Sequence Stratigraphic Features Of Paleogen And Their Control On Reservoir Formation In The West Of Liaohe Beach Area

Liaohe beach area, locating at the north of Bohai Bay Basin which is the transition area of Liaohe basin and Liaodong bay tectonic unit, is a Mesozoic-Cenozoic faulted basin. Combining the sequence stratigraphy theory and the prospecting practice in this area, the character and the distribution of the sequence boundaries, the sequence architecture and the filling evolution of paleogen were studied. Based on the summary of the sequence stratigraphy control on reservoir formation and the analysis of the oil and gas migration, the focal profitable area was evaluated.According to drilling, logging, seismic and palaeophyte data in the west of Liaohe beach, twelve third order sequence boundaries were identified in paleogen. They can be classified to three kinds: regional unconformity, erosional and overlapping surface and transition surface of deposition cycle. The configuration of second order sequence boundaries were vertically succeeded and horizontally zoned. The strata in the sag deposited continuously. There developed unconformities of overlapping in the slope and unconformities of truncation and overlapping in the upper part of slope. Based on this, the sequence stratigraphy framework was established and the original geologic zonation was adjusted. The attachment and profitable prospecting direction of breccia in Hainan area were confirmed.Totally, the sequences in the north and middle are thicker than that in south and both sides. The sequences in western slope developed completely, but they are relatively thin and the architecture of them is clear. In Hainan sag, SQ1 was not developed, but the sequences are thicker. The sediments are fine particles of lacustrine facies. In the middle swell, the sedimentation began at SQ5, every sequence overlapped on the Yuedong buried hill. Vertically, the filling area extended gradually. In fault depression period, the sequence units developed completely. The distribution of low stand system tracts extended continuously and controlled by synsedimentary faults and palaeotopography. In transition of fault-depression, SQ9-SQ11 did not develop the low stand system tract.There developed several profitable source rock-reservoir-cap rock assemblages in the sequence framework. The oil-gas accumulation units change vertically and horizontally. The oil-gas mainly distribute in low stand system tracts and high-stand system tracts of SQ4 and SQ6 in western slope and Bijialing area. In Hainan area, the low stand system tracts of SQ6-SQ8 are the main units of oil-gas accumulation. In Yuedong area, the oil-gas mainly distributes in transgressive system tracts and high stand system tracts of SQ9- SQ11.Mutual grade slope breaks of different kinds were developed in paleogen. The distribution of the profitable sand body and lithologic-stratigraphic traps were controlled by the slope breaks. The sand body of deltaic plain channel only developed above the high position slope breaks and the delta front sheet sand and underwater channel only developed below the high position slope breaks. The slope fan and overlap traps developed around the high position slope breaks while the distribution of the subsea apron, sublacustrine fan and fluxoturbidite were controlled by low position slope breaks. Obviously, the shape of sequence boundaries in different period influenced the migration of oil-gas. The faults and structure ridges became the beneficial pathway. Bijialing structural zone, Hudong slope and Hainan-Xianhe area were the main migration directive area. Combining reservoir-cap rock assemblage, trap condition and reservoir formation, they were evaluated comprehensively.