This week, six Oil and Gas stocks are improving their overall ratings on Portfolio Grader. Each of these stocks is rated an “A” (“strong buy”) or “B” overall (“buy”).

This week, Chesapeake Midstream Partners (NYSE:CHKM) is showing significant improvement as the company’s rating hops from a C (“hold”) to a B (“buy”). Chesapeake Midstream Partners owns, operates, develops, and acquires natural gas, natural gas liquids, and oil gathering systems, as well as other midstream energy assets in the United States. For more information, get Portfolio Grader’s complete analysis of CHKM stock.

This is a strong week for Western Gas (NYSE:WES). The company’s rating climbs to B from the previous week’s C. Western Gas engages in gathering, compressing, processing, treating and transporting Anadarko and third-party natural gas in the United States. The stock price has risen 6.6% over the past month, better than the 3.1% increase the S&P 500 has seen over the same period of time. For more information, get Portfolio Grader’s complete analysis of WES stock.

Access Midstream Partners LP (NYSE:ACMP) earns a B this week, jumping up from last week’s grade of C. Wall Street has pushed the stock higher by 9.5% over the past month. For more information, get Portfolio Grader’s complete analysis of ACMP stock.

This week, Global Partners (NYSE:GLP) pushes up from a C to a B rating. Global Partners owns, controls or has access to a terminal network of refined petroleum products in New England, located in the United States. Investors seem to agree with the upgraded status of the stock, and have pushed the stock up 21.9% over the past month. At present, the stock has a dividend yield of 2.1%. For more information, get Portfolio Grader’s complete analysis of GLP stock.

Teekay LNG Partners (NYSE:TGP) improves from a C to a B rating this week. Teekay LNG Partners provides marine transportation services for liquefied natural gas, liquefied petroleum gas and crude oil. Wall Street seems to agree with the upgrade and has propelled the stock up 8.1% over the past month. The current dividend yield is 2.7%. For more information, get Portfolio Grader’s complete analysis of TGP stock.

Scorpio Tankers’ (NYSE:STNG) ratings are looking better this week, moving up to a B from last week’s C. Scorpio Tankers provides seaborne transportation of crude oil and other petroleum products worldwide. Investors have pushed the stock price up 14.5% over the past month. For more information, get Portfolio Grader’s complete analysis of STNG stock.

Louis Navellier’s proprietary Portfolio Grader stock ranking system assesses roughly 5,000 companies every week based on a number of fundamental and quantitative measures. Stocks are given a letter grade based on their results — with A being “strong buy,” and F being “strong sell.” Explore the tool here.