Abstract:

 The research of some core, logging, seismic data from Quantou Formation in Sanzhao Sag has been finished and some Geological ideas have been got, that is, the provenance of Sanzhao Sag was located in the north and southwest of Songliao Basin during the period of Quantou Formation, and  the geological conditions were favorable for the formation of the largescale shallow delta: the basement sank stably and the palaeotopography was gentle and smooth slope; the physical weathering was strong due to the arid palaeoclimate(up to 37 ℃); the lake level changed cyclically because of two cycles of palaeoclimate change from arid to humid; the lake water was shallow less than 10 meters and lake level changed obviously.
The shallow delta of Sanzhao Sag  has obvious characteristics, that is the finer grain size, plenty of sedimentary structures formed by strong hydropower, wide purple mudstone with lots of  bioturbation structure, several noncontinuous normal cycles wertically, imbricated progradation seismic facies, large scale sandbody distribution with low percentage of sandstone to mudstone, and very unique distributary channels with few mouth bars. The sedimentation of the shallow delta was controlled by the climate change. When it was dry, the lake shrank, resulting in branched delta and branched distributary channels which were broad and shallow; the length of channels was more than 20 km, the width from 800 to 1 800 m, the average thickness of individual layer was 2.3 meters, and the average percentage of sandstone in strata was 35%. When it was wet, the lake expanded, the river branched off diverted frequently due to the lake water obstruction, formed reticular distributary channels that were narrow and deep, the length of the channel was less than 15 km, the width from 500 to 1 600 m, the average thickness of individual layer was 2.8 m, and the average percentage of sandstone  in strata was 40%. Therefore, the sandbodies of distributary channels are widely distributed, overlapped vertically, and continued laterally, and form favorable lithologic reservoirs.

Key words: Key words: the Sanzhao Sag, the Cretaceous Quantou Formation, largescale shallow delta, formation, sedimentary model