Part 6 Chula -- Hydrocarbon generation cracking expulsion.pptx

HC Generation

1. Hydrocarbon generation, cracking & expulsion Identify source rocks Characterize kerogen & SR richness Provide kinetic parameters for generation Choose cracking kinetics Develop expulsion model Calculate results

2. Identification of source rocks Geochemical data TOC, Rock-Eval pyrolysis Facies interpretation Sequence stratigraphy Geological models

3. Potential problems with source-rock data Bias toward richer rocks Bias from well locations Oil-based mud contamination Unpenetrated source rocks ?

4. Geochemical data requirements for possible source rocks TOC Oil SR: Minimum 1%; preferably >2% Gas SR: Minimum 0.5% (?)

5. Source-rock characteristics Kerogen quality Oil SR: Hydrogen Index (HI) >250 Type I = 600-900 Type II = 450-650 Type III (SE Asia) = 250-400 Gas SR: HI 100-200 Type III (traditional) = 80-150

6. Kerogen types for HC source rocks Types I, II III Types I-S, II-S, SE Asia III

7. Origins of kerogen types I: Algae from fresh water, brackish, hypersaline environments II: Marine algae; soft parts of terrestrial plants; resins; waxes III: Hard (structural) parts of terrestrial plants IV: Inertinite: oxidized residue; reworked material; fossil charcoal

8. Origins of kerogen types I-S: Algae from sulfate-rich anoxic lakes II-S: Type II kerogen rich in sulfur deposited in anoxic marine environments (usually nonclastic) of normal to elevated salinity

9. Origins of kerogen types SE Asian III: Mainly Tertiary age, tropical to subtropical, especially Australasia, strong contribution from terrestrial plants rich in resins and waxes, deposited in coal swamps and paralic settings.

10. Common source-rock facies Lacustrine (middle synrift phase) Lower coastal plain (LCP) coals Coals or shelf sediments redeposited in deep water Basinal marine facies (clastic or carbonate) Evaporite sequences

11. Kerogen types are associated with depositional facies This association leads to the concept of organic facies or organofacies , in which emphasis is partly on lithofacies and depositional environment, but even more on the origin and nature of the organic matter and how it was transported to and preserved in its final location.

12. Organofacies of Pepper & Corvi (1995) used to define kerogen types Marine carbonate Marine clastic Lacustrine Non-marine waxy Non-marine wax-poor

13. “Marine carbonate” Type II-S kerogen Organic matter of marine origin Non-clastic marine depositional environment Anoxic depositional conditions

14. “Marine clastic” Type II kerogen Organic matter of marine and possibly partly terrestrial origin Clastic marine depositional environment Anoxic depositional conditions

15. “Lacustrine” Type I or (rarely) I-S kerogen Organic matter of lacustrine algal origin, possibly mixed with minor hard (Type III and IV) and soft (Type II) terrestrial material Lacustrine depositional environment Anoxic depositional conditions

16. “Non-marine waxy” Type III (SE Asia) kerogen Organic matter of terrestrial origin, particularly from tropical Tertiary or possibly Late Cretaceous ages Non-marine (LCP) to shallow marine depositional environments. Can be reworked into deep-water mass-flow deposits. Can include everwet coals. Reducing to anoxic depositional conditions

17. “Non-marine wax-poor” Type III kerogen Organic matter of terrestrial structural origin Non-marine to marine depositional environments. Can be reworked into deep-water mass-flow deposits. Oxic depositional conditions

18. These are end-member organofacies Real organofacies can be more complicated

19. Example (A) A lacustrine environment with anoxic bottom waters in the center, dominated by freshwater algal contribution (Type I, lacustrine organofacies). But elsewhere in the lake (B) the bottom waters may be relatively oxic and there is contribution from land plants in addition to algae. What do we call environment (B)? What organofacies does it represent?

20. Once we have assigned kerogen types or organofacies to each source rock… we must assign kinetic parameters to those kerogens or organofacies

21. Kinetics parameters control… Absolute and relative quantities of oil and gas generated from kerogen Rates and timing of oil generation, gas generation, and oil cracking to gas Amount of gas produced by cracking a given amount of oil

22. Kinetic parameters for hydrocarbon generation Activation-energy distribution * Frequency factor * Total hydrocarbon yield % Oil, % Gas * Can be different for oil and gas

23. Kinetic parameters for cracking Activation energy distribution Frequency factor % Gas, % Residue

24. What is “activation energy”? Energy that must be put into a system in order to permit reactions to occur Like a barrier that must be overcome Higher Ea means slower reaction

25. Why a distribution of activation energies? Hydrocarbon generation consists of many different reactions. Each reaction has a different activation energy. We lump the reactions in groups for convenience. Groups consist of reactions having about the same Ea. The sum of all these different reactions represents HC generation. Therefore, we must include all the kinetic parameters that describe those many reactions.

