Paleoenvironment and Controlling Factors of Oligocene Source Rock

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Paleoenvironment and Controlling Factors of Oligocene Source Rock in the Eastern Deep-Water Area of the Qiongdongnan Basin: Evidences from Organic Geochemistry and Palynology Piao Wu,*,†,‡ Dujie Hou,*,†,‡ Jun Gan,§ Xing Li,§ Wenjing Ding,†,‡ Gang Liang,§ and Bibo Wu§ †

School of Energy Resources, China University of Geosciences, Beijing 100083, China Key Laboratory of Marine Reservoir Evolution and Hydrocarbon Accumulation Mechanism, Ministry of Education, China University of Geosciences, Beijing 100083, China § Exploration and Development Resource Institute, Zhanjiang Branch of China National Offshore Oil Corporation, Guangdong 524057, China

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ABSTRACT: Little research regarding the organic geochemistry of source rock deposited in marine or transitional environment in the eastern deep-water area of the Qiongdongnan Basin (QDNB) has been performed so far. Based on the organic geochemical and petrological data of mudstone samples collected from the eastern deep-water area in the QDNB, combined with palynological data of rock samples in the shallow-water area, this paper studies the paleoenvironment, hydrocarbon potential, and controlling factors of Oligocene source rock in different sedimentary facies in the eastern QDNB. Two distinct models of delta front source rock and neritic source rock are proposedaccording to organic matter sources and geochemical characteristics. Source rock in the delta front subface is of medium to high organic abundance and gas-prone-type kerogen, the geochemical biomarkers are characterized by high ratios of pristane/phytane, oleanane/C30 hopane, T-bicadinane/ C30 hopane, and low ratios of C23 tricyclic terpane/C30 hopane, C27 sterane/C29 sterane, (nC21 + nC22)/(nC28 + nC29). However, source rock in the neritic face has medium organic richness and kerogen type prone to generate both oil and gas; the biomarkers are characterized by low ratios of pristane/phytane, oleanane/C30 hopane, T-bicadinane/C30 hopane, and high ratios of C23 tricyclic terpane/C30 hopane, C27 sterane/C29 sterane, (nC21 + nC22)/(nC28 + nC29). During the Oligocene epoch, the paleoclimate in the QDNB was subtropical warm-humid, and paleovegetation around and within the sags was dominated by pteridophyte and angiosperm plants, causing terrestrial higher plants to be the main organic matter source. Source rock in the delta front subface is formed in oxidation fresh or brackish water environment, whereas source rock in the neritic face is formed in weak oxidization brackish or saline water conditions. Organic matter abundance in the delta front subface is mainly controlled by the terrigenous organic matter input, whereas organic matter abundance in the neritic face is controlled by aquatic organic matter input and influenced by the water redox environment.

1. INTRODUCTION Marine source rock and transitional source rock are two important source rock types. The formation of marine source rock with abundant organic matter is controlled by sedimentary bottom water conditions1−3 and paleoproductivity.4,5 However, the controlling factors and organic matter enrichment model of transitional source rock have been rarely investigated. Previous studies have shown that the transitional source rock is characterized by double sources from sea and land, stable lateral distribution, and favorable condition for hydrocarbon generation and accumulation.6 Research on transitional source rock in the Bonaparte Basin in Australia and the continental basins of northern South China Sea has also shown that transitional source rock possesses fair to good hydrocarbon potential and II2−III-type kerogen, and is obviously controlled by terrigenous organic matter (TOM) input.6−8 Currently, the transitional source rock has been discovered in various sedimentary basins around the world.9 It is of great theoretical significance to conduct research on the geochemistry of marine or transitional source rock. The Qiongdongnan Basin (QDNB) is a sedimentary basin spanning the shallow-water area and the deep-water area of © XXXX American Chemical Society

northern South China Sea. For the past 40 years, petroleum exploration has been active in the shallow-water area, while the deep-water area has always been in the low-exploration stage. With water depths of >300 m,10 the deep-water area covers the most area of the QDNB and could be further divided into the western and the eastern deep-water areas. At present, a total of six wells are drilled in this area, five of which are drilled into the Oligocene strata. Major breakthroughs of gas exploration were attained on the LS22-1 and LS17-2 structures in the western deep-water area in years 2010 and 2014, respectively,11 yet no major breakthroughs of petroleum exploration in the eastern deep-water area have been achieved so far.12 Hence, new gas exploration fields must be expanded in the western deep-water area, and determination of whether the source rock is favorable for petroleum accumulation must be confirmed in the eastern deep-water area.13 Many previous studies have been carried out regarding the sedimentary distribution,14−16 the geochemical characterReceived: April 4, 2018 Revised: June 11, 2018 Published: June 19, 2018 A

DOI: 10.1021/acs.energyfuels.8b01190 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 1. Structural outline of the Qiongdongnan Basin (QDNB) and the cross-section framework of the eastern deep-water area.

istics,17,18 and the gas-source rock correlations19−22 of Oligocene source rock in the western deep-water depressions. It is generally believed that the western deep-water area develops two types of hydrocarbon source rock, namely, Oligocene transitional coal-bearing source rock and Oligocene neritic source rock.18,20,22 The eastern deep-water area is unlike the western deep-water area in the deeper water depth, higher heat flow, and larger source rocks volumes, as well as shallower stratum buried depth and lower source rock maturity.13 Besides that, since the Songtao Uplift could act as an ancient island to provide an ideal environment for terrestrial higher plants grow, the amount of terrigenous supplement for source rock formation in the eastern deepwater area may be more abundant than that in the western deep-water area.18 Thus, source rock conditions in the eastern deep-water area are different from those in the western deepwater area. Currently, studies on the mechanisms for Oligocene source rock formation in the eastern deep-water area are few. Li et al.23,24 believed that, because of the dilution effect of terrigenous detrital matter (TDM), paleoproductivity during the Oligocene period was rather low, and the input of terrigenous organic matter (TOM) controlled the Oligocene source rock formation, whereas paleoproductivity or fresh oxidizing water conditions had minor influence on source rock formation. However, the research of Li et al.23,24 were mainly based on inorganic geochemical tests of trace elements. The organic geochemical study of Oligocene source rock in the

eastern deep-water area is scarce due to the contamination of oil-based mud. Hence, by means of washing pollutants on the cuttings and organic geochemical analyses of washed cuttings, the geochemical characteristics, paleoenvironment, and controlling factors of Oligocene source rock in the eastern deepwater area are finely researched. The results herein could help to evaluate the source rock potential and predict favorable exploratory areas in the eastern deep-water area of the QDNB.

