Geology/Deltaic setting with clastics starved
I am presently working in Baram delta, offshore Sarawak, Malaysia and Brunei NW Borneo. We have observed that Miocene to Pliocene sediments have abruptly clastic starved from one particular point of the offshore. (Very higher Net to Gross to very low NTG with dominant fine grained sandstone)
This makes the job of explorationists very challenging to justify the presence of reservoir quality sand beyond certain point within the shelf-slope-basin setting. Data of several drilled wells suggest that sandstone content is far less than initially estimated. Therefore, absence of sizeable thick sandstone is usually considered a significant risk despite presence of some large size prospects. Though there are some fields which are far away from the so called "That certain particular point" with reasonably thick sandstone to become a prolific reservoir.
Offshore sandstone may be expected through feeder channels and also through turbidites but still its not easily constrained through seismic data how that offshore sandstone deposited.
My questions are as follows;
Why so sudden sandstone is decreased and why only becoming fine sands?
Do we have any other example in the world with similar issue.
What are the possible tectonic, sedimentary or structural factors that control to reduce the sudden change in NTG in your opinion (regardless of Baram Delta example)to rationalize this phenomena. Is it due to its location i.e, beyond shelf edge setting in the slope- basin region where retention of clastic sediments are much complex and difficult.
I like to thank you in advance for your answer.
Thanks for posing this interesting question..... I am not an exploration geologist so I probably don't have the answers you are looking for. Consider though, that there might allogenic reasons for the sudden decrease in the sandstone and replacement with fine sands. In the deltaic environment you are used to looking at autogenic changes, things that would only affect the delta you are looking at. However, if there were global changes happening - whether this be major changes in sea level, or less likely in my view, some tectonic or perhaps structural change you might be to better explain your observations. Consider getting some data - if possible - from contemporaneous sediments further from your delta setting and see if there are noted changes at the same stratigraphic horizon.
Hope this helps.