Abstract:

Guided by the theories of high resolution sequence stratigraphy and modern sedimentology, sequence stratigraphic framework and depositional environment of Triassic Yanchang Formation in Longdong area of Ordos Basin were studied by making full use of outcrops, cores, logs and seismic data. Chang 8₁ -Chang 6₃ sublayer （LSC3） was defined as an integral long-term base level cycle and then divided into three mid-term base level cycle: MSC3-1, MSC3-2 and MSC3-3. MSC3-1 equals to Chang 8₁ depositional period, when the basin was in strong depression with mainly aggradation and retrogradation stacking pattern. MSC3-2 equals to Chang 7₃-Chang 7₂ depositional period, when the basin expanded rapidly and reached the maximum flood surface, and deposited dark shale in under-compensa-tion status. MSC3-1 equals to Chang 7₂-Chang 6₃ depositional period, when the basin began to shrink with mainly progradation stacking pattern. Sedimentary facies of Yanchang Formation in Longdong area was identified as braided river delta, meandering river delta, turbidite fan and lacustrine environments. In MSC3-1, there was mainly braided river delta with provenance of southwest. In MSC3-2, lacustrine environment occupied dominant position and there were turbidite deposits in deep lake. In MSC3-3, lake began to decrease, so both braided river delta from southwest and meandering river delta from northeast prograded strongly.

Key words: organic material ofcoal series , density , vapor phase , liquid phase , CO ₂ content