temperature limit to oil generation. When the temperature exceeds the upper limit of the oil window—in excess of approximately 150°C [300°F]—condensate and wet gas result. At higher temperatures, through a more extreme thermal process termed metagenesis, less com- plex gases are generated, and methane gas even- tually becomes the primary hydrocarbon produced (below right). In contrast to the limited window for oil
generation—restricted to certain kerogen types and a specific temperature range—natural gas originates under a variety of conditions. It is gener- ated from all source rocks and across a broad tem- perature range. During diagenesis (early burial), anaerobic microorganisms can convert source- rock organic matter into methane. During cata- genesis and metagenesis, significant amounts of natural gas are produced.11 The maturation process lends itself to poten-
tially complex fluid columns and compositional gradients. The natural forces of gravitational buoy- ancy and solubility can create asphaltene gradi- ents in the fluid column. Gravity drives the less dense hydrocarbons, especially gas, to the top of the reservoir. Because asphaltenes are not soluble in gas, the presence of a large GOR gradient results in an asphaltene gradient with higher concentra- tions at a lower point in the column. Transport processes of both convection and diffusion may also be active. Unlike diffusion,
4. Mullins OC: The Physics of Reservoir Fluids: Discovery Through Downhole Fluid Analysis. Sugar Land, Texas: Schlumberger (2008): 43.
5. Asphaltenes are organic materials consisting of aromatic and naphthenic ring compounds along with peripheral alkanes and contain small quantities of nitrogen, sulfur and oxygen molecules. They exist as a colloidal suspension in oil. Asphaltenes can be problematic in production whenever they precipitate as a result of pressure drop, shear (turbulent flow), acids, solution CO2, condensate charging, mixing of incompatible crude oils or other conditions that break the stability of the asphaltic suspension.
6. Welte DH: “Organischer Kohlenstoff und die Entwicklung der Photosynthese auf der Erde,” Naturwissenschaften no. 57 (1970): 17–23.
7. Tissot BP and Welte DH: Petroleum Formation and Occurrence. Berlin: Springer-Verlag, 1984.
8. Peters KE and Cass MR: “Applied Source Rock Geochemistry,” in Magoon LB and Dow WG (eds): The Petroleum System—From Source to Trap. Tulsa: AAPG, AAPG Memoir 60 (1994): 93–119.
9. Welte DH and Yukler MA: “Petroleum Origin and Accumulation in Basin Evolution—A Quantitative Model,” AAPG Bulletin 65, no. 8 (August 1981): 1387–1396.
10. Equilibrium, in this article, is defined as a condition in which fluids are stable and modest changes in conditions result in modest changes in fluid properties. In contrast, metastable conditions are those in which modest changes may produce dramatic changes in fluid properties.
11. Grunau HR: “Abundance of Source Rocks for Oil and Gas Worldwide,” Journal of Petroleum Geology 6, no. 1 (1983): 39–53.
Diagenesis
Cap rock Gas
Lighter oil Medium oil Heavier oil Water
Oil window, low- temperature limit
Low-maturity source rock
Medium-maturity source rock
High-maturity source rock
> Stainforth charge history model. According to the Stainforth model, charge history determines hydrocarbon distribution. In the early stage, low-maturity source rock (left) generates heavier oil, medium-maturity source rock (center) produces lighter oils along with gas and, finally, high-maturity source rock (right) generates light oil and gas. Lighter fluids rise to the top of the reservoir and push down fluids that migrated earlier. The extent of dissolved gas (as reflected in the GOR) in the hydrocarbon column is controlled by pressure and temperature. In this model the fluids are not in equilibrium. Whether the reservoir fluids attain equilibrium is a function of parameters such as vertical permeability and thermal gradients. (Adapted from Mullins, reference 4.)
Biogenic methane
Oilfield Review Autumn 09 FluidsLab Fig. 2
CO2, H2O
Increasing depth and temperature Catagenesis
Immature zone 50°C
Oil window 150°C
> Hydrocarbon maturation. Early-stage hydrocarbon creation occurs in immature source rock in a process of diagenesis, whereby organic materials are buried, compressed and undergo chemical alteration. Bacterial diagenesis can also occur through anoxic microbial conversion of organic material to methane. As temperatures rise above 50°C with deeper burial, microbes die off and catagenesis predominates. This process is similar to the high-temperature cracking and distillation in oil refineries, where heavy oils are converted to lighter petroleum products, but can occur at much lower temperatures over geologic time. Metagenesis is a later phase of hydrocarbon generation, occurring above 150°C, in which organic materials and previously generated petroleum are converted into natural gas, predominantly methane, at higher temperatures.
Winter 2009/2010 41
Metagenesis Gas window
ORWIN09/10-FluidsLab Fig. 2
Types l and ll kerogen
Oil,
Wet gas and condensate
Dry gas All kerogen types
Hydrocarbons generated
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