Facies Models Constrained by a High A/S

Zaixing Jiang and Lin Wang
China University of Geosciences, Beijing, China

The ratio of accommodation and sediment supply or A/S has a great influence upon the vertical succession and plane distribution of sedimentary facies. The classic facies sequence or a parasequence that has been well studied is shoaling-up (low A/S) and is seldom deepening-up (high A/S), although the deepening-up sequence has been reported recently. In this paper we took two rift basins, the Dongying depression in China and the Fula depression in Sudan to demonstrate coarse-clastic sub-lacustrine fans and fine-grained deltas in a context of a high A/S.

In the northern steep-slope zone of the Dongying depression, the Eogene episodic subsiding of the boundary fault of Shengbei produced a large accommodation. In this deep-water setting, coarse-grained clastics from the Chenjiazhuang uplift deposited in the area. The vertical sequence is fining-upward and consists of cobble-supported and chaostic conglomerate scouring the underlying dark mudstone at the base, followed by pebbly sandstone, and siltstone interbeded with bark mudstone. In the seismics, backstepping onlaps are seen. And the conglomerate lithofacies is predominant in plane view.

In the Cretaceous Fula depression, the retrogradational delta has an upward fining cycle generally against normal delta. Sands of underwater distributary channel were deposited at the bottom and is oil-prone. In the middle calcarious sandstones of mouth are interbedded with siltstones of distal bar, and fine graded sediments of underwater natural levee on the top.On the plane, retrogradational delta was distributed below the fault slope break and has the banded shape.

Reservoirs from the high A/S were developed as interlayers with lacustrine mudstone formed in deep water which can be fine source rock and caprock and thus are more oil-prone.

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas