The two armed men entered the store in Todmorden Road shortly before 10pm on Sunday, March 27.
Two staff members locked themselves in the storeroom as the offenders stole a quantity of cigarettes and alcohol.
The offenders then made off from the scene on foot along Todmorden Road in the direction of The Woodman pub.
A spokesman for Lancashire Police said: “An investigation is underway and we are appealing for anyone with information to contact us.
"We would like to hear from anyone who may have CCTV or dashcam footage to get in touch.”
The first man was wearing a black bubble coat and black bottoms.
The second offender was wearing a full grey tracksuit.
Anyone with information can contact the police by calling 101, quoting log number 1528 of March 27.
Alternatively, Crimestoppers can be contacted on 0800 555 111.