This week, the overall grades of 16 Oil and Gas stocks are lower, 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 April 19, 2013, 16.6% of outstanding PDC Energy shares were held short. For a full analysis of PETD stock, visit Portfolio Grader.

EOG Resources (NYSE:EOG) experiences a ratings drop this week, going from last week’s 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 price has fallen 7.3% over the past month, worse than the 1.1% decrease the S&P 500 has seen over the same period of time. The stock’s trailing PE Ratio is 55.10. To get an in-depth look at EOG, get Portfolio Grader’s complete analysis of EOG stock.

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.

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. For a full analysis of EEP stock, visit Portfolio Grader.

Slipping from a C to a D rating, PVR Partners L.P. (NYSE:PVR) takes a hit this week. 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 more information, get Portfolio Grader’s complete analysis of PVR stock.

This week, Green Plains Renewable Energy (NASDAQ:GPRE) drops from a C to a D rating. 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. The stock has a trailing PE Ratio of 25.60. To get an in-depth look at GPRE, get Portfolio Grader’s complete analysis of GPRE stock.

The rating of Chevron (NYSE:CVX) slips from a C to a D. 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) declines this week 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) earns an F this week, falling from last week’s grade of 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. To get an in-depth look at CLR, get Portfolio Grader’s complete analysis of CLR stock.

Teekay Corp. (NYSE:TK) gets weaker ratings this week as last week’s C drops 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 a full analysis of TK stock, visit Portfolio Grader.

Frontline (NYSE:FRO) experiences a ratings drop this week, going from last week’s 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. As of April 19, 2013, 14.3% of outstanding Frontline shares were held short. For more information, get Portfolio Grader’s complete analysis of FRO stock.

Slipping from a D to an F rating, Endeavour International (NYSE:END) takes a hit this week. 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 April 19, 2013, 23.2% of outstanding Endeavour International shares were held short. To get an in-depth look at END, get Portfolio Grader’s complete analysis of END stock.

The rating of North European Oil Royalty Trust (NYSE:NRT) slips from a D 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) 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 receives F’s in Earnings Growth, Earnings Momentum, and Equity. Cash Flow and Margin Growth also get F’s. As of April 19, 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 April 19, 2013, 16.7% of outstanding Gevo shares were held short. For a full analysis of GEVO stock, visit Portfolio Grader.

The rating of Teekay Offshore Partners (NYSE:TOO) declines this week from a C to a D. Teekay Offshore Partners LP provides marine transportation and storage services to the offshore oil industry. The stock also rates an F in Sales Growth. The stock currently has a trailing PE Ratio of 44.90. For more information, 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.