (1985) and Liu et al. (2013). Analytical results are shown in Table 2. 4.2.3.1. Bitumen reflectance (BRo). The BRo values of the 11 samples in the Shengli River black shale sequences range from 1.33–1.51% with an average of 1.41% (Table 2). These values were then substituted into VRo = 0.618BRo + 0.4 (Jacob, 1985), yielding VReq values of 1.22–1.33%Ro with an average of 1.27. This indicates that the OM within the sediments is highly mature. 4.2.3.2. LRM analysis. The molecular level vibrations in the Raman spectroscopy of OM can be used to evaluate the degree of thermal evolution of geological samples, especially rocks that are highly mature to over-mature (Liu et al., 2013; Zhou et al., 2014). Previous research has shown that two Raman peak shifts are

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Table 1 Basic organic geochemical parameters for samples of the Shengli River black shales. d13CPDB (‰)

Tmax (°C)

70 56 54 49 47 51 46 44 39 35 37

22.3 22.0 22.0 21.4 20.7 20.9 20.3 20.3 20.6 21.4 21.7

442 442 443 443 444 442 444 443 443 443 441

188

31

21.0

444

197 196 209 204 203 219 205 201 212 226 229 210 217 194 203 205

48 44 43 45 51 44 46 49 44 42 49 42 51 48 45 52

21.3 20.8 20.9 21.1 21.0 20.8 21.2 21.4 21.6 21.2 21.5 21.4 21.5 21.4 21.3 21.7

442 443 444 443 444 442 443 442 443 443 440 444 441 444 443 441

Sample

Lithology

TOC (wt%)

S1 (mg HC/g rock)

S2 (mg HC/g rock)

S3 (mg CO2/g rock)

S1 + S2 (mg HC/ g rock)

Extractable bitumen (ppm)

HI (mg HC/g TOC)

OI (mg CO2/ g TOC)

SLR-01 SLR-02 SLR-03 SLR-04 SLR-05 SLR-06 SLR-07 SLR-08 SLR-09 SLR-10 SLR-11

Marl Marl Marl Marl Shale Shale Shale Shale Shale Shale Micritic limestone Micritic limestone Shale Shale Shale Shale Shale Shale Shale Shale Shale Shale Shale Shale Shale Shale Shale Shale

1.96 1.74 3.71 3.77 6.29 6.59 7.71 7.19 5.80 6.24 6.46

0.06 0.07 0.09 0.16 0.31 0.27 0.32 0.35 0.39 0.38 0.28

2.98 2.83 6.24 6.84 11.43 12.32 14.48 14.47 13.07 15.73 11.84

1.38 0.98 1.99 1.83 2.97 3.37 3.58 3.15 2.27 2.2 2.36

3.04 2.90 6.33 7.00 11.74 12.59 14.80 14.82 13.46 16.11 12.12

816 896 1060 1352 1768 1781 1937 1852 2242 2110 1430

152 163 168 181 182 187 188 201 225 252 183

5.61

0.27

10.56

1.73

10.83

1536

5.80 5.68 5.77 5.98 5.93 5.71 5.20 5.54 4.91 4.56 4.67 4.44 4.50 4.27 4.12 4.24

0.24 0.25 0.27 0.25 0.18 0.28 0.24 0.24 0.25 0.26 0.22 0.17 0.18 0.15 0.17 0.14

11.43 11.15 12.08 12.17 12.06 12.48 10.68 11.14 10.43 10.32 10.7 9.34 9.77 8.28 8.36 8.69

2.79 2.52 2.50 2.68 3.02 2.54 2.39 2.72 2.17 1.93 2.3 1.85 2.31 2.04 1.84 2.22

11.67 11.40 12.35 12.42 12.24 12.76 10.92 11.38 10.68 10.58 10.92 9.51 9.95 8.43 8.53 8.83

1592 1621 1757 1823 1381 1700 1337 1312 1329 1403 1277 1443 1096 1195 1340 935

SLR-12 SLR-13 SLR-14 SLR-15 SLR-16 SLR-17 SLR-18 SLR-19 SLR-20 SLR-21 SLR-22 SLR-23 SLR-24 SLR-25 SLR-26 SLR-27 SLR-28

Note: Tmax = peak Rock–Eval pyrolysis temperature that generates the maximum S2 yield. Extractable bitumen refers to chloroform-extractable bitumen.

closely related to the maturity of OM (Liu et al., 2013). One peak is at ca. 1250–1450 cm–1 and represents peak D (the disorder band), which mainly provides information on defects in the lattice structure and vacancies in aromatic ring lamellae. The other peak is at ca. 1500–1605 cm–1 and represents peak G (order band), which mainly relates to the longitudinal stretching vibration mode of the C@C bond. The distance between peak D and peak G increases linearly with increasing maturity of the OM. This indicates that we can use variations in the distance between peaks D and G to measure the degree of thermal evolution of the OM; these variations can be expressed in vitrinite-reflectance-equivalent terms using Rmc (%Ro) = 0.0537d (G D) 11.21 (Liu et al., 2013). The analysis indicates that the Shengli River black shales have Rmc values of 1.27–1.39 %Ro, with an average of 1.34. These values are generally consistent with the BRo results (Table 2) and are also indicative of highly mature OM. 4.2.3.3. Rock–Eval pyrolysis Tmax. The Rock–Eval pyrolysis Tmax values of all of the Shengli River black shale samples fall in the range 440–444 °C, suggesting that the OM in these shales is mature (Table 1). This value is somewhat lower than that predicted from the BRo and Rmc values of the shales discussed above, both of which suggest these sediments are highly mature. This discrepancy probably relates to the heavy hydrocarbons in shales (3.6– 16.3 wt%; Fu et al., 2010), i.e. the oils and bitumens residual in the shales (shale oils; Wang et al., 2007) (Fig. 3c). As a consequence, the cracking of OM during pyrolysis is faster than under normal conditions, and Tmax values are relatively lower than the expected values (Zhang et al., 2006). In addition, the S2 values are expected to be slightly overestimated but would not change the general results and conclusions (Zhang et al., 2006). In summary, all of the Shengli River black shale samples are highly mature. This can be shown by the dominantly dark fluorescence of the samples (Fig. 4) and most of the geochemical

parameters (e.g., BRo and Rmc values discussed above, and OEP value in Section 4.3). This high maturity of the OM is also generally consistent with the results of previous studies (Qin, 2006c). Note that the results of other studies on the Upper Jurassic Suowa Formation (Wang et al., 2013) can be compared as the age indeed refers to the Lower Cretaceous in this study, which is relatively precisely characterized by isotopic dating and palynological data (Wang et al., 2007; Fu et al., 2009). The slight differences may arise from different samples and plotting errors. In addition, a few cases of golden yellow fluorescence from macroalgae, amorphous OM and sporopollen (Fig. 4) may imply special origins and/or preservation mechanisms, which need more detailed investigation. 4.3. Biomarker geochemistry Abundant biomarkers were identified in all of the shale, marl and micritic limestone samples, including n-alkanes, isoprenoid alkanes, terpanes and steranes (Tables 3 and 4; Fig. 6). They have important implications for the origin of OM, its maturity and the depositional environment (Murray et al., 1994; Peters et al., 2005; Riboulleau et al., 2007). 4.3.1. n-Alkanes The carbon number of the n-alkanes within the studied samples ranges between n-C12 and n-C35, with a generally unimodal distribution maximizing at n-C17 to n-C19. The abundance of n-alkanes in all of the samples, except sample SLR-01, shows a sharp decrease after n-C20. The C21/C+22 values of samples SLR-03 to SLR-28 are all > 1.5, whereas sample SLR-01 has a value slightly below 1.2, indicating a predominance of low molecular weight hydrocarbons. All of the n-alkane distributions have odd–even predominance (OEP, defined as [(Ci + 6Ci+2 + Ci+4)/(4Ci+1 + 4Ci+3)](–1)i+1, of which the ‘‘i + 2’’ denotes the n-alkane number with the highest abundance) values of 0.97–1.04, indicating no clear OEP predominance.

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Fig. 4. Stratigraphic column showing the basic organic geochemistry of samples from the Shengli River black shale sequences; data are from Table 1.

Fig. 5. Geochemical parameters indicating the type of organic matter within the Shengli River black shale sequences: (a) HI versus OI; (b) HI versus Tmax.

