Development Regularity of Shallow Water Delta Sand Bodies under the Control of Super-short-term Base Level Cycle：An Example From the Low Cretaceous in DT91 block, Fuyu Oilfields, Songliao Basin, China

Shallow water delta depositional system are widely existed in Meso-Cenozoic formation in Songliao Basin (NE China). Taking the fourth Member of the Low Cretaceous Quantou Formation (K1q4) in the DT91 Block of the Fuyu oilfield... [ view full abstract ]

Shallow water delta depositional system are widely existed in Meso-Cenozoic formation in Songliao Basin (NE China). Taking the fourth Member of the Low Cretaceous Quantou Formation (K1q4) in the DT91 Block of the Fuyu oilfield in Songliao Basin as an example, based on cores from 14 wells, well-log information of 789 wells and modern sedimentary phenomenon, this paper analyzed the genetic sandbodies and super-short-term base level cycles of shallow-water delta reservoirs, and discussed the controlling mechanism on the development and distribution of sandbodies.

Results indicated that: 1)six types of sandbodies including distributary channel sandbodies, overflowed sandboies, distributary bar, natural levee, crevasse splay and sheet sandbodies are widely developed in shallow-water delta reservoirs, especially the channel sandbodies and distributary bar. 2) Five physical interfaces of base level cycle including the erosion surface at the bottom of channel, progradation-to-retrogradation and retrogradation-to-progradation interfaces, color discontinuous surface of mudstone and flood surface can be recognized in the cores and well-log data. 3) Super-short-term base level cycle can be classified in three types: up-deepening non-symmetry, up-shallowing non- symmetry and symmetry base level cycles.

Unlike normal delta (Gilbert delta), the sedimentary facies belt of a shallow water delta are wider and obviously controlled by super-short-term base level cycle. For example, during the period of the 3th group of k1q4 in the research area, the base level mainly experienced three evolutionary stages: early to fall quickly stage, middle to rise slowly stage and later rise quickly stage; correspondingly, the sedimentary environment of this sand group experienced a series of changes from internal-delta-front to lower-delta-plain to upper-delta-plain. When super-short-term base level cycles changed from A/S<<1→A/S=1→A/S>>1, sandbodies stacking pattern changed from downcuting-aggradation to non-downcuting-aggradation to solitary, the lateral combination of sandbodies changed from complex multi-types to simple single-type ones, and the degree of overlap and connectivity of adjacent sandbodies decreasing (changed from carpet-type to sheet-type to dendritic-type) in the planar distribution.