Big oil is struggling to grow, that is no secret. With interest rates still at record lows, balance sheets still relatively free of debt, and production falling, acquisitions could be on the horizon.

Now would be as good a time as any for companies to get on with it and make a deal. The M&A market has boomed this year, reaching levels not seen since before the financial crisis.

Chevron (NYSE:CVX) and ExxonMobil (NYSE:XOM) are likely to be doing the bidding as both behemoths had net debt-to-equity ratios of less than 10% at the end of the first quarter. Each had tens of billions available to complete any deal.

So which companies would fit best in their portfolios?

Domestic focus 
First, there's Marathon Oil (NYSE:MRO). Marathon is a pure domestic U.S. play, which would work well with Chevron, one of the largest domestic oil producers. Marathon would be a good bolt-on addition to ExxonMobil as well, which after the acquisition of XTO is still overweight in domestic gas.

Further, the company's small size and attractive portfolio would make it a tasty bite for any major.

Over the past few years, Marathon has been reducing its international exposure and is instead focusing on domestic plays. The company has acreage within the Bakken, Oklahoma, and the Eagle Ford. In total, Marathon has 2.4 billion barrels of resources available to it and 4,500 well locations. 

The company does still have some international operations, however. These include prospects offshore Africa, the Gulf of Mexico, and Iraqi Kurdistan. That said, around 60% of the company's production is domestic.

Nevertheless, it's Marathon's size that makes the company attractive. Marathon reported production of 483,000 barrels of oil equivalent per day at the end of 2013, around 10% of Exxon's current daily production and 20% of Chevron's.

Moreover, the company's current market capitalization is only around $27 billion. This means that either Chevron or ExxonMobil could acquire Marathon for less than their annual capex budgets and add a double-digit boost to production. You can read more on Marathon's takeover prospects here.

Lots of oil
Hess (NYSE:HES) is no stranger to takeover speculation. With a market cap of just over $31 billion, the company could also be acquired by either Chevron or ExxonMobil for less than their annual capex budgets.

Like Marathon, Hess has also been selling off its non-core assets and concentrating on ramping up domestic production. North America is not Hess' most attractive quality, however; its oil-linked asset base is.

Specifically, around 80% of Hess' oil reserves are liquids. To put that in perspective, ExxonMobil's production during the first quarter was around 50/50 gas and oil.

This means that Hess has a significant advantage over its peers. With around 76% of its 2013 production being oil, Hess has been able achieve higher margins than many of its peers and reported a cash margin of $49 per barrel during 2013. This is an industry-leading figure, allowing Hess to achieve sector-leading returns on invested capital.

Hess' oil-linked asset base and sector-leading returns are an attractive asset for any buyers.

Last month, it was rumored that ExxonMobil was taking a look at Anadarko Petroleum (NYSE:APC) ahead of a possible buyout. It's easy to see why, since Anadarko and Exxon would make a perfect couple.

Anadarko has an attractive portfolio of assets and a superior five-year track record. Production has expanded at 7% per annum for the past five years and the company has achieved a reserve replacement ratio of 160%. Over the long term, Anadarko's management is looking for 7% to 9% annual output growth.

Anadarko also has an attractive portfolio of onshore assets within the U.S. Anadarko's U.S. onshore reported net sales volumes are expected to total 635,000 barrels of oil equivalent in 2014. That's year-on-year growth of 12%.

The company also has an attractive international asset base. Anadarko's international prospects are located within the Gulf of Mexico and Africa, equaling 8 billion barrels of low-risk development opportunities. This would make the company a huge asset boost for either Chevron or ExxonMobil.

Anadarko is also currently free cash flow positive, so any deal would be immediately earnings accretive to any buyer. That's not even getting into the cost savings that could be achieved through the combination of U.S. onshore and international assets.

Foolish summary
In conclusion, big oil companies could be about to embark on an acquisition spree. Anadarko, Hess, and Marathon could all be perfect targets.

It seems as if now could be the time for big oil to make a deal, before interest rates start to rise and the domestic plays of Anadarko, Marathon, and Hess fully mature. Doing so would likely result in an uplift of valuations as it would allow both Chevron and ExxonMobil to lock in low interest rates and extract synergies from domestic development plays without overpaying.