A number of studies have demonstrated that sediments with bio-precursors predominantly derived from higher plants often present a relatively high abundance of high molecular weight n-alkanes (e.g., n-C27, n-C29 and n-C31) and an odd–even carbon

predominance (OEP > 1). In contrast, sediments with bio-precursors derived mainly from marine or lacustrine algae often show a high abundance of relatively low molecular weight n-alkanes (e.g., n-C15, n-C17 and n-C19) with almost no odd–even

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Table 2 Parameters indicating the thermal maturity of organic matter within the Shengli River black shale sequences. Sample

BRo (%Ro)

Range of BRo (%Ro)

Measuring points

Standard deviation

VReq (%Ro)

D (G–D) (cm

SLR-01 SLR-03 SLR-06 SLR-10 SLR-11 SLR-13 SLR-17 SLR-20 SLR-23 SLR-25 SLR-28

1.51 1.37 1.43 1.42 1.47 1.42 1.36 1.33 1.36 1.36 1.47

0.92–2.08 1.11–1.59 0.96–1.77 1.01–1.70 1.10–1.72 1.05–1.74 1.00–1.66 1.02–1.57 1.18–1.62 1.02–1.69 1.10–1.72

51 50 50 50 52 50 50 50 57 50 50

0.28 0.12 0.18 0.17 0.18 0.18 0.16 0.14 0.12 0.12 0.16

1.33 1.24 1.28 1.28 1.30 1.28 1.24 1.22 1.24 1.24 1.31

233.87 234.51 234.43 234.68 233.27 233.67 234.43 233.85 233.45 233.27 232.42

1

)

Rmc (%Ro) 1.35 1.38 1.38 1.39 1.32 1.34 1.38 1.35 1.33 1.32 1.27

Note: BRo = bitumen reflectance; VReq = equivalent vitrinite reflectance calculated using the formula of Jacob (1985); D (G–D) represents the distance between G and D peaks in LRM spectra); Rmc = equivalent vitrinite reflectance values calculated using the formula of Liu et al. (2013).

Table 3 n-Alkane and isoprenoid parameters for samples from the Shengli River black shale sequences. Sample

Lithology

Major n-alkane

OEP

C21/C+22

Pr/Ph

Pr/n-C17

Ph/n-C18

SLR-01 SLR-03 SLR-06 SLR-10 SLR-11 SLR-13 SLR-17 SLR-20 SLR-23 SLR-25 SLR-28

Marl Marl Shale Shale Micritic limestone Shale Shale Shale Shale Shale Shale

n-C17 n-C18 n-C18 n-C18 n-C17 n-C18 n-C18 n-C19 n-C18 n-C17 n-C18

1.03 0.98 1.01 0.99 1.04 1.00 0.97 1.00 0.98 1.07 0.98

1.12 2.11 2.51 1.58 3.30 1.84 1.96 1.52 2.00 2.41 2.43

0.88 0.73 0.77 0.72 0.87 0.80 0.72 0.71 0.79 0.83 0.82

0.60 0.47 0.39 0.43 0.46 0.41 0.50 0.51 0.61 0.39 0.45

0.71 0.60 0.50 0.56 0.62 0.50 0.57 0.57 0.64 0.48 0.53

Note: OEP = [(Ci + 6Ci+2 + Ci+4)/(4Ci+1 + 4Ci+3)](–1)i+1, ‘‘i + 2’’ denotes the n-alkane number with the highest abundance; Pr/Ph = pristane/phytane.

Table 4 Terpane and sterane parameters indicating organic matter characteristics within Shengli River black shale sequences. Sample

C24TeT/(C24TeT + C26TT)

Ts/Tm

C3122S/(22S + 22R)

Gammacerane/C30H

Dia/Reg

C⁄30/C30H

C29abb/(abb + aaa)

C29aaa20S/(20S + 20R)

SLR-01 SLR-03 SLR-06 SLR-10 SLR-11 SLR-13 SLR-17 SLR-20 SLR-23 SLR-25 SLR-28

0.8 0.82 0.83 0.80 0.79 0.81 0.82 0.80 0.81 0.81 0.82

1.40 1.94 1.79 1.73 1.62 1.69 1.59 1.57 1.63 1.58 1.62

0.60 0.58 0.58 0.59 0.59 0.58 0.59 0.59 0.59 0.60 0.60

0.20 0.30 0.29 0.29 0.24 0.28 0.30 0.27 0.33 0.35 0.36

0.54 0.33 0.39 0.38 0.35 0.35 0.36 0.37 0.36 0.36 0.32

0.06 0.04 0.05 0.04 0.04 0.04 0.04 0.04 0.04 0.04 0.04

0.41 0.45 0.44 0.45 0.46 0.46 0.45 0.45 0.45 0.45 0.44

0.55 0.55 0.51 0.55 0.55 0.56 0.54 0.55 0.54 0.54 0.54

Note: TeT = tetracyclic terpane; TT = tricyclic terpane; gammacerane/C30H = gammacerane/C30 hopane ratio; Dia/Reg = C27–C29 diasterane/regular sterane ratio; C⁄30/ C30H = C30 diahopane/C30 hopane ratio.

