Depositional Characteristics and Front Facies ReservoirFramework Model in SongLiao Shallow Lacustrine Delta

Key words: 
• Songliao basin /
• shallow water delta /
• depositional characteristic /
• subaqueous channel /
• front facies

Abstract: Characterized by large area, shallow water, gentle slope, wide wave influencing belt, and weak energy, the SongLiao basin deposited a distinct large river delta in its northern depositional system. The delta is different from other marine delta in vertical sequence, types of framework sandbody and scale etc.. Instead, it presents more characteristics of shallow water delta. The delta is highly constructive fluvial dominated, characterized by directional ribbon like sandbody in front facies, and fluvial deposition can influences more than 20～30 kilometers under shoreline. Vertically, its sequence is discontinuous without three structure of typical delta. The thickness of sandbody is thin, with that of subaqueous channel 2～5m, and that of sheet sand less than 2m. In general, facies belts are well developed in which front facies usually covers more than 30～40 kilometers, and the inner front including the microfacies of subaqueous and sheet sand,and the outer front facies including only the sheet sand can be subdivided. Various kinds of sandbody association are formed because of frequently transgression and regression. The above characteristics have resulted in corresponding sandbody types in various stratigraphy section and subfacies area. Distinct framework models are formed while each kind of sandbody associate with mud in 3D space, these provides a sound genetic basis for establishing framework model. Based on the quantitative parameters such as density of sandbody, density of barrier, and coefficient of net sand,combining with the genetic features of sandbody associations and facies location, 5 types of framework models such as isolated channel pattern, stacked channel pattern, stable inter bedded pattern, unstable inter bedded pattern and isolated thin layer pattern are put forward. The sandbody in isolated channel pattern is very narrow, and shows puzzled shape in 3D space, so it is easy to form small ribbon like or lump like oil bearing area resulting from incompletely water driver under the condition of large well distance. Characterized by well developped subaqueous channel and main sheet sand, large sandbody density, good reservoir quality and connectivity between sandbodies, most of recovery oil in stacked channel pattern has been produced. The main features of unstable inter bedded pattern are its narrow subaqueous channel width and incised sheet sand, which result in good communication in later and poor connectivity in vertical. Remaining oil only exists in small area caused by local poor reservoir quality. For the stable inter bedded pattern, sandbody covers large horizontal area, sand layer and mud layer alternate in vertical. The sandbody possesses good connectivity in later and poor connectivity in vertical, some relative large area remaining oil exists resulting from poor reservoir quality. The sandbody of isolated thin layer pattern distribute on outer front facies of various types of delta, because of its very small sandbody density, the connectivity between sandbodies is very poor in 3D space, remaining oil can be produced only by density well pattern.