Platform Margin Styles And Forming Mechanism Of Reef-bank Facies In The Upper Ordovician In Tazhong Area, Tarim Basin

Carbonate Reservoirs of reef-bank facies played a key role in oil& gas fields. Due to the difference in exploration maturity in the Tazhong area of the Tarim Basin, the understandings of the styles of the Upper Ordovician carbonate platform edges were mainly limited to the northern edge of the Tazhong area. Taking the Upper Ordovician Lianglitage Formation in the Tazhong area of Tarim Basin as the major subject of study, this paper studied the formation mechanism of reservoirs of reef-bank facies developed on margin of carbonate platforms by integrating the latest seismic, drilling, logging and testing data, with a focus on the architectures of platforms in the southern, eastern and northwestern parts of the Tazhong area. The results can be used to guide exploration planning.Study based on tectonic-sea level changes showed that the Late Ordovician NW-SE trended Tazhong carbonate platform with narrow in the east and wide in the west, controlled by fault breaks on both sides of north and south, was formed in a persistent transgressive setting on the Tazhong ancient land that took shape at the end of Middle Ordovician. Both the southern and northern margins of the Late Ordovician platform in Tazhong area were steep in the central part and get gentle to the east and west. Three platform margin styles, including steep slope type rimmed platform margin, gentle slope type rimmed platform margin and step-type platform margin, were recognized according to the features of the Upper Ordovician platform margin and its adjacent slopes in the Tazhong area. The steep slope type rimmed platform margin was mainly distributed in central parts of the northern and southern margins of the Tazhong platform, gentle slope type rimmed platform in the northwestern part and the step platform margin in the eastern part. Development of different platform margins were mostly controlled by the fault activity in different locations during formation of the platform and the topographic difference resulted from the faults.Based on study of diagenesis, model for pore evolution in the Upper Ordovician Lianglitage Formation was established. The early fresh water dissolution was basically cemented and filled. The pores experienced four stages of burial dissolution. The burial dissolution of stage I was mainly filled by non-ferroan calcite, stage II by ferroan-bearing calcite, and stage III was half or totally filled by asphalt. And most of the dissolution of stage IV was the effective pore without filling. The Lianglitage Formation in the Tazhong area experienced karstification in the episode II of the Middle Caledonian which shows large regional variation of intensity. Karstification of the platform margin facies was strong along the Tazhong I fault zone, while was relatively weak in other areas.Based on synthetic analyses, it was showed that the sparry grain carbonates deposited in high-energy reef-bank facies was leached and cemented during the syngenetic period. However, the intra-grain calcite and inter-grain cement of grain carbonates was dissolved again in burial environment, which was related to the development of late effective pore. Early fresh water dissolution and cementation provided favorable conditions for burial dissolution. Hydrothermal activity in Tazhong area was mainly presented by filling of hydrothermal minerals to the pre-existing pores, with limited contribution to effective reservoirs by hydrothermal dissolution. Storage space of reservoir was divided into pore, vug and fracture. The dissolution pore and vug were principally formed by karstification of weathering crust and burial dissolution, with rare primary pores and early dissolution pores. The structural fractures formed effective storage space. Granulation of carbonate grains resulted from structural compression was favorable for burial dissolution. The quality of reservoirs was improved by multiple types and multistage fracturing.