Geochemical evidence of organic matter source input and depositional environments in the lower and upper Eagle Ford Formation, south Texas

Organic Geochemistry 98 (2016) 66–81

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Geochemical evidence of organic matter source input and depositional environments in the lower and upper Eagle Ford Formation, south Texas Xun Sun a, Tongwei Zhang a,⇑, Yongge Sun b, Kitty L. Milliken a, Dayang Sun b a b

Bureau of Economic Geology, University of Texas at Austin, 10100 Burnet Rd., TX 78758-4445, USA Department of Earth Science, Zhejiang University, Hangzhou 310027, PR China

a r t i c l e

i n f o

Article history: Received 24 November 2015 Received in revised form 30 May 2016 Accepted 31 May 2016 Available online 2 June 2016 Keywords: Eagle Ford shale Biomarkers Redox condition Organic matter conversion Organo-sulfur Thermal maturity

a b s t r a c t Late Cretaceous source rocks in the south Texas Eagle Ford Formation were characterized based on geochemical features of the Iona-1 core, which penetrates the lower and upper Eagle Ford Formation. This set of immature samples provides a detailed record of the variation of organic matter (OM) sources across different depositional facies. Our results show that organic matter rich shales are dominated by marine Type II kerogen. Lower Eagle Ford (LEF) contains high-sulfur kerogen (Type II-S) with S/C > 0.04, and a minor contribution of terrestrial OM is evident in the upper Eagle Ford (UEF). Combined geochemical data from Iona-1 core samples based on bulk geochemical rock properties, residual oil chromatography and biomarker analyses reveal that the total organic carbon content and extractable bitumen content in the LEF are higher than in the UEF; redox-sensitive biomarkers demonstrate that sulfur-rich photiczone anoxia/euxinia developed in the LEF and that persistent oxygenation occurs during deposition of the UEF. Sulfur-rich kerogen in the LEF, in turn, generates medium/high sulfur-containing oils (up to 2.6%). As a consequence of the elevated sulfur contents in the LEF, oil/bitumen generation initiated earlier and at lower maturity levels than in the UEF. High levels of anomalous 22S/(22S + 22R) homohopane ratios and 20S 5a,14a,17a-steranes have been observed in the LEF interval, which is controversial because of the low maturity level indicated by most bulk geochemical thermal maturity parameters, including Tmax-calculated %Ro (0.45) and Ts/(Ts + Tm), as well as C29Ts/(C29Ts + C29) hopane and C29bb/(bb + aa) sterane ratios. These anomalously high thermal maturity parameters are more likely caused by enhanced isomerisations due to high organic-sulfur content under reducing depositional environments in the LEF rather than by thermal maturation. Hypothetical diagenetic pathways for the sulfurisation of homohopanoids and steroids are proposed to explain their abnormal isomerisations in the LEF. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction In sedimentary analysis, the redox condition of the depositional setting is an important control on organic matter (OM) preservation and alteration, resulting in variation of OM composition across depositional facies. This difference, in turn, greatly affects OM conversion under geological conditions. For instance, the onset temperature of S-rich Type II-S kerogen conversion to oil is significantly lower than that of normal Type II kerogen because the carbon–sulfur (CAS) bond is relatively easier to break than the carbon–carbon (CAC) bond (Lewan et al., 2006). Edman and Pitman (2010) have documented that the OM deposited with the Eagle Ford shales and marls of the First Shot Field appears to be a Type II kerogen deposited in a marine environment under anoxic

⇑ Corresponding author. Tel.: +1 512 232 1496. E-mail address: [email protected] (T. Zhang). http://dx.doi.org/10.1016/j.orggeochem.2016.05.018 0146-6380/Ó 2016 Elsevier Ltd. All rights reserved.

conditions; it is likely that the Eagle Ford kerogen that generated the medium/high (up to 2.6%) sulfur oils of this field also has elevated sulfur contents. The elevated sulfur contents in the First Shot Eagle Ford kerogen suggest that oil generation may have begun earlier and at lower maturity levels than is characteristic of more typical Type II kerogens (Edman and Pitman, 2010). According to Little et al. (2012), oils in the Austin Chalk Formation of the Pearsall Field (southwest of the San Marcos Arch) are in the 30–40 API gravity range (The American Petroleum Institute gravity, is an inverse measure of a petroleum liquid’s density relative to that of water. If its API gravity is > 10, the oil is lighter and floats on water; if < 10, it is heavier and sinks), but have moderately higher sulfur contents than those of Giddings Field (northeast of the San Marcos Arch), whose oils are in the same API gravity range (Fig. 1c). It also has been documented that lithofacies changes between the Boquillas Formation and the Eagle Ford Group corresponding to different organic sulfur content, resulting in further produced oil compositional differences between these two fields (Little et al., 2012).

X. Sun et al. / Organic Geochemistry 98 (2016) 66–81

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Fig. 1. (a) Map showing the general region of the of the Iona-1and also well locations of 100819-3, 100819-5; (b) locations of Portland-1 and Iona (yellow square is the outline of map a); (c) formation fields (Pearsall and Giddings) (Little et al., 2012; Eldrett et al., 2014). The outline in the map is the resource assessment area by BEG Shale Play Team according to Hammes et al. (2016). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

To date, no detailed differentiation of the subunits of the Eagle Ford Group in terms of organic source and sulfur content has been presented. It is important to document which unit or units in the Eagle Ford Formation are favorable to S-rich kerogen formation, a subject which has not been addressed because of the lack of availability of a complete record of immature samples that cover both the UEF and LEF intervals. The differences in redox condition between the UEF and the LEF have been documented by high resolution records of stable carbon isotope ratios from OM, trace metal (TM), and biostratigraphic analysis of samples from the Iona-1 well (Eldrett et al., 2014). These results show a decoupling of the global positive d13C carbon isotope excursion (CIE) and OM preservation at a major site of organic-rich sediment deposition, as well as showing, paradoxically, a more oxygenated phase within Oceanic Anoxic Event 2 (OAE-2). Findings are also consistent with the USGS Portland-1 core data (Fig. 1b), which show restricted circulation and anoxia prior to the CIE, comparable to the southern Tethys (Trabucho Alexandre et al., 2010), suggesting that OM was not well sequestered globally in oceans during OAE-2, manifesting more complex carbon cycling during the Cenomanian–Turonian. Prior to the OAE2 event, the source rock in the LEF was enriched with redoxsensitive trace metals (TMs), whereas the CIE interval marking OAE-2 exhibits limited TMs (Eldrett et al., 2014). The LEF interval below the CIE is particularly enriched in Mo, which is thought to require the presence of free H2S (Helz et al., 1996), consistent with anoxic bottom waters (Eldrett et al., 2014). The coeval sharp increase in bioturbation and the appearance of diverse and abundant benthic foraminifera correspond with the benthic oxic zone, which is distributed in the lower part of the UEF (Keller and Pardo, 2004). However, no organic geochemical evidence is available to support or verify this observation, in particular, the difference in redox conditions that affects OM preservation. In this study, we analyzed the geochemical character of OM in samples from the Eagle Ford Formation of the Iona-1 core, which is the same core documented in Eldrett et al. (2014). The Iona-1

well contains a complete record of the Boquillas (Eagle Ford) Formation (Fig. 1a) and comprises organic-rich marls and limestones interbedded with bentonite layers. We conducted a series of geochemical analyses including TOC, Rock-Eval pyrolysis yields, kerogen elemental analyses, residual oil extraction and quantification, saturates, aromatics, resins, and asphaltenes (SARA) separation and quantification, and biomarker analyses. Our main objectives were: (1) to identify factors controlling TOC enrichment in the Eagle Ford Formation; (2) to constrain the difference in OM source input and depositional conditions between upper and lower Eagle Ford subunits; and (3) to quantify the difference in OM conversion to bitumen or oil at an immature stage. Our experimental results, which provide insights into variations of OM type with respect to lithofacies, allow a better understanding of OM accumulation and the nature of redox conditions during deposition of shale in the Eagle Ford Formation. This interpretation of the organic-rich Eagle Ford shale (EFS) could drive future exploration strategies for the formation.

