Investors looking for stocks in the Oil and Gas - Field Services sector might want to consider either Schlumberger (SLB) or Core Laboratories (CLB). But which of these two companies is the best option for those looking for undervalued stocks? Let's take a closer look.Everyone has their own methods for finding great value opportunities, but our model includes pairing an impressive grade in the Value category of our Style Scores system with a strong Zacks Rank. The proven Zacks Rank emphasizes companies with positive estimate revision trends, and our Style Scores highlight stocks with specific traits.Schlumberger and Core Laboratories are sporting Zacks Ranks of #2 (Buy) and #4 (Sell), respectively, right now. Investors should feel comfortable knowing that SLB likely has seen a stronger improvement to its earnings outlook than CLB has recently. But this is just one piece of the puzzle for value investors.Value investors also try to analyze a wide range of traditional figures and metrics to help determine whether a company is undervalued at its current share price levels.The Style Score Value grade factors in a variety of key fundamental metrics, including the popular P/E ratio, P/S ratio, earnings yield, cash flow per share, and a number of other key stats that are commonly used by value investors.SLB currently has a forward P/E ratio of 16.98, while CLB has a forward P/E of 28.88. We also note that SLB has a PEG ratio of 0.44. This popular figure is similar to the widely-used P/E ratio, but the PEG ratio also considers a company's expected EPS growth rate. CLB currently has a PEG ratio of 0.71.Another notable valuation metric for SLB is its P/B ratio of 2.89. The P/B ratio pits a stock's market value against its book value, which is defined as total assets minus total liabilities. For comparison, CLB has a P/B of 4.34.These metrics, and several others, help SLB earn a Value grade of B, while CLB has been given a Value grade of C.SLB is currently sporting an improving earnings outlook, which makes it stick out in our Zacks Rank model. And, based on the above valuation metrics, we feel that SLB is likely the superior value option right now. Free: Top Stocks for the $30 Trillion Metaverse Boom The metaverse is a quantum leap for the internet as we currently know it - and it will make some investors rich. Just like the internet, the metaverse is expected to transform how we live, work and play. Zacks has put together a new special report to help readers like you target big profits. The Metaverse - What is it? And How to Profit with These 5 Pioneering Stocks reveals specific stocks set to skyrocket as this emerging technology develops and expands.Download Zacks’ Metaverse Report now >>Want the latest recommendations from Zacks Investment Research? Today, you can download 7 Best Stocks for the Next 30 Days. Click to get this free report Schlumberger Limited (SLB): Free Stock Analysis Report Core Laboratories N.V. (CLB): Free Stock Analysis Report To read this article on Zacks.com click here. Zacks Investment Research