In the current commodity price environment, the best thing about a natural gas rig might be that it can stop drilling for gas, move somewhere else and start drilling for oil.

Put another way in a recent note by analysts at Goldman Sachs: “Where a rig is drilling is a more reliable indicator than its classification as a natural gas or oil rig in predicting if the well drilled will be a natural gas well or an oil well.”

Goldman analysts looked at well-level data to analyze productivity gains in the Eagle Ford Shale for natural gas, natural gas liquids (NGL), condensate and crude oil. They found:

Rig productivity refers to how many wells can be drilled by a rig over a given period. Well productivity refers to how much commodity can be produced from each well.

“Given the shift away from dry gas areas [in the Eagle Ford], drilling location has been more important than productivity gains in explaining Eagle Ford production growth, especially on the natural gas side, as gas well productivity gains have been modest when compared to more productive shale gas areas such as [the] Haynesville [Shale],” the analysts said. “On the other hand, Eagle Ford productivity gains have been more pronounced on the oil and condensate side, consistent with an increased focus on these production streams.”

Unlike in other shale plays, the Eagle Ford offers producers their choice of dry gas, wet gas and oil in regions that are fairly well delineated. Goldman said weighting rig counts for gas or oil based upon the South Texas county in which they are drilling improves their historical fit to well additions, particularly for gas wells, compared to relying upon reported rig classifications.

“We estimate that if drilling had not migrated to the oilier areas of the [Eagle Ford] play, and instead rig counts had increased proportionately across gas and oil areas, gas well production would be twice as high and oil production would be only a small fraction of current levels,” the analysts wrote.

And on the oily side of the play is where the productivity gain action is, Goldman found. This factor contributes to the fact that rig migration from the gassy Haynesville to the oily Eagle Ford means Eagle Ford gas production will not offset declines in gas production from the Haynesville, the analysts said. Overall, migration of rigs from the Haynesville to the Eagle Ford will slow U.S. gas production, they said.

“Specifically, keeping 10 more rigs in Haynesville over the course of 2012 would have added 0.4 Bcf/d to production in 2013, while adding another 10 rigs to Eagle Ford in 2012 would have added only 0.2 Bcf/d in the gas window and less than 0.1 Bcf/d in the oil window,” the analysts said.