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.