HARRISBURG — Pennsylvania has an estimated 200,000 abandoned oil and gas wells, but knows the location of only a tiny fraction of them — a potential problem for Marcellus Shale drillers trying to avoid the old wells, according to a published report.

Though it’s rare for a modern drilling operation to intersect with an abandoned well, it does happen. A subsidiary of Shell Oil was drilling in Tioga County last summer when a 30-foot geyser of methane gas and water erupted from the ground, prompting the evacuation of several homes until the leak was brought under control.

The gas most likely came from a shallow formation and made its way into an abandoned well drilled in 1932, according to StateImpact Pennsylvania, a collaboration of public radio stations WITF in Harrisburg and WHYY in Philadelphia.

Gene Pine, who runs a division of the state Department of Environmental Protection that is charged with finding and plugging abandoned wells, estimated that about 325,000 wells have been drilled in Pennsylvania since Edwin Drake sank the first commercial oil well in Titusville in 1959.