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Acetate, DIC and δ13CDIC evidence for acetoclastic methanogenesis in Songliao Basin, NE China
Journal of Petroleum Science and Engineering 131 (2015) 177–183
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Acetate, DIC and δ13CDIC evidence for acetoclastic methanogenesis in Songliao Basin, NE China Wenlong Jiang a, Haiping Huang a,b,n, Ningxi Li a, Haifeng Zhang a, Chao Yu a a b
School of Energy Resource, China University of Geosciences, Beijing 100083, China Department of Geoscience, University of Calgary, 2500 University Drive NW, Calgary, AB, Canada T2N 1N4
art ic l e i nf o
a b s t r a c t
Article history: Received 9 January 2015 Accepted 30 April 2015 Available online 9 May 2015
Extracted waters of thirty eight (38) shale samples from Songliao Basin, NE China have been determined their acetate and dissolved inorganic carbon (DIC) concentrations and carbon isotopic compositions of DIC. Wide range variations have been observed in the geochemical data, which seem not controlled by organic richness and burial depth. Systematically low acetate concentrations and low DIC content, coupled with relatively 13C enriched DIC in Pu–Ao area and west slope than these from north and east areas of the basin suggest different degree of acetoclastic methanogenesis. Moderate methanogenesis in Pu–Ao and west slope is in consistent with biogenic gas discovery in these areas. Methanogenesis in north and east is inhibited due to the loss of pore space and undeveloped fracture system, which is also supported by none encountered biogenic gas. However, overall methanogenesis in these fine grained shales is very limited and current ongoing methanogenesis is doubtful. & 2015 Elsevier B.V. All rights reserved.
Keywords: Acetate Acetoclastic methanogenesis Dissolved inorganic carbon (DIC) Songliao Basin
1. Introduction Biogenic methane is an important end-product of biogeochemical processes associated with the bacterial remineralization of labile organic matter in sediments (Zinder, 1993; Petscha et al., 2005). It is trapped typically in relatively shallow and immature sediments beneath the anaerobic sulfate-reducing zone (Rice and Claypool, 1981; Schoell, 1983; Whiticar et al., 1986). Although difficult to quantify, biogenic gases account for roughly 20% of the worldwide natural gas resource (Rice and Claypool, 1981; Rice, 1993). When gas hydrates, secondary biogenic gas from oil reservoirs, coals and organic rich shales are accounted, biogenic gas is far more important than previous thinking (Kotelnikova, 2002; Milkov, 2011). There are two main pathways responsible for biogenic methane generation. The first pathway is CO2 reduction via CO2-reducing prokaryotes, which use hydrogen as the electron donor or energy source and CO2 as the electron acceptor (hydrogenotrophic methanogenesis): CO2 þ4H2-CH4 þ 2H2O. The second pathway is by acetate fermentation (acetoclastic methanogenesis). In this case, acetate and hydrogen are used to produce methane and carbon dioxide: CH3COOH-CH4 þ CO2. In general, CO2 reduction is the n Corresponding author at: Department of Geoscience, University of Calgary, 2500 University Drive NW, Calgary, AB, Canada T2N 1N4. Tel.: þ 1 403 2208396; fax: þ1 403 2840074. E-mail address: [email protected] (H. Huang).
http://dx.doi.org/10.1016/j.petrol.2015.04.040 0920-4105/& 2015 Elsevier B.V. All rights reserved.
dominate pathway in marine environment where acetate is relatively depleted because of sulfate reducing bacteria consuming. While in terrestrial freshwater or brackish environment, acetate fermentation is initially significant, but decreases with increasing buried depth (Whiticar et al., 1986; Zinder, 1993; Conrad, 1999). However, the exact quantitative role of hydrogenotrophic and acetotrophic methanogenesis in deep subsurface is not yet fully understood. Recent investigations have revealed that hydrogenotrophic and acetoclastic methanogenesis are equally important in deep subsurface (Veto et al., 2004; Parkes et al., 2007; Shuai et al., 2007; Flores et al. 2008; Ulrich and Bower, 2008). Acetate is produced by fermentation of organic matter as well as by reduction of CO2 with H2 via the acetyl-CoA pathway (acetogenesis) (Drake et al., 2006; Hädrich et al., 2012). It can also be produced by low-temperature diagenesis of organic matter, and by thermal alteration of organic matter at temperatures higher than 80 1C (Wellsbury et al., 1997; Horsfield et al., 2006; Parkes et al., 2007). Acetate serves as an important substrate for a variety of microorganisms including sulfate reducing bacteria and methanogenic archaea (Krumholz et al., 2002; Kotelnikova, 2002; Drake et al., 2006). Acetate concentrations are generally low in formation water if methanogenesis is in operation. The threshold concentration for acetoclastic methanogenesis in sediments is approximately 5 μM (Lovley and Phillips, 1987). High accumulated acetate concentrations may reflect inhibition of acetoclastic methanogenesis (Routh et al., 2001; Waldron et al., 2007). In the case of the Antrim Shale, acetate concentration is commonly negligible because of the
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high levels of microbial activity (Martini et al., 1998; McIntosh et al., 2002; 2004). Dissolved inorganic carbon (DIC) is the sum of the dissolved carbon dioxide, bicarbonate ion, and carbonate ion species (CO2, HCO3 , and CO23 ) in the formation water. High concentration of DIC in the formation water is an important indicator of microbial activity (Martini et al., 1998; Claypool et al., 2006). During methanogenesis by acetate fermentation or CO2 reduction, the produced or the remaining CO2 is incorporated into the DIC pool. Martini et al. (1998) noticed that microbial gas accumulations contain high concentrations of CO2 (up to 14.82 mol%). Isotopic exchange between CO2 and CH4 produces heavier δ13C values of DIC as 12C is preferentially concentrated in methane while 13C is enriched in the surrounding water as DIC. The unusually 13C enriched values of CO2 ( þ22‰) coproduced with methane and dissolved inorganic carbon (DIC) in formation waters ( þ28‰) indicate extensive methanogenesis. Otherwise, thermogenic CO2 or DIC would have much more negative δ13C values, closer to that of the kerogen undergoing maturation (about 30‰) (Martini et al., 1998; Budai et al., 2002; McIntosh et al., 2004). Thermally immature organic rich source rocks that could potentially generate large quantities of biogenic gas are widely distributed in the Songliao Basin, NE China. However, the shallow biogenic gas occurrence is very limited, which does not match the wide distribution of potential source rocks. Only a few smaller gas pools have been discovered, which show close relationship to secondary biogenic gas from oil biodegradation (Zhang et al., 2011). Li et al. (2015) suggested petrophysical property of shale may exert significant impact on biogenic gas generation in the subsurface. Undeveloped fracture system, very low permeability and very small pores resulting from compaction and diagenesis restrict bacteria movement and activity, limit nutrient transport, diminish space availability, and lead to a reduced biodiversity. In the present study, we focus on indirect evidence for biologic activity through analyses of its by-products retained in pore water as direct evidence of microbial life is typically unreliable due to contamination issues. This study seeks to understand more in situ microbial activities in deep subsurface environments.