2. GEOLOGICAL SETTING The QDNB lies to the northeast of Hainan Island and is a Cenozoic extensional basin on the northern continental shelf of the South China Sea. The basin can be divided into longitudinal blocks in the S/N direction and cross zones in the E/W direction25 (see Figure 1). From north to south, there are northern depression belt, northern uplift belt, central depression belt, southern uplift belt, and southern depression belt, respectively. The deep-water area consists of the most areas of the central depression belt, all areas of the southern uplift belt, and southern depression belt.26 The eastern deepwater area is composed of Songnan sag, Baodao sag, and Changchang sag13 with an area size of ∼2.5 × 104 m2. Among these sags, the Oligocene source rock in the Songnan−Baodao sag is of higher thermal maturity than that in the Changchang sag, as the kerogen vitrinite reflectance (Ro) respectively lie in 0.5%−2.8% and 0.4%−1.4%.15 On the cross section, the B


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Figure 2. Generalized stratigraphy of the QDNB. extraction, gas chromatography (GC) and gas chromatography− mass spectrometry (GC-MS). Samples collected from the YL1, CC1 wells in the deep-water area and BD5 well in the shallow-water area were contaminated by the organic pollutants such as diesel or sulfonated bitumen in the oilbased mud. Thus, these samples were first disposed of pollutants via the following operation process: contaminated samples were dipped into the chloroform stilly for ∼2 h to soak out the pollutants on the appearance, and then stirred appropriately to further clean the residual organic pollutant by using n-hexane. After that, the cleaned samples were tested with Rock-Eval pyrolysis, and the samples were not considered to be completely cleaned up until the Rock-Eval results displayed a single smooth cracking hydrocarbons (S2) peak. 3.2. TOC Analysis and Rock-Eval Pyrolysis. The TOC and total sulfur content (TS) were measured on a LECO CS-744 analyzer, following the procedures of industry standard (GB/T 19145−2003). Rock-Eval pyrolysis were carried out using a Rock-Eval II instrument according to the industry standard (GB/T 18602−2012). 3.3. Macerals and Spore-Pollens Analysis. Maceral analysis was performed on a Leica DM4000B biomicroscope using transmission light and fluorescent light with the analysis method in accord with the industry standard (SY/T 5125−1996). About 60 rock samples from two wells were processed for spore-pollens analysis and followed the procedures of industry standard (SY/T 5915−2000). Samples of ∼10−30 g of sediments were treated with 10% HCl and 70% HF to remove carbonates and silica. Then, the samples were cleaned to neutral with clear water and the palyno-residues were sieved by using 10 μm mesh nylon sieves. Finally, the palyno-residues were identified on the biomicroscope. 3.4. GC and GC-MS Analysis. Mudstone samples were cleaned prior to crushing and powdering and then employed to study the saturated hydrocarbons in GC and GC-MS analysis. Soxhlet

Baodao and Changchang sags are both asymmetric double fault grabens, and the Songdong sag is mainly a single fault half-graben (Figure 1). In terms of tectonic evolution, the eastern deep-water area successively undergoes Paleogene rifting stage and Neogene to Quaternary post-rifting stage, causing the Cenozoic stratigraphy to have typical lower rifting and upper subsiding double structure layers. The lower structure layer consists of Eocene lacustrine stratum, lower Oligocene Yacheng Formation transitional sand-shale stratum, upper Oligocene Lingshui Formation littoral to neritic sand-shale stratum. The upper structure layer is composed of lower Miocene Sanya Formation, middle Miocene Meishan Formation, upper Miocene Huangliu Formation, Pliocene Yinggehai Formation and Quaternary Ledong Formation sedimentary strata of which the sedimentary facies gradually vary from littoral-neritic to abyssal15 (see Figure 2). The relative sea level change performs as an overall sea level rise cycle, which occurs transgression in the episode of E3ls, N1hl, N2ygh Formation27,28 (Figure 2).

3. SAMPLES AND METHODS 3.1. Samples. Twenty-one (21) cutting or core mudstone samples from the top of Yacheng Formation (E3yc top) and 140 cutting mudstone samples from the Lingshui Formation (E3ls) were provided by the China National Offshore Oil Corporation and were tested with total organic carbon (TOC), Rock-Eval pyrolysis, organic petrological analysis. Approximately 40 cutting samples were tested with C


