BIOGENIC GAS KULIAH

1. Sketch of a generic basin, comparing the location of shallow, biogenic gas accumulations above the floor with the location of deep, thermogenic accumulations below the ceiling.

2. Cartoons contrasting pod geometries in basin cross sections with biogenic gas accumulations above the biogenic floor that is shown as a dashed gray line. Numbers are generic formations discussed in the text. (A) Early-generation system has a blanket shape. (B) Late-generation system has a ring shape marked by black diagonal lines.

3. Events charts for shallow biogenic gas systems. Format is modified from Magoon and Dow (1994). (A) Early-generation system has all events in relatively close time correspondence. Asterisks mark four new dates discussed in a following section in this article (see also Figure 11). (B) Late-generation system has a wide separation between the time of deposition of source and reservoir rock and the critical moment of generation, migration, and accumulation.

4. Shallow biogenic gas is natural gas generated by anaerobic bacteria from organic-rich, thermally immature source rocks. Environmental constraints on the microbes, especially temperature and water composition, provide the biogenic floor that is analogous to the thermogenic ceiling over deep, basin-centered gas . Biogenic gas accumulations are located at shallow depths above the floor, especially around the margins of the basin. These shallow biogenic gas accumulations generally are underpressured and host large numbers of low-volume wells. (Law [2002]).

5. Shallow biogenic gas accumulations occur in a variety of unconventional reservoir types that also have deep thermogenic gas in the same basin. Low-permeability clastic reservoirs in the Alberta basin have biogenic gas on the southeastern margin (Ridgley et al., 1999) and prolific thermogenic gas production in the basin's center (Masters, 1984). Fractured shales on the northern margin of the Michigan basin have economic accumulations of biogenic gas, although the thermogenic gas at the basin's center has not yet been demonstrated to be economic (Walter et al., 1997). Low-permeability chalk reservoirs produce biogenic gas on the eastern margin of the Denver basin and thermogenic gas near the basin's center (Rice, 1984a). Coalbed methane in the San Juan basin is dominantly thermogenic but includes a component of secondary biogenic gas along the northern margin (Scott et al., 1994).

6. Most significant production of shallow biogenic gas comes from depths of less than 2000 ft (600 m), although the depth of the biogenic floor may vary from basin to basin and over time within a single basin. A summary of worldwide biogenic gas accumulations gives an average minimum depth of 1800 ft (550 m) (Rice, 1993a). A review of biogenic gas fields in the western United States indicates that the average minimum depth is about 1600 ft (490 m) (Rice and Claypool, 1981).

7. Biogenic gas is dominantly methane, but it may contain up to 2% ethane, propane, butane, and pentane (Rice and Claypool, 1981). Isotopic analyses are used to verify a biogenic origin because methane-rich gases are also produced by other processes. Isotopic compositions are expressed as ratios relative to analytic standards for 13C and for deuterium in the methane. Ranges in these values are used to distinguish fields of composition that commonly characterize biogenic and thermogenic gases . Isotopic compositions of gases from low-permeability clastic reservoirs in the northern Great Plains, low-permeability chalks in the Denver basin, and coal beds in the Powder River basin plot within the field for biogenic gas (Rice, 1993a).

8. Crossplot of carbon-isotope ratio ( 13C) and deuterium-isotope ratio (dD) for methane from several different reservoir types (modified from Rice, 1993a).