Oil & Gas Geology ›› 2006, Vol. 27 ›› Issue (1): 85-92.doi: 10.11743/ogg20060114

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Ordovician-Silurian sedimentation and diagenesis in northeastern Tarim basin

Zheng Bing1,2, Cheng Qiuquan2, Gao Renxiang2   

  1. 1. Chengdu University of Technology, Chengdu, Sichuan 610059, China;
    2. Institute of Petroleum Geology, Exploration and Production Research Institute, SINOPEC, Wuxi, Jiangsu 214151, China
  • Received:2005-09-23 Online:2006-02-25 Published:2012-01-16

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Abstract:

The Ordovician is composed of silty mudstone and limestone, and the Silurian is a typical flysch formation composed of four depositional cycles of glutinite in the northeastern Tarim basin. The reservoirs occur at the bottom of each cycle. All of the 4 cycles have been inverted. Diagenesis is obvious at a depth of 1 500 m where sparry calcite (Ⅰ) has been precipitated from water rich in atmospheric CO2 with δ13C in the range of -4‰ and -6‰, and 818O in the range of -5‰ and -6‰. At a burial depth of 2 400-2 600 m, δ13C and δ18O in the carbonate (Ⅱ) formed in the sulfate reduction zone are in the range -7‰ to -10‰ and -8‰ to -10‰, respectively, where the dissolution of feldspar constitutes the 1st secondary pore zone. Dolomites (Ⅲ) with wavy extinction have been formed at a burial depth of 3 800-4 000 m, along with the decarboxylation in the methane generating zone, the δ13C is in the range of -11‰ and -16‰, while in the carbonate directly formed from CO2, the δ13C is ±20‰, and the δ18O both in dolomite and carbonate range from -12‰ to -13‰, temperatures measured from secondary SiO2 inclusions are in the range of 110℃ and 120℃. This interval constitutes the 2nd secondary pore zone. The depth interval of 4 500-4 700 m is a wet gas generating zone, the δ13C and δ18O in carbonate (Ⅳ) formed in this interval are in the range of -27‰ to -31‰ and -13‰ to -14‰, respectively, and the temperature measured from inclusion thermometry is 130 -140℃; and the massive dissolution of ferroan calcite has resulted in the formation of the 3rd secondary pore zone. Large amount of dry gas (Ⅴ) were generated at a burial depth of over 5 000 m, where the 8 C ranges from -18‰ to -20‰ and δ18O ranges from -13‰% to -14‰, being accordant with the inclusion's homogenization temperature of 150-155℃. This interval constitutes the 4 th secondary pore zone with porosity in the range of 15% to 20%. The folded strata might have been inverted during Hercynian movement, uplifted and eroded about 5 000 m and associated with'retrodiagenesis'.

Key words: homogenization temperature of inclusion, secondary pore zone, isotope, generation, carbonate cement, Ordovician-Silurian, northwestern Tarim basin

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