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wt % and PG amounts varying from 0.15 mgHC/grock to 14.32 mgHC/grock. 4.2. Organic Petrology. As is shown in Figure 4, the sapropelinite and exinite contents in kerogens of neritic source rocks in the deep-water area are much higher than that in littoral, delta front source rocks. The proportions of type II2 kerogens in source rocks from the littoral face, delta front subface, neritic face in the shallow-water area, and neritic face in the deep-water area are, respectively, 4.2%, 5.6%, 10.5%, and 29.6% (see Figure 5). It is obvious that neritic source rocks in the deep-water area are composed of II2−III-type organic matter, whereas source rocks in other sedimentary facies are mainly of III-type organic matter. 4.3. n-Alkanes and Isoprenoids. The parameters of nalkanes and isoprenoids of 26 samples from different sedimentary facies on the northern slope of Baodao sag are tabulated in Table 2. Some representative gas chromatograms are displayed in Figure 6. Delta front source rocks display postpeak distribution of gas chromatograms (see Figure 6C) with the main carbon of nC25 or nC27. The pristane/phytane ratios (Pr/Ph) lie in the range of 2.45−6.18, with a mean of 3.85, and the (nC21 + nC22)/(nC28 + nC29) ratios lie in the range of 0.72−1.58, with a mean of 1.07. Neritic source rocks display prepeak distribution of gas chromatograms (Figure 6D) with the main carbon ranging from nC14 to nC23. The Pr/Ph ratios lie in the range of 1.11−3.37, with a mean of 1.87, and the (nC21 + nC22)/(nC28 + nC29) ratios lie in the range of 1.09− 2.56, with a mean of 1.81. Biomarker characteristics of source rocks in different subfacies of the littoral face vary greatly. In the gulf subface, source rocks display a pre-peak distribution of GC (Figure 6A) with Pr/Ph ratios that lie in the range of 1.58−2.95 (average = 1.98) and (nC21 + nC22)/(nC28 + nC29) ratios lie in the range of 1.62−3.14 (average = 2.35). However, in the foreshore or nearshore subfacies, source rocks display a post-peak distribution of GC (Figure 6B) with Pr/Ph ratios ranging from 2.49 to 5.57 (average = 3.87) and (nC21 + nC22)/ (nC28 + nC29) ratios ranging from 0.88 to 1.34 (average = 1.04). 4.4. Steranes and Terpanes. The parameters of steranes and terpanes of 30 samples from different sedimentary facies in the Baodao−Changchang sag are tabulated in Table 3. Some representative mass chromatograms of steranes (m/z = 217) and terpanes (m/z = 191, m/z = 412) are displayed in Figure 7. In the delta front subface, the C27, C28, C29 steranes display a “V”-shaped distribution, and the terpanes are characterized by low contents of tricyclic terpane and a high abundance of oleanane and bicadinane (Figure 7C). The ratios of C23 tricyclic terpane/C30 hopane (C23TT/C30H), C27 sterane/C29 sterane (C27ST/C29ST), oleanane/C30 hopane (OL/C30H), Tbicadinane/C30 hopane (T/C30H) in delta front source rocks, respectively, lie in the ranges of 0.05−0.3 (average = 0.14), 0.68−1.28 (average = 0.85), 0.05−0.34 (average = 0.19), and 0.37−4.42 (average = 1.91). Whereas, these parameters in littoral source rocks are, respectively, in the ranges of 0.11− 0.34 (average = 0.20), 0.72−1.30 (average = 1.01), 0.04−0.14 (average = 0.10), and 0.04−0.72 (average = 0.41) (see Figures 7A and 7B). Compared with the delta front source rocks, neritic source rocks in the shallow-water area have similar tricyclic terpanes content and regular steranes distribution, but lower contents of oleanane and bicadinane (see Figure 7D). The ratios of C23TT/C30H, C27ST/C29ST, OL/C30H, and T/ C30H, respectively, lie in the ranges of 0.08−0.18 (average = 0.12), 0.60−0.95 (average = 0.78), 0.04−0.47 (average =

4. RESULTS 4.1. Bulk Geochemical Parameters. The results of TOC and Rock-Eval pyrolysis of Oligocene source rocks in different sedimentary facies are plotted in Figure 3 and tabulated in

Figure 3. TOC and PG values scatter plot of Oligocene source rocks in different sedimentary facies in the eastern QDNB.

Table 1. Delta front source rocks are mainly distributed on the sag margin (BD3, BD4, SD1, LS1 wells) and consist of gray mudstone, silty mudstone, and carbonaceous mudstone as well as a thin coal seam in the Songdong sag. Mudstones from this environment have TOC contents in the range of 0.19−5.84 wt % with a mean of 1.07 wt %, and potential hydrocarbon generation (PG) amounts in the range of 0.32−18.51 mgHC/ grock with a mean of 1.8 mgHC/grock. Neritic source rocks are distributed not only on the sag margin (BD3, BD4, SD1, LS1 wells) but also in the deep-water area (CC1, YL1 wells). The lithology is mainly composed of bluish mudstone and the TOC contents mainly lie in 0.38−1.62 wt % with a mean of 0.72 wt %. However, PG amounts in this environment vary much between source rocks in the shallow-water area and source rocks in the deep-water area. Neritic source rocks in the shallow-water area have PG amounts with a mean of 1.56 mgHC/grock, whereas neritic source rocks in the deep-water area have PG amounts with a mean of 3.10 mgHC/grock. The littoral source rocks are mainly developed on the sag margin (BD2, SD1 wells) with TOC contents ranging from 0.31 wt % to 4.59 D


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Table 1. Geochemical Parameters of Oligocene Source Rocks in Different Sedimentary Facies in the Eastern Qiongdongnan Basin sedimentary face

well

TOC (%)

PG(mg/g)

HI(mg/g)

kerogen type

littoral (shallow water)

BD2

0.31−0.88 0.54(12) 0.64−4.59 1.65(6)

0.15−0.86 0.44(12) 1.3−14.32 4.28(6)

4.3−65.4 27.0(14) 151.6−290.8 203.1(5)

III

0.19−3.11 0.92(52) 0.36−5.84 1.48(19)

0.34−3.53 1.34(52) 0.32−18.51 3.21(19)

39.3−313.1 117.8(52) 67.5−298.5 174.7(19)

III

0.38−1.62 0.73(33) 0.43−0.8 0.62(5)

0.63−3.55 1.59(33) 1.05−2.14 1.36(5)

23.3−298.8 148.3(29) 122.5−229.1 183.6(5)

III

0.38−1.12 0.76(9) 0.38−1.08 0.64(26)

1.79−5.04 3.11(9) 1.33−6.75 3.09(26)

174.1−399.5 308.8(9) 33.8−287 122.6(26)

SD1

delta front (shallow water)

BD3, BD4, LS1 SD1

neritic (shallow water)

BD3, BD4, LS1 SD1

neritic (deep water)

YL1 CC1

III

III

III

II2−III II2−III

Figure 5. Frequency distribution histogram of kerogen type for Oligocene source rocks in different sedimentary facies in the eastern QDNB.

