Oil & Gas Geology ›› 2012, Vol. 33 ›› Issue (1): 50-60.doi: 10.11743/ogg20120107

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Significance of syndepositional volcanism to high quality reservoirs in the Xujiahe Formation,the western Sichuan Depression

Zeng Xiaoying1, Zhang Juan2, Liu Yuanyang1, Li Yong1   

  1. 1. Research Institute of Exploration & Development, SINOPEC Southwest Oilfield Company, Chengdu, Sichuan 610081, China;
    2. School of Petroleum Engineering, Southwest Petroleum University, Chengdu, Sichuan 610500, China
  • Received:2011-06-12 Revised:2011-12-12 Online:2012-02-28 Published:2012-03-22

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Abstract:

The Xujiahe Formation is a regional gas reservoir in western Sichuan Depression.Due to the great bu- rial depth,the reservoir generally experienced complicated diagenesis,resulting in tight to supertight reservoirs.Even so,porous reservoirs with relatively good storage capacity were also developed.From observation of more than 30 wells,over 7 000 ordinary thin sections and over 1 000 cast thin sections in the study area as well as the research of related reservoirs,it is demonstrated that volcanic debris dissolved pores are the major storage space of sandstones of of the 2nd and 4th member of Xujiahe Formation,and are the major controlling factor of high quality reservoirs in the area.Dissolution of feldspar deposited through weathering and transportation of parent rock is not the major reason for the forming of sandstone with medium-high porosity.Volcanic debris from syndepositional volcanic eruption contain many "soluble compositions",which tend to be dissolved and devitrified to form pores during diagenesis,thus controlling the distribution of high-permeability sandstone reservoirs with well-developed secondary dissolved pores.Therefore,the compositions of debries from the syndepositional volcanism and their dissolution intensity are the major factors controlling the development and distribution of high quality reservoirs of the Xujiahe Formation and the formation of sandstone with medium-high porosity.Ganzi-Litang inter-continental rift area where volcanisms were strong during the Late Triassic was the major source for volcanic debris in the study area.

Key words: syndepositional period, volcanism, soluble composition, secondary dissolution pore, pore genesis, high quality reservoir, Xujiahe Formation, western Sichuan Depression

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