Causes of Oil Accumulation of Isolated Bars in Lacustrine Delta

After over 20 years of development in Daqingzijiang Oilfield, isolated sand bodies under the delta front and pro-delta in the Qingshankou Formation(q n )have become important oil and gas reservoir bodies. However, the cause for large amounts of isolated bar sand bodies in the lake-basin delta sediment system has not been reported in China. This article, through core observation, reveals plenty of evidence that there storm waves once existed. Combined with paleogeography and hydrodynamic force analysis, it describes the transformation effect of waves on delta sand bodies and on the formation mechanism of bar sand bodies. Based on a study on paleogeomorphology and the statistics of sand body distribution, we consider ‘Storm waves conveying sand’ and ‘landform controlling sand’ as the cause and distribution model of the delta’s isolated bar sand body formation. We also think that the superposition of multiple bar sand bodies is the direct cause of the strong anisotropy in reservoirs and the complex relationship between oil and water in reservoirs. Most of these sand bodies have formed into lenticular lithologic hydrocarbon accumulations. On the basis of this integrated study on hydrocarbon accumulation, we set up an accumulation model of lenticular hydrocarbon accumulation involving the variables ‘Surrounded by source rocks to generate hydrocarbon’, ‘Driven by pressure difference’, ‘Migration through multi pathways’ and ‘Accumulation by filtering water’.