As per the agreement MarkWest will extend the NGL gathering pipeline from its Bluestone processing complex to XTO’s processing plant in Butler County.

The process plant is expected to become operational in late 2012.

MarkWest has three operational processing complexes and is constructing two more complexes in Pennsylvania and northern West Virginia to process Marcellus Shale gas production. The company is also constructing two processing complexes in Ohio for the Utica Shale gas production.

All the seven complexes will transport NGLs into the company’s extensive NGL gathering system which will then deliver the NGLs to the operating complexes or to the under construction Ohio fractionation and marketing complex.

MarkWest chairman, president and chief executive officer Frank Semple said there are vast potential reserves of rich-gas production in the Marcellus and Utica Shales in northwest Pennsylvania and northeast Ohio and one key to allowing the company’s producer customers to maximize the value of those reserves is to ensure that the NGLs can be effectively gathered, fractionated and marketed into the premium northeast market.

"Our NGL pipeline extension to the XTO processing facility also ensures that XTO will have access to all of the Marcellus ethane projects," Semple added.

"We have a great relationship with XTO and we are delighted to extend that relationship into the Marcellus."

Upon completion MarkWest is expected to operate about 3 billion cubic feet per day of gas processing capacity and approximately 270,000 barrels per day of fractionation capacity to support rich-gas production from the Marcellus and Utica Shale formations.

US based XTO Energy, a subsidiary of Exxon Mobil Corporation, is an energy producing company.