26. Example of an activation-energy distribution

27. Traditional kinetic scheme Simple model for generation Kerogen → Oil + Exhausted kerogen Kerogen → Gas + Exhausted kerogen Oil & Gas are assumed to be generated using the same kinetics , and thus are generated simultaneously and in constant proportions during the entire process

28. Problem with traditional kinetics We know that oil generation largely precedes gas generation. Therefore, kinetics for oil and gas generation should be different. Traditional kinetics models are not correct.

29. One solution: Traditional kinetic data are abundant and easy to obtain in laboratory: attractive method Possible to mathematically separate oil generation from gas generation in traditional kinetics to get different kinetics for oil and gas generation (Waples & Mahadir Ramly, 2001) This solution enables us to use existing data or acquire new data cheaply and still keep oil generation separate from gas generation

30. Example: Total hydrocarbon generation (traditional kinetics)

31. Example: Separated oil and gas generation (Waples & Mahadir Ramly, 2001)

32. There is considerable variation in activation-energy distributions among the main kerogen types and also within any given kerogen type

33. Ea distributions for standard kerogen types

34. Ea distributions for two Type III kerogens

35. Ea distributions for three Type II kerogens

36. Where do we get Ea distributions? Standard kerogen types: in software Standard organofacies: in software Published data Personalized kinetic analysis

37. Organofacies kinetics (Pepper & Corvi, 1995) Oil and gas generation have distinct kinetics. Gas generation overlaps with the later phase of oil generation, as we saw in an earlier example.

38. How important is it to have the correct kinetics?

39. Variation in timing of generation for three standard kerogen types

40. Type II kerogens from clastic SR

41. Type II-S kerogens from carbonate SR

42. In some cases these differences in timing and/or levels of generation could be of great exploration significance It pays to choose the correct kerogen type and kinetic parameters

43. Kinetics of oil cracking Many people use a single activation energy. Waples (2000) suggested using a distribution of activation energies. Waples (2000) suggested using the same kinetics for all oils. Most workers agree, but others use slightly different kinetics for different types of oils.

44. Cracking kinetics of Waples (2000) A = 1.78 * 10 14 s -1

45. Reaction networks An alternative to the traditional kerogen-oil-gas-residue system

46. Reaction networks: advantages Multiple products possible Normal oil, light oil (condensate) Wet gas, dry gas Different kinetics for each reaction Flexibility

47. Reaction networks can be as simple or complex as you wish

48. Example: Separated oil and gas generation (Waples & Mahadir Ramly, 2001)

49. Reaction network example Kero1 → Normal oil + Kero2 Kero1 → Wet gas + Kero3 Normal oil → Light oil + Residue Light oil → Wet gas + Residue Wet gas → Dry gas + Residue Kero2 → Wet gas + Kero4 Kero3 → Dry gas + Kero4

50. Reaction networks: disadvantages Require lots of kinetic parameters Kinetic parameters not readily available Is this level of detail worth the extra effort?

51. Kinetics: summary Numerous options available All have advantages & disadvantages Choosing correct generation kinetics can be important Good kinetic data often available Acquisition of personalized kinetics is sometimes an excellent option

52. Expulsion of hydrocarbons from the source rock after generation

53. We do not understand how expulsion occurs Proposed models Darcy flow through HC-saturated pores Diffusion through continuous kerogen network Flow through pressure-induced microfractures

54. Because we do not understand the expulsion process(es), employing a mechanistic model may be unwise

55. But we do know some facts that can help us develop a non-mechanistic empirical model that should give good results

56. Facts 1. Expulsion does not start when generation starts. Buildup of HC’s in SR is required before expulsion starts.

57. Facts 2. Cumulative expulsion efficiency at full maturity is known for different types of SR

58. Facts 3. Although we may not know the specific details, expulsion seems to occur by some sort of overflow mechanism. Once saturation is reached, all excess HC’s can flow out.

59. These facts can be combined to give a simple empirical model for oil expulsion that is probably fairly accurate

60. → → → → → → → →

61. Alternative view on expulsion threshold for oil Start of oil expulsion occurs when mass of oil generated exceeds a certain percentage of the mass of the kerogen 10% often used (e.g., Pepper & Corvi, 1995) Compatible with previous plot

62. Gas expulsion Threshold required, but lower than that for oil (2%: Pepper & Corvi, 1995) All gas above threshold is expelled Gas expulsion efficiency is quite high Gas expulsion can precede oil expulsion

63. Software programs often do not include these specific models. The software input can usually be adapted to fit these conceptual models, and thus to do a satisfactory job.

64. Bottom line: Check the calculated expulsion to make sure it makes sense and you are happy with it. Don’t assume that the software is automatically calculating expulsion correctly or in the way that you prefer.

65. Summary HC generation, expulsion, and cracking can be calculated with good confidence if adequate data about source rocks are available. Because these calculations normally cannot be checked, the main output of 1D modeling depends on the quality of the model constructed.