This study focuses on organic-rich shale strata in four formations in the Qamdo Basin and aims to evaluate their hydrocarbon potential. Stratigraphic sections were chosen on the basis of stratigraphic data, accessibility via roads and well-exposed conditions. Five stratigraphic sections (TB, BLL, ADL, DGL-E and DGL-W: Figure 2) were measured at outcrops alone the established roads.

At the TB section, about 489 m of the upper part of the Upper Permian Tuoba Formation is exposed, which is composed here of organic-rich shales, fine quartzose sandstones and bioclastic limestones (Figures 4, 5A). Palaeofusulina was found in the limestones, indicating that the formation is of Changhsingian (latest Permian) age (Sheng and Wang, 1981). The cumulative thickness of dark shale on the section is over120m, which is the potential source rock of this group. Eight source rock samples were collected within a 45 m thick organic-rich shale interval in the lowermost part of the measured section (Figure 3).

At the BLL outcrop section, the Upper Triassic Bolila Formation is > 480 m thick and mainly consists of limestones, dolomites and bioclastic limestones. Six dark limestone samples were collected for further analysis from the upper part of the formation (Figure 4). The carbonates consisted of fine-grained micrite and coarse sparry calcite in the pore spaces between grains and in larger cavity structures.

The ADL section spans the entire Upper Triassic Adula Formation with a thickness of approximately 635 m and the cumulative thickness of black shale over 290 m (Figures 4, 5B). The stratigraphically overlying DGL-W section (Figures 4, 5C) is dominated by Upper Truiassic Duogala Formation sandstones and shales with thin coal intervals, suggesting that facies pass from prodelta to delta-plain deposits corresponding to a drop of sea level. A total of 14 samples were collection from these two sections. Another outcrop section (Figure 5D) of the Duogala Formation was measured to the SE (the DGL-E section) where eight samples were collected.

For laboratory investigations, the samples were cleaned and weathered surfaces were removed. They were ground to analytical grain sizes (approximately 100 mg and 120 μm) and dried prior to further treatment. After the removal of carbonates using hydrochloric acid (HCl), a Leco CS-200 carbon–sulphur elemental analyzer was used to determine the total organic carbon (TOC) and total sulphur (TS) contents. The samples were also analysed by a Rock–Eval II instrument following the guidelines of Peters (1986).

A total of 29 samples were selected for further analysis as they contained abundant organic matter. The crushed samples were extracted with chloroform in a Soxhlet apparatus for 72 h. The extracted organic matter was further separated by a silica gel alumina column chromatography into saturated hydrocarbons, aromatic hydrocarbons and NSO (nitrogen, sulfur, and oxygen bearing compounds) compounds (Petersen et al., 2005). The saturated hydrocarbon fractions in the extracts were analysed by gas chromatography using an Agilent 6890N (GC) instrument with an injector temperature of 310°C. The GC was equipped with a 30 m × 0.20 mm internal diameter fused silica column (0.2 μm film) using helium as the carrier gas. The oven was gradually heated from 70 to 300°C at a rate of 8°C/min and followed by an isothermal period of 20 min.

To prepare kerogen material, carbonates were first removed using HCl. Then samples were washed several times with distilled water and treated with hydrofluoric acid (HF) for 12 h to remove silicates; this step was repeated until pure kerogens were isolated (c.f. Guthrie and Pratt, 1994; Ma et al., 2015). Strew-mounted kerogen slides were then prepared and used for further identification. A visual estimation of the relative abundance of macerals was conducted using an Axioskop2 plus microscope and a Swift point counter. To determine the relative abundance of macerals, at least 300 components were counted (Petersen et al., 2005). The vitrinite reflectance (% Ro) was determined using a MPM-80 microphotometer.

Elemental analyses were performed on a FLASH EA-1112 Series elemental analyzer according to National Standard of P.R. China (GB/T 19,143–2003).

Experimental work was performed at the Organic Geochemistry Laboratory, Institute of Exploration and Development, Huabei Oilfield Branch Company of the China National Petroleum Corporation (CNPC).

TOC, TS and Rock-eval data of the outcrop samples from the measured sections are presented in Table 1. The TOC content of the eight Tuoba Formation samples from the TB section varies between 1.66 wt% and 3.01 wt%, with an average of 2.33 wt%. The S1 and S2 values are in the range of 0.–0.22 mg HC/g rock and 0.29–4.29 mg HC/g rock, respectively. Low TOCs (<0.30 wt%) were recorded in the Bolila Formation limestone samples from the BLL section, and the Adula Formation samples (ADL section) also have relatively low TOC (0.43–0.75% wt%). The samples of the Duogaila Formation from the DGL-W section have TOC contents ranging from 0.48 to 3.47 wt%, with the exception of the coaly samples DGL-W-S6 (TOC = 25.60 wt%). In the DGL-E section, the TOC contents of the Duogaila Formation samples are 0.47–1.04 wt% (average: 0.70 wt%).