2. Geological background Songliao Basin with about 260,000 km2 is the largest continental sedimentary basin in northeast China (Fig. 1). It has a dual structure with complex fault depression developed in late Jurassic to early Cretaceous and intracratonic basin in late Cretaceous–Cenozoic (Huang et al., 2004). The depositional succession from Jurassic— Neogene is about 10 km thick in the depocenter. The tectonic evolution of the basin is mainly affected by the thermodynamic changes of deep-seated mantle material and the Pacific plate’s activity. According to Feng et al. (2010), basin evolution can be subdivided into a Late Jurassic to Early Cretaceous synrift stage, an Early to Late Cretaceous postrift stage and a Cenozoic stage of structural basin inversion. A stratigraphic column outlines the formations as well as sediment type and thickness (Fig. 2). The main source rocks are Cenomanian-Coniacian Qingshankou (K2qn) and Santonian Nenjiang (K2n) formations. The Qingshankou Formation contains three members (K2qn1 to K2qn3) while the Nenjiang Formation can be divided into five members (K2n1 to K2n5) based on their lithology variation. Thermally mature regions of these source rocks near the basin depocenter contribute petroleum to the giant Daqing Oilfield (Feng et al., 2011), while the immature and low maturity source rocks that are widely distributed at depths less than 1500 m on the basin margins appear to be excellent potential source rocks for biogenic gas generation.
3. Experimental methods Shale samples collected from the Songliao Basin were analyzed for concentration of acetate, DIC, and δ13C values of DIC. Because no formation water is available from shale succession, organic acids and bicarbonate ion are obtained from water extraction of shale. Samples were crushed in a steel ball mill to fine powder (100 mesh), 20 g of shale was dissolved in 50 ml deionized water, ultrasound for 1 h at 60 1C and room temperature, respectively, and then centrifugation to obtain clear solution. Concentrations of acetate were detected by Dionex ICS2000 ion chromatography (IC). The 60 1C ultrasound solution sample was injected into the loop of injection after getting through a 0.45 μm aqua filtration membrane, then the sample entered into the Dionex ICS2000 ion chromatography system and detected by conductivity detector after chromatographic column separation. The qualitative analysis of acetate in solution was determined by comparing the retention time of the sample spectra with that of the standard spectra. For quantitative purpose, an external standard calibration method was applied. The standard solution was injected and the standard curve was drawn based on the relationship between peak area and concentration. The concentration of the sample was calculated by mapping the peak area of the sample with the standard curve. The linear correlation coefficient of standard curve is greater than 0.98 and the standard deviation of measured concentration is smaller than 5%. DIC and its isotopes were detected by continuous flow- isotope ratio mass spectrometer (CF-IRMS) with the room temperature extraction sample using Isoprime 100TM continuous flux coupled to a MultiflowTM from Isoprime Corporation. Transfer 0.5 ml sample in to a round bottomed flask, and then 5–6 drops of anhydrous phosphoric acid was added after purging the pure helium gas into the flask. Reaction at 40 1C for more than 5 h till the equilibrium is reached. The generated CO2 was first led into the quantitative loop, and then separated through a chromatographic column and dehydrated by membranes. After that, the carbon and oxygen isotopes of CO2 were then tested on the mass spectrometer. Carbon isotopes of CO2 were corrected by using the national standard GBW04405 and international standard NBS19. DIC standards were prepared by using two NaHCO3 solutions whose carbon isotopes were known. One standard was inserted every five samples, thus the carbon isotopic value of the sample can be obtained.
4. Result and discussion Table 1 lists analytical data generated from water extraction of shales and their basic geochemistry derived from Rock–Eval pyrolysis. The Upper Cretaceous Nenjiang and Qingshankou formations were developed in deep and semi-deep lacustrine depositional environment which are very rich in organic matter. The TOC values in current analyzed sample suite range from 0.1 to 4.4%. Relatively lower TOC content than typical published data (Feng et al., 2011; Bechtel et al., 2012) is because our samples are mainly from Nenjiang Formation collected at basin margin. Samples are essentially siltstone adjacent to fine grained shale as these samples have large pore space for microbial activity. Nevertheless, majority studied shale samples have TOC content above 1%, which is far higher than minimum requirement of metabolizable organic matter equivalent to about 0.5% of TOC proposed by Rice and Claypool (1981). Abundant organic matter is absolutely necessary for formation of biogenic methane and shales from the Songliao Basin have sufficient quantities of organic matter to maintain microbial activity. The Rock–Eval Tmax of the studied samples are between 429 and 445 1C. Types II and III organic matter with Tmax less than 435 1C are considered thermally immature
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Fig. 1. Structural sketch map of the Songliao Basin NE, China with sample locations.
(Espitalié et al., 1985). Most of our studied samples are inferred to be thermally immature while some samples may reach early oil window. Overall, burial depth o1500 m in the Songliao Basin remains in the microbial active window. 4.1. Acetate Variable amount of acetate has been detected from shale extracts in the Songliao Basin (Table 1). Concentrations of acetate show no correlation to organic richness and burial depth, implying other factors dominating its occurrence (Fig. 3). As overall source rocks are organic rich, acetate concentration reflects most likely the whole shale system in certain succession rather than controlled by local organic richness. Meanwhile, acetate is easy mobile when connected pore system is available. Acetate concentration variations in pore water are generally controlled by relative intensity of acetate formation and consumption. Acetate is not only an important product in the anaerobic degradation of organic matter, but also formed through various acetogenesis processes. Detailed acetate generation mechanism is out the scope of present study, however, no obvious geological difference constrains acetate generation at different locations of the Songliao Basin. We assume acetate concentration is governed by varying degree of consumption. The consumption of acetate is performed by a broad spectrum of microorganisms including acetoclastic methanogens and sulfate reducing bacteria. As Songliao Basin is typical lacustrine freshwater system, sulfate reducing bacteria are restricted in very shallow depth. Acetoclastic methanogenesis seems the only regulator in the study areas. A distinct difference can be observed geographically as acetate concentrations from north and east area are systematically higher than these from West Slope and Pu–Ao area. Average acetate concentration from north and east is 940.2 μg/g, which is almost 6 times higher than that from West Slope and Pu–Ao area. Low acetate concentrations in West Slope and Pu–Ao area reflect a steady state between production and consumption. Acetate serves as key intermediate during methanogenesis, which is consistent with the occurrence of biogenic gas in these regions. High acetate concentrations in extracted water samples may prove acetate fermentation is
limited. Previous studies suggested that acetate concentrations 45 μM indicate inhibited acetoclastic methanogenesis (Lovley and Phillips, 1987; Schlegel et al., 2013). Although our data is difficult to correlate with published value due different analytical methods, large difference between different areas may suggest variable microbial activities. No commercial biogenic gas has ever been discovered at north and east regions of the Songliao Basin further support that inhibition of acetoclastic methanogens may occur. 4.2. DIC and its isotope The DIC concentrations of extracted water from shales in the Songliao Basin vary widely from 148.0 μg/g to 1076.1 μg/g, and the isotopic values of DIC vary accordingly ranging from 13.8‰ to 0.9‰. Plot of DIC concentration versus C isotope values of DIC in extracted waters shows a general positive correlation although the coefficient is fairly low. Relatively higher DIC concentration and heavier δ13CDIC value occur at Pu–Ao area and West Slope than these in the north and east regions. The average DIC concentration and δ13CDIC value of samples from Pu–Ao area are 581.6 μg/g and 6.6‰, respectively. An average DIC concentration of 571.0 μg/g and δ13CDIC of 8.9‰ have been observed from West Slope. The average DIC concentration and δ13CDIC value of samples from east area are 412.3 μg/g and 10.8‰, respectively, and these from north area are 483.5 μg/g and 11.2‰, respectively (Fig. 4). The change in the DIC concentrations is related to relative rates of alkalinity production and removal. While quantification of net DIC addition is very complicated as organic matter oxidation, CO2 reduction and authigenic carbonate precipitation all affect the DIC pore-water pool (Claypool et al., 2006), isotopic value of DIC can help to decipher its main origin and control factors. The δ13CCO2 value derived from decarboxylation reactions from organic matter during diagenesis is estimated to range from 25‰ to 10‰ (Schoell, 1983; Chung et al., 1988). As no carbonate has developed in shallow strata of Songliao Basin, carbonate dissolution released DIC to the pore water can be ignored. During methanogenesis by acetate fermentation or CO2 reduction, the produced or the remaining CO2 is incorporated into the DIC pool, which is accompanied by an
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Fig. 2. Generalized Upper Cretaceous stratigraphic column of the Songliao Basin (modified from Feng et al., 2010).