carbon predominance (OEP  1) (Eglinton and Hamilton, 1967; Scanlan and Smith, 1970; Tissot and Welte, 1984). This suggests that the precursors in the Shengli River OM originated from aquatic organisms, such as benthic algae, bacteria and amorphous OM, whereas sample SLR-01 may contain more organic input from higher plants. This interpretation is consistent with the organic petrological observations (Fig. 3). Alternatively, the high maturation of the OM may also contribute to these n-alkane characteristics because the high molecular weight n-alkanes tend to crack to low molecular weight n-alkanes with increasing maturation (Peters et al., 2005). 4.3.2. Isoprenoids Isoprenoids, especially pristane (Pr) and phytane (Ph), were identified in the studied samples. Pr/Ph values are known to be important indicators of the depositional environment of

sediments, with values of > 3.0 and < 1.0 generally indicating OM deposition in oxidized and reduced environments, respectively (Didyk et al., 1978). The samples analyzed in this study have Pr/Ph values of 0.71–0.88 with an average of 0.79, indicative of deposition in a generally reducing environment. The ratio of pristane to the adjacent n-alkane (Pr/n-C17) within these rocks varies between 0.39–0.61, with an average of 0.47, and the ratio of phytane to adjacent n-alkane (Ph/n-C18) is 0.48–0.71, with an average of 0.57. These values suggest that the shale sequences were deposited in a reducing, brackish environment (Fu et al., 1991). 4.3.3. Terpanes Abundant tricyclic and pentacyclic terpanes (hopanes) and a relatively small number of bicyclic sesquiterpanes and tetracyclic terpanes were identified in all of the studied samples. The C15 and C16 bicyclic sesquiterpanes were dominated by 8b(H)-drimane and

R. Yang et al. / Organic Geochemistry 86 (2015) 55–70

63

Fig. 6. Biomarker mass chromatograms for Shengli River black shale sequences; see Fig. 2 and Table 1 for the sample locations and characteristics, respectively.

8b(H)-homodrimane, respectively. The bacterial origin of the OM is indicated by the dominance of 8b(H)-drimane and 8b(H)homodrimane over the 8a(H)-drimane (Alexander et al., 1984). The tricyclic terpanes, mainly corresponding to cheilanthanes (Anders and Robinson, 1971), have complex origins, including marine algal sources as widely detected in Tasmanites-abundant sediments (Azevedo et al., 1992) and higher plants origin (i.e., C19 and C20 tricyclic terpanes; Noble et al., 1985). The tricyclic terpanes detected in this study range from C19 to C29 and are dominated by C23 terpanes, indicative of algal precursors (Azevedo et al., 1992). The relatively low abundance of C19 and C20 tricyclic terpanes indicates a minor component of higher plants in OM (Noble et al., 1985). The abundances of C20, C21, and C23 tricyclic terpanes follow the order of C20 < C21 < C23, suggesting that the OM was deposited in a brackish environment (Peters et al., 2005; Cao et al., 2006). A high abundance of the C24 tetracyclic terpane is thought to be indicative of saline water during sediment deposition (Clark and

Philp, 1989). The shale samples have C24 tetracyclic terpane/(C24 tetracyclic terpane + C26 tricyclic terpane) values of 0.79–0.84, implying that these sediments contain OM deposited in brackish to saline environments. The pentacyclic terpanes (e.g., hopanes) have complex origins and may be dominantly derived from bacteria (Ourisson et al., 1979; Peters et al., 2005). The wide variety of hopanes in the Shengli River black shale sequences suggests that the OM was derived from multiple sources, and in particular, hopanes with carbon numbers greater than C30 are indicative of a bacterial source. Terpanes provide evidence of the source, maturity and alteration of OM. For example, Ts/Tm values are influenced by both the environment of deposition and the degree of thermal evolution of the OM within the sediments; Ts/Tm values of < 1 usually reflect highly saline environments, whereas Ts/Tm values of > 1 are indicative of low salinity environments (Rullkötter and Marzi, 1988). In addition, Ts/Tm values positively correlate with the thermal evolution

