ExxonMobil (NYSE: XOM) has been able to combine an enviable status as the biggest member of Big Oil, with additional roles as the largest producer of natural gas in the U.S. and one of the industry's technological leaders.

For the past week, however, it's also been subject to ignominy resulting from its having been the subject to the most recent U.S. environmental mishap. The cause -- an oil-pipeline rupture under the Yellowstone River near Laurel, Montana -- remains a mystery. However, it's not unreasonable to assume that TransCanada (NYSE: TRP) could now find permission for its proposed Keystone XL line -- which would transport crude oil from Alberta to the U.S. Gulf Coast -- more difficult to obtain.

Rather than serving simply as a means of transportation for crude from north to south in the United States, the $7 billion Keystone XL line could alter U.S. crude prices. In essence, the portion of the line from its Cushing, Okla., storage site to Texas would carry 900,000 barrels of crude daily from the storage facility to the numerous refineries in the Lone Star State. A primary result would be a reduction of a crude glut at Cushing and a consequent lowering of the $20 spread between West Texas Intermediate crude and Europe's Brent.

However, U.S. regulators, led by Rep. Henry Waxman, the leading Democrat on the House Energy and Commerce committee, are intent on studying the effects of diluted bitumen moving from Canadian oil sands through the pipelines. Environmentalists who are opposed to the Keystone XL line maintain that the combination of the bitumen and the chemical properties of the mix make the pipelines more susceptible to mishaps than does carrying conventional crude oil. Further, a Senate bill passed last month also calls for a study of the effects of diluted bitumen on pipelines.

The Exxon rupture occurred in its 20-year-old, 12-inch Silvertip pipeline, which delivers 40,000 barrels a day to a refinery in Billings, along a path that runs beneath the river. The company estimates that the total spill amounted to approximately 1,000 barrels, or about 42,000 gallons, of oil.

It follows a 2010 Enbridge Energy (NYSE: EEP) pipeline mishap that sent 900,000 barrels of crude into Michigan's Kalamazoo River, along with a September Pacific Gas & Electric (NYSE: PCG) gas-line explosion and a Chesapeake Energy (NYSE: CHK) April well leak in Pennsylvania's Marcellus Shale. All such mishaps lend credence to those who advocate stricter regulations on the industry.

Clearly, none of us wishes for the occurrence of oil or natural-gas mishaps. Nevertheless, Exxon's Montana spill was relatively small and was limited to a company that remains solid in virtually all respects. For those reasons alone, I suggest that Fools continue to monitor the biggest member of Big Oil carefully by adding it to your personal version of My Watchlist.       

Motley Fool newsletter services have recommended buying shares of TransCanada and Chesapeake. Try any of our Foolish newsletter services free for 30 days.

We Fools don't all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. Fool contributor David Lee Smith doesn't own shares in any of the companies named in this article. The Motley Fool has a disclosure policy.