The ratings of 16 Oil and Gas stocks are down this week, according to the Portfolio Grader database. Each of these rates a “D” (“sell”) or “F” overall (“strong sell”).

PDC Energy (NASDAQ:PETD) is on the decline this week, earning a D (“sell”) after receiving a C (“hold”) last week. PDC is an oil and gas company with drilling and production operations in the Rocky Mountains, the Appalachian Basin, and Michigan. In Portfolio Grader’s specific subcategories of Earnings Revisions and Cash Flow, PETD also gets F’s. As of July 5, 2013, 20% of outstanding PDC Energy shares were held short. For a full analysis of PETD stock, visit Portfolio Grader.

The rating of EOG Resources (NYSE:EOG) slips from a C to a D. EOG Resources is in the business of the exploration, development, production, and marketing of natural gas and crude oil. The stock gets F’s in Earnings Growth, Earnings Momentum, and Margin Growth. The stock has a trailing PE Ratio of 50.60. For more information, get Portfolio Grader’s complete analysis of EOG stock.

Suncor Energy (NYSE:SU) ratings are on the decline this week as the company earns an F (“strong sell”). Last week, it received a D (“sell”). Suncor Energy is an integrated energy company in Canada. The stock gets F’s in Earnings Momentum and Earnings Surprise. To get an in-depth look at SU, get Portfolio Grader’s complete analysis of SU stock.

Enbridge Energy Partners (NYSE:EEP) experiences a ratings drop this week, going from last week’s D to an F. Enbridge Energy Partners transports crude oil and natural gas liquids to refineries in the midwestern United States and eastern Canada. The stock gets F’s in Earnings Growth, Earnings Revisions, and Earnings Surprise. Cash Flow and Sales Growth also get F’s. The trailing PE Ratio for the stock is 49.40. For more information, get Portfolio Grader’s complete analysis of EEP stock.

PVR Partners L.P. (NYSE:PVR) is having a tough week. The company’s rating falls from a C to a D. Penn Virginia Resource Partners owns and operates a network of natural gas pipelines and processing plants which provide gathering, transportation, compression, processing, dehydration and related services to natural gas producers. For a full analysis of PVR stock, visit Portfolio Grader.

Green Plains Renewable Energy (NASDAQ:GPRE) gets weaker ratings this week as last week’s C drops to a D. Green Plains Renewable Energy, Inc. was formed in June 2004 to construct and operate dry mill, fuel-grade ethanol production facilities. Ethanol is a renewable, environmentally clean fuel source that is produced at numerous facilities in the United States, mostly in the Midwest. The stock gets F’s in Earnings Growth, Earnings Revisions, and Margin Growth. To get an in-depth look at GPRE, get Portfolio Grader’s complete analysis of GPRE stock.

This week, Chevron’s (NYSE:CVX) rating worsens to a D from the company’s C rating a week ago. Chevron gives management and technological support to international subsidiaries that operate petroleum, chemicals, mining, power generation, and energy services. The stock also gets an F in Sales Growth. For a full analysis of CVX stock, visit Portfolio Grader.

The rating of ONEOK Partners (NYSE:OKS) slips from a C to a D. ONEOK Partners is engaged in the gathering, processing, storage, and transportation of natural gas in the United States. The stock also rates an F in Sales Growth. For more information, get Portfolio Grader’s complete analysis of OKS stock.

Continental Resources (NYSE:CLR) experiences a ratings drop this week, going from last week’s D to an F. Continental Resources explores for, develops, and produces oil and natural gas properties in the United States. In Earnings Growth, Earnings Momentum, Cash Flow, and Sales Growth the stock gets F’s. To get an in-depth look at CLR, get Portfolio Grader’s complete analysis of CLR stock.

Teekay Corp. (NYSE:TK) earns a D this week, falling from last week’s grade of C. Teekay is a provider of international crude oil and petroleum product transportation services. The stock receives F’s in Earnings Momentum, Earnings Revisions, and Earnings Surprise. Equity and Cash Flow also get F’s. For more information, get Portfolio Grader’s complete analysis of TK stock.

The rating of Frontline (NYSE:FRO) declines this week from a D to an F. Frontline owns a fleet of very large crude carriers and Suezmax tankers that transport crude oil and oil products between ports. The stock gets F’s in Earnings Revisions, Equity, Cash Flow, and Sales Growth. As of July 5, 2013, 14.1% of outstanding Frontline shares were held short. To get an in-depth look at FRO, get Portfolio Grader’s complete analysis of FRO stock.

Endeavour International (NYSE:END) earns an F this week, moving down from last week’s grade of D. Endeavour International is an international oil and gas exploration and production company that acquires, explores, and develops energy reserves. The stock gets F’s in Equity and Cash Flow. As of July 5, 2013, 24% of outstanding Endeavour International shares were held short. For a full analysis of END stock, visit Portfolio Grader.

North European Oil Royalty Trust’s (NYSE:NRT) rating weakens this week, dropping to an F versus last week’s D. North European Oil Royalty Trust is involved in gas and oil production. It holds overriding royalty rights in certain concessions or leases in the Federal Republic of Germany. The stock also gets an F in Sales Growth. To get an in-depth look at NRT, get Portfolio Grader’s complete analysis of NRT stock.

This week, SandRidge Energy (NYSE:SD) drops from a D to an F rating. SandRidge Energy explores and produces natural gas and crude oil. The stock gets F’s in Earnings Growth, Earnings Momentum, and Equity. Cash Flow and Margin Growth also get F’s. As of July 5, 2013, 10.5% of outstanding SandRidge Energy shares were held short. For more information, get Portfolio Grader’s complete analysis of SD stock.

This is a rough week for Gevo (NASDAQ:GEVO). The company’s rating falls to F from the previous week’s D. Gevo operates as a technology development company for biobutanol. The stock gets F’s in Equity, Cash Flow, and Sales Growth. As of July 5, 2013, 20.7% of outstanding Gevo shares were held short. For a full analysis of GEVO stock, visit Portfolio Grader.

Slipping from a C to a D rating, Teekay Offshore Partners (NYSE:TOO) takes a hit this week. Teekay Offshore Partners LP provides marine transportation and storage services to the offshore oil industry. The stock also gets an F in Sales Growth. The stock’s trailing PE Ratio is 35.40. To get an in-depth look at TOO, get Portfolio Grader’s complete analysis of TOO 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.