This week, seven oil and gas stocks are improving their overall rating on Portfolio Grader. Each of these rates an “A” (“strong buy”) or “B” overall (“buy”).

Chesapeake Midstream Partners (CHKM) is bumping up its rating from a C (“hold”) to a B (“buy”) this week. 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 week, PVR Partners, L.P.’s (PVR) ratings are up from a C last week to a B. PVR Partners owns and operates a network of natural gas pipelines and processing plants that provide gathering, transportation, compression, processing, dehydration and related services to natural gas producers. The stock has a dividend yield of 2.2%. For more information, get Portfolio Grader’s complete analysis of PVR stock.

World Fuel Services Corporation (INT) boosts its rating from a C to a B this week. World Fuel Services is engaged in the marketing and sale of marine, aviation and land fuel products and related services on a worldwide basis. For more information, get Portfolio Grader’s complete analysis of INT stock.

This is a strong week for Vertex Energy, Inc. (VTNR). The company’s rating climbs to B from the previous week’s C. Vertex Energy is a middle market consolidator, refiner and re-refiner of distressed petroleum streams, such as used oil, transmix, fuel oils and off-specification commercial chemical products. For more information, get Portfolio Grader’s complete analysis of VTNR stock.

TC PipeLines, LP (TCP) is progressing from last week’s rating of B (“buy”) as the company improves to an A (“strong buy”) this week. TC PipeLines acquires, owns and participates in the management of energy infrastructure businesses in North America. At present, the stock has a dividend yield of 3.4%. For more information, get Portfolio Grader’s complete analysis of TCP stock.

The rating of Sprague Resources LP (SRLP) moves up this week, rising from a B to an A. In the past week, trade volume fell significantly to half the previous rate. For more information, get Portfolio Grader’s complete analysis of SRLP stock.

U.S. Energy Corp. (USEG) improves from a C to a B rating this week. U. S. Energy explores for oil and natural gas. For more information, get Portfolio Grader’s complete analysis of USEG 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.