isotopic fractionation. Enriched δ13CDIC are recorded in reducing environments where the production of biogenic methane, either by acetate fermentation or by CO2 reduction, removes 12C, leaving 13Cenriched CO2 in the system, which accounts for the positive δ13CDIC in formation waters. The isotope fractionation between DIC and CH4 illustrated by the enrichment factor (ε‰¼ δ13CDIC δ13CCH4) can be used to characterize methane formation as well as degradation processes. Enrichment factors between 10 and 50‰ may show acetogenic methane formation and high enrichment factors (450‰) may point to CO2 reduction as the dominating process of methane formation (Whiticar et al., 1986; Conrad, 1999; Whiticar, 1999; Grossman et al., 2002). Most biogenic gas discovered from the Songliao Basin has methane isotopic value between 60‰ and 55‰, while the average isotopic value of CO2 in the Pu–Ao
biodegraded oil reservoirs is þ2.3‰. CO2 reduction is reasonably assumed in oil biodegradation related methanogenesis (Zhang et al., 2011). Isotopic value of DIC form extracted water is much lower than isotopic value of CO2 in biodegraded oil reservoir, acetoclastic methanogenesis might be expected if it occurs. Slightly higher DIC concentrations and heavier isotopic compositions in shale samples from Pu–Ao area and West Slope than these from north and east are interpreted to be related to in situ anaerobic methanogenesis. If methanogenic processes remain constant, δ13CDIC will be continually enriched. Strongly positive δ13CDIC ( 4 10‰) have been documented in produced water from several biogenic gas-producing fields with δ13CDIC approximately þ 26 to þ 30‰ in Antrim Shale (Martini et al., 1998; Budai et al., 2002; McIntosh et al., 2004). However,
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Table 1 Aerate and DIC concentrations and isotopic compositions of extracted water from shales in the Songliao Basin and their basic geochemistry derived from Rock–Eval pyrolysis. Area
Well
Agen
Depth (m)
Acetate (μg/g)
δ13C (‰)
δ18O (‰)
DIC (μg/g)
TOC (%)
Tmax (1C)
S1 (mg/g)
S2 (mg/g)
W. Slope W. Slope W. Slope W. Slope W. Slope W. Slope W. Slope W. Slope W. Slope Pu–Ao Pu–Ao Pu–Ao Pu–Ao Pu–Ao Pu–Ao Pu–Ao Pu–Ao Pu–Ao Pu–Ao Pu–Ao Pu–Ao Pu–Ao North North North North North North North North North North North North North East East East
J88 Du58 Du58 Du28 Du28 Du28 Du28 Du28 Du28 AQ2 AQ2 AQ2 AQ2 AQ2 AQ2 Da161 Da161 Da161 Da161 Da161 Da161 Da161 D401 D401 D401 B3 SY1 SY1 SY1 SY1 SY1 SY1 Yu12 Yu7 Yu7 Si103 C73-87 S35
K2y2 þ 3 K2n1 K2n1 K2n1 K2n1 K2n1 K2qn2 þ3 K2qn1 K2qn1 K2n4 K2n3 K2n3 K2n3 K2n3 K2n3 K2n3 K2n3 K2n3 K2n3 K2n3 K2n3 K2y1 K2n1 K2n1 K2n1 K2qn1 K2n2 K2n2 K2n1 K2n1 K2qn1 K2qn1 K2n1 K2qn1 K2qn1 K2n2 K2qn1 K2qn1
604.1 751.2 908.7 1085.4 1095.6 1256.1 1379.7 1384.5 1398.2 712.4 719.5 727.5 739.9 776.5 782.7 1079.2 1087.2 1092.0 1097.6 1106.9 1113.7 1553.7 481.9 490.0 525.3 726.1 713.9 724.5 832.4 875.6 1276.0 1302.6 1137.0 1541.5 1551.5 364.0 770.8 1447.0
36.8 695.1 776.0 27.8 266.5 199.6 5.9 23.2 66.9 5.2 16.9 114.3 70.8 177.2 269.7 218.4 100.7 118.5 120.4 151.0 58.2 67.5 972.9 568.7 1115.0 1156.8 1236.3 644.4 1262.6 1222.7 772.4 451.7 1132.0 652.7 1128.9 653.8 1403.0 669.9
7.1 11.3 10.3 7.9 6.3 9.2 10.3 8.6 10.6 7.8 10.3 5.2 7.6 7.1 8.9 7.4 0.8 7.6 4.5 5.8 7.3 5.7 10.1 8.4 7.3 13.7 11.1 12.4 12.6 11.0 12.9 10.1 11.4 10.3 13.8 10.1 11.2 11.2
7.3 7.2 6.6 6.3 6.2 7.9 6.7 6.3 5.2 6.2 6.2 5.6 6.6 6.6 7.1 6.2 6.9 5.8 7.4 6.4 6.7 6.8 7.7 6.8 7.4 8.2 8.2 7.4 7.7 7.6 7.7 7.7 7.8 8.2 8.2 8.4 7.4 7.4
891.9 492.8 193.9 878.8 807.5 148.0 579.2 426.9 893.7 573.9 587.5 655.5 353.0 669.3 192.7 466.8 1076.1 713.6 548.0 499.4 556.8 668.1 759.5 389.3 693.5 327.5 542.9 680.3 639.4 523.1 391.5 465.5 280.9 271.0 320.9 403.3 623.3 210.3
1.6 0.3 0.6 0.7 4.4 0.2 1.4 1.5 0.1 0.9 1.3 0.9 1.2 1.2 1.1 1.7 0.8 0.9 1.0 1.3 1.3 0.7
436 430 433 432 439 435 441 445 439 435 438 436 433 436 437 441 437 441 439 440 439 445 437
0.06 0.01 0.02 0.02 0.49 0.02 0.19 0.12 0.02 0.02 0.01 0.01 0.04 0.03 0.03 0.06 0.03 0.02 0.03 0.09 0.04 0.02 0.02
6.07 0.14 1.92 1.25 30.78 0.21 9.46 8.54 0.07 0.36 1.24 0.37 0.35 0.47 0.60 1.26 0.80 0.89 0.57 0.82 0.88 2.17 2.48
2.2 0.1 0.6 0.5 0.9 3.0 1.2
436 432 436 429 430 429 432 433
0.05 0.01 0.01 0.01 0.01 0.03 0.13 0.03
12.90 0.05 0.36 0.24 0.65 4.34 23.91 2.10
0.6 1.1
441 438
0.06 0.05
0.21 1.36
K2qn: Upper Cretaceous Qingshankou Fm.; K2n: Upper Cretaceous Nenjiang Fm.; K2y: Upper Cretaceous Yaojia Fm.