Figure 4. Maceral content triangular chart of Oligocene source rocks in different sedimentary facies in the eastern QDNB.

dominated by Pinuspollenites (6.6%−30.7%). The algae sporopollen occupy 1.9%−27.0% of the total assemblage and are mainly composed of dinoflagellate (1.0%−16.1%). In the angiosperm taxa, Quercoidites is the most common pollen (2.4%−22.0%), and dicolpopollis kockelii (1.8%−13.4%) is numerically subordinate. The pollen of deciduous broad-leaved trees (such as Caryapollenites, Salixpollenites, Ulmipollenites, Juglanspollenites, Liquidambarpollenites, Momipites coryloides, Alnipollenites, Ilexpollenites) totally occupy 2.7%− 10.3%. The Alnipollenites accounts for 0.9%−6.9% and the Ilexpollenites accounts for 0.7%−1.9% of the total assemblage. Palynological assemblages of N1sy2 rock samples in the BD3 well are represented by Polypodiaceaesporites−Polypodiisporites−Quercoidites (Figure 8). The sporopollen composition are basically similar to that of E3ls, but the main difference lies in the remarkable decrease of Pinuspollenites (2.7%−9.6%) and Alnipollenites (0.9%−2.0%), as well as the increase of Quercoidites (11.9%−23.1%) and Ilexpollenites (0.9%−5.6%).

0.10), and 0.02−0.47 (average = 0.18). However, the regular steranes in neritic source rocks in the deep-water area exhibit an “L”-shaped distribution, and the terpanes are characterized by high tricyclic terpane contents and a low abundance of oleanane and bicadinane (see Figure 7E), which are strikingly different from that in the delta front source rocks. Except for two samples, the C23TT/C30H, C27ST/C29ST, OL/C30H, and T/C30H ratios for most neritic source rock samples in the deep-water area respectively lie in the ranges of 0.45−1.24 (average = 0.67), 1.09−1.59 (average = 1.38), 0.08−0.15 (average = 0.11), and 0.03−0.05 (average = 0.04). 4.5. Palynological Assemblages. Palynological assemblages of E3ls rock samples in the BD3 well are represented by Polypodiaceaesporites-Quercoidites-Pinuspollenites (Figure 8). The pteridophyte spores and angiosperm pollen are the predominate taxa with contents that, respectively, lie in the ranges of 25.7%−51.4% and 17.3%−50.5%. The gymnosperm pollen occupies 8.7%−31.5% of the total assemblage and is E


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Table 2. Gas Chromatogram Parameters of Saturated Fractions for Oligocene Source Rocks in Different Sedimentary Facies in the Eastern QDNB well

sample

strata

depth (m)

BD2 BD2 BD2 BD2 BD2 BD2 BD2 BD2 BD4 BD4 BD4 BD4 BD4 BD4 BD1 BD1 ST1 BD4 BD4 BD4 BD4 BD4 BD4 BD4 BD4 BD5

cutting cutting cutting cutting cutting cutting cutting cutting cutting cutting cutting cutting cutting cutting sidecore sidecore cutting cutting cutting cutting cutting cutting cutting cutting cutting cutting

E3yc E3yc E3yc E3yc E3yc E3ls2 E3ls1 E3ls1 E3ls2 E3ls2 E3ls2 E3ls2 E3ls1 E3ls1 E3ls1 E3ls1 E3ls1 E3ls3 E3ls2 E3ls3 E3ls3 E3ls3 E3ls2 E3ls2 E3ls2 E3ls1

4112−4118 4140−4142 4188−4192 4216−4218 4260−4264 3502−3504 3184−3186 3708−3712 4092−4100 3878−3880 3980−3982 4020−4022 3460 3414−3416 2236−2240 2306−2310 3570−3572 4510−4558 4394−4432 4718−4760 4810−4812 4824−4828 4182−4186 4220−4258 4330 3886−3896

TOC (%)

main peak

0.36 0.32 0.36 0.39 0.35 0.35 0.29 0.29 0.54 1.07 0.8 0.93 0.88 0.88

nC20 nC23 nC20 nC19 nC21 nC27 nC25 nC25 nC22 nC23 nC25 nC25 nC27 nC21 nC21 nC29 nC27 nC18 nC19 nC20 nC18 nC14 nC21 nC21 nC23 nC21

1.60 0.63 1.02 0.54 0.58 0.81 0.68 0.78 0.38 0.76

Pr/ nC17

Ph/ nC18

1 1.06 0.97 0.96

0.64 0.49 0.42 0.67 1.85 4.52 0.89 0.78 1.58 1.2 3.13 3.91 1.41 2.24 1.96 6.75 1.62 1.09 1.74 1.73

0.2 0.23 0.21 0.29 0.39 0.67 0.28 0.17 0.27 0.23 0.54 0.76 0.60 0.51 0.52 1.52 0.61 0.54 0.83 0.74

1.03 1.05 1.12 1.11

1.22 1.36 2.32 1.06

0.25 0.29 0.54 0.24

CPI 1.11 1.12 1.1 1.1 1.07 1.18 1.26 1.15 1.04 1.07 1.08 1.07 1.1 1.61 1.31

nC21−/ nC22+

(nC21 + nC22) /(nC28 + nC29)

Pr/ Ph

face or subface

0.59 0.47 0.89 0.89 0.72 0.29 0.3 0.47 0.55 0.5 0.38 0.4 0.29 1.46 0.66 0.52 2.08 0.88 0.57 1.04 1.31 5.67 0.57 0.52 0.48 0.71

1.89 1.62 3.14 2.93 2.19 0.91 0.88 1.34 1.58 1.19 1.07 1.03 0.72 1.11 1.32 1.03 0.78 1.5 1.09 2.14 2.06 2.56 1.71 1.85 1.42 1.95

2.95 1.88 1.58 1.68 1.83 3.51 5.57 2.49 3.29 4.87 3.21 4.17 3.93 2.45 4.15 3.43 6.18 1.91 1.55 1.35 2.17 1.11 2.53 1.47 1.41 3.37

gulf gulf gulf gulf gulf littoral littoral littoral delta front delta front delta front delta front delta front delta front delta front delta front delta front neritic neritic neritic neritic neritic neritic neritic neritic neritic

Figure 6. Representative gas chromatograms of saturated fractions for Oligocene source rocks in different sedimentary facies in the eastern QDNB.

