This week, the overall grades of 17 Oil and Gas stocks are lower, according to the Portfolio Grader database. Each of these rates a “D” (“sell”) or “F” overall (“strong sell”).

Venoco‘s (NYSE:VQ) rating falls to a D (“sell”) this week, down from C (“hold”) the week prior. Venocois engaged in the acquisition, exploration, exploitation and development of oil and natural gas properties with a focus on properties offshore and onshore California and the Gulf Coast of Texas. In Portfolio Grader’s specific subcategories of Earnings Momentum and Cash Flow, VQ also gets F’s. As of Sept. 28, 2012, 11.3% of outstanding Venoco shares were held short. For a full analysis of VQ stock, visit Portfolio Grader.

Legacy Reserves (NASDAQ:LGCY) gets weaker ratings this week as last week’s C drops to a D. Legacy Reserves acquires and explores for oil and natural gas properties in the United States. The stock gets F’s in Earnings Momentum, Earnings Revisions, and Earnings Surprise. For more information, get Portfolio Grader’s complete analysis of LGCY stock.

This week, Rex American Resources‘s (NYSE:REX) rating worsens to a D from the company’s C rating a week ago. Rex American Resources produces ethanol. The stock gets F’s in Earnings Growth, Earnings Momentum, and Earnings Surprise. To get an in-depth look at REX, get Portfolio Grader’s complete analysis of REX stock.

The rating of Navios Maritime Acquisition (NYSE:NNA) declines this week from a C to a D. Navios Maritime Acquisition is an owner and operator of tanker vessels focusing in the transportation of petroleum products (clean and dirty) and bulk liquid chemicals. The stock also gets an F in Cash Flow. Shares of the company are down 1.9% from a month prior. This is worse than the S&P 500’s 2.6% increase for the same period. For a full analysis of NNA stock, visit Portfolio Grader.

Talisman Energy (NYSE:TLM) ratings are on the decline this week as the company earns a F (“strong sell”). Last week, it received a D (“sell”). Talisman Energy is a global diversified upstream oil and gas company. The stock gets F’s in Earnings Growth, Earnings Momentum, and Earnings Revisions. Earnings Surprise and Sales Growth also get F’s. Shares of the company are down 3% from last month. The stock’s trailing PE Ratio is 25.80. For more information, get Portfolio Grader’s complete analysis of TLM stock.

The rating of Gevo (NASDAQ:GEVO) slips from a D to a F. 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 45.8% over the past month. As of Sept. 28, 2012, 11.5% of outstanding Gevo shares were held short. To get an in-depth look at GEVO, get Portfolio Grader’s complete analysis of GEVO stock.

This is a rough week for Goodrich Petroleum (NYSE:GDP). The company’s rating falls to F from the previous week’s D. Goodrich Petroleum explores, develops, produces, and acquires oil and natural gas properties. The stock receives F’s in Earnings Momentum, Earnings Revisions, and Equity. Cash Flow and Sales Growth also get F’s. As of Sept. 28, 2012, 21% of outstanding Goodrich Petroleum shares were held short. The price of GDP has dropped 11.5% from a month ago. For more information, get Portfolio Grader’s complete analysis of GDP stock.

Martin Midstream Partners‘s (NASDAQ:MMLP) rating weakens this week, dropping to a D versus last week’s C. Martin Midstream Partners provides marine transportation, terminalling, distribution and logistical services for producers and suppliers of hydrocarbon products, specialty chemicals and other liquids. The stock gets F’s in Earnings Revisions, Earnings Surprise, and Cash Flow. The stock has a trailing PE Ratio of 37.90. For a full analysis of MMLP stock, visit Portfolio Grader.

Slipping from a C to a D rating, PDC Energy (NASDAQ:PETD) takes a hit this week. 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 Sept. 28, 2012, 17.9% 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.

Northern Oil & Gas (AMEX:NOG) experiences a ratings drop this week, going from last week’s D to a F. Northern Oil & Gas is an independent energy company that acquires, explores, exploits and develops oil and natural gas properties. The stock gets F’s in Earnings Momentum and Cash Flow. As of Sept. 28, 2012, 18.5% of outstanding Northern Oil & Gas shares were held short. For a full analysis of NOG stock, visit Portfolio Grader.

Teekay Tankers (NYSE:TNK) earns a F this week, moving down from last week’s grade of D. Teekay Tankers owns oil tankers and transports crude oil. The stock gets F’s in Earnings Revisions and Sales Growth. Investors seem to agree with the downgrade and have pushed down the share price 5.7% over the past month. To get an in-depth look at TNK, get Portfolio Grader’s complete analysis of TNK stock.

Tsakos Energy Navigation (NYSE:TNP) is having a tough week. The company’s rating falls from a C to a D rating. Tsakos Energy Navigation provides international seaborne crude oil and petroleum product transportation services. The stock gets F’s in Earnings Growth, Equity, Cash Flow, and Margin Growth. The price of TNP is down 4.8% since last month. For more information, get Portfolio Grader’s complete analysis of TNP stock.

This week, Imperial Oil (AMEX:IMO) drops from a C to a D rating. Imperial Oil produces and refines natural gas and petroleum products and manufactures petrochemicals. The stock also rates an F in Sales Growth. Over the last month, IMO shares have fallen 0.7%. For more information, get Portfolio Grader’s complete analysis of IMO stock.

Atlas Pipeline (NYSE:APL) earns a D this week, falling from last week’s grade of C. Atlas Pipeline provides services for gathering natural gas. The stock gets F’s in Cash Flow and Sales Growth. Shares of the company are down 4.2% from a month ago. To get an in-depth look at APL, get Portfolio Grader’s complete analysis of APL stock.

Panhandle Oil & Gas (NYSE:PHX) gets weaker ratings this week as last week’s C drops to a D. Panhandle Oil and Gas explores for and develops oil and gas properties, and produces and sells oil and natural gas. The stock also gets an F in Cash Flow. The stock’s trailing PE Ratio is 26.50. For a full analysis of PHX stock, visit Portfolio Grader.

Rex Energy‘s (NASDAQ:REXX) rating weakens this week, dropping to a D versus last week’s C. Rex Energy is an independent energy company engaged in the acquisition, production, exploration and development of oil and gas. The stock gets F’s in Earnings Revisions, Earnings Surprise, and Cash Flow. As of Sept. 28, 2012, 17.7% of outstanding Rex Energy shares were held short. For more information, get Portfolio Grader’s complete analysis of REXX stock.

The rating of Hallador Energy (NASDAQ:HNRG) declines this week from a C to a D. Hallador Energy processes, mines and sells coal to producers of electric power. For a full analysis of HNRG 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.