Acetate Concentration (μg/g) 200
0
500
1000
5
W. Slope
1500
Pu-Ao
4
North
Depth (m)
600 800
TOC (%)
400
East
3
2
1000 1 1200 1400 1600
0 0
500
1000
1500
Acetate Concentration (μg/g)
Fig. 3. Acetate concentration variation in the studies samples. (a) Acetate concentration versus burial depth; (b) acetate concentration versus TOC content.
positive carbon-isotope excursion of DIC does not occur in the Songliao Basin, δ13CDIC remains constantly low in all samples, which should be interpreted to indicate no significant methanogenesis on going.
4.3. Biogenic gas generation Small gas fields have been discovered in the Pu–Ao area and west slope. Detailed gas chemical and isotopic compositions have
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1200
DIC Concentration (μg/g)
1000 800 600
W. Slope
400
Pu-Ao
in addition to compaction, would have contributed to reductions in permeability and pore throat size over time. Biogenic gas is most likely generated during early diagenesis when pore throats may not have been as restrictive as they are presently. Alternatively, small scale fractures might be developed in Pu–Ao area and west slope as they are situated at structural extension region. Patchy microbial activity associated with the fracture zone may be still going on, however, overall biogenic gas generation potential is low. Similar conclusion for overall low microbial activity in shale system has been draw by other studies (Gao et al., 2013; Schlegel et al., 2013).
North
200
East
5. Conclusion
0 -15
-10
-5
0
δ13CDIC(‰) Fig. 4. Correlation between DIC concentration and carbon isotopic composition in the studied samples.
been reported by Zhang et al. (2011). Gas from Pu–Ao area is dry gas with 13C depleted methane, enriched C2 þ and enriched CO2, which indicates typical secondary biogenic gas from oil biodegradation. Gas from West Slope is also dry with low CO2 content. Some gases not only have 13C depleted methane, but also 13C depleted ethane and propane, indicating primary biological gas and possibly low maturity gas. Majority of biogenic gas (δ13CCH4 o 55‰) can be attributed to oil biodegradation but gases from dispersed immature source rocks are projected (Zhang et al., 2011). Data from present study are consistent with previous publications (Zhang et al., 2011; Li et al., 2015) that primary biogenic gas is limited. In the north and east areas of the Songliao Basin, no biogenic gas reservoirs have been encountered, which is consistent with high acetate and low DIC concentrations together with isotopically depleted DIC. On the contrary, acetoclastic methanogenesis may contribute to biogenic gases discovered at Pu–Ao area and West Slope on the basis of acetate and DIC concentrations and δ13CDIC. Unlike the Michigan and Illinois basins, there does not appear to be large accumulations of microbial CH4 in Cretaceous organic-rich shales at the Songliao Basin. Current active methanogenesis in these fine grained shales remain doubtful but it definitely occurs in history. Most studies regard temperature as main control for deep subsurface life since increasing temperatures lead to denaturation of bacterial tissues. However, methanogenic activity can prevail over a wide range of temperatures with the common range of 4– 100 1C (Kotelnikova, 2002; Rice, 1993). The upper temperature may determine the maximum depth at which subsurface methanogens can be active; however, temperature alone does not limit bacteria in nonhydrothermal sediments until about 4 km (Parkes et al., 2000). Thermal regime is obviously not a critical control factor for biogenic gas generation in the Songliao Basin as all studied samples have never reached the maximum life-tolerant temperature of microbe. The dominant control on biogenic gas generation is not the maximum burial temperature as others suggested (Grasby et al., 2008). The absence of a microbial consortium is unlikely as oil reservoir at similar burial depth is largely biodegraded (Zhang et al., 2011). Decreased bioavailability of shale organic matter may exert some impact but cannot be dominated controlled as deeply buried Cretaceous black shales still act as active bioreactors in great sediment depths has been demonstrated by transport-reaction model (Arndt et al., 2006). We propose that limited microbial activity is constrained by pore spaces as grain size in deep lacustrine shale is too small. Samples of this study have undergone considerable diagenesis including cementation of particles by quartz overgrowths and the formation of secondary minerals. These processes,
Extracted waters from Pu–Ao area and West Slope of Songliao Basin have lower concentrations of acetate, higher concentrations of DIC and carbon isotopically heavier DIC than these from north and east regions. The consumption of acetate, generation and isotopic enrichment of DIC from Pu–Ao area and West Slope of Songliao Basin are consistent to acetoclastic methanogenesis mediation. Overall high acetate concentrations and only slightly elevated DIC isotope point to limited methanogenesis in the Songliao Basin. Loss of pore space and undeveloped fracture rather than organic richness and burial depth take dominant controls on microbial activity in the Songliao Basin.
