Geological and Geochem ical Implications forMulti-period and Origin of Carbonate Karstif ication in the Northwestern Tazhong:taking Well Zhong 1 as an example

QIAN Yi-xiong; ZOU Yuan-rong; CHEN Qiang-lu; CHEN Yue

Well Zhong 1 is an important discovery well of oil and gas in Ordovician carbonate rocks located at the Northwestern Tazhong up lift. This paper p resents an updated review of the multi-period and origin of carbonate karstification, which took into account of core observation, palaeo-biological analysis, seismic section and mineralogical and petrological and geochemical of calcite cements and calcites in cleavages and cavities or vuges. The two tectonic-related truncation and sedimentary gap surfaces have been identified, which hare some extent of relationship with carbonate karstification in Caledonian, the fact that calcites in cleavages and cavities or vuges in Ordovician carbonate rockswith rather low contents of elements of Fe,Mn, Sr and Ba, and the normal value ofδ¹⁸O ( - 6. 4‰～ - 4. 5‰) andδ¹³ C( - 1. 4‰～0. 5‰) in comparison with that of the average value of seawater of Ordovicianare believed to occurr as a composite results of syngenesis affected by the mixed fluid at shallow buried condition and later meteoric invasion;while the calcite cements developed in Bachu,Donghe formations and Silurian sandstoneswith the ramiform structures of infiltration metasomatism related with magmatic hydrothermalism and the low value ofδ¹⁸O ( - 10. 5‰～ - 16. 1‰)andδ¹³ C ( - 1. 5‰～ - 9. 6‰) indicated it had undergone an intense hydrothermal buried aswell as Hercynian subaerialmeteoric diagenetic change; the low data of strontium isotop ic compositions (⁸⁷ Sr /⁸⁷ Sr less than 0. 7100) of filling calcites in cleavages and cavities or vuges in Kalashayi formation and Silurian and Ordovician system of Well Zhong-1 are exp lained as the results of the little effect of continental clastic sediment due to a rather short period of exposure; moreover, itmay be concluded that no similar geological condition or background of a large scale of strong ep igenic karstification exists in the Northwestern Tazhong just as occurred in the Tahei oilfield ( Shaya up lift) with Palocave systemswidely developed in forms of multiple passage caves in the Middle-Upper Ordovician carbonate

Geological and Geochem ical Implications forMulti-period and Origin of Carbonate Karstif ication in the Northwestern Tazhong:taking Well Zhong 1 as an example

1. Department of Resource and Environment , University of Petroleum, Beijing 102200;

2. Western Institute, Exploration & Production Research Institute, SINOPEC,Urumqi 830011

Received Date: 2004-11-12

Rev Recd Date: 2005-05-13

Publish Date: 2005-12-10

Key words:

Abstract: Well Zhong 1 is an important discovery well of oil and gas in Ordovician carbonate rocks located at the Northwestern Tazhong up lift. This paper p resents an updated review of the multi-period and origin of carbonate karstification, which took into account of core observation, palaeo-biological analysis, seismic section and mineralogical and petrological and geochemical of calcite cements and calcites in cleavages and cavities or vuges. The two tectonic-related truncation and sedimentary gap surfaces have been identified, which hare some extent of relationship with carbonate karstification in Caledonian, the fact that calcites in cleavages and cavities or vuges in Ordovician carbonate rockswith rather low contents of elements of Fe,Mn, Sr and Ba, and the normal value ofδ¹⁸O ( - 6. 4‰～ - 4. 5‰) andδ¹³ C( - 1. 4‰～0. 5‰) in comparison with that of the average value of seawater of Ordovicianare believed to occurr as a composite results of syngenesis affected by the mixed fluid at shallow buried condition and later meteoric invasion;while the calcite cements developed in Bachu,Donghe formations and Silurian sandstoneswith the ramiform structures of infiltration metasomatism related with magmatic hydrothermalism and the low value ofδ¹⁸O ( - 10. 5‰～ - 16. 1‰)andδ¹³ C ( - 1. 5‰～ - 9. 6‰) indicated it had undergone an intense hydrothermal buried aswell as Hercynian subaerialmeteoric diagenetic change; the low data of strontium isotop ic compositions (⁸⁷ Sr /⁸⁷ Sr less than 0. 7100) of filling calcites in cleavages and cavities or vuges in Kalashayi formation and Silurian and Ordovician system of Well Zhong-1 are exp lained as the results of the little effect of continental clastic sediment due to a rather short period of exposure; moreover, itmay be concluded that no similar geological condition or background of a large scale of strong ep igenic karstification exists in the Northwestern Tazhong just as occurred in the Tahei oilfield ( Shaya up lift) with Palocave systemswidely developed in forms of multiple passage caves in the Middle-Upper Ordovician carbonate

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Geological and Geochem ical Implications forMulti-period and Origin of Carbonate Karstif ication in the Northwestern Tazhong:taking Well Zhong 1 as an example

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