1. Modeling Shallow Pore Water Chemistry above Marine Gas Hydrate Systems Sayantan Chatterjee, Gerald Dickens, Gaurav Bhatnagar, Walter Chapman, Brandon Dugan, Glen Snyder, George Hirasaki Rice University, Houston, Texas, USA April 23, 2012 Rice University Consortium on Processes in Porous Media DE-FC26-06NT 42960

2. Torres et al., Earth Planet. Sci. Lett., (2004) 2 Gas hydrates Cage structure Ice that burns Core sample Source: USGS - Clathrates - Ice-like solids - Guest molecules (e.g., CH 4 ) encapsulated in H 2 O cages Stability - High pressure - Low temperature - Low salinity Occurrence - Marine sediments along continental margins - Permafrost regions

3. Motivation Potential energy resource Subsea geohazardGlobal climate change McIver, AAPG (1982) Westbrook et al., Geo. Res. Lett., (2009) A fundamental understanding of the dynamics of gas hydrate systems Recoverable and Non-recoverable fossil fuels (coal, oil, natural gas), 5,000

4. Hydrate dissociation due to burial below the GHSZ Free gas recycling Geologic time (Myr) Concentration (mM) Subsidence Hydrate layer extending downwards Solubility Hydrate Free gas Dissolved gas Organic carbon CH 4 SO 4 2- reduction zone Base of GHSZ GHSZ Temperature ( o C) Depth Seafloor Geotherm CH 4 3-phase equilibrium 0 10 20 30 Sedimentation fluid flux External fluid flux 0 100 200 300 T0T0 Phase relationships Steady state Transient state Components TOC o Sediment flux SO 4 2- Hydrotherm Transient state Steady state Schematic of hydrate formation and burial Bhatnagar et al., Am. J. Sci., (2007); Chatterjee et al., (2012) to be submitted

5. 5 Methods to quantify gas hydrate amount and distribution Dickens., Org. Geochem., (2001) Free gas 994 995 997 Paull et al., ODP Init. Repts., 164, (1996) Boswell et al., Mar. Pet. Geo., (2011)

6. Gas hydrate systems and the SMT 6 Bhatnagar et al., Geo. Res. Lett., (2008) SMT: Sulfate Methane Transition

7. SMT depth inversely proportional to upward methane flux 7 Borowski et al., Geology, (1996) Paull et al., Geo-Mar. Lett., (2005) Gas hydrate bearing sediment Gas hydrate free sediment Fault lineSMT Chemosynthetic community Depth below seafloor SMT

8. CH 4 solubility and phase equilibrium curves Sedimentation and compaction Mass balance equations: Sediment Water Organic carbon Methane (CH 4 ) Sulfate (SO 4 2- ) Bicarbonate (HCO 3 - ) Calcium (Ca 2+ ) Carbon isotopes of CH 4 and HCO 3 - CH 4 sources: In situ methanogenesis (biogenic) Deep external sources (thermogenic) Advection, diffusion and reaction Key model features 8 PDEs solved using finite-difference method (Explicit and implicit schemes)

9. Geologic sites known/inferred for gas hydrates Chatterjee et al., J. Geophys. Res., (2011)

10. Modeling pore water chemistry at 3 sites Chatterjee et al., J. Geophys. Res., (2011)

11. Flux balance across the SMT using steady-state simulations Chatterjee et al., J. Geophys. Res., (2011); Chatterjee et al., (2012) to be submitted At the SMT: Diffusive flux Advective flux = 0

12. A 1:1 flux balance across SMT implies dominant AOM at the SMT Chatterjee et al., J. Geophys. Res., (2011); Chatterjee et al., (2012) to be submitted flux Anaerobic Oxidation of Methane (AOM)

13. Paull et al., Geo-Mar. Lett., (2005) SMT depth: A useful proxy 13 Gas hydrate bearing sediment Gas hydrate free sediment Fault lineSMT Chemosynthetic community SMT depth Net fluid flux Top of gas hydrate Hydrate saturation Bhatnagar et al., Geochem. Geophys. Geosyst., (2011) Pe 1 Top of hydrate / SMT Normalized Scaled SMT depth Depth below seafloor SMT depth Hydrate saturation SMT depth Top of gas hydrate Rule of ten

14. Top of hydrate / SMT 14 Seafloor and geologic parameters Base of hydrate stability Local SMT depth Top of gas hydrate Local fluid flux Local hydrate saturation Modeling to quantify hydrate amount and distribution Depth Base of GHSZ Depth Geotherm T3PT3P Hydrotherm Scaled SMT depth Rule of ten Pe 1 Local fluid flux

15. Conclusions 15 A 1:1 flux balance across the SMT implies dominant AOM at the SMT SMT depth is a direct proxy to relate upward methane flux and hydrate saturation An empirical rule of ten established to relate SMT depth and top of hydrate Developed a model to evaluate hydrate amount and distribution

16. Back up slides 16

17. Pore water chemistry and reaction zones 17 Sulfate reduction zone Methanogenesis zone Snyder et al., J. Geochem. Explor., (2007)

