CAPILLARY SEALING’S ROLE AS AN OVERPRESSURE MECHANISM IN THE FRUITLAND FORMATION OF THE SAN JUAN BASIN

Located in the San Juan Basin, the Fruitland formation is a sedimentary formation composed mainly of sandstone, shale, and coalbeds. This area experiences high levels of overpressure well above the normal hydrostatic pressure. The overpressure reaches a maximum of 13.1 x 10⁶ Pa greater than the standard hydrostatic pressure (Kaiser et al., 1994). In older sedimentary basins where overpressure cannot be inferred from the lack of continuing sedimentation and compact disequilibrium there is a need to look elsewhere for explanations of the causes of the overpressure. A model of ‘self-sealing’ from gas generation can help explain overpressure within an older sedimentary basin (Deming et al., 2002).

This research paper suggests that the Fruitland formation’s methane production from coalbeds could possibly cause some of the overpressure in the basin by means of capillary sealing. Capillary sealing occurs when capillary forces act as gas-water interfaces between fine and coarse grained clastic rocks in a sedimentary basin (Cranganu and Villa, 2005). Twenty-seven coal and coal-bearing samples obtained from the New Mexico Bureau of Mines & Mineral Resources were analyzed from the Fruitland and Picture Cliff formations located within the San Juan Basin. Data obtained from injecting mercury into rock pores by use of a mercury injection porosimeter allows for calculation of pore throat radii of fine and coarse grained clastic rocks. Average pore throat radius for the coal-bearing layers was found to be about 4 x 10^-8m while for coarse-grained samples was about 424 x 10^-8m. Further calculations indicate that pressure drop transversely through each gas-water interface could be as great as ΔP_c= 1.2 x 10⁶ Pa. Six gas layers identified in the target area by well logs have 12 interfaces which could potentially generate a maximum pressure change of 14.2 x 10⁶ Pa potentially advocating capillary sealing as accounting for the entire overpressure observed in the basin of 13.1 x 10⁶ Pa. Capillary pressure increases as pore throat size decreases and, as capillary pressure increases it is plausible that the overpressure of the system increases as well. A capillary sealing mechanism may be the cause for the pressure change experienced within the samples with an implication for contributing to the overall overpressure within the San Juan Basin.