The E-M1 has abnormal image noise for long exposures.
To reduce this image noise, turn on Noise Reduction and the camera will take a second Dark Frame exposure equal to the actual exposure time for the first frame. This dark frame will contain the noise pattern only on a bed of black and the camera will subtract that pattern from the first frame. This affects both the raw image as well as the jpeg. Do not confuse with the menu item labeled Noise Filter - Noise Filter in Olympus speak is what we usually call Noise Reduction in computer software - this is the intentional blurring to cover the noise.
See
http://www.astropix.com/html/j_digit/jpg_dfs.html (includes an interactive image where you can see before and after by clicking the mouse)
http://www.ayton.id.au/wiki/doku.php?id=omd:dark_frame
https://unlockingolympus.com/2017/03/olympus-e-m1mkii-long-exposure-noise-reduction-comparison/