18. Pore water chemistry data: Sites 1244 and KC151 Chatterjee et al., J. Geophys. Res., (2011) 18

19. Pore water chemistry data: Site 1230 Chatterjee et al., (2012) to be submitted 19

20. Concentration crossplot of DIC and SO 4 2- 20 Chatterjee et al., J. Geophys. Res., (2011)

21. Steady state profiles: Site 1244 21 Chatterjee et al., J. Geophys. Res., (2011)

22. Steady state profiles: Site KC151 22 Chatterjee et al., J. Geophys. Res., (2011)

23. Physical property data: Site 1230 Chatterjee et al., (2012) to be submitted 23 Pressure (MPa)

24. Evidence of a 4.3 Myr hiatus implies Site 1230 is in transience 24 Hiatus

25. Seafloor 2.4 Myr ago Pre-hiatus steady state profiles: Site 1230 25 Chatterjee et al., (2012) to be submitted

26. Transient state profiles: Site 1230 26 Chatterjee et al., (2012) to be submitted

27. pH and activity correction 27 Chatterjee et al., (2012) to be submitted

28. Geotherm correction 28 Chatterjee et al., (2012) to be submitted

29. Effect of Da AOM 29 Chatterjee et al., J. Geophys. Res., (2011)

30. Effect of Da POC 30 Chatterjee et al., J. Geophys. Res., (2011)

31. 2:1 concentration crossplot 31 Chatterjee et al., J. Geophys. Res., (2011)

32. Concentration crossplot: Site 1244 Da = 0.22; C b,ext = 27 32 Chatterjee et al., J. Geophys. Res., (2011) Da = 1; C b,ext = 50

33. Concentration crossplot: Site KC151 33 Chatterjee et al., J. Geophys. Res., (2011)

34. Concentration crossplot: Site 1230 34 Chatterjee et al., (2012) to be submitted

35. Concentration crossplots CANNOT determine stoichiometry Chatterjee et al., J. Geophys. Res., (2011); Chatterjee et al., (2012) to be submitted Site 685/1230

36. Flux crossplot: Site 1244 Chatterjee et al., J. Geophys. Res., (2011) 36

37. Flux crossplot: Site 1230 Chatterjee et al., (2012) to be submitted 37

38. 1.Anaerobic Oxidation of Methane (AOM) (1:1) 2.Organoclastic Sulfate Reduction (OSR) (2:1) Two potential causes for SMT 38 = Dissolved Inorganic Carbon (DIC) ~ Alkalinity

39. = change from seawater AOM SO 4 2-, Alkalinity (mM) Depth (mbsf) (Alk+Ca+Mg) SO 4 39 OSR Site 1244 Kastner et al., Fire in the ice, (2008) Arguments for OSR: Stoichiometry and 13 C of DIC Depth (mbsf) 13 C DIC () OSR (2:1); δ 13 C DIC -25 AOM (1:1); δ 13 C DIC -60

40. Depth (mbsf) (Alk+Ca+Mg) 40 Site 1244 Dickens and Snyder., Fire in the ice, (2009) Depth (mbsf) Counterarguments for AOM and methanogenesis Methanogenesis; δ 13 C DIC 10 Flux units mol/m 2 kyr SO 4 2-, Alkalinity (mM)SO 4 +10 (methanogenesis) -60 (AOM) Deep DIC flux is enriched in 13 C OSR (2:1); δ 13 C DIC -25 AOM (1:1); δ 13 C DIC -60 13 C DIC ()

41. 13 C enriched DIC flux from depth impacts alkalinity and 13 C of DIC at SMT Chatterjee et al., J. Geophys. Res., (2011) AOM (δ 13 C DIC -60) Methanogenesis (δ 13 C DIC 10) OSR (δ 13 C DIC -25)

42. 42 OSR dominated systems CH 4 and SO 4 2- DIC (or HCO 3 - ) Ca 2+ δ 13 C in DIC

43. BHSZ Distinct zones of local fluid flux 43 High local fluid flux Low local fluid flux Seafloor Low local fluid flux

44. Local SMT depends on local fluid flux SMT depth Top of gas hydrate Low flux in sediment High flux in fracture BHSZ Pe Local = - 29 Pe Local = - 0.85 Normalized depth Normalized concentration BHSZ Local = 22% Local = 6% Normalized depth Hydrate and free gas Normalized depth Hydrate and free gas Normalized concentration 44

45. Generalized model to quantify amount and distribution of gas hydrates 45 Pe 1 Bhatnagar et al., Am. J. Sci., (2007) Biogenic sources only Biogenic sources and external flux Net fluid flux (Pe 1 ) and org C input at seafloor Hydrate saturation Net fluid flux (Pe 1 + Pe 2 ) Hydrate saturation

46. Kinetic and equilibrium reaction model Methanogenesis reaction: AOM reaction at the SMT: POC driven sulfate consumption above the SMT: Calcite precipitation reaction: 46

47. δ 13 C definition The isotopic carbon composition (δ 13 C ) in any sample is defined The isotope ratios usually reported in per mille, relative to an standard Pee-Dee-Belemnite (PDB) marine carbonate 47

48. Methanogenesis reaction: AOM reaction of biogenic methane at the SMT: Organoclastic sulfate consumption: Calcite precipitation reaction: Reactions with corresponding δ 13 C values 48

49. 1-D organic carbon mass balance Dimensionless mass balance 49

50. 1-D methane mass balance Dimensionless mass balance 50

51. 1-D sulfate mass balance Dimensionless mass balance 51

52. 1-D DIC mass balance Dimensionless mass balance 52

53. 1-D calcium mass balance Dimensionless mass balance 53

54. 1-D δ 13 C methane mass balance Dimensionless mass balance 54.

55. 1-D δ 13 C DIC mass balance Dimensionless mass balance 55