Genesis,distribution, and hydrocarbon implications of ostracod-bearing beds in the Songliao Basin (NE China)

The Upper Cretaceous Qingshankou Formation (K₂qn) and Nenjiang Formation (K₂n) in the Songliao Basin (SLB), China, contain discrete ostracod-bearing deposits. The ostracod-bearing layers range in thickness from 5 to 380 cm. Based on integrated analysis of seismic, drilling, core and logging data, the sedimentary characteristics, distribution patterns and hydrocarbon implications of the ostracod-bearing beds have been studied. Compared with mudstone and sandstone in lacustrine settings, ostracod-bearing beds have distinctive characteristics in terms of seismic response and logging features. Three types of depositional pattern of ostracod-bearing beds are recognised in different facies: mixed siliciclastic–ostracod deposits in the delta front, sheeted ostracod deposits, and dotted ostracod deposits in shallow or semi-deep lacustrine settings. In plan view, ostracod-bearing beds mainly occur in the Daqing area, the Longhupao–Honggang terraces and the Heidimiao depression, and are relatively less developed in the east of the SLB. Shallow and semi-deep lacustrine settings were favourable environments for the survival of ostracods. In the stratigraphic succession, ostracod-bearing beds mainly occur in the upper K₂qn¹, K₂qn², lower K₂qn³ and K₂n¹ units. High-frequency sequence cycles controlled ostracod reproduction and death, and mass mortalities of ostracods commonly occurred during periods in which the lake base-level fell, leading to water shallowing, increasing salinity and sand intrusion into the shore-shallow lakes. Ostracod mortality was not associated with expansion of the lake area, marine transgression or volcanic eruptions. The ostracod-bearing beds were widely formed in shallow and semi-deep lacustrine settings in the SLB. Tight oil/gas reservoirs can be developed in thick ostracod-bearing beds. In addition, areas containing ostracod can be regarded as sweet spots for mud or shale oil/gas exploration, because extensive ostracods can improve the content of brittle minerals in mudstone.