CHAPTER-IV CARBON ISOTOPE GEOCHEMISTRY

Transcription

1 CHAPTER-IV CARBON ISOTOPE GEOCHEMISTRY > Introduction > Isotopic properties of thermogenic gas > Isotopic properties of bacterial methane > Isotopic properties of secondary gas types > Stable carbon isotopic compositions > Results and discussion

2 t If MMhJUV 1 \rho\ ivoiopf I;f Oi Iff \inin 4.1 Introduction Carbon isotope studies (l3c/12c) have contributed widely in identification of organic source materials. These studies are also useful in understanding the influence of migration, recognition of bacterial degradation, quantitative determination of the maturity of producing organic source materials and characterization of light hydrocarbon gases. The presence of specific compounds (8 Ci, 8 C2, and 8 C3) and particular molecular (Ci, C2, C3 and C4) distribution patterns are employed in source identification of hydrocarbons. Sources are traditionally subdivided in two classes of processes: one process occurs during the thermal degradation of organic matter and is associated with the generation of large amounts of heavier gas molecules and liquid hydrocarbons (thermogenic); the other generation process occurs through bacterial methanogenesis with insignificant amounts of other hydrocarbon molecules (biogenic). The origins of methanogenesis in hydrocarbon reservoirs have been extensively studied through the last decades (Stahl, 1977; Bernard et al 1977; Schoell, 1980, 1983; Galimov, 1988; Faber et al., 1992). Primary type hydrocarbon gases, which are generated directly by a simple source rock (Whiticar, 1994) are mainly dependent on the type of the kerogen, burial history (temperature and time) and geothermal gradient. Most commercial quantities of oil form from sapropelic (Type 1 and II kerogen) 105

3 organic matter, whereas gas forms from humic (Types III and IV kerogen) source (Fig. 4.1). Primary hydrocarbon gas generation profiles from humic Type III kerogen are different from sapropelic Type I and II kerogen. During their maturation history, sapropelic kerogens generate a significant volume of C2+ hydrocarbons (Hunt, 1995). Humic kerogens results in high amounts of thermogenic methane relative to C2+ components, yielding mostly CO? at low thermal maturity levels. The biogenic gases are almost exclusively methane, with less than 0.5% C2+ gases (Stahl, 1974; Whiticar et al., 1986). The Fig. 4.1 Relative proportions of natural gas generated from different types of organic matter (after Hunt. 1996). 106

4 occurrence of bacterial methane under extreme conditions of temperature (-1.2 to 110 C) and depth (>1 kbar) has also been documented (Whiticar, 1992). In addition to surface environments, methane formed by microbes has been detected in oil reservoirs as a common component of oil field gas (Whiticar, 1994). Thermogenic gas may be formed by thermal degradation of kerogen or thermal cracking of crude oil with increasing maturity (Schoell, 1980, 1983; Hunt, 1995; Whiticar, 1999). Thermogenic gas may be wet or dry. Wet thermogenic gases are main products of the mature stage (between 70 C and 150 C, with peak generation 120 C) (Hunt, 1995) (Fig. 4.1 and Fig. 4.2). They are formed from sapropelic (Type I and II) kerogen and thermal cracking of oil (Tissot and Welte, 1978; Hunt, 1995). Moreover, wet thermogenic gas in limited amount may also be derived from humic Type III kerogen. Dry thermogenic gas is formed by all kerogen types during over mature stage from 150 C to over 200 C (Stahl, 1977; Schoell, 1983). At temperatures beyond 150 C, wet thermogenic gases decreases to very low values and dry thermogenic methane becomes dominant gas type (fig.4.2). 4.2 Isotopic properties of thermogenic gas Thermogenic gas may be formed by thermal degradation of kerogen or thermal cracking of crude oil with increasing maturity (Schoell, 1983; Hunt, 1995; whiticar, 1999). Thermogenic gas has a wide range of molecular and isotopic properties, which are affected by type, amount and maturation of organic matter (Tissot et al., 1974). Because of kinetic isotope effects, generation of thermogenic gases causes isotopic fractionation leading to a gas 107

5 Generation Intensity Oo 0) k_ 3 *-> re a> a. E.0) Mature C H A PTER - IV CARBON ISOTOPE GEOCHEM ISTRY Overmature Immature Fig.4.2 Diagram showing generation of natural gas and its types with increasing temperature (after Rice and Claypool, 1981). 1 ^ depleted in C and leaves the residue enriched in C. Therefore during thermal maturation, later formed gas will be enriched in 13C, although still depleted with respect to the residual kerogen (Clayton, 1991). Consequently with increasing maturation of the source, the most mature gas will be the most enriched in C. Therrmogenic gas has geochemical properties, which gradually change via the effects of thermal maturation history of the source 108

