The ratings of 14 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”).

Northern Tier Energy LP Class A’s (NTI) rating falls to a D (“sell”) this week, down from C (“hold”) the week prior. Northern Tier Energy is an independent downstream energy company. To get an in-depth look at NTI, get Portfolio Grader’s complete analysis of NTI stock.

Blueknight Energy Partners, L.P. (BKEP) earns an F (“strong sell”) this week, moving down from last week’s grade of D (“sell”). Blueknight Energy Partners provides integrated terminalling, storage, processing, gathering, and transportation services for companies engaged in the production, distribution, and marketing of crude oil and asphalt products in the United States. The stock gets F’s in Earnings Momentum, Cash Flow and Sales Growth. The stock has a trailing PE Ratio of 86.60. For more information, get Portfolio Grader’s complete analysis of BKEP stock.

Antero Resources Corporation (AR) earns an F this week, falling from last week’s grade of D. The stock gets F’s in Earnings Revisions, Earnings Surprise, Cash Flow and Margin Growth. To get an in-depth look at AR, get Portfolio Grader’s complete analysis of AR stock.

This week, Teekay Corporation’s (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 Revisions, Equity and Cash Flow. For more information, get Portfolio Grader’s complete analysis of TK stock.

This is a rough week for KNOT Offshore Partners LP (KNOP). The company’s rating falls to D from the previous week’s C. The stock also gets an F in Earnings Momentum. To get an in-depth look at KNOP, get Portfolio Grader’s complete analysis of KNOP stock.

Slipping from a D to an F rating, Atlas Resource Partners, L.P. (ARP) takes a hit this week. Atlas Resource Partners engages in the production of natural gas, crude oil, and natural gas liquids in basins across the United States. The stock gets F’s in Equity and Cash Flow. For more information, get Portfolio Grader’s complete analysis of ARP stock.

Atlas Pipeline Partners, L.P. (APL) experiences a ratings drop this week, going from last week’s C to a D. Atlas Pipeline provides services for gathering natural gas. The stock gets F’s in Earnings Revisions and Cash Flow. To get an in-depth look at APL, get Portfolio Grader’s complete analysis of APL stock.

This is a rough week for Amyris, Inc. (AMRS). The company’s rating falls to F from the previous week’s D. Amyris provides alternatives to a range of petroleum-sourced products used in specialty chemical and transportation fuel markets worldwide. The stock gets F’s in Earnings Revisions and Cash Flow. For more information, get Portfolio Grader’s complete analysis of AMRS stock.

Slipping from a C to a D rating, DCP Midstream Partners, LP (DPM) takes a hit this week. DCP Midstream Partners engages in gathering, compressing, treating, processing, transporting, storing, and selling natural gas in the United States. The stock also rates an F in Cash Flow. The trailing PE Ratio for the stock is 29.60. To get an in-depth look at DPM, get Portfolio Grader’s complete analysis of DPM stock.

This week, Gevo’s (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. For more information, get Portfolio Grader’s complete analysis of GEVO stock.

This week, PDC Energy (PETD) drops from a C to a D rating. PDC Energy 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. To get an in-depth look at PETD, get Portfolio Grader’s complete analysis of PETD stock.

Pembina Pipeline Corporation (PBA) experiences a ratings drop this week, going from last week’s C to a D. Pembina Pipeline transports, stores, and markets petroleum products. The stock also gets an F in Earnings Surprise. The stock’s trailing PE Ratio is 31.90. For more information, get Portfolio Grader’s complete analysis of PBA stock.

Whiting Petroleum Corporation’s (WLL) rating weakens this week, dropping to an F versus last week’s D. Whiting Petroleum acquires, exploits, develops and explores for crude oil, natural gas and natural gas liquids. The stock gets F’s in Earnings Revisions and Cash Flow. To get an in-depth look at WLL, get Portfolio Grader’s complete analysis of WLL stock.

Arch Coal, Inc. (ACI) is having a tough week. The company’s rating falls from a D to an F. Arch Coal produces coal and sells it to power plants, steel mills, and industrial facilities. The stock gets F’s in Equity, Cash Flow and Sales Growth. As of Jan. 23, 2015, 23.7% of outstanding Arch Coal, Inc. shares were held short. For more information, get Portfolio Grader’s complete analysis of ACI 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.