柯坪硫磺沟奥陶系（纹层状）古钙华的特征及石油地质意义

天山南缘从柯坪阿恰经巴楚三叉口到巴楚五道班的100余公里的316国道北侧的奥陶系中发育了一条1~2 km宽的碳酸盐岩岩溶带。在该带的鹰山组中发育了一条900 m宽的风化壳沉积（古土壤）和约30多个大型古溶洞。其中以硫磺沟2号溶洞最为典型，该溶洞位于一个很窄的倾角大于70°的走滑断裂带内，古钙华一般产在溶洞和裂缝中，作为充填体出现。硫磺沟2号溶洞的古钙华多呈黄褐色-黑色，普遍含石油。古钙华纹层发育，根据结构、颜色及形态等可以分为六种，分别为层状纹层、波状纹层、包壳状纹层、束状纹层、丘状纹层或半球形丘状纹层和杂乱纹层，这些纹层的形态和结构特点反映了其成因与菌藻或微生物有关；构成纹层的方解石主要为针状、柱状、细晶状和粗晶状等。钙华方解石晶体之间缝隙和孔隙非常发育，是良好的储集空间和运移通道。物性研究表明，硫磺沟2号溶洞古钙华的孔隙度介于3.81%~13.91%之间，平均7.21%，从孔隙度值来看属于致密储层（10%）；但渗透率很大，介于（2.99~87.60）×10-3μm2之间，平均48.99×10-3μm2，从渗透率看是一种较好的储层。出现这种“反常”的情况与古钙华特殊的内部结构有密切关系，钙华以晶间缝为主，具有紧密堆积的结构，虽然孔隙度不大，但是晶间缝在三维上构成了网络系统，导致渗透性很好，被称之为“致密高渗”储层。通过对塔里木盆地北部柯坪奥陶系露头硫磺沟2号溶洞及现代九寨沟溶洞的研究，发现岩溶过程中的化学沉积形成的钙华具有重要的石油地质意义，主要表现在两个方面:一是钙华是很好的储集空间；二是钙华具有一定的生烃能力。文章旨在探讨塔里木盆地奥陶系露头钙华相、钙华岩石学及钙华地球化学证据，以支持钙华的形成过程和形成环境的解释、及对塔河油田奥陶系缝洞油气藏的储集空间和油气成藏意义。 

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.