Abstract In the Late Sinian, the Tarim Basin experienced regional tectonic uplift and subaerial exposure. A regional unconformity was extensively distributed during the succession from the Sinian to Cambrian. As a result, a large number of secondary pores were developed in the Upper Sinian Qegebulake Formation. An integrated study based on petrographical and petrophysical analysis, geochemistry, karstology and fluid inclusions was performed to provide insight into the characteristics and formation mechanism of the Upper Sinian carbonate reservoirs. The reservoir rocks mainly include microbial dolomite, crystalline dolomite and breccias. Reservoir matrix porosity and permeability range from 0.30% to 11.06% and from 0.00156 mD to 1.49 mD respectively. Geological and geochemical characteristics suggest that three phases of dissolution were determined to be responsible for the development of the reservoirs within about 70 m beneath the unconformity. They are meteoric dissolution during subaerial exposure, and deep-seated hydrothermal dissolution and organic acid dissolution during the burial stage. The Upper Sinian dolomite underwent strong meteoric dissolution, which is supported by the appearance of the unconformity and the associated caves. The geochemical characteristics of the cave-filling calcite further indicate the diagenetic overprint from meteoric water, including the relatively negative values of δ18O (−18.46 to −8.5‰, VPDB) and wide range of δ13C (−8.11 to −0.24‰, VPDB), low ƩREE (1.77–4.23 ppm) and Yb–La and Yb–Ca variation. The occurrence of saddle dolomite as well as analytical results such as δ13C (0.67–2.52‰, VPDB) and δ18O (−13.21 to −9.81‰, VPDB) isotope signature and high homogenization temperature of fluid inclusions in quartz (87.2 to > 220 °C) and saddle dolomite (82.3–135.4 °C) suggest that the reservoirs rocks experienced interaction with hydrothermal fluids. Only bitumen occurred in the edge of secondary pores formed by dissolution of quartz and saddle dolomite, which suggests that organic acid dissolution likely occurred during the evolution of reservoirs. A geological model for the evolution of the paleokarst reservoirs is proposed through the analysis of major reservoir controlling factors. The primary pore types and reservoir petrophysical property of the Qigebulake Formation mainly depend on the sedimentation. Supergene karstification is the most important process for reservoir formation. Deep-seated hydrothermal activity has both positive and negative attribution on the formation of reservoirs. Organic acid dissolution plays an active role in the formation and preservation of reservoirs. This study provides a useful reference for hydrocarbon exploration in the deep buried Sinian carbonate reservoirs.

中文翻译：

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塔里木盆地西北部上震旦统不整合面古岩溶储层特征及形成机制

摘要 晚震旦世，塔里木盆地经历了区域性构造隆升和地表出露。震旦系至寒武系的演替过程中，区域性不整合面广泛分布。因此，上震旦系奇格布拉克组发育大量次生孔隙。开展了基于岩石学和岩石物理分析、地球化学、岩溶学和流体包裹体的综合研究，以深入了解上震旦统碳酸盐岩储层的特征和形成机制。储集岩主要包括微生物白云岩、结晶白云岩和角砾岩。储层基质孔隙度和渗透率范围分别为 0.30% 至 11.06% 和 0.00156 mD 至 1.49 mD。地质和地球化学特征表明，不整合面下约 70 m 内的储层发育由三个溶解阶段决定。它们是地表暴露时的大气溶蚀，埋藏阶段的深部热液溶蚀和有机酸溶蚀。上震旦系白云岩经历了强烈的陨石溶解，这得到了不整合面和相关洞穴的支持。充填洞穴方解石的地球化学特征进一步表明了大气水的成岩作用，包括δ18O相对负值（-18.46～-8.5‰，VPDB）和δ13C范围较广（-8.11～-0.24‰，VPDB） , 低 ƩREE (1.77–4.23 ppm) 和 Yb-La 和 Yb-Ca 变化。鞍状白云岩的产状以及分析结果，如 δ13C (0.67–2.52‰, VPDB) 和 δ18O (-13.21 to -9.81‰, VPDB) 同位素特征和石英中流体包裹体的高均质化温度 (87.2 to > 220 ° C) 和鞍状白云岩 (82.3–135.4 °C) 表明储层岩石经历了与热液流体的相互作用。石英和鞍状白云岩溶蚀形成的次生孔隙边缘仅存在沥青，这表明有机酸溶蚀可能发生在储集层演化过程中。通过对储层主要控制因素的分析，提出了古岩溶储层演化的地质模型。奇格布拉克组的原生孔隙类型和储层物性主要取决于沉积作用。表生岩溶作用是储层形成的最重要过程。深部热液活动对储集层的形成既有积极的作用，也有消极的作用。有机酸溶蚀作用对储层的形成和保存起着积极的作用。该研究为深埋震旦系碳酸盐岩储层油气勘探提供了有益参考。