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

Gevo’s (NASDAQ:GEVO) rating falls this week to an F (“strong sell”), down from last week’s D (“sell”). Gevo operates as a technology development company for biobutanol. In Portfolio Grader’s specific subcategories of Equity, Cash Flow, and Sales Growth, GEVO also gets an F. As of March 28, 2013, 18.3% 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 Endeavour International (NYSE:END) slips from a D to an F. 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 March 28, 2013, 18.9% 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, 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 March 28, 2013, 11% of outstanding SandRidge Energy shares were held short. For more information, get Portfolio Grader’s complete analysis of SD stock.

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. As of March 28, 2013, 13.5% of outstanding Frontline shares were held short. To get an in-depth look at FRO, get Portfolio Grader’s complete analysis of FRO stock.

The rating of North European Oil Royalty Trust (NYSE:NRT) declines this week 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 more information, get Portfolio Grader’s complete analysis of NRT stock.

This week, PDC Energy (NASDAQ:PETD) falls to a D (“sell”), worse than last week’s grade of C (“hold”). PDC is an oil and gas company with drilling and production operations in the Rocky Mountains, the Appalachian Basin, and Michigan. The stock gets F’s in Earnings Revisions and Cash Flow. As of March 28, 2013, 21% of outstanding PDC Energy shares were held short. For a full analysis of PETD stock, visit Portfolio Grader.

Teekay Offshore Partners’ (NYSE:TOO) rating weakens this week, dropping to a D versus last week’s C. Teekay Offshore Partners LP provides marine transportation and storage services to the offshore oil industry. The stock also rates an F in Sales Growth. Trade volume is up 387.1% from the previous week. The stock’s trailing PE Ratio is 46.00. For more information, get Portfolio Grader’s complete analysis of TOO stock.

This week, EOG Resources (NYSE:EOG) drops from 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 has a trailing PE Ratio of 59.80. For a full analysis of EOG stock, visit Portfolio Grader.

Suncor Energy (NYSE:SU) gets weaker ratings this week as last week’s D drops to an F. 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.

Enbridge Energy Partners (NYSE:EEP) earns an F this week, moving down from last week’s grade of 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.

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 rates an F 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, falling 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. The trailing PE Ratio for the stock is 26.70. 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 gets an F in Sales Growth. For a full analysis of CVX stock, visit Portfolio Grader.

This is a rough week for ONEOK Partners (NYSE:OKS). The company’s rating falls to D from the previous week’s C. ONEOK Partners, L. P. is a publicly traded Delaware master limited partnership that was formed in 1993. The stock also gets an F in Sales Growth. To get an in-depth look at OKS, get Portfolio Grader’s complete analysis of OKS stock.

Continental Resources (NYSE:CLR) is having a tough week. The company’s rating falls 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 more information, get Portfolio Grader’s complete analysis of CLR stock.

This week, Teekay Corp.’s (NYSE:TK) rating worsens to a D from the company’s C rating a week ago. 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.

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.