6 material. Immature thermogenic gas is typically depleted in 13C and dry to very dry (<5% C2+) (Rice et al., 1989; Hunt, 1995). With increasing maturity, mature thermogenic gas is enriched in 13C and the gas is wetter (5%<C2+<15%)(Rice et al., 1989). Over mature thermogenic gas is significantly enriched in 13C and dry (<5% C2+) (Stahl, 1977; Schoell, 1983; Whiticar, 1994). The isotopic properties for wet thermogenic gas range from - 30%o to -60%o (PDB) and -120%o to -300%o (SMOW) (Hunt, 1996). Dry thermogenic gas isotopic properties are varying from -15%o to -40%o PDB and -70%o to -150%o (SMOW) (Hunt, 1995). Thermogenic gas from terrestrial environments (humic source) is enriched in 13C relative to the gas from marine environments (sapropelic source) (Stahl, 1975; Fuex, 1977). Fuex (1977) and Stahl (1977) proposed an empirical differentiation of thermogenic gas based on C2+ composition and carbon isotope properties of methane. Thermogenic gas associated with oil generation has 513C values in - CHAPTER- IV CARBON ISOTOPE 40%o to -58%e (PDB) range and significant quantities of C2+ components (>5%) (Fuex, 1977). The wet thermogenic gas may be derived from a mature oil source rock or from crude oil itself (Stahl, 1975; Fuex, 1977). Deep dry thermogenic gas from either terrestrial humic or marine sapropelic organic matter is characteristically dry (<5% C2+) and has methane enriched in 13C ranging from -25%o to -40%o (PDB) (Stahl, 1977). The dry thermogenic gas may be derived from over mature oil source rocks or from thermal cracking of crude oil at over maturity (Fuex, 1977). The first methane or thermogenic gas associated with crude oil formed from cracking reactions in petroleum has been suggested as approximately -50%o to -60%o (PDB) and -245%c to 340%o 109

7 (SMOW) (Schoell, 1983). Methane of thermogenic gas associated with condensates is enriched than -409% (PDB) indicating higher source maturities relative to that of oil associated thermogenic gas (Schoell, 1983). 4.3 isotopic properties of Bacterial Methane Roughly 20% of the worldwide natural gas reservoirs are estimated as originated from microbial sources (Rice and Claypool, 1981; Rice, 1992). Bacterial methane is formed in shallow marine environments by microbial reduction of CO2 and in freshwater environments by near surface microbial acetate fermentation (Whiticar et al., 1986; Whiticar, 1999). The occurrence of bacterial methane under extreme conditions of temperature (-1.2 to 110 C) and depth (<1 kbar) has also been documented (Whiticar, 1992). In addition to surface environments, methane formed by microbes has been detected in oil reservoirs as a common component of oil field gas (Whiticar, 1994). Bacterial gas may be recognized by unique molecular and isotopic properties. Bacterial gas is defined as exclusively methane depleted in 13C, with less than 0.5% higher C2+ components (Whiticar, 1994). It is possible to differentiate bacterially formed methane from thermogenic dry gas formed by thermal cracking reactions by carbon isotopic properties. The isotopic properties vary from -60%c to -1109% (PDB) and from -150%0 to -4009% (SMOW) (Schoell, 1983; Hunt, 1995; Whiticar, 1999). Anomalous values may occur outside these ranges because of variable sources of l3c and because of secondary processes such as migration or microbial alteration. Some specific environments with anomalous I3C values of sources (interstitial C02 in sediments with already enriched,3c values) may lead enrichment of l3c in 110