2. Materials and methods 2.1. Sample selection 2.1.1. Geologic setting Since the drilling of the STS-241 #1H discovery well by Petrohawk in the fall of 2008 in what would become the Hawkville Field in LaSalle County, Texas, there has been considerable interest by the oil and gas industry in the Eagle Ford Formation (Durham, 2010). The Eagle Ford Formation, one of the most active U.S. shale plays, is a self-sourced oil reservoir that produces oil, gas condensate and dry gas. Maps generated by Tian et al. (2013) show the average gas-oil ratio (GOR) in the first 3 months of production and identify four fluid regions, including black oil, volatile oil, gas condensate and dry-gas zones. The thermal maturity and structural setting of the EFS strongly influence fluid types and

TOC: total organic carbon; S1 and S2 are obtained from Rock-Eval analysis; S1: volatile hydrocarbon; S2: degraded hydrocarbon from kerogen; HI: hydrogen index, S2  100/TOC; OI: oxygen index, S3  100/TOC; PI: production index, S1/[S1 + S2].

0.03 0.03 0.03 0.04 0.05 0.04 0.04 18.9 23.3 26.4 29.7 43.5 35.2 28.1 12.7 11.6 8.6 15.1 7.6 9.0 6.3 686.1 711.4 737.0 738.6 779.3 780.0 738.2 0.5 0.4 0.5 0.4 0.5 0.4 0.5 424 422 424 420 425 421 423 0.7 0.6 0.6 0.8 0.9 0.7 0.8 39.9 38.8 47.2 40.0 92.0 62.1 96.7 1.1 1.3 1.7 1.6 5.1 2.8 3.7 5.8 5.5 6.4 5.4 11.8 7.9 13.1 43.9 48.3 56.6 46.9 46.4 59.6 28.9 11 10 8 7 4 2 1

115.5 122.9 128.9 133.7 139.8 149.5 151.9

16.5 18.6 13.3 11.8 12.3 16.2 16.2 21.0 21.0 28.2 28.0 26.3 16.9 14.8 660.3 653.2 491.9 450.0 510.1 724.8 664.5 0.4 0.4 0.5 0.5 0.5 0.4 0.4 422 421 424 428 426 417 419 0.6 0.7 0.7 0.5 0.6 0.8 0.8 18.0 22.5 11.5 8.4 11.6 32.2 33.7 0.5 0.6 0.3 0.2 0.3 0.7 0.8 2.7 3.4 2.3 1.9 2.3 4.4 5.1 54.5 70.4 71.9 65.6 60.7 62.6 48.5

Lower EF

(m)

23 22 20 19 17 16 12 Upper EF

56.6 57.1 79.7 81.3 92.8 98.1 110.0

S1/TOC Norm. Oil Content Oxygen Index (S3  100/TOC) Hydrogen Index (S2  100/TOC) Calculated %Ro (RE Tmax) Rock-Eval Tmax (°C) Rock-Eval S3 (mg CO2/g) Rock-Eval S2 (mg HC/g) Rock-Eval S1 (mg HC/g) Leco TOC (wt%) Percent Carbonate (wt%) Depth

2.2.1. Solvent extraction and quantification About 3–5 g of core sample powders were loaded into thimbles and extracted (3 h) using a FOSS Soxhlet extraction system with CH2Cl2. The extraction procedure is as follows: (1) submerge thimbles into a 200 ml aluminium beaker loaded with 40 ml CH2Cl2, and refluxed at 85 °C (3 h); (2) lift thimbles in an upward position and rinse 30 min with CH2Cl2; (3) close cooling circulator valve and air-dry thimbles (10 min); (4) remove thimbles from extraction system and leave them in a fume hood to dry completely; (5) weigh the aluminium cup with extracts after solvent evaporates completely; (6) transfer extracts to 2 ml centrifuge vials; (7) centrifuge vials to separate asphaltenes, using excess pentane addition

Sample ID

2.2. Experimental methods

Table 1 Rock-Eval bulk geochemical data for samples from Iona-1 core well.

2.1.2. Sample information Of the 17 core samples analyzed, 6 came from the LEF and 11 from the UEF. The average TOC content of the LEF (7.8%) is much higher than that of the UEF (3.1%) (Table 1; Fig. 2a). Additional background stratigraphic and sedimentological information on the Eagle Ford Group and adjacent sections of the Maverick Basin can be found elsewhere (Pessagno, 1969; Denne et al., 2013; Lowery et al., 2014; Eldrett et al., 2015; Milliken et al., 2016).

Production Index (S1/[S1 + S2])

production rate, and the vertical and lateral heterogeneity in reservoir properties of the EFS play a critical role in the regional extent of fluid types (Tian et al., 2013). The Upper Cretaceous EFS is an oil/gas shale play in the Maverick Basin and the adjacent western San Marcos Arch in south Texas, potentially extending at least 400 km farther northeastward to the East Texas Basin (Hentz and Ruppel, 2010). The EFS is an organicrich calcareous shale, in places transitioning to an organic, argillaceous lime mudstone. Most, if not all, of the oil found in the Austin Chalk and Buda Formations is generally thought to be derived from the EFS (Sassen, 1990; Kennicutt et al., 1992). The Eagle Ford succession comprises two depositional units in the Maverick Basin/Rio Grande Embayment area: (1) a lower unit containing marine transgressive and condensed intervals dominated by dark welllaminated shales, and (2) an upper regressive unit consisting of thinly interstratified shales, limestones and carbonaceous quartzose siltstones (Dawson, 1997; Dawson and Almon, 2010). The more calcareous, upper EFS exists almost solely south of the San Marcos Arch, whereas the more organic-rich LEF unit, which is characterized by generally high gamma-ray values, extends continuously from south Texas across to the northeast flank of the San Marcos Arch (Hentz and Ruppel, 2010). Detailed sedimentological and geochemical analyses of the Eagle Ford Group have documented substantial lithologic and organic geochemical variability that includes differences in TOC and kerogen types (Liro et al., 1994; Grabowski, 1995; Robison, 1997). The Iona-1 well is located in northern Kinney County, Texas, at the north margin of Maverick Basin and the southern gateway of the Cretaceous Western Interior Seaway (KWIS) near the margin of the Gulf of Mexico carbonate shelf. The well contains a complete record of the Boquillas (Eagle Ford) Formation (Fig. 1). The Eagle Ford section of the Iona Core is mainly composed of interbedded marls (74%), limestones (20%), and bentonites (6%). The UEF section is dominated by limestones, whereas the LEF section is enriched in marls (Eldrett et al., 2015). The marls are dark organic-rich laminae which consist of OM, sparse planktonic foraminifera and common fecal pellets, and show less bioturbation. The limestones show a wide morphological variability including nodular, lenticular and sheet-like facies (Minisini et al., 2014). Lithologic variation in the Iona core displays the typical differences between the LEF and UEF.

0.02 0.03 0.03 0.03 0.02 0.02 0.02

X. Sun et al. / Organic Geochemistry 98 (2016) 66–81

FM

68

X. Sun et al. / Organic Geochemistry 98 (2016) 66–81

69

Fig. 2. Vertical variations of geochemical parameter samples analyzed from UEF and LEF in Iona-1 core well. (a) total organic carbon (TOC wt%), (b) extractable organic matter (mg/g rock), (c) hydrogen index (HI, mg HC/g TOC, (d) ratio of pristane vs phytane, (e) ratio of dibenzothiophene vs phenanthrene, (f) ratio of aryl isoprenoids C13–17 vs phenanthrene. (Subunits of upper and lower Eagle Ford after Eldrett et al. (2015). Oceanic Anoxic Event 2 (OAE-2); Cenomanian–Turonian boundary (CTB)).

at room temperature; and (8) collect pentane solution after separation of asphaltenes for column separation of maltenes.

