Although we don't believe in timing the market or panicking over market movements, we do like to keep an eye on big changes -- just in case they're material to our investing thesis.

What: Shares of TransGlobe Energy (NASDAQ:TGA) fell 10% today after the oil and gas explorer issued an operations update.

So what: Gross proved oil reserves (1P) fell 3% from a year ago to 31.6 million barrels as of Dec. 31, while proved plus probable plus possible (3P) gross reserves fell 11% to 55.3 million barrels. The present value of 1P reserves fell from $782 million a year ago to $663 million and 3P value dropped from $1.30 billion to $1.05 billion.  

Now what: Oil producers strive to replace the oil they produce in a year and TransGlobe Energy clearly didn't do that in 2013. Shares appear to be a valued at just slightly more than seven times this year's earnings, but with reserves falling this could be a value trap for investors. I'm not buying into this stock even with the discount, particularly with oil prices trending lower recently.