There are many places in US where oil and gas are producted simultaneously (you can not get one without the other). High oil prices stimulate investments in oil exploration and this causes also increased natural gas production. Then if LNG export infrastructure is insuffcient natural gas prices can get into negative region.

"An important feature of tight oil is that both oil and natural gas are present in the same formations and are produced from the same wells."

https://www.ucs.org/sites/default/files/attach/2016/05/The-T...