Here's a section out of an analyst's review of Exxon strategy. The last paragraph is probably the key. Rather than buying a CHK, HK, etc., which Exxon could do and not even dent their wallets, they're trying to sink $4 billion into a deepwater west aftrica development.

In contrast to ConocoPhillips’ strategy, ExxonMobil stayed on the sidelines during the last high and low cycle, quietly accumulating stock and not much else. The financial crowd has waited impatiently since the downturn last fall for the countercyclical Exxon to start spending on acquisitions with speculation they might seek a larger presence in North American shales by buying a large independent. Exxon finally opened its wallet, but
not in a way that many expected. They have ignored the shale acquisition game and instead entered the Big Horn and Piceance through additional land grabs.

On the oil front, Exxon is now offering $4.0 billion for a chunk of Ghana’s Jubilee
oil field from Kosmos Energy, a Dallas-based independent backed by private equity
firms Blackstone Group and Warburg Pincus. Unfortunately, Exxon’s acquisition is not
a done deal, since the Ghana National Petroleum Corporation remains interested in
the Kosmos holdings. The GNPC has the backing of the Ghanaian government who
reportedly is working on financing a Kosmos purchase in concert with another foreign
player (rumored to be BP, Shell or CNOOC).Should the Ghanaian government negate the Kosmos deal, Exxon always has the option of just ponying up to buy fellow Kosmos partners Tullow Oil or Anadarko Petroleum, though there are no credible indications Exxon would take this step.

The deepwater Jubilee Field has attracted interest as part of a greater offshore play stretching from Sierra Leone to Ghana. The play offers the majors a reprieve from the land-based political issues that have hampered the industry in Africa, especially in Nigeria, and virulent resource nationalism in Russia, Venezuela and the Middle East.

Exxon’s move for Kosmos also contradicts market anticipation. For example, the headlines surrounding last year’s troubles at Chesapeake, Devon, and other U.S. unconventional gas players spawned speculation over just whom ExxonMobil (or one of the other majors) might buy. The theory was the majors would return to the U.S. after being escorted out of the major oil plays globally by the National Oil Companies.

Shale gas represented the newest opportunity set and purchasing a Devon or Chesapeake involved just a quarter’s worth of free cash flow for majors like ExxonMobil. The fact that ExxonMobil is looking to throw $4.0 billion at foreign deepwater oil rather than North American shale gas is a situation pregnant with implications
and is seconded by ConocoPhillips reversal of fortune post Burlington. Simply stated, Exxon’s bid for deepwater assets is a glaring example that, next to its own stock, Exxon still views international deepwater as more opportunistic than North American shale gas.