Hydrogeological Controls on Coalbed Methane Accumulation in Eastern Surat Basin, Australia

Abstract

AbstractThe Middle Jurassic Walloon Coal Measures (CM) is a prolific, low-rank coalbed methane (CBM) resource in the Surat Basin, Australia. Depositional environment, structural background, coal quality, and hydrogeology have been identified as determining CBM accumulation. It is the basis for the evaluation and optimization of CBM favorable areas to clarify the main controlling factors of CBM accumulation. Taking the eastern Surat Basin in Australia as an example, based on clarifying the geological characteristics of CBM, the gas content versus depth profiles of 88 CBM core wells revealed the gas content changes in the eastern Surat Basin. Within individual wells, gas content either increases; increases, then decreases; or decreases with depth as the result of the variable coring intervals. The gas content maxima coincides with the Tangalooma Sandstone, regardless of depth, which is mainly controlled by the secondary biogenic recharge from above Tangalooma sandstone. It is proposed that hydrogeological conditions are the main controlling factors of gas content distribution and CBM accumulation in the eastern Surat Basin, mainly including groundwater hydrodynamic conditions and surface water system. In terms of groundwater hydrodynamics, from the eastern edge to the center of the Surat Basin, the degree of salinity gradually increased, the anion content of groundwater changed from mainly HCO3− to mainly Cl−, and the hydrodynamic region was changed from strong runoff zone, medium runoff zone to a weak runoff-retention zone; the favorable water type for CBM accumulation in Walloon CM is mainly Cl•HCO3-Na type, which is a weak runoff-pressured zone hydrodynamically. In terms of surface water system, the relationship between the direction of surface water runoff and the formation tendency in the eastern Surat Basin controls CBM accumulation. When the direction of runoff is consistent with the formation tendency, the formation is overpressured or hydrodynamically blocked, indicating favorable accumulation conditions, and the direction of water flow is conducive to the methanogen-producing bacteria to carry into the coal seam, thereby generating biogas and increasing the gas content. When the direction of surface water runoff is opposite to the formation tendency, the formation underpressure or hydrodynamic erosion will cause the loss of CBM, indicating unfavorable accumulation conditions, and it is not conducive to the introduction of methanogenic bacteria, thus, there is no biogas supply.KeywordsSurat BasinCoalbed MethaneHydrogeologyGas contentAccumulation