Vespene Gas is the secondary resource in StarCraft. It is more difficult to harvest than Minerals, requires additional buildings, and has a limited worker capacity. Since only one worker can mine Vespene Gas at a time, much fewer workers are usually relegated to this task than mineral harvesting. As such, Vespene Gas is more essential for late game units, upgrades, and special abilities. As the game progresses, the player who controls more geysers will often emerge victorious, as he or she can muster more powerful armies. In order to begin mining gas, a player will have to build a Terran Refinery, Zerg Extractor or Protoss Assimilator on top of the geyser. Workers will gather 4 Vespene Gas units per trip. Most Vespene Geysers have 2500 gas.
Vespene gas has an unusually high potential energy rating, and as such it is highly valued for use as a fuel in high-performance engines and energy reactors. The Zerg have adapted themselves to use raw Vespene as a source of nourishment to drive their greatly accelerated metabolisms. Even the Protoss require Vespene to work as a catalyst for their psi-driven machines. Geysers of this powerful green gas can be found on many planets along the Rim, and several space stations have been built over Vespene-rich asteroid belts. A refinery of some sort must be built over an active geyser to process and package the gas for its collection by the different species. While an individual geyser has an effectively unlimited amount of gas within it, after a certain point the geyser “collapses” and only trace amounts of Vespene can be refined from it. Most commanders would do well to seek fresh geysers to ensure that they have enough fuel to maintain their military.
As with Minerals, there are some new twists on Vespene Gas gathering in StarCraft 2. Most importantly, workers now gather only half as much gas per trip (4 as opposed to 8 in SC1). However, most starting locations and expansions will have 2 Vepene Geysers rather than just 1, as was usually the case in StarCraft 1 maps. Also, the improved AI pathing of the workers is said to somewhat make up this rate reduction, so that gathering is about as fast as it was in StarCraft 1.
Blizzard considered a "recharging" ability, in which a player could invest Minerals in order to replenish some gas in their geyser. This ability has since been removed from the game due to balancing issues. Another change is the geyser's behaviour once it has been depleted. In StarCraft 1 a depleted geyser could be harvested indefinitely, at a much lower rate of 2 gas units per trip. In StarCraft 2, however, a depleted geyser cannot be mined at all, and the player will have to manually re-assign his workers to a new task.
Early in development, Blizzard hinted at the possiblity of High Yield vespene geysers, similar to High-yield mineral patches, but this idea was scrapped.