8 bacterial methane although these particular conditions do not exist with commercial quantities of bacterial gas (Nissenbaum et al., 1972). Bacterial methane is delineated with a field of values in Ci/^+C.fi concentrations ranging from 103 to 105 and in 13C properties vary from -60%c to -90%c (PDB) using Bernard plot (Bernard, 1978). 4.4 Isotopic properties of secondarj gas tvpes The emphasis on the unaltered gases (thermogenic and biogenic) was discussed above. In many cases geochemical analyses may help in determining the characteristics of a primary gas, some secondary (post generative) processes may alter their initial geochemical characteristics. The original molecular and isotopic properties of natural gas may suffer alteration as bio degradation or migration fractionation (Thompson and Kennicutt, 1990). A mixture of more than one gas type in a natural gas sample is a common phenomenon. This may be a bacterial gas within a thermogenic gas pool, indicating a mixed situation. Molecular and isotopic properties of a natural gas help to recognize that the gas is a mixture and to determine possible end members of the mixture and their relative contributions (Schoell, 1983; Chung et al., 1988; Whiticar, 1994; Prinzhofer and Hue, 1995). Mixtures of bacterial and thermogenic gas may be recognized by using 5,3C of methane and Ct/(C2+C3) molecular concentration of the gas on the Bernard plot. The relationship between 513C of methane and 813C of ethane data pair has also been widely used to define the mixed gas properties (Schoell, 1983; Berner, 1989). For cogenetic methane ethane pairs in thermogenic gas, it has been 111

9 observed that generally ethane is enriched in ' C between 5%c (PDB) and 10%c (PDB) relative to methane (Silverman, 1971; Deines, 1980). If bacterial methane is added to a thermogenic methane, the 8UC value of the methane changes accordingly and the 5 ' C value of the ethane remains constant (arrow Ms) (Schoell, 1983). 4.5 Stable carbon isotopic compositions Carbon has two stable isotopes, ~C and C in the global abundance of 98.89% and 1.11% respectively. During chemical, physical, biological process, these relative ratios may be transformed slightly, leading to different isotopic compositions for certain carbon pools. In one such example, 12C is preferred in photosynthesis on land leading to a general depletion of,3c in all terrestrial organic matter. Conversely the lower vapour pressure of C02 results in the dissolved inorganic carbon being enriched in 13C in marine organic matter relative to terrestrial organic materials. Provided that organic compounds retain the carbon isotopic compositions of their biosynthetic precursors, variations in isotopic compositions may be used to distinguish hydrocarbons of different origin and trace the movement of carbon through organic and inorganic reservoirs (Murphy and Abrajano, 1994; Dowling et al., 1995). In order to obtain the stable carbon isotopic composition of individual compounds, the components of the sample mixture are separated by GC. The organic carbon of the sample is then quantitatively combusted and converted into CO2 and the three major isotopic masses (44, 45 and 46) of CO2 measured. Results are reported in conventional delta (8) notation. 112

10 5 C = 1000 X (Rsample/Rstandard - \)%c Where R represents the ratio l3c/12c. All reported analyses are referred to the internationally accepted standard Pee Dee Belemnite (PDB) a calcium carbonate fossil of Belemnite Americana form Cretaceous Pee Dee formation in South Carolina which is assigned a 513C value of 0%c. 4.6 Results and discussions The carbon isotopic composition of methane (5 CO, ethane (8 'C2) and propane (813C;?) for seventeen samples was selected on the basis of concentration of methane and the presence of heavier hydrocarbons is given in Table 3.1 (chapter 3). Selected soil sample locations (Table 4.1) have been marked in Figure 4.3. Table 4.1 Selected soil sample locations for carbon isotopic ratio analysis S.No. Sample Id Latitude Longitude 1 GNR " 29 56' 40" 2 GNR ' 50" " 3 GNR " " 4 GNR ' 00" 29 59' 10" 5 GNR " " 6 GNR " 30 00' 50" 7 GNR " 30 00' 00" 8 GNR ' 20" 30 00' 50" 9 GNR ' ' 40" 10 GNR " " 11 GNR " " 12 GNR " " 13 GNR ' 50" 30 00' 00" 14 GNR " 113

11 15 GNR " 30 00' 00" 16 GNR ' 10" 29 59' 10" 17 GNR ' 10" " The methane carbon isotope (513C,) compositions range from to -37.0% PDB (Table 4.1b). It might be noted, however, that if partial microbial oxidation has occurred the residual gas will be isotopically enriched (i.e. heavier) and the mode of formation and/or thermal maturity of the source may be misinterpreted (Coleman, Risatti & Schoell, 1981). Such enrichment would suggest a stronger thermal signature (i.e. a more elevated level of thermal maturity) than would be observed in an unaltered gas. The wet gas components also display significant variability in their stable carbon isotope compositions (Tabele 5.1). The ethane (813C2) values have a range from %c to %. The propane (813C3) values have a range of % to %e (PDB). The mean carbon isotopic composition of methane (-32.4% ), ethane (-23.39(0 and propane (-22.89ic). It has long been known that methane (C,) is always isotopically lightest among gaseous hydrocarbons followed by ethane (C2), propane (C3), butane (C4) and pentane (C5) if the gas is pristine. Such an orderly distribution was explained from theoretical calculation by Waples and Torheim (1978) as well as laboratory heating experiments by McCarty and Felbeck (1986). Generally, carbon isotopes of methane, ethane and propane show 61 C <6MC:<(5''Cs trend in natural gas. The present study shows almost similar trend like 51,Ci<8!3C2<8I3C3. 114