2.2.2. Small-scale column separation and SARA quantification Following a standard rapid small-scale column-separation procedure (Bastow et al., 2007), small-scale columns were prepared in disposable 5.0 ml serological pipettes by filling with silica gel activated at 120 °C overnight. Saturated hydrocarbons were eluted using 1.8 ml of pentane; aromatic hydrocarbons were eluted using a 1.8 ml C5H12 and CH2Cl2 mixture (7:3, v:v); and the polar fraction was eluted using a 1.8 ml CH2Cl2 and CH3OH mixture (9:1, v:v). The asphaltene fraction, saturate, aromatic, and polar eluents were weighed after the solvent evaporated completely.

2.2.3. GC–MS analyses for saturates and aromatics Gas chromatography–mass spectrometry (GC–MS) was performed on a Shimadzu GC-2010 connected to a Shimadzu QP2010S mass spectrometer, to determine the distributions of n-alkanes, isoprenoids, terpenoids and steranes. The GC was equipped with 30 m  0.25 mm i.d. fused-silica Restek Rxi-5 ms capillary column coated with 0.25 lm thick diphenyl dimethyl polysiloxane. For saturated hydrocarbon fraction analysis, the starting temperature of the GC oven was 50 °C, programmed to 140 °C at 12 °C/min and then from 140 °C to 310 °C at 3 °C/min, and held for 15 min at 310 °C. For aromatic hydrocarbon fraction analysis, the starting temperature of the oven was 50 °C, programmed to 310 °C at 3 °C/min, and held for 15 min at 310 °C. The column effluent was monitored in either full-scan or selected ion monitoring (SIM) mode. To improve the signal-to-noise ratio, hopanes and steranes were measured in SIM mode using m/z 191 for terpanes, m/z 217, 218 and 231 for steranes and m/z 133 and 134 for aryl isoprenoids at a scan rate of 0.8 s/scan. Compounds were identified using published mass spectra and retention times based on NIST 08 software. The relative abundance of biomarkers was determined from peak areas in the relevant mass chromatograms.

2.2.4. Kerogen elemental analyses The kerogen was isolated from the pulverized rock by a series of acid treatments based on the procedure in Lewan (1986): carbonates were removed with HCl (6 M) at 60 °C and then concentrated HCl at 100 °C, followed by addition of 48% HF (100 °C) to remove silicates. Repeated washing with distilled water was carried out until the solution reached neutral pH. Pyrite was removed by a hot, acidified Cr(II) solution in a nitrogen atmosphere according to the method of Tuttle et al. (1986). C, H and N analyses of the samples were conducted using a Perkin Elmer 2400 elemental analyzer equipped with a thermal conductivity detector. Determination of sulfur was done via an oxygen-flask combustion method and subsequent gravimetric titration by BaCl2 solution according to the method of Levaggi and Feldstein (1963).

3. Results 3.1. TOC and Rock-Eval pyrolysis Rock-Eval pyrolysis was conducted by GeoMark’s Rock Source Laboratory, Houston. Rock-Eval data and parameters such as hydrogen index (HI), oxygen index (OI), and production index (S1/[S1 + S2]) are listed in Table 1. The TOC content of the 17 samples ranges from 0.4–13.6%; the average TOC content (7.8%) of the LEF interval is much higher than that of the UEF (3.1%). The HI value for the LEF ranges from 686–780 mg HC/g TOC, with an average value of 739 mg HC/g TOC. The HI value for the UEF ranges from 450–725 mg HC/g TOC, with an average value of 594 mg HC/g TOC. LEF HI values are higher than those of the UEF, suggesting the greater hydrocarbon generation potential of the LEF. In Fig. 3a, a plot of HI vs OI, all samples from the LEF interval and some samples from the UEF interval fall in the threshold area. Three UEF samples, whose OI values are the same as the other samples but with HI values dropping to around 450 mg HC/g TOC, fall in the mixed Type II/III kerogen area, probably indicating varied OM type within the UEF interval. The rest of the samples from LEF and UEF fall along the Type I kerogen trendline which is unex-

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X. Sun et al. / Organic Geochemistry 98 (2016) 66–81

Fig. 3. Plots of (a) hydrogen index vs oxygen index, and (b) hydrogen index vs Tmax for studied Eagle Ford Shale samples from the Iona-1 well. Solid curves of I, II, and III represent typical Type I, Type II, and Type III kerogen.

pected for this OM type (i.e., marine source). This probably results from relatively low OI for these samples. A reasonable explanation of low OI is substitution of sulfur for oxygen in the kerogen for those samples. The Type II-S kerogen of Montery Shale is an example where the HI vs OI plot falls along the Type I kerogen trendline (Williams, 1984). In the plot of HI vs Tmax (Fig. 3b), the LEF samples are near the typical Type II kerogen curve, while the UEF samples show variable degrees of mixing of Type II and Type III kerogen, but are dominated by Type II kerogen. Three outlier values from the UEF appear at slightly higher Tmax values than all the other samples. Because these three samples from the UEF are stratigraphically younger than the LEF samples (Table 1), the slightly elevated Tmax values are probably due to the variation in OM type. The Tmax value, which is generally considered a thermal maturity indicator, ranges from 417–428 °C, with an average of 422 °C. The production index (S1/[S1 + S2]) is < 0.05, and S1/TOC is < 44 mg HC/g TOC. These values indicate that the OM conversion to oil and gas is low and that the OM is at the immature stage (Fig. 3b). The calculated vitrinite reflectance values (%Ro) based on Tmax values according to the formula of Jarvie et al. (2001) ranges from 0.38–0.54, with an average of 0.45. In the Tmax to vitrinite reflectance conversion scheme of Jarvie et al. (2001), the %Ro calculation from Tmax can be applied to Type II and III kerogens, but not Type I kerogen. In our study, we simply assume Type II kerogen and calculate the %Ro values from Tmax (Jarvie et al., 2001). Vertical variations of geochemical parameters in the Iona-1 core well are presented in Fig. 2. The compositional differences between

the UEF and LEF and the potential causes of the differences are discussed below. 3.2. Kerogen C, H, N, S elemental compositions C, H, N, S elemental analyses of the kerogen isolated from four samples (two from LEF and two from UEF) were conducted and the results are listed in Table 2. The H/C atomic ratio is from 1.19–1.30 with an average of 1.27, and it is slightly higher for the LEF than the UEF. In contrast, S/C atomic ratios for two LEF kerogen samples are high and range from 0.04–0.065, and those for the two UEF kerogen samples are obviously low and vary from 0.016 to 0.020. The LEF kerogen contains high sulfur and belongs to Type II-S kerogen, and the UEF kerogen belongs to low-sulfur kerogen according to the criteria of Durand and Monin (1980). Sulfur content in kerogen plays an important role in OM accumulation, preservation and the early conversion (Baskin and Peters, 1992). 3.3. Residual oil and SARA quantification Table 3 gives the residual oil and SARA measurements for the sample set. The distribution of saturate, aromatic, polar and asphaltene fractions for the UEF and LEF from the Iona-1 shale samples are presented in a ternary plot (Fig. 4). In both the UEF and LEF intervals, residual oil is dominated by polar and asphaltene components; a high yield of polar compounds is typical of lowmaturity OM (Tissot and Welte, 1984). The LEF is slightly more

Table 2 C, H, N, S elemental compositions of the kerogens isolated from the Eagle Ford core samples in the Iona-1 well. FM

Sample

Depth (m)

TOC (wt%)

C%

H%

N%

S%

Atomic Sorg/C

Atomic H/C

Upper EF

22 21

57.1 69.9

3.44 1.53

71.38 75.38

7.72 7.48

2.99 3.21

3.72 3.13

0.020 0.016

1.30 1.19

Lower EF

4 1

139.9 151.9

11.8 13.1

66.62 72.74

7.19 7.81

2.43 2.58

11.59 7.71

0.065 0.040

1.30 1.29

71

X. Sun et al. / Organic Geochemistry 98 (2016) 66–81 Table 3 Basic information and bulk properties of residual oil samples analyzed at Eagle Ford. FM

Sample

Depth (m)

TOC (wt%)

EOM/rock (mg/g)

EOM/TOC (mg/g)

SAT %

ARO %

NSO %

Asph %

SAT/ ARO

SAT/TOC (mg/g)

ARO/TOC (mg/g)

