Shares of Diamondback Energy (FANG 0.20%) slumped 11.6% in September, according to data provided by S&P Global Market Intelligence. Lower crude oil prices weighed on the oil company's stock price last month. That offset some positive news for the oil producer.

Down despite some positive catalysts

Crude oil prices continued to cool off last month. West Texas Intermediate (WTI), the primary U.S. oil price benchmark, fell 7.3% for the month, closing at a little more than $68 per barrel. WTI has now lost 16.4% over the past quarter. Weighing on oil are growing expectations that supplies will rise as OPEC members might boost their production in December. That would come amid demand concerns about a potential slowdown of the global economy. Lower oil prices will impact the cash flows of oil companies like Diamondback Energy, which weighed on oil stock prices last month.

Lower oil prices overshadowed some positive strategic developments for the oil company last month. The biggest was closing its $26 billion merger with Endeavor Energy Resources. The transaction will make Diamondback bigger and better. It enhances its premier position in the Permian Basin. The highly accretive deal should increase its free cash flow per share by about 10% next year.

Diamondback primarily funded the deal with stock (it paid $8 billion in cash and issued 117.3 million shares to the sellers). Those new investors sold some of their shares (nearly 12.8 million) shortly following the deal's closing in September via a secondary offering, of which Diamondback directly repurchased 2 million. The share sales also put some downward pressure on the stock price last month.

The Endeavor deal wasn't the only transformational transaction the company closed last month. Diamondback, Kinetic Holdings, and Epic Midstream announced a series of deals for Epic Crude. Diamondback and Kinetic bought an additional 30% interest in Epic Crude and now own 27.5% apiece. In addition, Diamondback increased its committed volumes following the Endeavor merger, which made it the third-largest producer in the Permian. The deal enhances the company's ability to transport crude oil out of the region.

Growing stronger

Last month was a transformational period for Diamondback Energy. It closed its merger with Endeavor Energy Resources and bolstered its midstream position. Those transactions will enhance its scale and reduce its costs, putting it in a better position to handle lower oil prices in the future. Because of that, its oil-driven stock slump looks like a potentially attractive buying opportunity for investors seeking a high-upside play in the oil patch.