Deep marine reservoirs with fresh water – how come?

It was on the back of an article we published about CSEM that Brian Frost, an experienced ex­plorer from the USA, com­mented on the LinkedIn post that deep marine res­ervoirs are sometimes char­acterised by fresh formation water. This was a new thing for me, and it triggered my interest.

It is obviously no coin­cidence that this comment was made following a post about CSEM. A reservoir that has a fresh water fluid will be more resistive than one with a brine, poten­tially leading to the drill­ing of a “dry” well. It just shows that CSEM, as all geophysical methodologies, are non-unique and should ideally be used as part of a wider mix of tools to de-risk a prospect. But that is noth­ing new.

Again, what was new to me was the notion that deep marine reservoirs, deposited far away from shore, can have fresh for­mation waters. This article is no serious attempt to ex­plain why this happens, but rather to raise awareness of the phenomenon and possi­bly cause some more debate in case more people have observed the same thing in exploration wells they have been involved with.

Finding fresh water reservoirs offshore is not something new, though. In 2019, Forbes published an article referring to work done offshore New Eng­land, east coast USA. Re­searchers found a shallow reservoir that was dominat­ed by fresh formation water, mapped through the use of CSEM. Oil companies had already proven the existence of it earlier when drilling pilot holes for exploration. The presence of fresh water in this setting can be well explained, as the area was subaerially exposed during the last ice age and was well saturated with fresh water through the work of rivers, glaciers and the resulting groundwater flow.

But to explain the presence of fresh water in deeper realms, in areas that were not subaerially ex­posed during the late Gla­cial Maximum, is another matter. “I don’t know yet how to precisely explain this”, said Brian to me in an email we exchanged. “The most sensible explana­tion I can come up with is that the fresh water became entrapped when the sedi­ments were shed into the basin at the time of deposi­tion,” he wrote.

“And this is not some­thing rare and utterly unique,” he continued. “The Pomboo well drilled offshore Kenya a number of years ago found a fresh-wa­ter dominated reservoir, to the extent that the water could be bottled and con­sumed. It was found in other places along the East African margin too. And in Nigeria, in the Niger delta, this is a common observa­tion as well, even though this has not been extensive­ly published about at all.”

If there is someone who has worked on this mat­ter before and is willing to share some thoughts on how to explain this phe­nomenon, please do get in touch with us.