Acknowledgements This research is supported by National Natural Science Foundation (41273062). We would like to acknowledge Shuichang Zhang and Guoyi Hu from Research Institute of Petroleum Exploration and Development, PetroChina and Zihui Feng, Wei Fang, Jingkun Li, Xue Wang, Zhongliang Dong, Ting Liu and Qiuli Huo from Daqing Oilfield Company Ltd for valuable assistance in sampling, data collection. Dr Yildiray Cinar and three anonymous reviewers are gratefully acknowledged for their constructive comments that substantially improved the quality of this manuscript. References Arndt, S., Brumsack, H.J., Wirtz, K.W., 2006. Cretaceous black shales as active bioreactors: a biogeochemical model for the deep biosphere encountered during ODP Leg 207 (Demerara Rise). Geochim. Cosmochim. Acta 70, 408–425. Bechtel, A., Jia, J.L., Strobl, S.A.I., Sachsenhofer, R.F., Liu, Z.J., Gratzer, R., Puttmann, W., 2012. Palaeoenvironmental conditions during deposition of the Upper Cretaceous oil shale sequences in the Songliao Basin (NE China): implications from geochemical analysis. Org. Geochem. 46, 76–95. Budai, J.M., Martini, A.M., Walter, L.M., Ku, T.C.W., 2002. Fracture-fill calcite as a record of microbial methanogenesis and fluid migration: a case study from the Devonian Antrim Shale, Michigan Basin. Geofluids 2, 163–183. Chung, H.M., Gormly, J.R., Squires, R.M., 1988. Origin of gaseous hydrocarbons in subsurface environments: theoretical considerations of carbon isotope distribution. Chem. Geol. 71, 97–103. Conrad, R., 1999. Contribution of hydrogen to methane production and control of hydrogen concentrations in methanogenic soils and sediments. FEMS Microbiol. Ecol. 28, 193–202. Claypool, G.E., Milkov, A.V., Lee, Y.-J., Torres, M.E., Borowski, W.S., Tomaru, H., 2006. Microbial methane generation and gas transport in shallow sediments of an accretionary complex, southern Hydrate Ridge (ODP Leg 204), offshore Oregon, USA. In: Proceedings of the Ocean Drilling Program Science Results. Drake, H.L., Küsel, K., Matthies, C., 2006. Acetogenic prokaryotes. In: Dworkin, M., Falkow, S., Rosenberg, E., Schleifer, K.-H., Stackebrandt, E. (Eds.), The Prokaryotes: An Evolving Electronic Resource for the Microbiological Community, 2. Springer, New York, pp. 354–420. Espitalié, J., Deroo, G., Marquis, F., 1985. Rock–Eval pyrolysis and its applications. Revue De L Institut Francais Du Petrole 40, 563–579. Feng, Z.Q., Jia, C.Z., Xie, X.N., Zhang, S., Feng, Z.H., Cross, T.A., 2010. Tectonostratigraphic units and stratigraphic sequences of the nonmarine Songliao basin, northeast China. Basin Res. 22, 79–95. Feng, Z.H., Fang, W., Li, Z.G., Wang, X., Huo, Q.L., Huang, C.Y., Zhang, J.H., Zeng, H.S., 2011. Depositional environment of terrestrial petroleum source rocks and
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Journal of Petroleum Science and Engineering journal homepage: www.elsevier.com/locate/petrol
Acetate, DIC and δ13CDIC evidence for acetoclastic methanogenesis in Songliao Basin, NE China Wenlong Jiang a, Haiping Huang a,b,n, Ningxi Li a, Haifeng Zhang a, Chao Yu a a b
School of Energy Resource, China University of Geosciences, Beijing 100083, China Department of Geoscience, University of Calgary, 2500 University Drive NW, Calgary, AB, Canada T2N 1N4
art ic l e i nf o
a b s t r a c t
Article history: Received 9 January 2015 Accepted 30 April 2015 Available online 9 May 2015
Extracted waters of thirty eight (38) shale samples from Songliao Basin, NE China have been determined their acetate and dissolved inorganic carbon (DIC) concentrations and carbon isotopic compositions of DIC. Wide range variations have been observed in the geochemical data, which seem not controlled by organic richness and burial depth. Systematically low acetate concentrations and low DIC content, coupled with relatively 13C enriched DIC in Pu–Ao area and west slope than these from north and east areas of the basin suggest different degree of acetoclastic methanogenesis. Moderate methanogenesis in Pu–Ao and west slope is in consistent with biogenic gas discovery in these areas. Methanogenesis in north and east is inhibited due to the loss of pore space and undeveloped fracture system, which is also supported by none encountered biogenic gas. However, overall methanogenesis in these fine grained shales is very limited and current ongoing methanogenesis is doubtful. & 2015 Elsevier B.V. All rights reserved.
Keywords: Acetate Acetoclastic methanogenesis Dissolved inorganic carbon (DIC) Songliao Basin
1. Introduction Biogenic methane is an important end-product of biogeochemical processes associated with the bacterial remineralization of labile organic matter in sediments (Zinder, 1993; Petscha et al., 2005). It is trapped typically in relatively shallow and immature sediments beneath the anaerobic sulfate-reducing zone (Rice and Claypool, 1981; Schoell, 1983; Whiticar et al., 1986). Although difficult to quantify, biogenic gases account for roughly 20% of the worldwide natural gas resource (Rice and Claypool, 1981; Rice, 1993). When gas hydrates, secondary biogenic gas from oil reservoirs, coals and organic rich shales are accounted, biogenic gas is far more important than previous thinking (Kotelnikova, 2002; Milkov, 2011). There are two main pathways responsible for biogenic methane generation. The first pathway is CO2 reduction via CO2-reducing prokaryotes, which use hydrogen as the electron donor or energy source and CO2 as the electron acceptor (hydrogenotrophic methanogenesis): CO2 þ4H2-CH4 þ 2H2O. The second pathway is by acetate fermentation (acetoclastic methanogenesis). In this case, acetate and hydrogen are used to produce methane and carbon dioxide: CH3COOH-CH4 þ CO2. In general, CO2 reduction is the n Corresponding author at: Department of Geoscience, University of Calgary, 2500 University Drive NW, Calgary, AB, Canada T2N 1N4. Tel.: þ 1 403 2208396; fax: þ1 403 2840074. E-mail address: [email protected] (H. Huang).
http://dx.doi.org/10.1016/j.petrol.2015.04.040 0920-4105/& 2015 Elsevier B.V. All rights reserved.
dominate pathway in marine environment where acetate is relatively depleted because of sulfate reducing bacteria consuming. While in terrestrial freshwater or brackish environment, acetate fermentation is initially significant, but decreases with increasing buried depth (Whiticar et al., 1986; Zinder, 1993; Conrad, 1999). However, the exact quantitative role of hydrogenotrophic and acetotrophic methanogenesis in deep subsurface is not yet fully understood. Recent investigations have revealed that hydrogenotrophic and acetoclastic methanogenesis are equally important in deep subsurface (Veto et al., 2004; Parkes et al., 2007; Shuai et al., 2007; Flores et al. 2008; Ulrich and Bower, 2008). Acetate is produced by fermentation of organic matter as well as by reduction of CO2 with H2 via the acetyl-CoA pathway (acetogenesis) (Drake et al., 2006; Hädrich et al., 2012). It can also be produced by low-temperature diagenesis of organic matter, and by thermal alteration of organic matter at temperatures higher than 80 1C (Wellsbury et al., 1997; Horsfield et al., 2006; Parkes et al., 2007). Acetate serves as an important substrate for a variety of microorganisms including sulfate reducing bacteria and methanogenic archaea (Krumholz et al., 2002; Kotelnikova, 2002; Drake et al., 2006). Acetate concentrations are generally low in formation water if methanogenesis is in operation. The threshold concentration for acetoclastic methanogenesis in sediments is approximately 5 μM (Lovley and Phillips, 1987). High accumulated acetate concentrations may reflect inhibition of acetoclastic methanogenesis (Routh et al., 2001; Waldron et al., 2007). In the case of the Antrim Shale, acetate concentration is commonly negligible because of the
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high levels of microbial activity (Martini et al., 1998; McIntosh et al., 2002; 2004). Dissolved inorganic carbon (DIC) is the sum of the dissolved carbon dioxide, bicarbonate ion, and carbonate ion species (CO2, HCO3 , and CO23 ) in the formation water. High concentration of DIC in the formation water is an important indicator of microbial activity (Martini et al., 1998; Claypool et al., 2006). During methanogenesis by acetate fermentation or CO2 reduction, the produced or the remaining CO2 is incorporated into the DIC pool. Martini et al. (1998) noticed that microbial gas accumulations contain high concentrations of CO2 (up to 14.82 mol%). Isotopic exchange between CO2 and CH4 produces heavier δ13C values of DIC as 12C is preferentially concentrated in methane while 13C is enriched in the surrounding water as DIC. The unusually 13C enriched values of CO2 ( þ22‰) coproduced with methane and dissolved inorganic carbon (DIC) in formation waters ( þ28‰) indicate extensive methanogenesis. Otherwise, thermogenic CO2 or DIC would have much more negative δ13C values, closer to that of the kerogen undergoing maturation (about 30‰) (Martini et al., 1998; Budai et al., 2002; McIntosh et al., 2004). Thermally immature organic rich source rocks that could potentially generate large quantities of biogenic gas are widely distributed in the Songliao Basin, NE China. However, the shallow biogenic gas occurrence is very limited, which does not match the wide distribution of potential source rocks. Only a few smaller gas pools have been discovered, which show close relationship to secondary biogenic gas from oil biodegradation (Zhang et al., 2011). Li et al. (2015) suggested petrophysical property of shale may exert significant impact on biogenic gas generation in the subsurface. Undeveloped fracture system, very low permeability and very small pores resulting from compaction and diagenesis restrict bacteria movement and activity, limit nutrient transport, diminish space availability, and lead to a reduced biodiversity. In the present study, we focus on indirect evidence for biologic activity through analyses of its by-products retained in pore water as direct evidence of microbial life is typically unreliable due to contamination issues. This study seeks to understand more in situ microbial activities in deep subsurface environments.