64

R. Yang et al. / Organic Geochemistry 86 (2015) 55–70

of OM within a sediment, especially under high thermal stress (Moldowan et al., 1986). The samples in this study have Ts/Tm values of 1.40–1.95 with an average of 1.65. Because the OM was deposited in a reducing and brackish environment, as evidenced by the organic petrology and geochemistry discussed above, these high Ts/Tm values suggest high thermal maturity, consistent with other maturity data for these samples (see Section 4.2.3). High maturity of the OM is also supported by the C31 hopane 22S/(22S + 22R) values (0.58–0.60; Seifert and Moldowan, 1980). Volkman et al. (1983) and Peters et al. (2005) proposed that the 17a(H)-diahopanes originate from microbial hopanoids catalyzed by clay minerals under sub-oxic conditions, with C30 diahopane/C30 hopane ratio values generally positively correlating with the maturity, primarily as a result of the higher stability of C30 diahopane compared with C30 hopane. Alternatively, previous studies suggested another interpretation as this compound has been identified in sediments containing significant amounts of benthic algae (Zhang et al., 2007; Cao et al., 2009). Considering that the samples in these studies are quite mature, the maturity may also affect the abundance of this compound. As such, the widespread occurrence of diahopanes in the present samples provide clues that the sediments in the study area may be rich in benthic algae and/or have high maturity. Abundant gammacerane is generally indicative of deposition in saline water, often related to stratification of the water column (Sinninghe Damsté et al., 1995). The samples in this study have gammacerane indices of 0.20–0.35, indicating deposition in a reducing, marine and brackish water environment (Fu et al., 1991).

4.3.4. Steranes Abundant steranes were identified in all of the Shengli River shale samples, mainly C21 and C22 steranes (pregnanes and diginanes), regular steranes C27–C29 and diasteranes C27–C29. The relative abundances of regular C27–C29 steranes is commonly used to evaluate the source of precursors. C27 and C29 steranes are thought to be mainly indicative of algae and higher plant precursors, respectively (Huang and Meinschein, 1979). The samples analyzed in this study yield an asymmetrical V-shaped distribution of C27–C29 regular steranes, with abundances of 44%–51% C27, 14%–17% C28, and 32%–40% C29. The apparent C27 sterane predominance of these samples suggests algae-dominated organic macerals within the Shengli River black shales. This distribution pattern of regular steranes can also be observed in diasteranes, i.e., a C27 diasterane predominance among the C27–C29 diasteranes. Ratios of C29 sterane isomers C29 aaa20S/(20S + 20R) and C29 abb/(abb + aaa) indicate the maturity of OM, especially within low to moderate maturity rocks with %Ro values < 1.0 (Seifert and Moldowan, 1980), whereas the ratios may be reversed in sediments that are highly to over mature in phosphate-rich samples (Lewan et al., 1986; Peters et al., 1990). All of the samples analyzed in this study have C29 aaa20S/(20S + 20R) values of 0.51–0.56 and C29 abb/(abb + aaa) values of 0.41–0.46. Considering that a certain number of phosphate minerals were found in the samples of this study as shown in SEM images (Fig. 3d) and the P2O5 content ranges in 0.4–0.7%, we infer that the OM within these sediments has undergone thermal maturation and the ratios may be reversed because the OM has entered the high maturation stage according to petrographic and organic geochemical analyses (see Section 4.2.3). In summary, the biomarker data of the Shengli River black shales provide clues that dominant precursors in the shale sequences are aquatic organisms, bacteria and amorphous OM with minor amounts from higher plants. The black shales were deposited in a generally reducing and brackish water environment, and have reached moderate to high thermal maturity. These

