Forming Conditions and Depositional Characteristics of Shallowwater Deltas in Depression Basins:  A case study of K₂y¹ in the south of Songliao Basin

Key words: 
• shallowwater basin /
• shallowwater delta /
• K2y1 /
• Songliao Basin /

Abstract: Shallowwater delta in large lacustrine depression basins is different from conventional delta in aspects of framework, microfacies, sandbody, scale and shape. In this article, forming conditions and deposition characteristics of K₂y¹ shallowwater deltas in the southern Songliao basin has been analyzed. Songliao basin are characterized by stable structure, slow subsidence, flat palaeotopography, dry climate and sufficient material source during Putaohua Formation, thus formed a distinctive shallowwater delta deposition. Unlike conventional terrigenous basins, its shoreline often migrated in large scale and few deep lacustrine and semideep lacustrine facies developed .
 The shallowwater delta can be distinguished by following characteristics:(1)The channel of shallowwater delta extended far away, up to 160 km from delta plain to lake center; Divaricating and confluence were often happen in delta . Sand bodies show obvious direction and birds footlike distribution on the isopach map.(2)Mouth bar in the front of delta was not developed as restricted by accommodation and washing action. The sandstone reservoirs were obviously controlled by fluvial system.(3)Channel on the delta front were reformed to sheetlike sandstone by lake water and can be divided into three styles which include poorly sheetlike, moderately sheetlike and intensely sheetlike styles;(4)The sedimentary models of shallowwater delta differ from Gilbert delta style which have topset bed, bottomset bed and foreset bed. During K₂y¹ deposition period, the base level increased gradually,  I and II  sand group provided a favorable reservoir storage space, III sand group transgress provided a good seal for Putaohua reservoir.