The average S1, S2, potential yield (PY) values are 0.12, 0.32 and 0.44 mgHC/g rock for samples from the Tuoba Formation, 0.04, 0.11 and 0.15 mgHC/g rock for samples from the Adula Formation, and 0.06, 0.33, and 0.39 mgHC/g rock for samples from the Duogaila Formation, respectively (Table 1). The T_max values fall within the range of 424–538°C and an average of 492°C.

The maceral composition (vol%) and vitrinite reflectance (Ro) values for selected samples from the Tuoba, Adula and Duogaila Formations are shown in Table 1. About 47–53 vol% of the total organic matter (OM) in the Tuoba Formation samples from the TB section is composed of amorphous organic matter (AOM). Vitrinite and inertinite account for about 20–38 vol% and 14–30 vol% of the kerogen assemblages, respectively (Figure 6A).

In Adula Formation samples from the ADL section, AOM dominates the maceral composition with proportions of more than 65 vol% of total OM. Inertinite group macerals accounts for 28%-31 vol% of the kerogen, and vitrinite up to 4 vol%.

In the DGL-E section samples, AOM is present as 40–64 vol%, whereas vitrinite and inertinite range from 15 to 35 vol% and 17 to 30 vol%, respectively (Figures 6B,C).

Within the eight Duogaila Formation samples from the DGL-W location, the samples from lower part of the section are dominated by AOM (65–68 vol%), followed by 28–32 vol% inertinite and 0–5 vol% vitrinite. Samples DGL-W-S5 and DGL-W-S6 have a different maceral composition which is dominated by inertinite and vitrinite. Coal seam occurs in the upper part of the DGL-W section and the photomicrograph showing an inertinite dominated composition (Figure 6D).

Vitrinite reflectance (Ro) values for samples from the four measured sections are listed in Table 1, including the number of points measured for each sample. All of the samples have Ro values > 1.3%, and some have R_o > 2.0%.

Elemental analysis provides a measure of the bulk chemical properties of isolated kerogen (Katz, 1988; Wang et al., 2011b) and data are summarized in Table 2. In general proportions of major elements are similar in all samples. Atomic O/C ratios and H/C ratios range from 0.05 to 0.16 and 0.48 to 1.38, respectively (Table 2).

Gas chromatograms of saturated hydrocarbon fractions in extracts of the samples are shown in Figure 7, and calculated parameters are given in Table 2. The chromatograms display a wide range of n-alkanes and isoprenoids between n-C₁₅ and n-C₃₅. The n-alkane distribution shows a bimodal pattern with a predominance of high molecular weight compounds (n-C₂₃ -- n-C₃₅); most of the samples have a peak at n-C₂₅ or n-C₃₃. In addition, all samples show odd-over-even predominance with a moderate carbon preference index (1.09 < CPI <1.72). These results indicate a high input of terrigenous organic matter.

The maceral composition can be interpreted to indicate the origin and type of kerogen. Previous studies (e.g., Hutton et al., 1994; Petersen et al., 2005; Bechtel et al., 2014; Ma et al., 2015) have suggested that alginate, liptinite, and inertinite and vitrinite can be correlated to Types I, II and III kerogens, respectively. For most of the samples, AOM derived from lipid-rich precursors accounts for the largest proportion (nearly 50 vol%) of OM relative to vitrinite and inertinite macerals, suggesting that samples from all of the measured sections contain mostly Type II kerogen with smaller proportions of Type III kerogen. A cross-plot (Figure 8) shows that the organic matter in the samples is mainly Type II with the exceptions of two Duogaila Formation samples from the DGL-W section which contain Type III OM. The results are consistent with the relative proportions of the maceral groups.

Vitrinite reflectance (R_o) is the most reliable parameter for measuring the thermal maturity of sedimentary organic matter (Tissot and Welte, 1978; Grassmann et al., 2005; Opera et al., 2013). In general, Ro values of 1.3 and 2.0% are accepted as the threshold values for highly mature and over-mature Type II kerogen (Killops and Killops, 2005). Therefore, the thermal maturity of the measured samples is high-to overmature (i.e., wet gas to dry gas zone). The results are consistent with the high T_max value.

Overall, the samples collected from Tuoba Formation exhibit the best hydrocarbon potential of the formations investigated with TOC >1.0 wt%. By contrast, the samples from the BLL section have TOC <0.3 wt%. Samples from the Adula and Duogala Formations have poor to medium organic contents, with the exception of a few coaly samples of the latter formation from the DGL-W section. PY (PY = S1 + S2) represents the potential yield and a quantity of the transformation potential of the total organic matter (Demaison and Huizinga, 1991; Tissot and Welte, 1978). According to evaluation criterion of the hydrocarbon source rock established by the China National Petroleum Corporation (Wang et al., 2009; Ding et al., 2013), PI value over 0.25 is classified as good source rock, and 0.15 to 0.25 for medium source rock.