In the N1sy1 rock samples, the content of algae sporopollen increases obviously, occupying 20.5%−87%, and the contents of pteridophyte taxa, angiosperm taxa, and gymnosperm taxa decrease significantly. Palynological assemblages of E3ls rock samples in ST1 well are also represented by Polypodiaceaesporites−Quercoidites− Pinuspollenites (Figure 9). This assemblage is dominated by the angiosperm pollen (28.8%−71.6%), followed by the pteridophyte spores (21.6%−61.6%). The gymnosperm pollen occupies 1.5%−17.8% of the total assemblage and are

dominated by Pinuspollenites (1.5%−16.4%). The algae taxa are rare in the assemblage. In the angiosperma taxa, Quercoidites occupies 2.3%−34.2% of the total palynological assemblage, and the pollen of deciduous broad-leaved trees (mainly Juglanspollenites and Momipites) are totally occupying 2.7%−31.1%. Palynological assemblages of N1sy2 rock samples in the ST1 well are represented by Polypodiaceaesporites−Stenochlaena− Pinuspollenites. This assemblage is dominated by the pteridophyte spores (38.8%−70.2%), followed by the F


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Table 3. Mass Chromatograms (m/z = 191,217,412) Parameters of Saturated Fractions for Oligocene Source Rocks in Different Sedimentary Facies in the Eastern QDNBa well

sample

strata

depth (m)

BD2 BD2 BD2 BD2 BD2 BD2 BD2 BD2 BD4 BD4 BD4 BD4 BD4 BD4 ST1 BD3 BD4 BD4 BD4 BD4 BD4 BD4 BD4 BD4 BD5 YL1 YL1 YL1 YL1 CC1

cutting cutting cutting cutting cutting cutting cutting cutting cutting cutting cutting cutting cutting cutting cutting cutting cutting cutting cutting cutting cutting cutting cutting cutting cutting cutting cutting cutting cutting cutting

E3yc E3yc E3yc E3yc E3yc E3ls2 E3ls1 E3ls1 E3ls2 E3ls2 E3ls2 E3ls2 E3ls1 E3ls1 E3ls1 E3ls2 E3ls3 E3ls2 E3ls3 E3ls3 E3ls3 E3ls2 E3ls2 E3ls2 E3ls1 E3ls1 E3ls2 E3ls3 E3ls1 E3yc

4112−4118 4140−4142 4188−4192 4216−4218 4260−4264 3502−3504 3184−3186 3708−3712 4092−4100 3878−3880 3980−3982 4020−4022 3460 3414−3416 3570−3572 4012−4014 4510−4558 4394−4432 4718−4760 4810−4812 4824−4828 4182−4186 4220−4258 4330 3886−3896 3714−3726 3828−3852 3924−3984 3702−3708 3960−3970

A

B

C

D

E

F

G

faces

0.72 0.82 1.23 0.73 1.25 1.3 0.98 1.09 1.15 0.77 0.7 0.72 0.83 0.7 0.68 1.28 0.95 0.76 0.72 0.72 0.6 0.77 0.69 0.84 0.92 1.12 1.56 1.59 1.09

0.05 0.11 0.13 0.04 0.12 0.13 0.14 0.07 0.13 0.2 0.05 0.19 0.25 0.34 0.13 0.21 0.05 0.04 0.06 0.05 0.07 0.04 0.06 0.04 0.47 0.08 0.09 0.56 0.1 0.15

0.22 0.19 0.34 0.23 0.26 0.11 0.12 0.15 0.09 0.28 0.05 0.13 0.07 0.14 0.06 0.3 0.18 0.08 0.17 0.18 0.09 0.08 0.12 0.1 0.12 0.45 0.63 0.54 0.47 1.24

0.4 0.56 0.19 0.23 0.68 0.46 0.72 0.04 0.47 4.42 0.79 2.82 1.21 3.53 0.37 1.64 0.06 0.05 0.07 0.1 0.02 0.47 0.26 0.42

0.18 0.14 0.09 0.21 0.13 0.12 0.08 0.12 0.09 0.16 0.23 0.13 0.06 0.12 0.15 0.17 0.17 0.23 0.19 0.2 0.23 0.23 0.19 0.13 0.13 0.15 0.18 0.12 0.15 0.11

1.2 0.96 0.59 1.41 1.36 0.82 0.36 0.62 0.56 0.78 0.74 0.15 0.46 0.4 0.43 0.55 0.57 0.54 0.75 0.88 1.02 0.72 0.7 0.58

0.42 0.41 0.47 0.46 0.44 0.38 0.38 0.41 0.39

gulf gulf gulf gulf gulf littoral littoral littoral delta front delta front delta front delta front delta front delta front delta front delta front neritic neritic neritic neritic neritic neritic neritic neritic neritic neritic neritic neritic neritic neritic

