PALAEO-TUFA FROM LIUHUANGGOU CAVE 2 IN THE ORDOVIAN CARBONATE, TARUM BASIN, CHINA: FEATURES AND SIGANIFICANCES

Limestone palaeokarst cave and related other problems occur extensively in the Ordovician limestones of south edge of Tianshan, located in a belt 3-4 km wide and more than 100 km length from Aqia near Keping, past Yijianfang, through Sanchakou of Keping County, to just north of Wudaoban. In this belt, up to roughly 900 m thickness of weather crust deposits (Speleothems ) and at least more than 30 large or small typical palaeokarst caves are present in the Yingshan Formation. Nevertheless, a well-developed palaeokarst cave, Liuhuanggou cave 2, with a great deal of tufas containing oil has been discovered and was formed by dissolution of the underlying Ordovian carbonates and organism and inorganism precititation under ambient temperature freshwater environment.

The Liuhuanggou cave 2 develops in a narrow sliding slip faulted zone bordered by steeply dipping faults with a angle of more than 70°, lithologically characterized mainly by the occurrence of Ordovian carbonates rocks. The palaeo-tufa has a color ranging from white to black, mostly yellowish brown. The white tufa is free of oil, the yellowish brown has proper oil and the black is fundamentally filled with oil. The palaeo-tufas are further divided on the basis of petrological features, mainly color, textures, and resulting morphology: laminated tufa, columnar tufa, wavy tufa, coated tufa, needle-like tufa, sheaf-like tufa, fine crystal tufa, coarse crystal tufa, and so on.

This study has confirmed the existence of palaeo-tufa in early Ordovician limestone deposited as a mainly chemical matters in kastification . It was confirmed that the tufa reservoir layer is both good reservoir body itself and hierarchically shield the primary pores in loose particals (such as pebbles, conglamerate) of the sediments by chemical cementing to form a good reservoir space.