NSO/TOC (mg/g)

Asph/TOC (mg/g)

Upper EF

22 21 20 19 17 16 15 14 13 12

57.1 69.9 79.7 81.3 92.8 98.1 99.8 104.8 106.4 110.0

3.4 1.5 2.3 1.9 2.3 4.4 2.0 1.7 2.9 5.1

1.7 1.4 1.7 1.3 1.5 2.0 1.3 0.7 1.4 2.9

49.1 94.1 72.2 69.1 64.2 45.6 62.8 42.2 47.4 56.7

12.0 26.5 18.8 17.7 8.3 12.2 12.0 5.3 13.7 12.2

12.7 16.5 14.7 13.4 7.5 18.1 8.0 2.7 13.7 16.9

33.9 37.5 34.4 30.5 20.0 36.6 28.0 15.0 22.6 38.0

35.4 19.5 32.1 38.4 53.3 30.7 53.3 74.3 54.0 28.2

0.9 1.6 1.3 1.32 1.1 0.7 1.5 2.0 1.0 0.7

5.9 25.0 13.5 12.2 5.4 5.6 7.5 2.2 6.5 6.9

6.2 15.5 10.6 9.3 4.8 8.2 5.0 1.1 6.5 9.6

16.7 35.3 24.8 21.1 12.8 16.7 17.6 6.3 10.7 21.6

17.4 18.4 23.2 26.5 34.2 14.0 33.5 31.3 25.6 16.0

Lower EF

11 10 8 7 4 2 1

115.5 122.9 128.9 133.7 139.9 149.5 151.9

5.8 5.5 6.4 5.4 11.8 8.0 13.1

4.0 5.2 7.2 6.4 22.6 9.7 13.0

69.0 95.3 112.8 117.4 191.6 122.0 99.3

21.4 10.6 9.1 8.3 6.1 7.2 12.5

17.3 8.6 23.1 11.6 11.9 13.1 18.4

36.4 16.3 38.9 23.4 14.7 22.7 33.1

20.0 62.5 21.7 48.5 53.1 47.1 36.3

1.2 1.2 0.4 0.7 0.5 0.6 0.7

14.7 10.1 10.3 9.7 11.7 8.8 12.4

11.9 8.2 26.0 13.6 22.9 16.0 18.2

25.1 15.6 43.8 27.5 28.3 27.7 32.9

13.8 59.5 24.5 56.9 101.7 57.4 36.0

EOM: extractable organic matter.

Fig. 4. SARA separation showing the distribution of saturate, aromatic, polar, and asphaltene fractions for the UEF and LEF from Iona-1.

enriched in the aromatic, polar and asphaltene fractions (Table 3), but there is little difference in the proportion of saturate fractions between the UEF and the LEF. The LEF has more extractable OM (EOM)/TOC than the UEF does. The average EOM/TOC is 58.2 mg/g TOC for the UEF and 105 mg/g TOC for the LEF (Table 3), which indicates that either OM conversion to bitumen is greater in the LEF than in the UEF, or oil-prone maceral content is proportionally greater in the LEF relative to the UEF.

with a slightly even carbon-numbered preference, indicating low-maturity OM (Fig. 5). Pristane and phytane are present at abnormally high values in both of the UEF and LEF intervals, resulting in exceptionally high Pr/n-C17 and Ph/n-C18 ratios (Table 4). The relatively high concentration of biomarkers and isoprenoids also suggests the low thermal maturity of the Iona-1 well (Peters et al., 2005). The average Pr/Ph ratio is slightly < 1.0 for the LEF and about 1.4 for the UEF (Table 4).

3.4. Molecular composition of hydrocarbons

3.4.2. Terpane biomarker assemblage The distribution of tricyclic to pentacyclic terpanes of the UEF which is the limestone-dominated interval and the LEF which is the marl-dominated interval were assessed from their m/z 191 chromatograms (Fig. 6a and b). All samples contain tricyclic terpanes ranging from C20 to C28. All extracts from both the UEF and

3.4.1. n-Alkanes and isoprenoids characteristics The Total Ion Chromatogram (TIC) for residual oil extracted from the LEF interval shows n-alkanes ranging from n-C14 to n-C35 with a unimodal n-alkane distribution maximizing at n-C17

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X. Sun et al. / Organic Geochemistry 98 (2016) 66–81

a

Pristane

UEF Iona 21 (69.87 m) Phytane

C17

C18

Biomarker C24

C26

Phytane Pristane

LEF Iona 8 (128.88 m)

C17

b

C18 C24

Biomarker

C26

Fig. 5. Total ion current gas chromatograms of saturated hydrocarbon fractions. (a) Sample Iona 21 (subunit UEF; 69.87 m depth), and (b) sample Iona 8 (subunit LEF; 128.88 m depth). n-alkanes are labeled according to carbon number.

LEF contain low concentrations of tricyclic terpanes when compared to C30 hopane and C29 norhopane. Dinorhopane, a compound probably associated with a sulfur-rich anoxic depositional environment, was identified in most of the samples from the LEF. Dinorhopane has been found in Miocene and Cretaceous oils derived from the Monterey shale, and is related to sulfur content in the source rock (Curiale and Odermatt, 1989; Peters et al., 2005). As shown in Fig. 6, our Eagle Ford samples contains a high concentration of extended hopanes (C31–C35). High values of the extended hopane Iindex (C35/C34 > 0.8) have been found to be associated with marine carbonate environments (Mello et al., 1988; Clark and Philp, 1989). Gammacerane was detected in most of our samples, and the low gammacerane index (< 0.3) is consistent with marine carbonate environments in the Eagle Ford Formation (Moldowan et al., 1991; Sinninghe Damsté et al., 1995; Tulipani et al., 2015). 3.4.3. Sterane biomarkers assemblage Steranes were determined from m/z 217 chromatograms (Fig. 6c and d). In most samples from the LEF shales, the regular steroid distribution is dominated by the C28 5a(H),14a(H),17a(H) -20R sterane, followed by the C29 homologue. Conversely, most samples from the UEF are enriched in the C29 5a(H),14a(H), 17a(H)-20R regular sterane, followed by the C28 5a(H),14a(H),

17a(H)-20R sterane. The 4-methylsteranes, determined from the m/z 231 chromatogram (Fig. 6c and d), are relatively abundant in both UEF and LEF shales, suggesting a marine dinoflagellate source input (Robinson et al., 1984; Volkman, 1988; Thomas et al., 1993). 4. Discussion 4.1. Difference in UEF and LEF OM input The abundant lower molecular weight n-alkanes without obvious odd–even preference suggest that a marine algal source was dominant during the deposition of the EFS (Matsuda and Koyama, 1977; Cranwell, 1981). The TICs of the saturate fractions from the UEF and LEF extracts are dominated by lower molecular weight n-alkanes and are enriched in biomarkers. A slight predominance of even-over-odd hydrocarbons is observed in both intervals. Terrestrial source input is normally characterized by high molecular weight n-paraffins and an odd-carbon predominance in n-alkane fractions (Stevenson, 1961; Cocker and Shaw, 1963; Clark and Blumer, 1967; Blumer et al., 1971; Colombo et al., 1989). However, this is not the case for our UEF and LEF samples, indicating a dominant marine source throughout the whole interval, which is consistent with dinoflagellate cysts being the

Pr: pristane; Ph: phytane; Pr/C17: pristane/n-C17; Ph/C18: phytane/n-C18; Pr/Ph: pristane/phytane; Ts/(Ts + Tm): 18a-trisnorhopane/(18a-trisnorhopane + 17a-trisnorhopane; C29Ts/(C29Ts + C29H): 18a(H)-30-norneohopane/(18a (H)-30-norneohopane + 17a(H)-30-norhopane); S/T: steranes/17a-hopanes; CxS/(S + R): ratio of 17a(H),21b(H)-homohopane epimers at C22; Cx20R%: percentage proportion of 5a(H),14a(H),17a(H)-22R- Cx steranes; C29S/(S + R): ratio of 20S and 20R epimers of 5a(H),14a(H),17a(H)-ethylsterane; C29bb/(bb + aa): ratio of 5a(H),14b(H),17b(H) and 5a(H),14a(H),17a(H) 20R ethylsteranes; DBT/P: dibenzothiophene/phenanthrene ratio; GI: 10  gammacerane/17a,21b-hopane; aryl isoprenoid (C13–C17)/P: aryl isoprenoid (C13–C17)/phenanthrene.

