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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 sizable 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.

Regards

Bodiarto Rayuf

Answer
The situation you describe sounds somewhat similar to what is described in the following thesis which covers one of the more productive basins in Oklahoma.

digital.library.okstate.edu/etd/umi-okstate-2251.pdf

I am not familiar with the area you are interested in but I can pose some possible explainations for what you are seeing.

In the deltaic system, a channel change could have resulted in sudden sediment starvation in one area with a subsequent enrichment locally elsewhere.  

Or as in the example in the thesis, a local basin became a sediment trap, catching the bulk of the sediment being transported while starvation occurred in other areas.

A change in sea level might have led to reworking of delta front redepositing pods of finer sandstone farther off shore in submerged bars that were later buried as transgressive sea levels rose further.  

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Keith Patton

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I can answer questions concerning physical and historical geology, environmental geology/hydrology, environmental consulting, remote sensing/aerial photo interpretation, G&G computer applications, petroleum exploration, drilling, geochemistry, geochemical and microbiological prospecting, 3D reservoir modeling, computer mapping and drilling.I am not a geophysicist.

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