2. Geological background Songliao Basin with about 260,000 km2 is the largest continental sedimentary basin in northeast China (Fig. 1). It has a dual structure with complex fault depression developed in late Jurassic to early Cretaceous and intracratonic basin in late Cretaceous–Cenozoic (Huang et al., 2004). The depositional succession from Jurassic— Neogene is about 10 km thick in the depocenter. The tectonic evolution of the basin is mainly affected by the thermodynamic changes of deep-seated mantle material and the Pacific plate’s activity. According to Feng et al. (2010), basin evolution can be subdivided into a Late Jurassic to Early Cretaceous synrift stage, an Early to Late Cretaceous postrift stage and a Cenozoic stage of structural basin inversion. A stratigraphic column outlines the formations as well as sediment type and thickness (Fig. 2). The main source rocks are Cenomanian-Coniacian Qingshankou (K2qn) and Santonian Nenjiang (K2n) formations. The Qingshankou Formation contains three members (K2qn1 to K2qn3) while the Nenjiang Formation can be divided into five members (K2n1 to K2n5) based on their lithology variation. Thermally mature regions of these source rocks near the basin depocenter contribute petroleum to the giant Daqing Oilfield (Feng et al., 2011), while the immature and low maturity source rocks that are widely distributed at depths less than 1500 m on the basin margins appear to be excellent potential source rocks for biogenic gas generation.
3. Experimental methods Shale samples collected from the Songliao Basin were analyzed for concentration of acetate, DIC, and δ13C values of DIC. Because no formation water is available from shale succession, organic acids and bicarbonate ion are obtained from water extraction of shale. Samples were crushed in a steel ball mill to fine powder (100 mesh), 20 g of shale was dissolved in 50 ml deionized water, ultrasound for 1 h at 60 1C and room temperature, respectively, and then centrifugation to obtain clear solution. Concentrations of acetate were detected by Dionex ICS2000 ion chromatography (IC). The 60 1C ultrasound solution sample was injected into the loop of injection after getting through a 0.45 μm aqua filtration membrane, then the sample entered into the Dionex ICS2000 ion chromatography system and detected by conductivity detector after chromatographic column separation. The qualitative analysis of acetate in solution was determined by comparing the retention time of the sample spectra with that of the standard spectra. For quantitative purpose, an external standard calibration method was applied. The standard solution was injected and the standard curve was drawn based on the relationship between peak area and concentration. The concentration of the sample was calculated by mapping the peak area of the sample with the standard curve. The linear correlation coefficient of standard curve is greater than 0.98 and the standard deviation of measured concentration is smaller than 5%. DIC and its isotopes were detected by continuous flow- isotope ratio mass spectrometer (CF-IRMS) with the room temperature extraction sample using Isoprime 100TM continuous flux coupled to a MultiflowTM from Isoprime Corporation. Transfer 0.5 ml sample in to a round bottomed flask, and then 5–6 drops of anhydrous phosphoric acid was added after purging the pure helium gas into the flask. Reaction at 40 1C for more than 5 h till the equilibrium is reached. The generated CO2 was first led into the quantitative loop, and then separated through a chromatographic column and dehydrated by membranes. After that, the carbon and oxygen isotopes of CO2 were then tested on the mass spectrometer. Carbon isotopes of CO2 were corrected by using the national standard GBW04405 and international standard NBS19. DIC standards were prepared by using two NaHCO3 solutions whose carbon isotopes were known. One standard was inserted every five samples, thus the carbon isotopic value of the sample can be obtained.
4. Result and discussion Table 1 lists analytical data generated from water extraction of shales and their basic geochemistry derived from Rock–Eval pyrolysis. The Upper Cretaceous Nenjiang and Qingshankou formations were developed in deep and semi-deep lacustrine depositional environment which are very rich in organic matter. The TOC values in current analyzed sample suite range from 0.1 to 4.4%. Relatively lower TOC content than typical published data (Feng et al., 2011; Bechtel et al., 2012) is because our samples are mainly from Nenjiang Formation collected at basin margin. Samples are essentially siltstone adjacent to fine grained shale as these samples have large pore space for microbial activity. Nevertheless, majority studied shale samples have TOC content above 1%, which is far higher than minimum requirement of metabolizable organic matter equivalent to about 0.5% of TOC proposed by Rice and Claypool (1981). Abundant organic matter is absolutely necessary for formation of biogenic methane and shales from the Songliao Basin have sufficient quantities of organic matter to maintain microbial activity. The Rock–Eval Tmax of the studied samples are between 429 and 445 1C. Types II and III organic matter with Tmax less than 435 1C are considered thermally immature
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Fig. 1. Structural sketch map of the Songliao Basin NE, China with sample locations.