understandings are generally consistent with those obtained from organic petrology and geochemistry. 4.4. Hydrocarbon resource potential 4.4.1. Conventional hydrocarbon resources The generation potential of source rocks is generally dependent on the quality and quantity of OM in the source rocks (Tissot and Welte, 1984; Katz, 2005; Wu et al., 2014). The source rock quality of the Shengli River black shale sequences can be constrained by the data of the present study. The Shengli River black shales occur across the northern Qiangtang depression and to a lesser degree in parts of the southern Qiangtang depression (Zhao et al., 2000; Hu et al., 2001; Li et al., 2005, 2010; Du and Chen, 2008); all of these shales formed in the same lagoonal environment, including shales outcropping in the Changshe Mountain, Nagde Kangri and Tuonamu areas (Zeng et al., 2012; Fig. 7). This indicates that these shales are sufficiently widespread to be a significant hydrocarbon source rock. The source rock quality is discussed below. 4.4.1.1. Hydrocarbon resource potential of the Lower Cretaceous black shales in the Qiangtang Basin. The present study indicates that the Shengli River black shale sequences are generally high quality source rocks having abundant OM. Source rock quality decreases from the black shales to the micritic limestones and marls, with the latter being the lowest quality source rocks in the study area. These rocks contain dominantly type II kerogen (tending to type III), and almost all of these samples are highly mature. Consequently, the black shales are predicted to generate significant amounts of natural gas. The fact that these shales are generally highly mature indicates that the TOC and hydrogen index would have decreased during hydrocarbon generation and expulsion (Espitalié et al., 1977; Daly and Edman, 1987; Kalkreuth and McMechan, 1988; Baskin, 1997), meaning that the original quality of the OM in these rocks was higher than indicated by the present results. The northern Qiangtang depression may also have oil generation potential as a result of variations in OM maturity and type across the study area. The Lower Cretaceous equivalent to the black shales in the study area reaches differing levels of thermal maturity across the northern Qiangtang depression, although most of these source rocks are moderately to highly mature (Fig. 8; Qin, 2006c). The black shale samples in this study are located at the border between areas with VRo values of < 1.3 and 1.3–2.0%Ro (Fig. 8), consistent with our finding that these black shales are mature to highly mature. Source rocks with moderate maturity are present to the east and west of the study area (Fig. 8), indicating that these areas may have oil potential if there are source rocks with relatively good kerogen type (Xu and Qin, 2004; Qin, 2006c) which seems likely. It has been proposed that the Shengli River and its neighboring area were deposited in a restricted lagoon environment during the Late Jurassic–Early Cretaceous, when transgression periodically took place (Xu and Qin, 2004; Qin, 2006c). Therefore, the locality, area and depth of the lagoon were always changing, making the sedimentary facies and the OM richness, type and maturity of the Shengli River shales differ from neighboring areas such as Changshe Mountain and Tuonamu, but to varying degrees. As a consequence, it is likely that the neighboring black shales within the same stratigraphic interval as the Shengli River shales, if having moderate OM maturity and oil-prone kerogen, can be expected to generate oil. Nevertheless, the specific environments and associated OM natures require further detailed work. 4.4.1.2. Comparison with other Upper Jurassic–Lower Cretaceous source rocks in the Qiangtang Basin. The Lower Cretaceous black

R. Yang et al. / Organic Geochemistry 86 (2015) 55–70

65

Fig. 7. Stratigraphic correlation of Lower Cretaceous rocks within the northern Qiangtang depression (modified after Zeng et al., 2012). See Fig. 1a for the location of these outcrops.

Fig. 8. Distribution of the maturity of organic matter within Lower Cretaceous sediments of the Qiangtang Basin (modified after Qin, 2006c).

shale sequences studied here occur widely across the northern Qiangtang Basin and to a lesser degree in the southern basin. In addition, these shales are in close contact with the underlying Upper Jurassic shales (Zhao et al., 2000; Hu et al., 2001). Thus, to have a more comprehensive understanding, we compare here the source rock properties of the Lower Cretaceous black shales and the underlying Upper Jurassic rocks across the study area (Table 5). Lower Cretaceous black shales in the study area are generally much better potential source rocks than the Upper Jurassic rocks

in terms of OM abundance as measured by TOC. The black shales contain OM that has type II kerogen and is less mature (Table 5). In contrast, the Upper Jurassic source rocks are more widespread, across the Qiangtang Basin and are thicker than the Lower Cretaceous source rocks that are predominantly located in the northern Qiangtang depression. As such, both the Lower Cretaceous and Upper Jurassic source rocks in the Qiangtang Basin have significant hydrocarbon generation potential.

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Table 5 Generalized properties of Lower Cretaceous and Upper Jurassic source rocks within the Qiangtang Basin. Strata

Lithology

Area (104 km2)

Thickness (m)

TOC⁄ (wt%)

S1 + S⁄2 (mg HC/g Rock)

Kerogen type

%Ro

References

Lower Cretaceous (Shengli River outcrop) Lower Cretaceous (Changshe Mountain outcrop) Lower Cretaceous (Tuonamu outcrop)

Shale, marl, and micritic limestone Oil shale

nd

15–40

5.06 (11)

11.4 (9)

III

1.22–1.33

This study

12

9.76 (17)

40.2 (17)

II

0.37–0.90

Li et al. (2010)

18

9.32 (4)

nd

II

0.72

Li et al. (2005)

Upper Jurassic (Open platform facies) Upper Jurassic (Restricted platform facies) Upper Jurassic (Deep-water basin facies)

Micritic limestone

270

0.20 (4)

0.13 (9)

II

0.94–2.70

Micritic limestone

0.23 (16)

0.11 (16)

II

Zhao et al. (2000), Hu et al. (2001), Du and Chen (2008)

Limestone, marl, and mudstone

1.08 (43)

2.48 (9)

II

Oil shale 11.47

Note: ‘‘nd’’ = no data available, ‘‘*’’ = average (number of analyzed samples).