0.03 0.04 0.05 0.04 0.92

0.55 0.51 0.58

0.4

0.38 0.46 0.41 0.43 0.41 0.42 0.42 0.42 0.41 0.4 0.39 0.21 0.22 0.21 0.17 0.38

Legend: A, C27/C29ST = C27(ααα20S + αββ20R + αββ20S + ααα20R) steranes/C29(ααα20S + αββ20R + αββ20S + ααα20R) steranes; B, OL/ C30H = oleanane/C30 hopane; C, C23TT/C30H = C23 tricyclic terpane/C30 hopane; D, T/C30H = T-bicadinane/C30 hopane; E, Ga/C30H = gammacerane/C30 hopane; F, C35/C3422S = αβ20SC35 homohopane/αβ20SC34 homohopane; G, C29ββ/(αα + ββ) = C29(αββ20R + αββ20S) steranes/C29(ααα20S + αββ20R + αββ20S + ααα20R) steranes

a

(Figure 9). These characteristics indicate the algae blooming did not occur and paleoproductivity in the delta front subface was limited during the period of E3ls. Overall, organic matter in the delta front subface of eastern QDNB mostly originated from the higher plants around the sag, and marine algae did not take the main part of organic matter composition in the epoch of E3ls. Statistics of sporopollen in the N1sy2 section of the BD3 well are similar to that of the E3ls section, but Pinuspollenites from the conifer trees and Alnipollenites from the deciduous broadleaved trees are remarkably decreased, and the algae content increases obviously in the N1sy1 section (see Figures 8 and 9). In the N1sy section of the ST1 well, Quercoidites and pollen of deciduous broad-leaved trees decrease remarkably, whereas the pteridophyte spores increase obviously. Such difference indicates that the higher plants in the N1sy epoch is not as prosperous as that in the E3ls epoch and there exist algae blooming or transgression in the late N1sy epoch, causing marine algae to be an increasing important part for the formation of source rock in the neritic face. That may also be the reason why the coal seam develops in the Oligocene epoch but is scarce in the Miocene epoch.32 5.1.2. Biomarker Analysis of Organic Matter Sources. The distribution of n-alkanes is a classical organic geochemical

angiosperm pollen (23.4%−48.7%). The gymnosperm pollen content and algae sporopollen content are similar to that of E3ls, but Quercoidites (2.0%−17.9%) and the pollen of deciduous broad-leaved trees (3.9%−19.7%) decrease remarkably (Figure 9).

5. DISCUSSION 5.1. Organic Matter Sources and Its Control on Source Rock Formation. 5.1.1. Palynological Analysis of Organic Matter Sources. The quantitative analysis of sporopollen data could approximately reflect the paleovegetation.29 In the BD3 and ST1 wells (Figures 8 and 9), sporopollen in the delta front subface of E3ls section are dominated by pteridophyte spores or angiosperm pollen. Furthermore, the conifers (Pinuspollenites), evergreen broadleaved trees (Quercoidites), deciduous broad-leaved trees all take up large proportions in the assemblages. These characteristics indicate that the paleovegetation of QDNB was a mingled forest of conifers and broad-leaved trees, as well as a mingled forest of evergreen and deciduous broad-leaved trees in the epoch of E3ls. The floating algae, which is mainly produced in situ in a marine or lake environment and reflects the paleoproductivity,30,31 occupies ∼10% in the assemblage of the BD3 well and is scarce in the assemblage of ST1 well G


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Figure 7. Representative mass chromatograms (m/z = 191, 217, 412) of saturated fractions for Oligocene source rocks in different sedimentary facies in the eastern QDNB.

are mainly terrestrial higher plants on the northern slope of Baodao sag. Delta front source rocks generally have (nC21 + nC22)/(nC28 + nC29) ratios of 3.0 (see Figure 11B, as well as Table 2). However, neritic source rocks generally have (nC21 + nC22)/(nC28 + nC29) ratios of >1.5 and Pr/Ph ratios of 3) could also be used to reflect organic matter sources.33 As is shown in Figure 10A, all of the samples from different sedimentary facies on the northern slope of Baodao sag fall into the TOM field, which means the organic matter sources H


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Figure 8. Palynological assemblages and its palaeoclimate significance for E3ls and N1sy source rocks in the BD3 well in the eastern QDNB. Legend: Thermophile sporopollen = (Alnipollenites + Juglanspollenites + Ulmipollenites + Salixpollenites + Cupuliferoipollenites + Quercoidites + Pinuspollenites + Momipites + Taxodiaceaepollenites); Philotherm sporopollen = (Liquidambarpollenites + Caryapollenites + Ilexpollenites + Polypodiaceaesporites); Hygrophyte sporopollen = (Alnipollenites + Salixpollenites + Cupuliferoipollenites + Quercoidites + pteridophyte spores).

Figure 9. Palynological assemblages and its palaeoclimate significance for E3ls and N1sy source rocks in ST1 well in the eastern QBND. Legend: Thermophile sporopollen = (Alnipollenites + Juglanspollenites + Ulmipollenites + Salixpollenites + Cupuliferoipollenites + Quercoidites + Pinuspollenites + Momipites + Taxodiaceaepollenites); Philotherm sporopollen = (Liquidambarpollenites + Caryapollenites + Ilexpollenites + Polypodiaceaesporites); Hygrophyte sporopollen = (Alnipollenites + Salixpollenites + Cupuliferoipollenites + Quercoidites + pteridophyte spores).

ratios of C27ST/C29ST and C23TT/C30H successively increase in the following order: neritic face in the shallow-water area < delta front subface < littoral face < neritic face in the deepwater area (see Figure 11, as well as Table 3). These characteristics indicate that different sedimentary facies are of different organic matter sources. Source rock in the delta front subface is of strong TOM input and weak aquatic organic matter (AOM) input, whereas source rock in the neritic face in the deep-water area is of weak TOM input and increased AOM input. The TOM input decrease gradually and the marine AOM input increase gradually from land to sea. 5.1.3. Control of Organic Matter Sources on Source Rock Formation. In order to elucidate the effect of organic matter sources on source rock formation, the correlations between TOC and some parameters mentioned above are analyzed. In

steranes are derived mainly from phytoplankton and metazoan, and C29 regular steranes mainly originate from terrestrial higher plants.40,41 Whereas C23 tricyclic terpane is often the dominant homologue of marine or lacustrine source rocks.42−44 The ratios of OL/C30H and T/C30H are often used to estimate the contribution from higher plants,45,46 whereas the ratios of C27/C29ST and C23TT/C30H could be used to determine the contribution of phytoplankton or metazoan.45,47 Source rocks from different sedimentary facies could be distinctly divided in the crossplots of C27ST/C29ST vs C23TT/C30H and OL/C30H vs T/C30H (see Figures 10C and 10D). The average ratios of T/C30H in source rocks successively decrease in the following order: delta front subface > littoral face > neritic face in the shallow-water area > neritic face in the deep-water area. However, the average I


