Palaeo-tufa from the Liuhuanggou cave 2 in the Ordovician carbonate, North Tarim basin, China:Features and petroleum-geologic significances

There occurs karst belt with extensive palaeokarst caves and other related phenomena in the Ordovician limestone along the south edge of the Tianshan. The belt is 3-4 km width and more than 100 km in length, extending from Aqia near Keping, passing Yijianfang through Sanchakou of Bachu County to the north of Wudaoban. In Yingshan Formation of the karst belt, a roughly 900 m thick weathered crust deposit (Speleothems ) and more than 30 palaeokarst caves are present. Among them, the Liuhuanggou cave 2 is a typical one, with a great deal of tufa containing oil has been discovered, which was formed by dissolution of the underlying Ordovician carbonates and organism and inorganism precipitation under an ambient temperature freshwater environment. This cave developed along a narrow and steeply dipped slip fault with a dipping angle of more than 70°in the Ordovician carbonate rocks. The palaeo-tufa is compositionally and texturally a special substance, with colors from white to black, mostly yellowish brown. The white tufa is free of oil, the yellowish brown has proper oil content and the black is fundamentally filled with oil. The palaeo-tufas are further divided on the basis of petrological feature, color, texture, and morphology into laminated tufa, columnar tufa, wavy tufa, coated tufa, needle-like tufa, sheaf-like tufa, fine crystal tufa, coarse crystal tufa（sand-size detrital tufa）, and so on. Besides, in the edge of the north Tarim basin, from Aqia of Keping to Wudaoban of Bachu, gypsum karst occurs sometimes within the early Ordovician limestone. Natural sulfur ore has been observed in the outcrops. Morphologically and mineralogically, the fissions and pores in tufas have developed between calcite crystals are good reservoir spaces and migrating paths of hydrocarbon liquid. The porosity of the tufa is considerably diversified, from 3.81% to 13.91% (on average 7.21%), which belongs to compact reservoirs (10%). Nevertheless, its permeability is relatively good, ranging from 2.99 to 87.60 ×10-3μm2 and 48.99×10-3μm2 on average, the tufa is generally a good reservoir, because of a great deal of micro-fissures between calcite crystals. Although the highly compacted calcite crystals of the tufa have very low porosity, the abundant network fissures between the calcites crystals appear to be dominant in terms of reservoir permeability and fissure connectivity. It is hence termed a lower porosity and higher permeability reservoir. This study has confirmed the existence of palaeo-tufa in early Ordovician limestone deposited as a primary chemical matter in karst processes. It is confirmed that the tufa layer is both a good reservoir body itself and hierarchically a shield of the primary pores in loose particles (such as pebbles, conglamerate) of the sediments by chemical cementing to form a good reservoir space. In addition, because of containing relatively abundant organic matter, such as microbes like cyanobacteria and algae, the tufa is also considered having somewhat hydrocarbon-forming capability.