0.6 0.8 0.9 1.3 2.9 1.7 2.9 1.7 2.1 2.1 1.8 2.3 2.0 1.7 1.0 1.1 1.4 1.3 1.7 1.6 1.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.1 0.2 0.3 0.3 0.4 0.4 0.3 35.1 29.1 31.3 31.8 30.6 32.9 33.6 40.2 44.1 41.5 38.1 41.7 38.5 41.0 24.7 26.8 27.2 30.1 27.8 28.7 25.5 0.4 0.4 0.5 0.6 0.6 0.6 0.6 3.5 3.7 2.9 2.0 1.2 1.9 1.9 11 10 8 7 4 2 1 Lower EF

115.5 122.9 128.9 133.7 139.9 149.1 151.9

1.1 0.9 0.8 0.9 0.6 0.8 0.9

2.5 2.1 2.5 1.9 2.0 2.3 2.0

1.6 2.1 2.5 1.6 2.4 2.3 2.1

0.2 0.2 0.3 0.3 0.2 0.2 0.2

0.2 0.2 0.2 0.3 0.2 0.2 0.2

0.5 0.5 0.6 0.6 0.6 0.6 0.6

0.1 0.2 NA. 0.1 0.3 0.2 NA. NA. 1.0 0.3 1.9 1.6 1.7 1.5 1.2 2.2 1.3 1.5 1.0 1.7 0.8 0.6 0.6 0.7 0.8. 0.9 0.8 0.5 0.9 1.1 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 37.4 37.8 38.9 39.6 41.5 42.6 45.5 42.2 41.5 37.2 39.5 38.5 36.0 34.7 33.2 33.7 30.5 29.3 34.0 36.6 23.1 23.7 25.1 25.7 25.3 23.7 24.0 28.5 24.4 26.2 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.4 0.2 0.3 0.3 0.3 0.4 0.3 0.3 0.4 0.3 0.4 0.4 0.4 3.8 2.8 3.7 3.2 3.2 3.8 2.8 2.1 2.8 4.3 0.2 0.2 0.2 0.2 0.3 0.3 0.2 0.3 0.2 0.2 22 21 20 19 17 16 15 14 13 12 Upper EF

57.1 69.9 79.7 81.3 92.8 98.1 99.8 104.8 106.4 110.0

1.3 1.1 1.3 1.3 2.0 1.4 1.7 1.7 1.8 1.6

2.6 2.2 2.7 2.6 3.5 3.7 3.2 2.6 3.0 3.5

1.9 1.1 1.3 1.1 1.3 2.5 1.5 1.0 1.2 2.0

0.3 0.3 0.2 0.3 0.3 0.3 0.3 0.3 0.3 0.3

S/H C29Ts/ (C29Ts + C29H) Ts/(Ts + Tm) Ph/C18 Pr/C17 Pr/Ph Depth (m) Sample FM

Table 4 Molecular marker parameters and selected biomarker ratios for extractions from Iona-1 rock samples.

C31S/(S + R)

C35S/(S + R)

C27% 20R

C28% 20R

C29% 20R

C29 S/(S + R)

C29bb/(aa + bb)

DBT/P

GI

Aryl isoprenoid (C13–17)/P

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73

dominant algal constituent of the Eagle Ford Group samples (Eldrett et al., 2014, 2015). Biomarkers provide additional insight regarding OM sources. Regular steranes/17a-hopanes ratio provides a measure of the input of eukaryotic (mainly algae, higher plants) vs bacteria to the OM (Peters et al., 2005). Total steranes/hopanes values from the EFS are all P 1, consistent with marine OM, with major contributions from planktonic and/or benthic algae (Moldowan et al., 1985). Ratios fall in the range 1.2–3.7 for the LEF and 2.1–4.3 for the UEF, which clearly indicates a variation in OM inputs (Table 4). A minor contribution of terrigenous OM in the limestone interval from UEF can be recognized based on diterpane, triterpane and tetraterpane distributions, implying a dominant marine source with some mixed terrigenous OM input in the UEF (Noble et al., 1986; Woodhouse et al., 1992; Dzou et al., 1999; Samuel et al., 2010) (Fig. 6a). A steroid biomarker ternary distribution plot has been used to determine OM source input and depositional environment (Palmer, 1984; Moldowan et al., 1985; Curiale and Odermatt, 1989). C27 sterols derive mainly from zooplankton and C28 sterols mainly from phytoplankton, which are the main precursors of marine OM (Huang and Meinschein, 1979; Volkman, 1986, 1988). In the ternary diagram of the 20R epimers of C27, C28, and C29 steranes (Fig. 7), a dominant marine input in the EFS is consistent with the marine depositional setting. The higher proportion of C28 steranes in the LEF may relate to the increased diversification of the marine algae sources, including phytoplanktonic assemblages, diatoms, coccolithophores and dinoflagellates (Grantham and Wakefield, 1988). The increasing proportion of C29 steranes in the UEF might be an indication of an elevated supply of terrigenous or benthic algal OM (Volkman, 1986, 2003). The 4-methylsteranes having a C30 dinosterol structure are considered biomarkers of dinoflagellates, specifically related to marine dinoflagellate source input, which appears to be abundant in both upper and lower EF shales (Robinson et al., 1984; Volkman, 1988; Thomas et al., 1993) (Fig. 6c and d). Although Bird et al. (1971) proposed an additional source of 4-methylsteranes from methanotrophic bacteria, notably Methylococcus capsulatus, the amounts of sterols in eubacteria are usually quite small (Volkman, 2003) and lack the 23,24-dimethyl side-chain structure. Furthermore, Eldrett et al. (2015) reports that the recovered palynological assemblages contain abundant and diverse marine palynomorphs, with dinoflagellate cysts being the dominant constituent of the Eagle Ford Group samples in the Iona-1 well and there is no significant variation between marl (organic-rich interval) and limestone (organic-lean interval) samples. This finding is consistent with our biomarker analysis.

4.2. Difference in UEF and LEF depositional environment Redox biomarker parameters show that the LEF water column was less oxygenated during deposition than that of the UEF. The Pr/Ph ratio is commonly used as a redox indicator during early diagenesis. Pr/Ph ratios < 1.0 indicate anoxic conditions, whereas values > 1.0 reflect suboxic to oxic environments (Didyk et al., 1978). However, Pr/Ph ratios can be affected by maturation (Tissot and Welte, 1984) and there are various precursors for these isoprenoids (Goossens et al., 1984; Volkman, 1986; ten Haven et al., 1987). Both of these latter influences can be excluded for the Iona-1 well samples because of the limited thickness of the shale and the dominance of a marine OM source. Therefore, an environment with strictly anoxic bottom waters is interpreted for the LEF, in which Pr/Ph ratios are < 1. The UEF shifts to less-reducing (i.e., suboxic to anoxic) conditions since Pr/Ph ratios are > 1 (Table 4; Fig. 2d). The vertical redox trend suggested by Pr/Ph ratios is confirmed by bioturbation patterns (Eldrett et al., 2014); small-scale bioturbation is visible in the UEF (Eldrett et al., 2014, 2015).

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Fig. 6. Mass chromatograms of hopanes (m/z 191) in saturated hydrocarbon fractions of: (a) sample Iona 21 (subunit UEF; 69.87 m), and (b) sample Iona 4 (subunit LEF; 139.85 m). Y, Y1, Z, and Z1 are terpanes from higher plants. Mass chromatograms of steranes (m/z 217) and 4-methylsteranes (m/z 231) in saturated hydrocarbon fractions of (c) sample Iona-21 (subunit UEF; 69.87 m), and (d) sample Iona-4 (subunit LEF; 139.85 m).

