High-Frequency Sequence Stratigraphy, Depositional Systems, and Hydrocarbon Trapping Style of Upper Cretaceous QY Formations in HTZ 3-D SSurvey, Southwest Songliao Basin, China

Zhao, Wenzhi¹, Caineng Zhou¹, Yingliu Chi¹, Hongliu Zeng² (1) PetroChina, Beijing, China (2) The University of Texas at Austin, Austin, TX

The southwest margin of the Giant Cretaceous Songliao Basin is currently undergoing active stratigraphic-trap prospecting for additional hydrocarbon resources in the region. A successful prediction of thin sandstone reservoirs requires high-resolution (5- to10-m) geologic and seismic mapping. In the 800-km² HTZ 3-D seismic-survey area, the Upper Cretaceous Qingshankou and Yaojia (QY) Formations are composed of five 3rd-order lacustrine sequences (100-m scale). Wireline-log correlation made it possible to recognize 3rd-order maximum flooding surfaces and sequence boundaries that can also be correlated through seismic profiles. At isolated well locations, 4th- and 5th-order sequences (5- to 30-m scale) are also recognizable using high-resolution wireline logs. However, these high-frequency sequences are difficult to correlate between wells because of the lack of seismically resolved correlation marks in low-frequency seismic data. We unraveled the problem by first doing seismic lithology and tying seismic amplitudes to the sandstone/shale ratio and then making seismic stratal slices for the seismic geomorphologic signature of high-frequency depostional systems tracts. Eighteen 4th-order sequences and even more 5th-order sequences have been identified. Two main depositional systems have been interpreted on the basis of residual sedimentary-rock images on stratal slices. The Lower Qingshankou Formation is characterized by transgressive, lacustrine deltaic systems dominated by fluvial processes and modified by wave processes. The Upper Qingshankou and Yiaojia Formations are dominated by highstand, lacustrine, wave-dominated deltas with highly linear, sandy barrier-bar development. With few easy-to-map local structures, traps formed by pinch-out or fault sealing of distributary-channel and barrier-bar sandstones are major drilling targets for the near future.