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 July 19, 2013, 20.2% 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.

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 currently has a trailing PE Ratio of 53.40. 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. For a full analysis of SU stock, visit Portfolio Grader.

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 stock’s trailing PE Ratio is 52.00. To get an in-depth look at EEP, get Portfolio Grader’s complete analysis of EEP stock.

PVR Partners L.P. (NYSE:PVR) earns a D this week, moving down from last week’s grade of 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 gets F’s in Earnings Growth, Earnings Revisions, and Equity. Cash Flow, Margin Growth, and Sales Growth also get F’s. For a full analysis of PVR stock, visit Portfolio Grader.

This is a rough week for Green Plains Renewable Energy (NASDAQ:GPRE). The company’s rating falls to D from the previous week’s 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. 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 more information, get Portfolio Grader’s complete analysis of CVX stock.

Slipping from a C to a D rating, ONEOK Partners (NYSE:OKS) takes a hit this week. 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.

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 receives 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.

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 more information, get Portfolio Grader’s complete analysis of TK stock.

Frontline (NYSE:FRO) gets weaker ratings this week as last week’s D drops 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 19, 2013, 12.8% of outstanding Frontline shares were held short. For a full analysis of FRO stock, visit Portfolio Grader.

Endeavour International’s (NYSE:END) rating weakens this week, dropping to an F versus last 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 July 19, 2013, 24.3% of outstanding Endeavour International shares were held short. For more information, get Portfolio Grader’s complete analysis of END stock.

This week, North European Oil Royalty Trust’s (NYSE:NRT) rating worsens to an F from the company’s D rating a week ago. 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.

Slipping from a D to an F rating, SandRidge Energy (NYSE:SD) takes a hit this week. 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 19, 2013, 10.1% of outstanding SandRidge Energy shares were held short. For more information, get Portfolio Grader’s complete analysis of SD stock.

This week, Gevo (NASDAQ:GEVO) drops from a D to an F rating. Gevo operates as a technology development company for biobutanol. The stock gets F’s in Equity, Cash Flow, and Sales Growth. The stock price has dropped 8.9% over the past month, worse than the 1.3% decrease the Nasdaq has seen over the same period of time. As of July 19, 2013, 19.5% 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 gets an F in Sales Growth. The stock currently has a trailing PE Ratio of 36.90. 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.