The plot of Pr/n-C17 vs Ph/n-C18 is used to indicate not only OM origin, but also source rock facies and maturity (Shanmugam, 1985; ten Haven et al., 1987). As shown in Fig. 8, the LEF lies toward a reducing marine environment that is more restricted than that of the UEF. Despite various possible sources of the variation of the Pr/n-C17 and Ph/n-C18 ratios, more oxidizing conditions were prevalent during the deposition of the UEF interval than during that of the LEF. This interpretation is supported by the sterane distribution as shown in Fig. 7 and the Pr/Ph variations as shown in Fig. 2d. The decreasing trend of gammacerane indices from the LEF to the UEF reflects a more stratified water column in the LEF (Fig. 9). Lower gammacerane indices in the UEF suggest increasing currents and water-mass ventilation in the water column during the lower to upper Eagle Ford depositional transition; the LEF is more restricted than the UEF (Sinninghe Damsté et al., 1995). As

shown in the plot of the Pr/Ph vs gammacerane index for UEF and LEF shales (Fig. 9), although the gammacerane indices are relatively low throughout the whole EF group, consistent with normal marine salinities for the Eagle Ford Formation, the decreasing trend of gammacerane indexes from the LEF to the UEF reflects an increasing oxygen content in bottom waters. Two data points (Iona 22 and Iona 16) show elevated values of the gammacerane index indicating short periods of anoxia in the UEF section. Although a dysoxic to oxidizing condition has been identified for the UEF section, the gammacerane indices suggest episodically anoxic bottom water possibly appeared during and after OAE-2 (Table 4). The oxygenation difference between the LEF and the UEF also has been clearly recorded by the Pr/Ph vertical profile. The postulated paleoredox trend is also visible in a plot of Pr/Ph vs DBT/P (Fig. 10). The dibenzothiophene (DBT) to phenanthrene

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C29%

C28%

C27%

Fig. 7. Relative abundance of C27, C28, and C29 steranes from m/z 217 distributions from UEF and LEF shales of Iona-1.

Fig. 8. Pr/n-C17 and Ph/n-C18 plot to differentiate depositional redox condition, source input, and thermal maturity of the LEF and UEF shales from Iona-1. Light blue circles are UEF samples and dark blue circles are LEF samples where lines and fields are modified from Shanmugam (1985). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

(P) ratios and Pr/Ph ratios were combined to infer source rock depositional environments and lithologies (Hughes et al., 1995; Maslen et al., 2011; Tulipani et al., 2015). The origin of DBT must ultimately be attributed to the interaction of hydrogen sulfide or polysulfides with an organic substrate during early diagenesis (Sinninghe Damsté and de Leeuw, 1990; Hurtgen et al., 1999). Increases in the DBT/P ratio could reflect the availability of reduced sulfur for interacting with OM in the depositional environment, further implying the presence of sulfur-rich, anoxic bottom waters. Therefore, a high S/C atomic ratio of kerogen and a higher concentration of organosulfur compound (OSC) in the LEF indicates a marine-carbonate, sulfur-rich, reducing depositional environment. A low terrigenous mineral input is also required in the LEF to prevent H2S from preferentially reacting with detrital iron to form pyrite (Hurtgen et al., 1999).

Based on the plot of Hughes et al. (1995), samples fall into Zone 1B, which originates from marine carbonate or marine marls having DBT/P ratios ranging from 1 to 3 (Table 4; Fig. 10). These moderate DBT/P values always correlate with a 0.5–2% total sulfur content of the sample sets in Hughes et al. (1995). Thus, according to DBT/P ratios (> 1) from the LEF, a moderate total sulfur content could be inferred, which is confirmed by high S/C ratios (0.04–0.065) of the LEF kerogen samples. Furthermore, a previous geochemical study of dense marlstone from the Boquillas Formation also identified Type II-S kerogen (wells 100819-3 and 100819-5) (Fig. 1c) based on average atomic Sorg/C values of isolated kerogen (0.045 and 0.029) (Little et al., 2012). Total organic sulfur content values obtained by CRS (chromium [II]-reducible sulfur) analysis for the same samples are both 0.04 (Little et al., 2012). Kerogen can be classified as Type II-S when its sulfur

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X. Sun et al. / Organic Geochemistry 98 (2016) 66–81

3.00

more strafied water column UEF Iona

hypersalinity

2.50

Gamma × 10 index

LEF Iona 2.00

1.50

1.00

less strafied water column 0.50 0.00

0.50

1.00

1.50

2.00

2.50

Pr/Ph Fig. 9. Gammacerane index vs Pr/Ph, reflecting a stratified water column and hypersalinity in UEF and LEF intervals of Iona-1. Light blue circles are UEF samples; dark blue circles are LEF samples. (Gamma  10 index, 10  gammacerane/(gammacerane + C30 hopane)). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 10. Dibenzothiophene to phenanthrene (DBT/P) and the ratio of pristane to phytane coupled together, inferring source rock depositional environments and lithology difference between UEF and LEF of Iona-1. Light blue circles are UEF samples and dark blue circles are LEF samples where lines and fields are modified from Hughes et al. (1995). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

content (atomic Sorg/C) is > 0.04. The evidence from Little et al. (2012) and the Iona-1 well enable us to conclude that kerogen of the LEF is a Type II-S kerogen. Aryl isoprenoids were identified from the LEF samples. These compounds are thought to be derived largely from carotenoids specific to photosynthetic green sulfur bacteria (Summons and Powell, 1986). When aryl isoprenoids are used as indicators of paleo environment, caution must be taken since the origin of aryl isoprenoids is not only from Chlorobi, which is a direct evidence of high green sulfur bacteria activity and thus photic zone euxinic (PZE). Other sources of aryl isoprenoids, e.g., b- and c-carotene, and actinomycetes are reported in sedimentary environments (Hartgers et al., 1994; Koopmans et al., 1996b; Brocks and Grice, 2011). To establish PZE conditions in the LEF, it is necessary to demonstrate that the aryl isoprenoids are derived from isorenier-

atene precursors, which are derived from the main carotenoid made by green sulfur bacteria, thus reflecting PZE condition in the LEF. Fig. 11b shows the distribution of aryl isoprenoids and isorenieratane in the m/z 133 chromatogram from the LEF, and confirms that Chlorobi is the main precursor of the aryl isoprenoids in the LEF. The existence of Chlorobi that produced specific aryl isoprenoids (isorenieratene) requires light penetration in the water column as well as water column euxinia; the total concentration of aryl isoprenoids reflects the amount of H2S in the water column that could incorporate the functionalised precursors to be bound into kerogen matrix during diagenesis (Summons and Powell, 1986; Grice et al., 1996; Koopmans et al., 1996b). Elevated concentrations of aryl isoprenoids observed in the LEF from the vertical profile of aryl isoprenoids coincides with those of the DBT/P profile (Table 4; Figs. 2e, f and 11). The similarity of the DBT/P and aryl

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77

Fig. 11. Mass chromatograms of aryl isoprenoids (m/z 133, 134) in aromatic fractions of (a) Iona 21 (subunit UEF; 69.87 m), and (b) Iona 4 (subunit LEF; 139.85 m); isorenieratene is marked.

isoprenoid profiles suggests that the formation of DBT and development of a green sulfur bacterial biomass were simultaneous (Kuypers et al., 2002). This further confirms sulfur-rich, PZE conditions existed in the bottom waters during the deposition of the LEF. 4.3. Effect of depositional environment on biomarker thermal maturity parameters of EFS Thermal maturity level can be estimated by Rock-Eval (Tmax, PI) and biomarker proxies. A very consistent low thermal maturity is indicated by low Tmax and PI values for all samples of the Iona-1 core. An average maturity of 0.45 %Ro is calculated from Tmax values according to the equation VR = 0.018  Tmax 7.16 for Type I and Type II kerogen (Peters et al., 2005). Biomarker thermal

maturity parameters provide additional insight into the maturity assessment of the EFS. Ratios of terpane and sterane biomarkers, such as ratios of 22S- and 22R-17a(H),21b(H)-homohopanes, C29bb/(bb + aa) sterane, 20S- and 20R-5a,14b,17b-steranes, Ts/(Ts + Tm), and C29Ts/(C29Ts + C30 hopane) are the most commonly applied thermal maturity molecular indicators. Both cracking reactions and configurational isomerisation at certain asymmetric carbon atoms affect biomarker thermal maturity parameters (Mackenzie et al., 1980; Peters et al., 2005). Ts/(Ts + Tm) and C29Ts/(C29Ts + C30 hopane) are the most reliable maturity indicators when evaluating oils from a common source of consistent organic facies (Moldowan et al., 1986; Peters et al., 2005). The ratios of Ts/(Ts + Tm) for the UEF and LEF are all in the range 0.19–0.3 (Table 4; Fig. 12b). Values of

Fig. 12. Biomarker thermal maturity parameters versus depth in Iona-1. (a) C29Ts/(C29Ts + C29H), (b) Ts/(Ts + Tm), (c) C29bb/(bb + aa) steranes, (d) C2920S/(20S + 20R) steranes, (e) 22S/(22S + 22R) homohopanes C35, and (f) 22S/(22S + 22R) homohopanes C31.