(Espitalié et al., 1985). Most of our studied samples are inferred to be thermally immature while some samples may reach early oil window. Overall, burial depth o1500 m in the Songliao Basin remains in the microbial active window. 4.1. Acetate Variable amount of acetate has been detected from shale extracts in the Songliao Basin (Table 1). Concentrations of acetate show no correlation to organic richness and burial depth, implying other factors dominating its occurrence (Fig. 3). As overall source rocks are organic rich, acetate concentration reflects most likely the whole shale system in certain succession rather than controlled by local organic richness. Meanwhile, acetate is easy mobile when connected pore system is available. Acetate concentration variations in pore water are generally controlled by relative intensity of acetate formation and consumption. Acetate is not only an important product in the anaerobic degradation of organic matter, but also formed through various acetogenesis processes. Detailed acetate generation mechanism is out the scope of present study, however, no obvious geological difference constrains acetate generation at different locations of the Songliao Basin. We assume acetate concentration is governed by varying degree of consumption. The consumption of acetate is performed by a broad spectrum of microorganisms including acetoclastic methanogens and sulfate reducing bacteria. As Songliao Basin is typical lacustrine freshwater system, sulfate reducing bacteria are restricted in very shallow depth. Acetoclastic methanogenesis seems the only regulator in the study areas. A distinct difference can be observed geographically as acetate concentrations from north and east area are systematically higher than these from West Slope and Pu–Ao area. Average acetate concentration from north and east is 940.2 μg/g, which is almost 6 times higher than that from West Slope and Pu–Ao area. Low acetate concentrations in West Slope and Pu–Ao area reflect a steady state between production and consumption. Acetate serves as key intermediate during methanogenesis, which is consistent with the occurrence of biogenic gas in these regions. High acetate concentrations in extracted water samples may prove acetate fermentation is
limited. Previous studies suggested that acetate concentrations 45 μM indicate inhibited acetoclastic methanogenesis (Lovley and Phillips, 1987; Schlegel et al., 2013). Although our data is difficult to correlate with published value due different analytical methods, large difference between different areas may suggest variable microbial activities. No commercial biogenic gas has ever been discovered at north and east regions of the Songliao Basin further support that inhibition of acetoclastic methanogens may occur. 4.2. DIC and its isotope The DIC concentrations of extracted water from shales in the Songliao Basin vary widely from 148.0 μg/g to 1076.1 μg/g, and the isotopic values of DIC vary accordingly ranging from 13.8‰ to 0.9‰. Plot of DIC concentration versus C isotope values of DIC in extracted waters shows a general positive correlation although the coefficient is fairly low. Relatively higher DIC concentration and heavier δ13CDIC value occur at Pu–Ao area and West Slope than these in the north and east regions. The average DIC concentration and δ13CDIC value of samples from Pu–Ao area are 581.6 μg/g and 6.6‰, respectively. An average DIC concentration of 571.0 μg/g and δ13CDIC of 8.9‰ have been observed from West Slope. The average DIC concentration and δ13CDIC value of samples from east area are 412.3 μg/g and 10.8‰, respectively, and these from north area are 483.5 μg/g and 11.2‰, respectively (Fig. 4). The change in the DIC concentrations is related to relative rates of alkalinity production and removal. While quantification of net DIC addition is very complicated as organic matter oxidation, CO2 reduction and authigenic carbonate precipitation all affect the DIC pore-water pool (Claypool et al., 2006), isotopic value of DIC can help to decipher its main origin and control factors. The δ13CCO2 value derived from decarboxylation reactions from organic matter during diagenesis is estimated to range from 25‰ to 10‰ (Schoell, 1983; Chung et al., 1988). As no carbonate has developed in shallow strata of Songliao Basin, carbonate dissolution released DIC to the pore water can be ignored. During methanogenesis by acetate fermentation or CO2 reduction, the produced or the remaining CO2 is incorporated into the DIC pool, which is accompanied by an
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Fig. 2. Generalized Upper Cretaceous stratigraphic column of the Songliao Basin (modified from Feng et al., 2010).
isotopic fractionation. Enriched δ13CDIC are recorded in reducing environments where the production of biogenic methane, either by acetate fermentation or by CO2 reduction, removes 12C, leaving 13Cenriched CO2 in the system, which accounts for the positive δ13CDIC in formation waters. The isotope fractionation between DIC and CH4 illustrated by the enrichment factor (ε‰¼ δ13CDIC δ13CCH4) can be used to characterize methane formation as well as degradation processes. Enrichment factors between 10 and 50‰ may show acetogenic methane formation and high enrichment factors (450‰) may point to CO2 reduction as the dominating process of methane formation (Whiticar et al., 1986; Conrad, 1999; Whiticar, 1999; Grossman et al., 2002). Most biogenic gas discovered from the Songliao Basin has methane isotopic value between 60‰ and 55‰, while the average isotopic value of CO2 in the Pu–Ao
biodegraded oil reservoirs is þ2.3‰. CO2 reduction is reasonably assumed in oil biodegradation related methanogenesis (Zhang et al., 2011). Isotopic value of DIC form extracted water is much lower than isotopic value of CO2 in biodegraded oil reservoir, acetoclastic methanogenesis might be expected if it occurs. Slightly higher DIC concentrations and heavier isotopic compositions in shale samples from Pu–Ao area and West Slope than these from north and east are interpreted to be related to in situ anaerobic methanogenesis. If methanogenic processes remain constant, δ13CDIC will be continually enriched. Strongly positive δ13CDIC ( 4 10‰) have been documented in produced water from several biogenic gas-producing fields with δ13CDIC approximately þ 26 to þ 30‰ in Antrim Shale (Martini et al., 1998; Budai et al., 2002; McIntosh et al., 2004). However,
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Table 1 Aerate and DIC concentrations and isotopic compositions of extracted water from shales in the Songliao Basin and their basic geochemistry derived from Rock–Eval pyrolysis. Area
Well
Agen
Depth (m)
Acetate (μg/g)
δ13C (‰)
δ18O (‰)
DIC (μg/g)
TOC (%)
Tmax (1C)
S1 (mg/g)
S2 (mg/g)
W. Slope W. Slope W. Slope W. Slope W. Slope W. Slope W. Slope W. Slope W. Slope Pu–Ao Pu–Ao Pu–Ao Pu–Ao Pu–Ao Pu–Ao Pu–Ao Pu–Ao Pu–Ao Pu–Ao Pu–Ao Pu–Ao Pu–Ao North North North North North North North North North North North North North East East East
J88 Du58 Du58 Du28 Du28 Du28 Du28 Du28 Du28 AQ2 AQ2 AQ2 AQ2 AQ2 AQ2 Da161 Da161 Da161 Da161 Da161 Da161 Da161 D401 D401 D401 B3 SY1 SY1 SY1 SY1 SY1 SY1 Yu12 Yu7 Yu7 Si103 C73-87 S35
