APPLICATION OF SEQUENCE STRATIGRAPHY TO THE PREDICTON OF SUBTLE RESERVOIRS IN THE BIYANG DEPRESSION

Sequence stratigraphy provides an effective theory and method for studying subtle reservoirs e-xisting in strata. The distribution of oil and gas is mainly controlled by surfaces of sequence, i.e. sequence boundary (SB), first flooding surface (FFS) and maximum flooding surface (MFS). The FFS is the most important control factor, and the range of the distribution of subtle reservoirs in the lowstand system tract (LST) is greatly controlled by the range of effective enclosing of the first flooding mudstones for oil and gas. A method for forecasting subtle reservoirs,"four confirmations and one integration", is put forward on the basis of sequence stratigraphy and sedimentation, and five subtle reservoirs are summarized in the end. Among the five types, the reservoir formed by the"faults cutting sausage"and sealed by FFS mudstone is always occurred in LST and is strictly controlled by FFS, and the onlap reservoir and the unconformity reservouir are developed in transgressive system tract (TST) and highstand system tract (HST) respectively and both have close relationship with SB. The lithologic reservoirs are developed in various different system tracts.