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C29Ts/(C29Ts + C30 hopane) are in the range 0.18–0.25 (Table 4; Fig. 12a), which corresponds to the immature thermal range of % Ro < 0.4 (Peters et al., 2005), and is consistent with the calculated %Ro values based on Tmax based on the formula of Jarvie et al. (2001). The ratios of C29bb/(bb + aa) sterane, ranging from 0.20–0.33, also reflect low thermal maturity (Table 4; Fig. 12c). No obvious thermal maturity difference was observed for the UEF and LEF intervals based on the above biomarker thermal maturity indicators and Tmax calculated %Ro values. Surprising levels of isomerisation of steranes and extended hopanes occur in our LEF samples. As shown in Table 3 and Fig. 12d–f, almost fully epimerised 20S- and 20R-5a,14b, 17b-steranes and 22S- and 22R-17a(H),21b(H)-homohopanes (C31 and C35) were identified, indicating the mature characteristic of the LEF (Seifert and Moldowan, 1980; Grantham, 1986). Based on calibrations to calculated %Ro for these biomarker thermal maturity parameters the entire oil window appears to be present in just 100 m of section for the Iona well (Peters et al., 2005). The unusual isomerisation seems to conflict with the characters of typical Type II kerogen at an immature/low maturity stage, based on bulk and other biomarker maturation parameters such as Tmax-calculated %Ro and Ts/(Ts + Tm), C29Ts/(C29Ts + C29 hopane), and C29bb/(bb + aa) steranes ratios (Table 4; Fig. 12). The ‘‘anomalies” of high levels of isomerisation for steranes and extended hopanes have been observed occasionally in hypersaline environments or in the presence of reducing sulfur (Mackenzie et al., 1980). Fully isomerised homohopanes in very immature carbonate rock extracts and early diagenetic formation of bb-steranes in hypersaline environments could be due to the reaction of OM with reducing sulfur (ten Haven et al., 1986; Rullkötter and Marzi, 1988; Moldowan et al., 1991). Salinity and/or lithotype variations are regarded as possible triggers of sterane and homohopane ratio variations; high salinity in the Perticara Basin results in high 20S/(20S + 20R) sterane and 22S/(22S + 22R) homohopane ratios (ten Haven et al., 1986). All such ‘‘anomalies” of isomerisation for steranes and extended hopanes that we are aware of from previous studies are associated with carbonate hypersaline and high sulfur-content reducing environments. A reducing sulfur-rich environment with low pH has usually been proposed as providing conditions that result in the more mature extended homohopane and sterane patterns, which caused by unusual diagenetic pathways (Moldowan et al., 1985; ten Haven et al., 1987). Because no evidence for hypersalinity has been observed in the Iona well, high sulfur content of the LEF is a more plausible cause of the observed high level of isomerisation. The increase in the 22S/(22S + 22R) hopane ratio can be explained by reduction of the corresponding hop-17(21)-enes, which are intermediates of extended hopanes and which reach a 22S/(22S + 22R) ratio of equilibrium in early diagenesis because of sulfur incorporation. Hopene isomerisation at C22 can be attributed to either a migration of unsaturation to intermediate hopanoids at the D17(21) or the D21 position, hopanoids which are then preferentially rearranged to the thermally more stable 22S configuration (ten Haven et al., 1985, 1986; Volkman et al., 2015). The 22S/(22S + 22R) hopane ratio for the C31 homohopane in the LEF is shown in Fig. 12. A similar elevated trend has been observed with burial depth for the C31 homohopane 22S/(22S + 22R) ratio. Based on earlier research, C31 homohopane isomerisation should show a lower value compared with that on C35 due to the different reaction pathways under high sulfur and hypersalinity conditions (Köster et al., 1997). According to this study, two reactions are involved: side-chain degradation from C35 homohopane and sulfurisation of homohopanes, the formation of C31 homohopane is considered to be dominated by side-chain cleavage of the C35 homohopane prior to sulfur incorporation in a high sulfur,

reducing and hypersaline environment. However, the competition between side-chain degradation and sulfurisation should be affected by multiple constraints such as redox conditions, pH and reduced sulfur concentration. The dominant role of sulfur incorporation prior to side-chain cleavage of the C35 homohopane cannot be ruled out in other occasions, which could result in the similar C31 and C35 22S/(22S + 22R) ratios as observed in our study. Sterane maturity ratios are also unusual in our samples. According to Moldowan et al. (1985) based on research on the Lower Toarcian of southwestern Germany, sterols, which contain a sidechain double bond, may count for unusually ‘‘mature” 22S/22R sterane ratio. Sterenes and spirosterenes could be formed as an intermediate through a series of reactions, including dehydrogenation, elimination reactions and sulfurisation/desulfurisation from unsaturated sterols. Intermediates can be formed easily in a low pH and high-sulfur reducing environment, which is consistent with the initial precursor of the 20S steranes (Anastasia et al., 1978). The sulfurisation of double bond isomers formed by a sequence of H2S addition and elimination reactions could easily form hopanoid and spirosterene intermediates that further lead to anomalous maturation indicators of steranes and hopanes (Köster et al., 1997; Hebting et al., 2006). Thus, normally formed 20R steranes would initially be mixed with some diagenetic 20S steranes, leading to a non-zero 20S/(20S + 20R) starting point for steranes at the beginning of catagenesis. In contrast to some previous research, unlike the anomalous 20S/(20S + 20R) ratios, we observed no anomalous C29bb/(bb + aa) sterane ratios in the LEF. These two thermal maturity parameters seem to vary in a similar pattern, but based on the chemical mechanism reaction pathway of sterane isomerisation, racemic reaction at chiral carbon C-20 and rearrangement of C-17 and C-14 are not concerted reactions. The isomerisation at C-20 is thought to occur prior to the rearrangement at C-17 and C-14 (Brassell et al., 1984; Peakman and Maxwell, 1988). Considering the depositional and redox condition of the EFS, we do not consider it as either a high-salinity or a very low pH environment at the time of deposition. Because of these ‘‘reaction conditions” differences, sterene isomerisation in the EFS may not be that intense. The extent of hopanoid and steroid isomerisations was controlled, we believe, mainly by pH and redox conditions during early diagenesis (Moldowan et al., 1985). In summary, fully epimerised 20S- and 20R-5a,14b,17b-steranes and 22S- and 22R-17a(H),21b(H)-homohopanes (C31 and C35) in the LEF are due to the sulfurisation/desulfurisation pathway that interferes with the ‘‘normal” thermal stress-dependent isomerisation of steranes and hopanes under low pH, high-sulfur reducing conditions. We consider bulk and other molecular geochemical thermal maturity parameters, such as Tmax-calculated %Ro, Ts/(Ts + Tm), C29Ts/(C29Ts + C30 hopane), and C29bb/(bb + aa) steranes ratios, to be more reliable for thermal maturity calibration of the EFS. Ratios of 22S/(22S + 22R) homohopane and 20S/(20S + 20R) sterane reflect lithofacies and depositional environment variations between the UEF and LEF units rather than thermal maturation. 4.4. Organic matter productivity, preservation and enhanced soluble OM transformation in LEF The high TOC abundance in the LEF could result from either an increased primary productivity rate in the overlying water column or enhanced preservation due to a euxinic stratified water column, or both. The LEF contains a much higher relative concentration of aryl isoprenoids than the UEF, indicating that the photic zone of the water column contained H2S resulting in an excellent environment for primary productivity and OM preservation prior to the onset of the C/T OAE-2. The study from Eldrett et al. (2014) also provides evidence of reducing conditions in the LEF by the