K2y2 þ 3 K2n1 K2n1 K2n1 K2n1 K2n1 K2qn2 þ3 K2qn1 K2qn1 K2n4 K2n3 K2n3 K2n3 K2n3 K2n3 K2n3 K2n3 K2n3 K2n3 K2n3 K2n3 K2y1 K2n1 K2n1 K2n1 K2qn1 K2n2 K2n2 K2n1 K2n1 K2qn1 K2qn1 K2n1 K2qn1 K2qn1 K2n2 K2qn1 K2qn1
604.1 751.2 908.7 1085.4 1095.6 1256.1 1379.7 1384.5 1398.2 712.4 719.5 727.5 739.9 776.5 782.7 1079.2 1087.2 1092.0 1097.6 1106.9 1113.7 1553.7 481.9 490.0 525.3 726.1 713.9 724.5 832.4 875.6 1276.0 1302.6 1137.0 1541.5 1551.5 364.0 770.8 1447.0
36.8 695.1 776.0 27.8 266.5 199.6 5.9 23.2 66.9 5.2 16.9 114.3 70.8 177.2 269.7 218.4 100.7 118.5 120.4 151.0 58.2 67.5 972.9 568.7 1115.0 1156.8 1236.3 644.4 1262.6 1222.7 772.4 451.7 1132.0 652.7 1128.9 653.8 1403.0 669.9
7.1 11.3 10.3 7.9 6.3 9.2 10.3 8.6 10.6 7.8 10.3 5.2 7.6 7.1 8.9 7.4 0.8 7.6 4.5 5.8 7.3 5.7 10.1 8.4 7.3 13.7 11.1 12.4 12.6 11.0 12.9 10.1 11.4 10.3 13.8 10.1 11.2 11.2
7.3 7.2 6.6 6.3 6.2 7.9 6.7 6.3 5.2 6.2 6.2 5.6 6.6 6.6 7.1 6.2 6.9 5.8 7.4 6.4 6.7 6.8 7.7 6.8 7.4 8.2 8.2 7.4 7.7 7.6 7.7 7.7 7.8 8.2 8.2 8.4 7.4 7.4
891.9 492.8 193.9 878.8 807.5 148.0 579.2 426.9 893.7 573.9 587.5 655.5 353.0 669.3 192.7 466.8 1076.1 713.6 548.0 499.4 556.8 668.1 759.5 389.3 693.5 327.5 542.9 680.3 639.4 523.1 391.5 465.5 280.9 271.0 320.9 403.3 623.3 210.3
1.6 0.3 0.6 0.7 4.4 0.2 1.4 1.5 0.1 0.9 1.3 0.9 1.2 1.2 1.1 1.7 0.8 0.9 1.0 1.3 1.3 0.7
436 430 433 432 439 435 441 445 439 435 438 436 433 436 437 441 437 441 439 440 439 445 437
0.06 0.01 0.02 0.02 0.49 0.02 0.19 0.12 0.02 0.02 0.01 0.01 0.04 0.03 0.03 0.06 0.03 0.02 0.03 0.09 0.04 0.02 0.02
6.07 0.14 1.92 1.25 30.78 0.21 9.46 8.54 0.07 0.36 1.24 0.37 0.35 0.47 0.60 1.26 0.80 0.89 0.57 0.82 0.88 2.17 2.48
2.2 0.1 0.6 0.5 0.9 3.0 1.2
436 432 436 429 430 429 432 433
0.05 0.01 0.01 0.01 0.01 0.03 0.13 0.03
12.90 0.05 0.36 0.24 0.65 4.34 23.91 2.10
0.6 1.1
441 438
0.06 0.05
0.21 1.36
K2qn: Upper Cretaceous Qingshankou Fm.; K2n: Upper Cretaceous Nenjiang Fm.; K2y: Upper Cretaceous Yaojia Fm.
Acetate Concentration (μg/g) 200
0
500
1000
5
W. Slope
1500
Pu-Ao
4
North
Depth (m)
600 800
TOC (%)
400
East
3
2
1000 1 1200 1400 1600
0 0
500
1000
1500
Acetate Concentration (μg/g)
Fig. 3. Acetate concentration variation in the studies samples. (a) Acetate concentration versus burial depth; (b) acetate concentration versus TOC content.
positive carbon-isotope excursion of DIC does not occur in the Songliao Basin, δ13CDIC remains constantly low in all samples, which should be interpreted to indicate no significant methanogenesis on going.
4.3. Biogenic gas generation Small gas fields have been discovered in the Pu–Ao area and west slope. Detailed gas chemical and isotopic compositions have
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1200
DIC Concentration (μg/g)
1000 800 600
W. Slope
400
Pu-Ao
in addition to compaction, would have contributed to reductions in permeability and pore throat size over time. Biogenic gas is most likely generated during early diagenesis when pore throats may not have been as restrictive as they are presently. Alternatively, small scale fractures might be developed in Pu–Ao area and west slope as they are situated at structural extension region. Patchy microbial activity associated with the fracture zone may be still going on, however, overall biogenic gas generation potential is low. Similar conclusion for overall low microbial activity in shale system has been draw by other studies (Gao et al., 2013; Schlegel et al., 2013).
North
200
East
5. Conclusion
0 -15
-10
-5
0
δ13CDIC(‰) Fig. 4. Correlation between DIC concentration and carbon isotopic composition in the studied samples.
been reported by Zhang et al. (2011). Gas from Pu–Ao area is dry gas with 13C depleted methane, enriched C2 þ and enriched CO2, which indicates typical secondary biogenic gas from oil biodegradation. Gas from West Slope is also dry with low CO2 content. Some gases not only have 13C depleted methane, but also 13C depleted ethane and propane, indicating primary biological gas and possibly low maturity gas. Majority of biogenic gas (δ13CCH4 o 55‰) can be attributed to oil biodegradation but gases from dispersed immature source rocks are projected (Zhang et al., 2011). Data from present study are consistent with previous publications (Zhang et al., 2011; Li et al., 2015) that primary biogenic gas is limited. In the north and east areas of the Songliao Basin, no biogenic gas reservoirs have been encountered, which is consistent with high acetate and low DIC concentrations together with isotopically depleted DIC. On the contrary, acetoclastic methanogenesis may contribute to biogenic gases discovered at Pu–Ao area and West Slope on the basis of acetate and DIC concentrations and δ13CDIC. Unlike the Michigan and Illinois basins, there does not appear to be large accumulations of microbial CH4 in Cretaceous organic-rich shales at the Songliao Basin. Current active methanogenesis in these fine grained shales remain doubtful but it definitely occurs in history. Most studies regard temperature as main control for deep subsurface life since increasing temperatures lead to denaturation of bacterial tissues. However, methanogenic activity can prevail over a wide range of temperatures with the common range of 4– 100 1C (Kotelnikova, 2002; Rice, 1993). The upper temperature may determine the maximum depth at which subsurface methanogens can be active; however, temperature alone does not limit bacteria in nonhydrothermal sediments until about 4 km (Parkes et al., 2000). Thermal regime is obviously not a critical control factor for biogenic gas generation in the Songliao Basin as all studied samples have never reached the maximum life-tolerant temperature of microbe. The dominant control on biogenic gas generation is not the maximum burial temperature as others suggested (Grasby et al., 2008). The absence of a microbial consortium is unlikely as oil reservoir at similar burial depth is largely biodegraded (Zhang et al., 2011). Decreased bioavailability of shale organic matter may exert some impact but cannot be dominated controlled as deeply buried Cretaceous black shales still act as active bioreactors in great sediment depths has been demonstrated by transport-reaction model (Arndt et al., 2006). We propose that limited microbial activity is constrained by pore spaces as grain size in deep lacustrine shale is too small. Samples of this study have undergone considerable diagenesis including cementation of particles by quartz overgrowths and the formation of secondary minerals. These processes,
Extracted waters from Pu–Ao area and West Slope of Songliao Basin have lower concentrations of acetate, higher concentrations of DIC and carbon isotopically heavier DIC than these from north and east regions. The consumption of acetate, generation and isotopic enrichment of DIC from Pu–Ao area and West Slope of Songliao Basin are consistent to acetoclastic methanogenesis mediation. Overall high acetate concentrations and only slightly elevated DIC isotope point to limited methanogenesis in the Songliao Basin. Loss of pore space and undeveloped fracture rather than organic richness and burial depth take dominant controls on microbial activity in the Songliao Basin.
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