断裂控制的非暴露型大气水岩溶作用模式——以塔北英买2构造奥陶系碳酸盐岩储层为例

摘要

Karst reservoir takes an important role in carbonate reservoir, but it had been long regarded the leading reason that eogenetic carbonatic rock, after lifted to the ground, suffers phreatic water table-controlled karstification. Based on the understanding of geological setting of the YM2 area in the northern Tarim uplift and integrated analysis of lithology, geochemistry and the controlling factors of karstification, this study suggests that karstification may have resulted in the formation fracture-cavern reservoir in the areas with the continued depositional sequence. The calcites of various structures and textures were studied in detail to understand the properties and provenance of diagenetic fluids using a series of geochemical analyses such as C, O and Sr isotopes, REE, electron microprobe, cathode-ray luminescence, and fluid inclusion, in combination to observation of core and thin sections. Meteoric water is regarded to be the main karst fluid responsible for the formation of porosities, whereas deep hydrothermal fluid just modified reservoirs at late stage. Analysis of the features and development of strike-slip faults in the YM 2 shows that the structure forming and strike-slip fault development provide prerequisite for karsitification. Furthermore, a new meteoric karstification controlled by steep structure and faulting is proposed, in which carbonatic rock sequences, although covered by thick clastic strata, is uplifted at the late stage, accompanied with development of strike-slip faulting, a large amount of fractures is formed within the top of carbonatic rock sequence, and meteoric water penetrates along and results in karstification within fractures, thus forming large-scale fracture-cavern reservoirs. This model has broken through the restriction of the exposure-type buried hill karst model, improved our understanding of karst reservoir formation in the areas with whole carbonatic rock sequence, and finally broadened our thought of oil and gas exploration in karst reservoirs.%岩溶储层在碳酸盐岩储层中占有重要的地位,但过去一直认为表生期碳酸盐岩地层抬升至地表受大气水潜水面控制的岩溶作用是其主要成因.本文在充分认识塔北英买2地区构造地质背景的基础上,通过岩石学、地球化学及其与岩溶成因和控制因素关系的综合分析,提出在地层沉积序列完整的内幕区也可形成大气水岩溶作用改造的缝洞型储层.以岩芯和薄片观察为基础,进行不同组构方解石的碳氧同位素、锶同位素、稀土元素、电子探针、阴极发光和包裹体等地球化学分析,综合岩石学与地球化学特征,分析了岩溶流体性质和来源,认识到大气水是主要的岩溶流体,对储层形成起决定性作用；深层热液在后期起到了改造作用.通过对英买2高陡构造和走滑断裂发育特征与演化分析,认识到构造形成和断裂的发育为岩溶作用提供了必要条件.进而,建立了一种受高陡构造和断裂控制的新型大气水岩溶作用模式,即碳酸盐岩地层虽上覆厚层碎屑岩地层,但在后期抬升形成隆起,并伴生走滑断裂发育的背景下,大量裂缝发育于隆起顶部的碳酸盐岩地层上部,大气水可沿断裂下渗并在裂缝带发生岩溶作用,从而形成大型缝洞型储层.该模式突破了暴露型潜山岩溶模式的局限,深化了地层沉积序列完整的碳酸盐岩内幕区岩溶储层成因机理的认识,扩宽了岩溶储层油气藏勘探的思路.