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

PDC Energy’s (NASDAQ:PETD) rating falls to a D (“sell”) this week, down from C (“hold”) the week prior. 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 May 10, 2013, 16.6% of outstanding PDC Energy shares were held short. To get an in-depth look at PETD, get Portfolio Grader’s complete analysis of PETD stock.

The rating of EOG Resources (NYSE:EOG) declines this week 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 trailing PE Ratio for the stock is 58.80. For a full analysis of EOG stock, visit Portfolio Grader.

Suncor Energy’s (NYSE:SU) rating falls this week to an F (“strong sell”), down from last week’s D (“sell”). Suncor Energy is an integrated energy company in Canada. The stock gets F’s in Earnings Momentum and Earnings Surprise. For more information, get Portfolio Grader’s complete analysis of SU stock.

Slipping from a D to an F rating, Enbridge Energy Partners (NYSE:EEP) takes a hit this week. 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 stock currently has a trailing PE Ratio of 45.90. For a full analysis of EEP stock, visit Portfolio Grader.

This week, PVR Partners L.P. (NYSE:PVR) drops from a C to a D rating. 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. To get an in-depth look at PVR, get Portfolio Grader’s complete analysis of PVR stock.

Green Plains Renewable Energy (NASDAQ:GPRE) experiences a ratings drop this week, going from last week’s C 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. For more information, 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. To get an in-depth look at CVX, get Portfolio Grader’s complete analysis of CVX stock.

ONEOK Partners’ (NYSE:OKS) rating weakens this week, dropping to a D versus last week’s C. ONEOK Partners is engaged in the gathering, processing, storage, and transportation of natural gas in the United States. The stock also gets an F in Sales Growth. For a full analysis of OKS stock, visit Portfolio Grader.

This is a rough week for Continental Resources (NYSE:CLR). The company’s rating falls to F from the previous week’s D. Continental Resources explores for, develops, and produces oil and natural gas properties in the United States. The stock gets F’s in Earnings Growth, Earnings Momentum, Cash Flow, and Sales Growth. For more information, get Portfolio Grader’s complete analysis of CLR stock.

The rating of Teekay Corp. (NYSE:TK) slips from a C to a D. 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 a full analysis of TK stock, visit Portfolio Grader.

Frontline (NYSE:FRO) is having a tough week. The company’s rating falls 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 receives F’s in Earnings Revisions, Equity, Cash Flow, and Sales Growth. The stock price has fallen 9.3% over the past month, worse than the 2.1% increase the S&P 500 has seen over the same period of time. As of May 10, 2013, 14.3% 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, falling 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 May 10, 2013, 23.3% of outstanding Endeavour International shares were held short. For more information, get Portfolio Grader’s complete analysis of END stock.

North European Oil Royalty Trust (NYSE:NRT) earns an F this week, moving down from last week’s grade of 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 rates an F in Sales Growth. To get an in-depth look at NRT, get Portfolio Grader’s complete analysis of NRT stock.

SandRidge Energy (NYSE:SD) gets weaker ratings this week as last week’s D drops to an F. 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 May 10, 2013, 13.3% of outstanding SandRidge Energy shares were held short. For a full analysis of SD stock, visit Portfolio Grader.

Gevo (NASDAQ:GEVO) earns an F this week, moving down from last week’s grade of D. Gevo operates as a technology development company for biobutanol. The stock gets F’s in Equity, Cash Flow, and Sales Growth. As of May 10, 2013, 16.7% of outstanding Gevo shares were held short. For more information, get Portfolio Grader’s complete analysis of GEVO stock.

Teekay Offshore Partners (NYSE:TOO) gets weaker ratings this week as last week’s C drops to a D. Teekay Offshore Partners LP provides marine transportation and storage services to the offshore oil industry. The stock also gets an F in Sales Growth. For a full analysis of TOO stock, visit Portfolio Grader.

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.