Abstract:

The discovery of several large to mediumscale gas fields, such as Puguang gas field, Yuanba gas field, Longgang field etc., reveals that carbonate reefbeach zones have great petroleum exploration potential in the platform margin of the east and west sides of KaijiangLiangping shelf and the west side of ChengkouExi shelf. Previous exploration has confirmed that the characteristics of gas fields, sedimentation and reservoirs of reefbeach are obviously different in different parts of platform margins. However the regularity of the differences needs to be disclosed by further studies. Based on the study of highresolution sequence stratigraphy, sedimentology and reservoir evaluation, the paper analyzes the differences. Two thirdorder sequences have been identified which are shoaling upward, and all of them are composed of transgressive and highstand system tracts. It reveals that the characteristics of sedimentary facies and reservoirs are obviously different in different parts of platform margin. During the period of Changxing, the palaeogeomorphology of platform margins was similar in west and east sides of KaijiangLiangping shelf. The strata with main aggradational stacking patterns were strongly dolomitized at the top of Changxing Formation in TiechanghePuguangGaofengchang platform margin, which was the characteristics of steep ramp, thin sedimentation of beach and developing reef. And the YuanbaLonggang platform margin was characterized by gentle slope, obvious forest and lateral accretion stacking pattern, development both reef and beach, and stronger dolomitization at the top of reef and beach; porosity reservoir with good physical property was developed. In the period of Feixianguan, the asymmetry of the platform margins was clear. In TiechanghePuguangGaofengchang platform margin, the stacking pattern of stratigraphy was still aggradational, and thick oolitic beach with strong dolomitization was developed, and its porefractured type reservoir with good physical properties was developed in the second member of Feixianguan Formation. The YuanbaLonggang platform margin was developed thin oolitic beach with weak dolomitization in the second member of Feixianguan Formation, and the reservoir was typical of pore reservoir with poor physical property. The PanlongdongJiantianba platform margin grew reef and beach in Changxing Formation, and developed oolitic beach in the second member of Feixianguan Formation, both of them were strongly dolomitized and were easy to form available reservoirs. Through the comprehensive study of the characteristics of sedimentation and reservoir, we conclude that the sedimentation and the distribution of reservoir are obviously controlled by the integrated factors of synsedimentary palaeogeomorphology, dolomitization, faults, and the diagenesis and fluids in the burial period.

Key words: reef and beach in platform margin, sequence stratigraphy, sedimentation, reservoir, Northeastern Sichuan Basin