Title
Distribution and origin of tectonic fractures in ultra-deep tight sandstone reservoirs: A case study of Keshen gas field, Kuqa foreland thrust belt, Tarim Basin
摘要
综合采用岩心、薄片及成像测井等资料，对塔里木盆地克深气田巴什基奇克组超深层致密砂岩储层构造裂缝的基本特征与形成序列进行了分析，在此基础上讨论了裂缝产状、力学性质、密度、开度、充填性和充填物的差异性分布及其成因，最后探讨了不同气藏应采取的储层改造措施和有利勘探目标。结果表明：克深气田主要发育3期构造裂缝，各个气藏之间构造裂缝具有明显差异性，主要受构造变形时间和成岩孔隙水介质环境控制。北部克深5和克深6气藏为早期缓慢挤压变形，主要形成与背斜长轴近似平行的直立或高角度张性裂缝；中部克深2气藏变形时间稍晚，近NS走向剪切裂缝和近EW走向张性裂缝共存，并且前者略占优势；南部克深8和克深9气藏变形时间最晚，主要形成近NS走向的直立剪切裂缝。自北向南，主差应力与岩石抗压强度的比值先增大后减小，裂缝密度随之先增大后减小，同时裂缝优势走向与现今水平最大主应力之间由大角度相交逐渐变为近似平行，并且裂缝形成时间越来越晚，充填率逐渐降低，因此裂缝有效开度逐渐增大。巴什基奇克组沉积期的水体总体呈咸化状态，北部克深5、克深6和克深2区块为淡水-半咸水介质成岩环境，裂缝充填物主要为方解石，储层改造措施应以酸化为主；南部克深8和克深9区块为偏碱性水成岩环境，裂缝充填物主要为硬石膏和白云石，储层改造措施应以压裂为主。克深8气藏的构造裂缝有效性最好，其次为克深2和克深9气藏，克深5和克深6气藏较低。克深18、克深19、克深20和克深24号构造的裂缝可能比较发育，可作为克深气田下一步勘探的重点目标。
Abstract
Observations of cores and thin sections combined with logging data interpretation were used to analyze the basic properties and forming sequence of tectonic fractures in the ultra-deep tight sandstone reservoirs of the Bashijiqike Formation in Keshen gas field.The differences in the fracture properties, including occurrence, mechanical characteristics, density, aperture, filling and fills, were then studied and the causes for the differences were also investigated to select suitable reservoir stimulation measures and determine exploration targets for the field.The results show that the fractures were developed in 3 stages (early, middle and late) in Keshen gas field and constitute reservoirs of distinctive characteristics due to the control of tectonic deformation stage and aqueous diagenesis environment.The Keshen-5 and -6 gas reservoirs in the north of the field are dominated by vertical fractures or high-angle tensile fractures which are nearly parallel to the long axis of the anticline under the effect of slow tectonic deformation at the early stage; the Keshen-2 gas reservoir in the center was under the influence of the middle deformation stage and contains both near NS-trending shearing fractures and near EW-trending tensile fractures with the former being relatively superior in number; the Keshen-8 and -9 gas reservoirs in the south of the field is dominated by vertical shearing fractures of nearly NS trending under deformation of the late stage.In general, from the north to the south, both the major differential stress to rock compressive strength ratios and the fracture density show a rise-fall tendency; meanwhile, the dominant strike of fractures gradually runs parallel to the present major principal stress from a high-angle intersection along with shrinking facture forming time, which in turn reduces filling rate and results in greater effective aperture.In addition, the Bashijiqike Formation (K) was deposited in a saline water as a whole: the Keshen-5, -6, and -2 blocks in the north of the field were formed in a fresh-brackish setting with fractures mainly filled by calcites, making acidification a suitable reservoir stimulation choice; and the Keshen-8, and -9 blocks in the south of the field were formed in a alkaline-prone water setting with fractures primarily filled with anhydrite and dolomite, making fracturing a more applicable stimulation option.The Keshen-8 block ranks first in terms of tectonic fracture effectiveness, followed by the Keshen-2 and -9 blocks, and then by the Keshen-5 and -6 gas pools.The Keshen-18, -19, -20, and -24 blocks are suggested to have well-developed tectonic fractures and shall be targeted in future exploration.
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