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”).

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. In Portfolio Grader’s specific subcategories of Earnings Revisions and Cash Flow, PETD also gets F’s. As of Oct. 18, 2013, 17.3% of outstanding PDC Energy shares were held short. For more information, get Portfolio Grader’s complete analysis of PETD stock.

Slipping from a C to a D rating, EOG Resources, Inc. (NYSE:EOG) takes a hit this week. 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 76.90. 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. The stock’s trailing PE Ratio is 47.50. 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, L.P. Class A (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 trailing PE Ratio for the stock is 73.90. For a full analysis of EEP stock, visit Portfolio Grader.

PVR Partners, L.P. (NYSE:PVR) is having a tough week. The company’s rating falls from a 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 currently has a trailing PE Ratio of 442.70. For more information, get Portfolio Grader’s complete analysis of PVR stock.

This week, Green Plains Renewable Energy, Inc.’s (NASDAQ:GPRE) rating worsens to a D from the company’s C rating a week ago. Green Plains Renewable Energy constructs and operates dry mill, fuel-grade ethanol production facilities. The stock gets F’s in Earnings Growth, Earnings Revisions, and Margin Growth. As of Oct. 18, 2013, 14.3% of outstanding Green Plains Renewable Energy, Inc. shares were held short. To get an in-depth look at GPRE, get Portfolio Grader’s complete analysis of GPRE stock.

Chevron Corporation (NYSE:CVX) experiences a ratings drop this week, going from last week’s 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 more information, get Portfolio Grader’s complete analysis of CVX stock.

ONEOK Partners, L.P.’s (NYSE:OKS) rating weakens this week, dropping to a D versus last week’s C. 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, Inc. (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. The stock has a trailing PE Ratio of 31.40. To get an in-depth look at CLR, get Portfolio Grader’s complete analysis of CLR stock.

Teekay Corporation (NYSE:TK) earns a D this week, moving down from last week’s grade of C. 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) 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. The stock price has fallen 19.1% over the past month, worse than the 1.7% decrease the S&P 500 has seen over the same period of time. As of Oct. 18, 2013, 13% of outstanding Frontline shares were held short. For more information, get Portfolio Grader’s complete analysis of FRO stock.

This week, Endeavour International Corporation (NYSE:END) drops from a D to an F rating. 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 Oct. 18, 2013, 19.2% of outstanding Endeavour International Corporation 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) 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 rates an F in Sales Growth. For more information, get Portfolio Grader’s complete analysis of NRT stock.

SandRidge Energy, Inc. (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. For a full analysis of SD stock, visit Portfolio Grader.

Gevo (NASDAQ:GEVO) gets weaker ratings this week as last week’s D drops to an F. Gevo operates as a technology development company for biobutanol. The stock gets F’s in Equity, Cash Flow, and Sales Growth. As of Oct. 18, 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.

Teekay Offshore Partners L.P. (NYSE:TOO) earns a D this week, moving down 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.