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accumulation of redox-sensitive TMs, high HI index, and low bioturbation. On the other hand, based on the bio-assemblage study of Iona-1 by Eldrett et al. (2014), ‘‘the limestone intervals record significant reductions in both the abundance and preservation of amorphous OM and the significant decrease in the dinocyst P:G ratio suggesting decreased organic paleoproductivity in the limestone compared to the marls beds”. As discussed earlier, the UEF is dominated by limestone and the LEF is dominated by marls (Eldrett et al., 2014). The observation from this study is consistent with Eldrett et al. (2014): the LEF exhibits greater primary productivity and better OM preservation than the UEF. Soluble OM in the LEF is much higher than that in the UEF, based on the profile of EOM/TOC ratios vs depth (Table 2). Variation in OM transformation between the LEF and the UEF cannot be attributed to the difference in thermal maturity in this 100 m thick section (Eldrett et al., 2014). Tmax values, as a measure of thermal maturity, are almost identical for the UEF and the LEF (average values are 422.4 °C and 422.8 °C for the LEF and the UEF, respectively). Comparing the yields of saturates, aromatics, polar compounds, and asphaltene fractions, the increased EOM per gram of TOC is mainly derived from the aromatic, polar and asphaltene fractions (Table 3). It can therefore be deduced that cyclization and aromatization reactions are more extensive during OM transformation in the LEF compare to the UEF. Cyclization and aromatization reactions could be enhanced by the presence of high OSC in the LEF. OSC are considered to be created by incorporation of inorganic sulfur in the forms of elemental sulfur or sulfides into functionalised lipids in an intermolecular (cross-link of different molecules) or intramolecular fashion (Sinninghe Damsté and de Leeuw, 1990). The nature of the macromolecular organic sulfur compounds formed through sulfide cross-linking or intramolecular sulfurisation prevents, or at least significantly hinders, microbial attack and aerobic degradation by binding individual functionalised compounds into a macromolecular matrix, thereby preserving organic compounds over geologic time scales (Werne et al., 2004) and offering a supplementary interpretation for the better OM preservation in the LEF. Furthermore, the formation of OSC can occur at very early stages during diagenesis and has been observed in modern/living ecosystems (Hebting et al., 2006). The presence of OSC can significantly reduce the onset temperature of kerogen thermal decomposition to bitumen at early maturation (Aizenshtat et al., 1981; Kohnen et al., 1991; Lewan et al., 2006). CAS bonds cleave more easily than CAC bonds (Aizenshtat et al., 1995). The lower average bond energy of CAS bonds (275 kJ/ mol) compared to that of CAC bonds (350 kJ/mol) may account for the lower onset temperature of soluble OM generation in the LEF (Sinninghe Damsté et al., 1989). A number of hydrous pyrolysis studies have utilized artificial maturation of natural organic sulfur-rich kerogens and shales (Lewan, 1993) to quantify the effect of sulfur–sulfur (SAS) bonds on petroleum formation (Krein and Aizenshtat, 1995; Nelson et al., 1995; Tomic´ et al., 1995; Koopmans et al., 1996a; Putschew et al., 1998; Sinninghe Damsté et al., 1998). Experimental simulation has shown that the thermal degradation of sulfur-rich kerogen (i.e. Monterey kerogen) occurs at a faster rate than that of typical Type II marine kerogen (i.e. Toarcian kerogen) (Peters et al., 1990; Tomic´ et al., 1995). All of these studies clearly demonstrate that sulfur-rich kerogens produce petroleum at appreciably lower temperatures than their low-sulfur counterparts, supporting the theory that early soluble OM generation in the LEF is enhanced by sulfurisation of the OM.

5. Conclusions Detailed geochemical characterization for immature Eagle Ford core samples from the Iona-1 well reveals a clear difference in OM

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input and depositional environments between the lower and upper Eagle Ford subunits. The OM of the Eagle Ford Formation is dominated by Type II kerogen. OM in the LEF contains high S/C atomic ratios (0.04–0.065) and is characterized as Type II-S kerogen. The extracts of the LEF have high DBT/P ratios and high concentration of aryl isoprenoids, possibly due to Type II-S kerogen as suggested by biomarker studies. A minor contribution of terrigenous OM or benthic algae is inferred for the UEF subunit. Within the Eagle Ford succession, the depositional environment changes from sulfur-rich, anoxic–euxinic reducing conditions in the LEF into sulfur-poor, to persistently oxygenated marine carbonate conditions in the UEF. A more stratified water column, as indicated by higher gammacerane indices, was developed during LEF deposition. The reducing redox condition in the LEF provides better preservation of OM. A high concentration of aryl isoprenoids in the LEF further supports the existence of a high H2S-containing water column that extended to the photic zone. Water column anoxia and sulfur incorporation into OM during early diagenesis played a significant role in OM preservation. The formation of organosulfur compounds accounts for the earlier conversion of kerogen to bitumen in the LEF. The presence of these compounds in the LEF enhances the isomerisation of homohopanes and steranes. As a result, biomarker thermal maturity parameters were substantially affected, and an overestimation of thermal maturity could be made if using only the 22S/(22S + 22R) homohopane and 20S/(20S + 20R) sterane biomarker maturity parameters. Acknowledgements This study was supported by Shell/The University of Texas at Austin Unconventional Research (SUTUR) project, under Subtask 7, ‘‘Investigation of Oil Storage and Migration in the Eagle Ford Formation by Integrated Geochemistry and Petrography,” Tongwei Zhang, PI. We thank Shell colleagues Aysen Ozkan and Robert Dombrowski, as well as David Chapman of The University of Texas at Austin, Bureau of Economic Geology, for their help with sampling and discussions of Eagle Ford geology. Dr. Alon Amrani from The Institute of Earth Sciences, The Hebrew University conducted kerogen isolation and elemental analyses. Also, Yongge Sun thanks the financial support from Chinese Natural Science Foundation (Grant Nos. 41172112 and 41372131). This publication is authorised by the Director, Bureau of Economic Geology. We thank all the reviewers for their constructive suggestions. In particular, we are sincerely grateful to Joseph A. Curiale, Associate Editor Kliti Grice and Co-Editor in Chief John Volkman for their time, comments and corrections to improve the original manuscript. Associate Editor—Kliti Grice References Aizenshtat, Z., Stoler, A., Cohen, Y., Nielsen, H., 1981. The geochemical sulphur enrichment of recent organic matter by polysulfides in the Solar Lake. In: Bjorøy, M., Albrecht, P., Cornford, C. (Eds.), Advances in Organic Geochemistry, 1981. Wiley, Chichester, pp. 279–288. Aizenshtat, Z., Krein, E., Vairavamurthy, M.A., Goldstein, T., 1995. Role of sulfur in the transformations of sedimentary organic matter: a mechanistic overview. In: Geochemical Transformations of Sedimentary Sulfur. ACS Symposium Series. Washington, DC, pp. 16–37. Anastasia, M., Fiecchi, A., Scala, A., 1978. Side-chain inversion of steroidal olefins promoted by hydrogen chloride. The Journal of Organic Chemistry 43, 3505– 3508. Baskin, D.K., Peters, K.E., 1992. Early generation characteristics of a sulfur-rich Monterey kerogen. American Association of Petroleum Geologists Bulletin 76, 1–13. Bastow, T.P., van Aarssen, B.G.K., Lang, D., 2007. Rapid small-scale separation of saturate, aromatic and polar components in petroleum. Organic Geochemistry 38, 1235–1250.

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