It was announced last week that Exxon Mobil has purchased Phillips Resources and TWP Inc. at a price of $1.7 billion.

These are both Pennsylvania-based exploration and production companies with significant Marcellus Shale reserves and likely exposure to the emerging Utica Shale as well. This acquisition provides Exxon with 317,000 acres prospective for the Marcellus.

Last week’s purchase follows on the heels of Exxon’s acquisition of XTO Energy in 2010. While the XTO purchase gave Exxon a diversified portfolio of natural gas reserves in various conventional and unconventional plays, the Phillips acquisition more narrowly targets the Marcellus.

Exxon’s most recent investment confirms the interest of the major oil and gas companies in Pennsylvania’s unconventional shale deposits. European companies Royal Dutch Shell and Statoil have both invested heavily in Keystone State shale, while Chevron recently paid $3.2 billion for Pennsylvania based Atlas Energy.

Perhaps the most interesting aspect of this deal is the price.

Exxon has paid approximately $5,000 per acre for the Phillips – TWP acquisition. A New York Times article notes that this is barely half the price companies were paying for comparable acreage last year. For instance, Japan’s Mitsui and India’s Reliance Industries both paid $14,000 per acre for Marcellus assets in 2010, while EXCO purchased acreage in Pennsylvania at the rate of $9,000 per acre last December.

This decline in price mirrors what this Commentator and others have observed with lower cash-bonus prices offered for gas leases in Central and North Eastern PA.

It seems that dry gas reserves are no longer the hot commodity. Instead, liquids-rich plays such as the Eagle Ford Shale in Texas are gaining increased prominence in terms of energy investment. The NYT article mentioned above cites a recent purchase of Texas oil acreage by Marathon Oil for $20,000 per acre.

With the current disparity between the price of oil and the price of natural gas, this shift of interest makes perfect sense. It may also make sense for the majors to invest further in the Marcellus and the Utica now, taking advantage of today’s bargain prices for Pennsylvania shale.