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 May 3, 2013, 16.6% of outstanding PDC Energy shares were held short. For more information, get Portfolio Grader’s complete analysis of PETD stock.

EOG Resources’ (NYSE:EOG) rating weakens this week, dropping to a D versus last week’s C. 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’s trailing PE Ratio is 56.80. 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 a full analysis of SU stock, visit Portfolio Grader.

The rating of Enbridge Energy Partners (NYSE:EEP) slips from a 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 receives 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 29.40. To get an in-depth look at EEP, get Portfolio Grader’s complete analysis of EEP stock.

PVR Partners L.P. (NYSE:PVR) experiences a ratings drop this week, going from last week’s 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. 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.

Green Plains Renewable Energy (NASDAQ:GPRE) earns a D this week, moving down from last week’s grade of C. 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 a full analysis of GPRE stock, visit Portfolio Grader.

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

ONEOK Partners (NYSE:OKS) gets weaker ratings this week as last week’s C drops 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. The stock price has dropped 11.2% over the past month, worse than the 2.4% increase the S&P 500 has seen over the same period of time. For more information, get Portfolio Grader’s complete analysis of OKS stock.

The rating of Continental Resources (NYSE:CLR) declines this week from a D to an F. 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) is having a tough week. The company’s rating falls from a 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.

Frontline (NYSE:FRO) earns an F this week, falling from last week’s grade of D. 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 May 3, 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.

This is a rough week for Endeavour International (NYSE:END). The company’s rating falls to F from the previous week’s 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 3, 2013, 23.3% of outstanding Endeavour International shares were held short. For a full analysis of END stock, visit Portfolio Grader.

Slipping from a D to an F rating, North European Oil Royalty Trust (NYSE:NRT) takes a hit this week. 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 more information, get Portfolio Grader’s complete analysis of NRT stock.

This week, SandRidge Energy’s (NYSE:SD) rating worsens to an F from the company’s D rating a week ago. 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 May 3, 2013, 13.2% 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 3, 2013, 16.7% of outstanding Gevo shares were held short. To get an in-depth look at GEVO, get Portfolio Grader’s complete analysis of GEVO stock.

The rating of Teekay Offshore Partners (NYSE:TOO) slips 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. 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.