The proneness towards oil or gas generation is indicated by the kerogen type and the maceral composition. Fluorescent macerals of the liptinite group are here considered as the oil-prone fraction, and the sum of inertinite and vitrinite group macerals are the gas-prone fraction (Killops and Killops, 2005; Furmann et al., 2015; Hakimi and Ahmed, 2016). The analysed samples contain mainly Type II OM with nearly equal oil- and gas-prone fractions. However, all the samples are highly to over-mature in terms of R_o, as discussed above. The maturity stage of studied samples (high to over mature) is high enough to generate and expel large amount of hydrocarbons and cracked into dry gas (Killops and Killops, 2005; Liang et al., 2017), this reveals that the measured strata were underwent deep burial after deposition. Therefore, the combined evidence suggests that gas generation can be expected in the Tuoba, Adula and Duogaila Formations.

Total sulphur is considered to be a good measure of the degree of marine influence (Sykes, 2004; Sykes et al., 2014; Sarki Yandoka et al., 2015). In samples from the Tuoba Formation, TS contents varied between 0.60 and 1.12 wt%; values for samples from the ADL, DGL-W and DGL-E sections (Adula and Duogaila Formations) were much lower (0.10–0.40 wt%). Thus, compared to the Tuoba Formation, the samples from the Adula and Duogaila Formations are interpreted to have been deposited under more freshwater-influenced conditions which consistent with delta facies.

Pristane (Pr) and phytane (Ph) occur in significant amounts in all the studied samples (Figure 7). The most abundant source of pristane and phytane is the phytyl side-chain of chlorophyll a in phototrophic organisms, and bacterio chlorophyll a and b in purple sulphur bacteria (Powell and McKirdy, 1973). The phytyl side-chain is prone to be converted into phytane in reducing or anoxic conditions, whereas oxic conditions favour pristane (Peters et al., 2005). Therefore, the value of the Pr/Ph ratio is considered to indicate the redox conditions during sedimentation and diagenesis (Didyk, 1978; Chandra et al., 1994; Escobar et al., 2011). High Pr/Ph (>3.0) indicates oxic conditions often associated with terrigenous organic matter input, while low values (<0.8) typify anoxic, commonly hypersaline or carbonate environments (Peters et al., 2005; El Diasty and Moldowan, 2012). Phytane occurs in high relative concentrations in the analyzed samples, resulting in Pr/Ph ratios in the range of 0.44–0.85 (Table 2), indicating a suboxic to relatively anoxic depositional setting. Furthermore, Pr/n-C₁₇ and Ph/n-C₁₈ ratios are in the range of 0.64–0.94 and 0.58–2.54, respectively. A cross-plot of Pr/n-C₁₇ versus Ph/n-C₁₈ (Figure 9) also suggests that marine-derived organic matter was an important input to the source rock and was preserved under a reducing environment.

The Qamdo Basin experienced frequent sea-level fluctuations, especially in the Late Triassic, and four transgressive-regressive cycles have been recognized according to the sedimentary record (Du et al., 1997; Du et al., 1998). Based on sequence stratigraphy analyses for the outcrops in the Qamdo area, Peng et al. (2005) constructed a curve of sea level change for the Late Triassic (Figure 10). Compared with the eustatic cycle of Haq et al. (1988), the curve in the Qamdo area contains more low-amplitude signals relating to short-term sea-level change. Figure 10 shows that the source rock intervals in the Adula and Duogala Formations were deposited during transgressive episodes. Instead, peat deposits (coal beds of Duogaila Formation) and coarse-grained clastic deposits may be preserved in response to relative sea-level fluctuations (Diessel, 1992; Lv and Chen, 2014).

Various reasons have been proposed to account for the positive association between transgressions and the deposition of organic-rich facies (Klemme and Ulmishek, 1991; Arthur and Sageman, 1994; Arthur, 2005). including: 1) transgressive periods may enhance seasonal or longer-term water-column stratification and development of anoxia; 2) rising sea levels may promotes near-shore trapping of terrigenous clastic material, creating condensed intervals which are characterized by enrichment in organic carbon (Arthur, 2005); and 3) land-derived nutrients give rise to surface plankton booms, resulting in high biological productivity (Hao et al., 2011; Li et al., 2016).

The transgressive episodes in the Jiapila and Bilila Formations have quite different sedimentary characteristics. During Jiapila Formation deposition, intense tectonism resulted in widespread conglomerates which are have little source rock potential (Du et al., 1997; Du et al., 1998). In addition, evidence for an arid palaeoclimate in the Qamdo basin area is provided by evaporitic lacustrine deposits (Du et al., 1997; Du et al., 1998). By contrast, the Bolila Formation is characterised by the development thick platform carbonates during a period of tectonic and volcanic quiescence, during which terrigenous input was greatly reduced (Peng et al., 2005). Consequently, the accumulation of organic matter just depended on the productivity of the ocean with the insufficient nutrients.

Clearly, the development of source rock is governed by a variety of factors, such as a warm and moist climate, profitable regional tectonics and eustasy. Among them, sea level fluctuations have important implications for organic productivity of the oceans and sediment distribution patterns for the Qamdo Basin. The rise of sea level usually results in a reduced environment, and indicators of redox (such as Pr/Ph ratio) in accord with these samples were deposited in an anoxic and reducing environment.