Oil & Gas Geology ›› 2017, Vol. 38 ›› Issue (6): 1087-1097.doi: 10.11743/ogg20170609

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Evaluation of Ordovician source rocks and natural gas accumulation patterns in west flank of a paleo-uplift,Ordos basin

Xiao Hui1,2, Zhao Jingzhou1,2, Xiong Tao3, Wu Weitao1,2, Mi Jingkui4, Liu Sutong1,2   

  1. 1. School of Earth Sciences and Engineering, Xi'an Shiyou University, Xi'an, Shaanxi 710065, China;
    2. Key Laboratory of Hydrocarbon Accumulation of Shannxi Province, Xi'an Shiyou University, Xi'an, Shaanxi 710065, China;
    3. No.5 Oil Production Plant, PetroChina Changqing Oilfield Company, Xi'an, Shaanxi 710200, China;
    4. PetroChina Research Institute of Petroleum Exploration & Development, Beijing 100083, China
  • Received:2016-12-14 Revised:2017-10-08 Online:2017-12-28 Published:2018-01-08

Abstract: The existence of a new gas accumulation type sourced from the source rocks within the Ordovician was confirmed at the lower slope in the west flank of a paleo-uplift in Ordos Basin based on effective source rock assessment results.Analyses of 347 samples taken from the source rock intervals in 11 wells revealed high-quality source rocks developed in marginal slopes of the middle and upper Ordovician platform in the Basin.Measurements showed that the source rocks in the Wulalike and Kelimoli Formations had relative higher TOC values,with marlite TOC averaged at 0.33% and 0.39%,and mudstone TOC averaged at 0.51% and 0.65%,respectively.The accumulative effective thicknesses of source rocks of the two formations were 12 to 40 m and 7 to 14 m,respectively.Areas with thick source rocks were assessed to have a gas generation capacity of 14.0×108 to 15.5×108 m3/km2.Gas accumulations in the Kelimoli Formation experienced two charging stages:the early stage in the Middle Jurassic,during which paleo-oil reservoirs were formed;and the late stage in the Late Jurassic to Early Cretaceous,during which the paleo-oil reservoirs were cracked into petroliferous gas reservoirs.Carbon isotopes of gas-bearing inclusions from the later stage were measured to be -38.84‰ in average,indicating late petroliferous gas charging characteristics.Coal-derived gas reservoirs were distributed widely in the upper slope in the west flank of the paleo-uplift.The fact that the massive detrital calcareous berccias formed in the Kelimoli Formation at the lower slope had a close contact with the overlying effective source rocks of the Middle and Upper Ordovician and that they stretched away laterally from the top Ordovician weathering crust,made it possible for the formation of self-sourced gas accumulations,like Yutan1 gas reservoir in the area.Fractured-vuggy karst reservoirs developed in the lower slope of the platform margin were considered to be potential exploration targets for such kind of gas reservoirs.

Key words: accumulation pattern, source rock, Ordovician, Lower Paleozoic, Ordos Basin

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