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In any natural gas play, the degree of saturation defines, in percentage terms, how much gas could be in the reservoir versus how much is actually present. For coalbed methane (CBM) plays, adsorption isotherms need to be measured to determine maximum gas holding capacity. The gas charge of a reservoir is determined through measurement of the rate and amount of gas desorbed from samples of freshly cored coal. These two measures (adsorption and desorption) allow the % gas saturation to be calculated and will give vital clues as to how, when and to what degree a reservoir will produce gas.The level of gas saturation in a coal reservoir is most notably affected by rank (i.e. level of maturation). In under-mature coal seams, methane is produced purely biogenically and because the conditions must be just right for microbes to produce gas, there can be large variations in the gas produced, and thus also the saturation, within a single seam. With increasing rank, geochemical processes are predominant, with methane production and saturation levels tending to be consistently high. However, at certain depths and therefore temperatures, the gas holding capacity in coal can decrease, effectively expelling gas. If the coal is later uplifted, the gas holding capacity will increase but the reservoir will be under-saturated. Saturation can be restored through secondary biogenic recharge. The effect that saturation has on gas flow is second only to permeability. Low gas saturation will result in uneconomic flow rates and low gas recoverability. In low rank coals, such as found predominantly in Indonesia, gas saturation may be the governing reservoir property on the economics of gas development. A basin may contain significant gas in-place (GIP) values but what must be remembered is that GIP takes no account of saturation.