Abstract:

The pore,vadose and fissure reservoirs associated with epigenic Karstification of carbonate rocks of  Lianglitaga Formation in northern margin of Katake uplift,Tarim basin, is one of important areas of hydrocarbon exploration. Well Zhong-2 is the first and foremost important exploring well for identification of structure of platform margin and reservoir of reefshoal of Lianglitaga Formation of the Upper Ordovician, southern margin of Katake uplift. The carbonate rocks of Lianglitaga Formation in Well Zhong-2 were studied on the assemblage of para-facies and sedimentary geochemical characteristic with comparable data to that of Yijianfang and Yingshan formations of the Middle and Lower Ordovician, the results show Lianglitaga Formation of Well Zhong-2 consists of a serial of stacking pattern of reef, shoal and bioherm mound with predominant development of bryozoans and coralline framestone, bindstone and bafflestone. The geochemical data indicated that the carbonate of Lianglitoga Formation primarily precipitated in the normal marine with less contents of siliciclastic input. The total contents of SiO₂,Al₂O₃,Fe₂O₃ are less than 0.5%, the content of MgO less than 0.40%, the contents of CaO values vary from 53.4% to 54.25%. The average total rare earth elements of Lianglitaga Formation carbonate rocks is 8.65×10^-6 with average ratio of LREE and HREE 620（vary from 4.71 to 6.90）. The average value ⁸⁷Sr/ ⁸⁶Sr is 0.708,43 with no significantly  change between 0.708,00 and 0.708,82 for carbonate rocks of Lianglitaga Formation. The stable isotopic and rare earth elements suggest the precipitation is in shallowing upward succession carbonate cycles and in the suboxidization or weak reduction depositional environment, with the average value  δEu 0.67（values vary from 0.49 to 0.75）and δCe 0.80（values vary from 0.76 to 0.84）respectively.The δ¹⁸O and δ¹³C values of carbonate rocks of Lianglitaga Formation vary from -7.3‰ to -3.2‰VPDB （the average value -4.4‰,n=13） and from +1.8‰ to +3.1‰ VPDB （the average value 2.7,n=13） respectively, and both values show a similar to that of the global sea-water and carbonate rocks in the late Ordovician,indicating that the stable isotope composition and  record have been kept in the weak diagensis process

Key words: sub-facies analysis, elements analysis, isotope value, Lianglitaga Formation, well Zhong-2, southern margin of Katake uplift