Table 6 Generalized properties of Cretaceous source rocks in China and elsewhere. Country

Age

Basin

Formation/member

Area (104 km2)

Thickness (m)

TOC (wt%)

Kerogen type

%Ro

References

International

K2

Sirte

Sirte Shale

23

100–900

0.5–4.0

II

0.7–1.6

K1 K1

Potiguar Gabon

Alagamar Melania

200–350 500

4.0–6.0 6.1

I, II I, II

< 0.5 0.5–1.0

Parsons et al. (1980), Futyan and Jawzi (1996) Mello et al. (1995) Teisserenc and Villemin (1989)

K2

Songliao

26

0–112

1.6

I, II

0.5–0.8

Wu et al. (2009), Zhong et al. (2009)

K1 K1

Hailar Qiangtang

First member of Qingshankou First member of Nantun Upper Suowa

7 nd

50–70 15–40

1.9 5.1

II II

0.6–0.7 1.2–1.3

Li et al. (2011), Cui et al. (2012) This study

China

4.8 9

Note: ‘‘nd’’ = no data available.

All of the Lower Cretaceous black shales in the study area contain significant amounts of oil-prone kerogen, and are moderately mature (except for the shales in this study). In comparison, the Upper Jurassic source rocks deposited in deep water environments are higher quality than those deposited in platform environments, as is evidenced by variations in OM abundance (Table 5). This provides evidence of a sedimentary environment control on source rock quality within the Qiangtang Basin (Tissot and Welte, 1984; Katz, 2005). 4.4.1.3. Comparison with other representative Cretaceous source rocks worldwide. To better understand the quality and hydrocarbon potential of the Shengli River black shale source rocks, we compared them with other Cretaceous source rocks worldwide to get a global perspective (Table 6). The Shengli River black shales contain similar amounts of OM to other source rocks formed in similar depositional environments (i.e., transitional lagoons, including the Potiguar and Gabon basins; Table 6), but have more TOC than the Sirte and Songliao shales, both of which were deposited in transgressional environments (Van Houten, 1983; Jia et al., 2013). In addition, the Shengli River black shales are slightly less oil-prone than the other source rocks in terms of kerogen type, consistent with environmental control on the source rock properties of these sediments (Demaison and Moore, 1980; Pedersen and Calvert, 1990). Finally, the OM within the Shengli River black shale sequences is of higher maturity than the other source rock sequences listed in Table 6. 4.4.2. Unconventional hydrocarbon resource potential To advance exploration and research into unconventional shale oil and gas within the study area, we provide a preliminary assessment of the unconventional hydrocarbon resource potential of the Shengli River black shale sequences and outline the conventional resource potential of these rocks, as discussed above. Key

additional properties that need to be evaluated include the production potential of the rocks (e.g., generation and expulsion) and the reservoir space within the shale (Curtis, 2002; Jarvie et al., 2007; Zou et al., 2011). The hydrocarbon enrichment and expulsion potential of source rocks can be roughly evaluated from the correlation between S1 and chloroform-extractable bitumen values and the TOC of source rocks, as shown by recent research on a large dataset for shales in China (Lu et al., 2012), including sediments from the Songliao, Yitong, Bohai Bay and Hailar basins. Using this approach, the S1 and chloroform-extractable bitumen contents of the Shengli River black shale sequences rise rapidly when TOC contents reach ca. 4.0 wt% (Fig. 9), indicating that these rocks have good unconventional hydrocarbon potential (Lu et al., 2012). This suggests that the Shengli River black shale sequences with TOC > 4.0 wt% are nearly saturated with petroleum and thus may have significant shale-oil production potential. The adsorption and storage of hydrocarbon within the shale sequences commonly depends on micro- to nanoscale pores formed during the maturation of kerogen and subsequent hydrocarbon expulsion (Curtis, 2002), as evidenced by the relationship between gas content and TOC for North American gas shales (Li et al., 2009; Zou et al., 2010; Chalmers et al., 2012). In addition, shale can contain micropores and fractures that enable the accumulation of oil and gas as a result of kerogen adsorption and capillary pressure (Slatt and O’Brien, 2011). The Shengli River black shale sequences contain various types of micropores and fractures, which can be divided into four types (Slatt and O’Brien, 2011): (1) intraparticle pores/intergranular pores, which were most frequently found in the Shengli River black shales, as they are often associated with carbonates such as calcites and dolomites. Most of such pores are at nanometer scale (Fig. 10a and b); (2) floccule-derived pores which often occur with clay minerals, especially illites. The diameter of such pores can reach tens of