12 Table 4.2 Stable carbon isotope composition with wetness (t.t/fc b+(b)] of ight hydrocarbons S.No. Sample Id 813Ci(%c) 813C2(%e) 813C3(%c) Ci/(C2+C3) 1 GNR nd nd 15 2 GNR nd nd 13 3 GNR nd nd 9 4 GNR nd nd 12 5 GNR nd nd 16 6 GNR GNR nd nd 20 8 GNR GNR nd nd GNR nd nd GNR nd GNR GNR GNR nd nd GNR nd nd 8 16 GNR GNR The molecular ratio of Ci/fCs+C.O and carbon isotope properties of methane (8i3Ci) are used in order to delineate its origin. Bernard (1978) proposed a genetic classification diagram combining molecular and isotopic properties of gases from vents, seeps and sediments in various areas. Schoell (1983) provides a plot relating variations in the carbon isotopic properties of ethane and methane based on the hypothesis that mixing of various proportions of two gases results in a linear change of their isotopic properties. It has recently been proposed that some isotopically lighter methane accumulations are not 115

13 biogenic in origin but are the result of isotopic fractionation during hydrocarbon migration (Prinzhofer and Pematon, 1997). Prinzhofer and Pernaton (1997) suggested that when the Q/Ci ratio is plotted against the methane stable carbon isotope (8I3Ci) composition curve display a positive concavity if diffusion had occurred during migration resulting in isotopically light methane. 116

14 I Kilometers,4.3 Loo; Sample Location for Isotope analyses town l'.6to Z InQUPoU 3«0S,Sfo Zl 3 0l.tt.CZ 3 M,Zt*ZL L ) ID o m N Kesrisinghpur # # ChakB12Q Mirzawala Mohanpura Prithwfrajpur at ion of soil sample selected tor Carbon isotope anal SO 0 3-OS.OtoCi IM&ZoZL luozizou i i i i N) < SI w o w oo o

15 A classic Bernard plot has been used to differentiate the gases produced from biogenic activity to that from thermogenic processes (Bernard, 1978). Microbial degradation produces hydrocarbon gas with Ci/(C2+C3) ratios greater than 1000 and contains almost exclusively methane with very low concentrations of ethane, propane, butane and pentane (C2+). Methane usually a quite isotopically depleted 13C ratio of less than -60%0. Thermogenic processes produce a wide spectrum of low molecular weight hydrocarbons with C,/(C2+C3) ratios ranging from 0 to 50 with significant contribution from ethane and higher hydrocarbons. The light hydrocarbon gases from the Shri Ganganagar area are shown on the Bernard plot (Fig. 4.4) and the values for the ratio (Ci/C2+C3) are provided in the Table4.2 along with the 813C value of methane. The adsorbed alkane gases from all of the seventeen soil samples composed of methane with considerable fractions of ethane and propane and their C]/(C2+C3) values are below 50. The isotopic value of methane is ranging from %0 to %0 (PDB). None of the isotopic values is a characteristic of the biogenic range. Thus the two important parameters, the gas wetness ratio Ci/(C2+C3) and 8I3C of methane and the trend between these two are highly diagnostic of the thermal origin of hydrocarbon gases from the Shri Ganga Nagar area. 118

16 = A Fig. 4.4 Log C4 A ;>+C:i versus <'>"' 4 (Bernard plot) for die adsorbed light gaseous hydrocarbons from the Shri Ganganagar area. Bikaner- Nagure basin (alter Bernard et al ). Abrams (1996b, 2007) used the methane isotope and compositional data to classify the gas into one of three categories: Type I is sediment gases with small concentrations of methane (usually <200 ppb) and isotopically enriched in l3ci (>-45%c) and is bacterially altered; Type II is sediment gases with large concentration of methane (usually >1000ppb) with depleted methane carbon isotope ratios (<-55%e) and is bacterial sourced gas or mixed with in situ bacterially derived gases, and Type A is sediment gases with elevated concentrations of methane and carbon isotope ratios within the tehrmogenic range (-35 to -55%c), indicating the presence of thermogenic seepage. The 119

