How to extend minimum shutter speed in Olympus PM2

Hey guys,

I am not very good at photo shooting techniques, I want to take some nice blurry shots from sea and cloud movements. There are many shots described that has been taken like in 400 seconds to get a decent blurred shots. I have an Olympus PM2 with a 60S minimum shutter speed. I wonder how can I reach such slow speeds with this camera since my camera does not have infinite time for shutter speed or maybe I dont know!

Here is a shot I took with PM2 at 60seconds shutter speed , ISO 200 , F22 and ND filter of 16.

Thanks in advance for your help.