This week, the ratings of three energy services stocks on Portfolio Grader are down. Each of these rates a “D” (“sell”) or “F” overall (“strong sell”).

North American Energy Partners’ (NOA) rating falls this week to an F (“strong sell”), down from last week’s D (“sell”). North American Energy Partners is a resource services provider to oil and natural gas, and other natural resource companies, with a primary focus in the Canadian oil sands. NOA also rates an F in Portfolio Grader’s specific subcategory of Earnings Revisions. For more information, get Portfolio Grader’s complete analysis of NOA stock.

This week, Basic Energy Services, Inc.’s (BAS) rating worsens to an F from the company’s D rating a week ago. Basic Energy Services provides oil and gas drilling and production companies with a range of well site services. The stock gets F’s in Earnings Momentum, Earnings Revisions and Earnings Surprise. As of July 23, 2015, 27.3% of outstanding Basic Energy Services, Inc. shares were held short. To get an in-depth look at BAS, get Portfolio Grader’s complete analysis of BAS stock.

This week, Helix Energy Solutions Group, Inc. (HLX) drops from a D to an F rating. Helix Energy Solutions is a marine contractor and operator of offshore oil and gas properties and production facilities. 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 stock are changing hands at twice the rate they were a week ago. For more information, get Portfolio Grader’s complete analysis of HLX 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.