R. Yang et al. / Organic Geochemistry 86 (2015) 55–70

67

Fig. 9. Correlation between TOC and Rock–Eval S1 and chloroform-extractable bitumen values for the Shengli River black shale sequences: (a) TOC versus S1; (b) TOC versus chloroform-extractable bitumen.

Fig. 10. SEM images showing the development of micro-pores and fractures within the Shengli River black shale sequences; (a) intraparticle pores (sample SLR-10); (b) intergranular pores (sample SLR-20); (c) floccules with pores (sample SLR-17); (d) organic matter-related pores (sample SLR-16); (e) microfractures (sample SLR-13); (f) brittle minerals (e.g., calcites) (sample SLR-07). See Fig. 2 and Table 1 for the sample locations and geochemical characteristics, respectively.

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R. Yang et al. / Organic Geochemistry 86 (2015) 55–70

microns and can be connected (Fig. 10c); (3) OM-related pores which were prevalent in shale samples and accompany fragments of benthic algae. Pores are generally isolated with nanometer scale (Fig. 10d); (4) microfractures, which were the least abundant, but are indispensable for unconventional hydrocarbon resource potential. Such microfractures often occur with brittle minerals and can be at nanometer or larger scales (Fig. 10e). Abundant brittle minerals in shales, such as quartz, feldspar, dolomite and calcite are important indicators of shale oil and gas resource potential (Passey et al., 2010). Brittle minerals within a shale allow increased microfractures with good connectivity, which is beneficial for the accumulation of oil and gas and for their recovery using fracturing technology (Zou et al., 2010; Chalmers et al., 2012). In contrast, abundant clay within a shale makes exploitation more difficult; for example, North American gas shales generally contain 46–60% brittle minerals (Zou et al., 2010). The Shengli River black shales have a high content of CaO (28.6– 38.1 wt%), equivalent to 50–68 wt% CaCO3 in mineral composition. This means that the black shales in this study are rich in brittle minerals (e.g., calcites and dolomites, Fig. 10f). All these data suggest that the shales are expected to have good unconventional hydrocarbon potential. In summary, black shales with relatively high TOC > 4.0 wt% within the Shengli River region and equivalent areas within the Qiangtang Basin may be highly prospective for unconventional hydrocarbon exploration. This preliminary investigation needs more detailed studies of porosity types and a detailed petrographic identification of the minerals present in the rocks (Curtis, 2002; Jarvie et al., 2007; Zou et al., 2011). 5. Conclusions The Shengli River black shale sequences in this study are organic-rich and have a high hydrocarbon (gas in particular) resource potential. They were deposited in a generally reducing and transitional platform–lagoonal environment and contain precursors that are dominated by input from aquatic benthic algae, bacteria and amorphous OM. Terrigenous plants are also present, but in lower abundance. The Shengli River black shale sequences are high quality source rocks with high TOC (generally > 2.0 wt%), contain type II kerogen (tending to type III), and they are moderately to highly mature. They are most likely to generate gas. Equivalent rocks elsewhere in the basin have better kerogen type and lower maturity, suggesting the potential for oil generation. The black shales contain well developed micropores and cracks, as well as significant amounts of brittle minerals, indicating that these shales, especially shales with high TOC > 4.0 wt%, can have significant unconventional oil and gas potential. Acknowledgements We thank Drs. Andrew Murray, Armelle Riboulleau and John Volkman and an anonymous reviewer for their thorough reviews and comments. Prof. Lizeng Bian and Dr. Xiaolin Wang from the School of Earth Sciences and Engineering, Nanjing University are thanked for their help in organic petrological and LRM analyses, respectively. This work was jointly funded by the Major State Basic Research Development Program (973 project, Grant No. 2012CB214803), National Natural Science Foundation of China (Grant Nos. 41322017, 41472100 and 41472099), and Scientific and Technological Supporting Program of Jiangsu Province (Grant No. BE2013115). Associate Editor—Andrew Murray

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