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Figure 10. Scatter plots of organic matter source biomarker parameters for Oligocene source rocks in different sedimentary facies in the eastern QDNB.

controls the organic matter abundance of neritic source rocks. The low TOC values of littoral source rocks, as well as the poor correlations between TOC and organic matter source biomarker parameters, illustrate that the organic matter source is not the controlling factor for source rocks formation on the littoral face (Figure 12). 5.2. Paleoclimate, Paleoenvironment, and Its Influences on Source Rock Formation. 5.2.1. Paleoclimate and Its Influence on Source Rock Formation. The theoretical basis for the study of paleoclimate in palynology is the botanical affinities with the sporopollen and the climate, as well as the ecological environment.48 Pinuspollenites belong to conifer trees, which generally grow in the upland forests of the northern temperature zone in warm or cold climates.49 Alnipollenite is considered as vegetation that mainly distributed in warm humid condition of the temperate or northern temperate zone.49 Juglanspollenites and Momipites are also the vegetation of the temperature zone.50 However, the ferns such as Polypodiaceaesporites are mainly distributed in the tropical or subtropical zone and prefer shady and humid habitats.49 Ilexpollenites and Quercoidites were mainly distributed in the subtropical zone.49 As is seen in Figures 8 and 9, in the E3ls section, the thermophile, philotherm, and hygrophyte in the BD3 well and ST1 well respectively occupy 33.3%, 42.6%, and 54.1% and 43.2%, 36.8%, and 57.6%, on average. Based on the type of palynological climate and humidity proposed by Zhao,29 the paleoclimate of E3ls in the QDNB was of the subtropical warmhumid type. Under such climate conditions, higher plants flourished on land and could provide a large amount of terrigenous organic matter for Oligocene source rock formation. However, in the section of N1sy, the values of

Figure 11. Chart of biomarker parameter average values for Oligocene source rocks in different sedimentary facies in the eastern QDNB.

the delta front source rocks, TOC generally increase as the Pr/ Ph ratios increase (see Figure 12A) and decrease as (nC21 + nC22)/(nC28 + nC29) ratios, C27/C29ST ratios increase (see Figures 12B and 12E), higher ratios of OL/C30H or T/C30H are often meant to have higher TOC values (see Figures 12C and 12D). These correlations indicate that the TOM input controls the organic matter abundance of delta front source rocks, and the greater the TOM input, the higher the TOC of delta front source rocks. However, there is positive correlation between TOC and (nC21 + nC22)/(nC28 + nC29) ratios in neritic source rocks in the shallow-water area (Figure 12B), and positive correlations between TOC and C27/C29ST ratios, C23TT/C30H ratios in neritic source rocks in the deep-water area (Figure 12E, 12F), which indicates that the input of AOM J


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Figure 12. Correlations between TOC and organic matter source biomarker parameters for Oligocene source rocks in different sedimentary facies in the eastern QDNB.

intercept on the plot of TS vs TOC. As shown in Figures 13A, 13B, and 13C, there are obvious positive correlations between TOC and TS in source rocks from different sedimentary facies of the BD3, SD1, and YL1 wells, and the intercepts on the TS coordinate axis are negative or very small positive values. The positive correlations and low intercept values indicate that the enrichment of sulfur is closely related to the total organic matter content, which could be explained by the redox reaction of sulfate with organic matter in the early diagenetic stage. Thus, the water condition of different depositional environment in the E3ls period exhibited as oxidization or weak oxidization. C35/C3422S hopane is an effective oxicity parameter; it is generally believed that a high abundance of C35 homohopane is an indication of a strong reductive marine sedimentary environment.33 As shown in Figure 13D and Table 3, the C35/C3422S ratios of samples in the delta front subface lie in the range of 0.15−0.78, with a mean of 0.51, whereas the C35/ C3422S ratios of samples in the neritic face lie in a range of

thermophile, philotherm, hygrophyte change to 29.2%, 49%, and 63% in the BD3 well and 25.1%, 56.7%, and 62.5% in the ST1 well. The thermophile plants such as Pinuspollenites and Alnipollenites are remarkably decreased, whereas the philotherm plants increased obviously, which means the paleoclimate of N1sy is hotter than that of E3ls. Generally, the paleoclimate of N1sy epoch is not as good as that of E3ls epoch for higher plants growth. 5.2.2. Redox Condition and Its Influence on Source Rock Formation. The total sulfur (TS) content in the sedimentary rocks characterizes the abundances of organic sulfur, pyrite sulfur, and sulfate sulfur, as well as elemental sulfur, and is often used as an indication to reflect the sedimentary environment.51 According to studies,52−54 marine sediments deposited in the oxygenated conditions often show strong positive correlation between TOC and TS and a zero intercept on the crossplot of the two variables. However, marine sediments deposited in the anoxic or euxinic environment will show no correlation between TS and TOC and a positive K


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Figure 13. (A−C) Carbon−sulfur relationships and (D) correlation between TOC and C35/C3422S for Oligocene source rocks in different sedimentary facies in the eastern QDNB.