17 adsorbed gases from the Shri Ganganagar area cover the range of Type-A category with almost no samples typical of Type-I and Type-II zone (Fig. 4.5). All the samples in Type-A zone are characterized by large concentration of Ci along with the presence of ethane plus higher hydrocarbons (C2+) and 8 Ci values between -26.2'G to -31%,c, (PDB). These samples show the isotopic compositions similar to the subsurface methane (Type-A) and could be considered as unaltered thermogenic gas migrating upward and reaching the near surface environment. Genetic characterization of the hydrocarbon gases has been carried out by Schoell (1983a, 1983b) by means of incorporating their specific genesis (terrestrial or marine source), which serves to further the understanding of origin and occurrence of these gases. Seven samples from the Shri Ganganagar area, which show the 813C values for both methane and ethane, have been used to genetically typify the hydrocarbon gases based on the Schoelfs diagram (Fig. 4.6). The 5l3Ci - S' Tb signatures of theses samples indicate the gases to be non associated, originating from sapropelic liptinitic organic matter i.e TT(m) and humic organic matter TT(h) of Schoelfs classification and represent an admixture of thermally generated gases from two sources. To elaborate; TT is the second stage of thermogenic gas formation which follows the principal stage of oil formation resulting in dry or deep dry gases and (m) is the marine or sapropelic source organic matter and (h) symbolizes the humic source. The compositional shifts due to migration 120

18 10000 Methane (ppb) 10 i , n(XMcthane)%o Fig.4.5 Isotopic classification of adsorbed methane extracted from the sitrficial soil samples of the Sliri Ganganagar. Bikaner Nagaur Basin (after Abrams. 1996b). are indicated by arrows, Md (deep migration ) and Ms (shallow migration), respectively. Mixing of gases is a common phenomenon (Whiticar, 1994), which indicates that gas components are not necessarily co genetic, and is observed in the alkane gases from the Shri Ganganagar area. The four samples (GNR 59, GNR 93, GNR 66and GNR 75) have 813C,-S,3C2 values 121

19 characteristic of the sapropelic organic matter where as three samples (GNR 44, GNR 37, GNR 95) show enriched 513C values of methane, represent the contributions from humic source. The stages of maturity ranging from immature (bacterial/diagenesis) to mature and over-mature are defined according to Dow (1997). The maturity of organic matter in terms of vitrinite reflectance (Ro) reflects the maturity of the gas source rock at the time of generation and migration. It is known that the type and maturity of source influences the isotopic composition of the produced light hydrocarbon gases. Based on the relationship between maturity and the,3c concentration in methane, 813C can be used as a qualitative maturity parameter. Despite the constraints of mixing and bacterial alterations etc., an estimate of source rock type and maturity of thermogenic gas can be determined by comparing the stable isotope signature of methane versus ethane and of ethane verses propane (Berner and Faber, 1993; James, 1983; Stahl and Carey, 1975; Whiticar and Faber, 1986; Whiticar, 1994). In the Fig. 4.7 the 813C values of adsorbed methane and ethane from the Shri Ganganagar area are plotted together with the semi-empirical maturity relationship of Berner and Faber (1996). The 8 C values of methane and ethane display a trend similar to that of thermogenic gas derived from terrestrial source rock whose maturity ranges between 0.5 to 1.5 vitrinite reflectance and is represented by GNR-44, GNR- 37 and GNR-95. The adsorbed alkane gases from samples GNR-59, GNR-93, GNR-66 and GNR-75 indicate a shift towards the terrestrial source from that 122

20 of a marine one and show a mixing trend between the two different sources. The published model of Berner and Faber (1996) is dependent on the original CHAPTER - IV CARBON ISOTOPE GEOCHEM ISTRY Fig.4.6 Plot showing carbon isotope variations in methane related to carbon isotope variations in ethane extracted from the surficial soil samples of the Shri Ganganagar. Bikaner-Nagaur basin, (after Schoell. 1983a). carbon isotopic composition and maturity of kerogen. However the gases extracted from sediments are immature, the alkane gases are presumed to have been migrated from deeper, more mature part of the sedimentary column and their 51?C values provide a general indication for the level of maturity. Based on maturity scale applied to the surfacial sediment samples of the Shri 123

21 Ganganagar area, the source material might be capable of generating oil and Carbon isotopic ratios of methane, ethane and propane indicate the thermogenic source of light hydrocarbons in the study area. Prominent signature of carbon isotopes observed is in northern part of the study area. 124