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 June 7, 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.

This week, EOG Resources (NYSE:EOG) drops from a C to a D rating. 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 price has fallen 5.6% over the past month, worse than the 0.6% decrease the S&P 500 has seen over the same period of time. The stock’s trailing PE Ratio 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. To get an in-depth look at SU, get Portfolio Grader’s complete analysis of SU stock.

This is a rough week for Enbridge Energy Partners (NYSE:EEP). The company’s rating falls to F from the previous week’s D. Enbridge Energy Partners transports crude oil and natural gas liquids to refineries in the midwestern United States and eastern Canada. The stock receives 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.

PVR Partners L.P.’s (NYSE:PVR) rating weakens this week, dropping to a D versus last week’s C. 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. The stock receives F’s in Earnings Growth, Earnings Revisions, and Equity. Cash Flow, Margin Growth, and Sales Growth also get F’s. For more information, get Portfolio Grader’s complete analysis of PVR stock.

Slipping from a C to a D rating, Green Plains Renewable Energy (NASDAQ:GPRE) takes a hit this week. 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 rates 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 gets an F in Sales Growth. For more information, get Portfolio Grader’s complete analysis of OKS stock.

Continental Resources’ (NYSE:CLR) rating weakens this week, dropping to an F versus last 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 a full analysis of CLR stock, visit Portfolio Grader.

Teekay Corp. (NYSE:TK) experiences a ratings drop this week, going from last week’s C to a D. Teekay is a provider of international crude oil and petroleum product transportation services. The stock gets 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.

This week, Frontline’s (NYSE:FRO) rating worsens to an F from the company’s D rating a week ago. 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. As of June 7, 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, 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 June 7, 2013, 23.4% 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) gets weaker ratings this week as last week’s D drops to an F. 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. For a full analysis of NRT stock, visit Portfolio Grader.

SandRidge Energy (NYSE:SD) is having a tough week. The company’s rating falls from a D to an F. SandRidge Energy explores and produces natural gas and crude oil. The stock receives F’s in Earnings Growth, Earnings Momentum, and Equity. Cash Flow and Margin Growth also get F’s. As of June 7, 2013, 13.2% of outstanding SandRidge Energy shares were held short. To get an in-depth look at SD, 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 June 7, 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) earns a D this week, falling from last week’s grade of C. 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.