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 12, 2013, 20% of outstanding PDC Energy shares were held short. For a full analysis of PETD stock, visit Portfolio Grader.

EOG Resources (NYSE:EOG) earns a D this week, falling from last week’s grade of 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 52.60. For more information, get Portfolio Grader’s complete analysis of EOG stock.

Suncor Energy (NYSE:SU) earns an F (“strong sell”) this week, moving down from last week’s grade of D (“sell”). 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) rating weakens this week, dropping to an F versus last 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 stock’s trailing PE Ratio is 52.50. For more information, get Portfolio Grader’s complete analysis of EEP stock.

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 receives F’s in Earnings Growth, Earnings Revisions, and Equity. Cash Flow, Margin Growth, and Sales Growth also get F’s. To get an in-depth look at PVR, get Portfolio Grader’s complete analysis of PVR stock.

Green Plains Renewable Energy (NASDAQ:GPRE) experiences a ratings drop this week, going from last week’s C to a D. Green Plains Renewable Energy, Inc. was formed in June 2004 to construct and operate dry mill, fuel-grade ethanol production facilities. The stock gets F’s in Earnings Growth, Earnings Revisions, and Margin Growth. For a full analysis of GPRE stock, visit Portfolio Grader.

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

Continental Resources (NYSE:CLR) earns an F this week, moving down 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. For more information, get Portfolio Grader’s complete analysis of CLR stock.

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

Slipping from a D to an F rating, Frontline (NYSE:FRO) takes a hit this week. 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 July 12, 2013, 12.8% of outstanding Frontline shares were held short. For more information, 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 July 12, 2013, 24.3% of outstanding Endeavour International shares were held short. For a full analysis of END stock, visit Portfolio Grader.

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. To get an in-depth look at NRT, get Portfolio Grader’s complete analysis of NRT stock.

SandRidge Energy (NYSE:SD) earns an F this week, falling from last week’s grade of D. 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 July 12, 2013, 10.5% of outstanding SandRidge Energy shares were held short. For more information, get Portfolio Grader’s complete analysis of SD stock.

This week, Gevo’s (NASDAQ:GEVO) rating worsens to an F from the company’s D rating a week ago. Gevo operates as a technology development company for biobutanol. The stock gets F’s in Equity, Cash Flow, and Sales Growth. As of July 12, 2013, 19.5% of outstanding Gevo shares were held short. For a full analysis of GEVO stock, visit Portfolio Grader.

This is a rough week for Teekay Offshore Partners (NYSE:TOO). The company’s rating falls to D from the previous week’s 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. The stock has a trailing PE Ratio of 35.60. 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.