Figure 14. Correlations of biomarker parameters reflecting the depositional environment and its influence on Oligocene source rocks formation in the eastern QDNB.

ratios and TOC in the neritic samples (see Figure 13D). This phenomenon indicates that the water redox condition has minor influence on organic matter enrichment in the delta front or the littoral environment, whereas the weak oxidization water condition was favorable for organic matter enrichment in the neritic face. 5.2.3. Water Salinity and Its Influence on Source Rock Formation. Water salinity can be assessed utilizing gammacerane content (Ga/C30H).55,56 As shown in Figure 14A, the neritic source rocks have higher Ga/C30H ratios (lie in the range of 0.11−0.23, average = 0.19), compared with that in the delta front subface (lie in the range of 0.06−0.23, average = 0.14), and there exists positive correlation between Ga/C30H and C35/C3422S ratios. These features indicated sedimentary water in the delta front subface was fresh or brackish water, whereas sedimentary water in the neritic face was brackish or

0.51−1.02, with a mean of 0.67. Source rocks in the neritic face have slightly higher C35/C3422S ratios than that in the delta front subface, which means the degree of water reduction is heavier in the neritic face, compared with that in the delta front subface. This conclusion could also be evidenced by the lower Pr/nC17, Pr/Ph ratios in the neritic source rocks, compared with that in the delta front subface. The littoral source rocks display a wide range of Pr/nC17, Pr/Ph, and C35/C3422S ratios, which indicate that the water redox condition fluctuates in different littoral subfacies. Thus, sedimentary water in the delta front subface was of strong oxidization, whereas sedimentary water in the neritic face was of weak oxidization or weak reduction. Besides that, there is no correlation between C35/C3422S ratios and TOC in the delta front or littoral source rock samples, but mild positive correlation between C35/C3422S L


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Figure 15. Developmental models of Oligocene source rocks in the eastern QDNB.

C29ST (average = 1.38), (nC21 + nC22/nC28 + nC29) (average = 1.8). Neritic source rocks with high organic matter abundance are greatly controlled by AOM input and also influenced by the weak oxidation brackish or saline water environment. Source rock developed in the littoral face is generally of poor to medium quality and is seldom influenced by organic matter sources. Thus, we considered the littoral source rock model as a destructive model, which is unfavorable for organic matter enrichment.

saline water. The water salinity is closely related to the water redox conditions. Besides that, there is mild positive correlation between Ga/ C30H ratios and TOC in the neritic source rocks (see Figure 14B), which indicates that the brackish or saline water in the neritic face is favorable for organic matter enrichment. 5.3. Source Rock Depositional Model. The Oligocene source rocks in the eastern deep-water area of QDNB were deposited in the basin rifting stage. During the sedimentary period, the climate was subtropical warm humid. There developed Pinus in the surrounding mountains, deciduous and evergreen broad-leaved trees in the plain, and pteridophyte plants in the low land habitats, but floating algae did not flourish in the water. These features of climate and vegetation caused the terrestrial higher plants to be the main organic matter source. However, source rocks in different sedimentary facies have different hydrocarbon potential, different biomarker features, and different controlling factors. Based on these differences, the organic matter enrichment model of Oligocene source rocks could be divided into a transitional delta front model and a neritic model, as shown in Figure 15. Source rock of the delta front model is of medium to high organic abundance (TOC lie in the range of 0.19−5.84 wt %, with a mean of 1.07 wt %; the hydrogen index lies in the range of 39.3−313.1, with a mean of 133.1 mg/g TOC), and gasprone-type kerogen. The biomarkers are characterized by high ratios of Pr/Ph (average = 3.85), OL/C30H (average = 0.19), T/C30H (average = 1.91), and low ratios of C23TT/C30H (average = 0.14), C27ST/C29ST (average = 0.85), (nC21 + nC22/nC28 + nC29) (average = 1.1). Delta front source rock with high organic matter abundance is greatly controlled by TOM input. Whereas source rock of neritic model is of medium organic abundance (TOC lie in the range of 0.38− 1.12 wt %, with a mean of 0.67 wt %; hydrogen index lie in the range of 33.8−399.5, with a mean of 170.4 mg/g TOC) and kerogen type prone to generate both oil and gas. The biomarkers are characterized by low ratios of Pr/Ph (average = 1.87), OL/C30H (average = 0.11), T/C30H (average = 0.04), and high ratios of C23TT/C30H (average = 0.67), C27ST/

6. CONCLUSIONS (1) Oligocene source rocks in different sedimentary facies in the eastern QDNB are of different hydrocarbon potential: the delta front source rock is of medium to high organic abundance and gas-prone-type kerogen, whereas the neritic source rock is of medium organic abundance and kerogen type prone to generate both oil and gas, the littoral source rocks is of poor to medium organic abundance and gas-prone-type kerogen. (2) Biomarker features of Oligocene source rocks in delta front subface and neritic face in the eastern QDNB are distinctly different: delta front source rock is characterized by high ratios of Pr/Ph, OL/C30H, T/C30H, and low ratios of C23TT/C30H, C27ST/C29ST, (nC21 + nC22/nC28 + nC29), whereas neritic source rock is characterized by low ratios of Pr/Ph, OL/C30H, T/C30H, and high ratios of C23TT/C30H, C27ST/C29ST, (nC21 + nC22/nC28 + nC29). During the sedimentary period of E3ls, the subtropical warm humid paleoclimate provided nice conditions for higher plant growth, causing the TOM to be the main organic matter source. The delta front subface was of oxidation fresh or brackish water environment, whereas the neritic face was of weak oxidization brackish or saline water environment. (3) The delta front model with strong TOM input and the neritic model with weak TOM input were presented to explain the Oligocene source rocks depositional process. Organic matter abundance of source rock in the delta front model was controlled by TOM input, whereas organic matter abundance of neritic model was controlled by AOM input and also M


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influenced by the weak oxidization brackish or saline water environment.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail (P.W.): [email protected]. *E-mail (D.H.): [email protected]. ORCID

Piao Wu: 0000-0002-5345-5450 Dujie Hou: 0000-0003-2001-4082 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank the Zhanjiang Branch of CNOOC, Ltd. for kindly providing part of the data in this study. We also would like to thank Weilai Zhang (China University of Petroleum) for GC-MS analysis of the studied samples and Dr. Chen Xiong for providing valuable comments that helped to greatly improve the quality of the manuscript.

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