Geological characteristics and sweet spot selection of Permian organic-rich sedimentary tuff series in northern Fuling, Chongqing
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摘要: 重庆涪陵北部二叠系吴家坪组二段(吴二段)沉凝灰岩是一种新型富有机质烃源岩,是非常规气藏勘探的新领域。明确吴二段沉凝灰岩层系关键甜点参数的分布是勘探评价的首要问题。通过钻测录井、实验分析及地震等资料,从沉凝灰岩层系的有机地球化学特征、岩性特征、厚度分布、地层压力及裂缝发育等成藏富集地质条件入手,聚焦岩性品质及可压缝网两个关键因素,优中选优,评价目标甜点区。研究结果表明:涪陵北部吴二段沉凝灰岩层系处于深水陆棚相,有机质丰度高,TOC含量主要为5%~6%,Ro为1.90%~2.44%,有机质类型以Ⅰ和Ⅱ型为主,TOC含量大于4%的优质沉凝灰岩段厚度在15~20 m之间,分布面积广;优质沉凝灰岩段储集空间多样,孔隙度较高,脆性矿物含量较高,地层压力系数为高—超高压级别,同时转折端发育小型—微型裂缝带,体现了良好的“自生自储”含气勘探潜力;涪陵北部吴二段沉凝灰岩层系气藏具有优质控富、超压控藏及缝网控甜的规律。结合主控因素初步建立吴二段沉凝灰岩层系甜点评价标准,优选出3个目标甜点区,面积达186.4 km2,其中研究区西北部L2-L1井区油气显示良好,可以作为近期勘探的有利目标。Abstract: The sedimentary tuff in the second member of Permian Wujiaping Formation in the northern Fuling area of Chongqing is a new type of organic-rich source rock, which is a new field of unconventional gas reservoir exploration. Clarifying the distribution of key sweet spot parameters of the sedimentary tuff series in the second member of Wujiaping Formation is a primary issue in exploration and evaluation. Based on drilling, logging, experimental analysis and seismic data, this paper evaluated the target sweet spot starting from the sedimentary tuff series' organic geochemistry, lithological characteristics, thickness distribution, formation pressure, fracture development and other geological conditions for reservoir formation and enrichment, and focusing on the two key factors of lithology quality and compressible fracture network. The results show that: (1) The sedimentary tuff series in the second member of Wujiaping Formation in the northern Fuling area is in the deep-water shelf facies, with high organic matter abundance. The TOC content is mainly 5%-6%, the Ro value ranges 1.90%-2.44%, the organic matter types are mainly Ⅰ and Ⅱ, and the thickness of high-quality sedimentary tuff section (TOC≥4%) is 15-20 m, with a wide distribution area. (2) The reservoir space of the high-quality sedimentary tuff section is diversified, the porosity is high, the brittle mineral content is high, the formation pressure coefficient is high to ultra high pressure level, and small-micro fracture zone is developed at the hinge zone, reflecting a good gas exploration potential of "self-generation and self-storage". (3) The sedimentary tuff gas reservoir in the second member of Wujiaping Formation in the northern Fuling area has the laws of high quality control of enrichment, overpressure control of reservoir and fracture network control of sweet spot. The sweet spot evaluation criteria for sedimentary tuff series in the second member of Wujiaping Formation are preliminarily established in combination with the main control factors, and three target sweet spots are selected, with an area of 186.4 km2. The L2-L1 well area in the northwest of the exploration area has good oil and gas shows, which can be a favorable target for recent exploration.
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图 3 重庆涪陵北部FT1井吴家坪组二段沉凝灰岩层系储集空间特征
据参考文献[16]修改。
Figure 3. Reservoir space characteristics of sedimentary tuff series in second member of Wujiaping Formation in FT1 well in northern Fuling, Chongqing
表 1 重庆涪陵北部及周缘地区吴家坪组二段沉凝灰岩层系有机地化特征
Table 1. Geochemical characteristics of sedimentary tuff series in second member of Wujiaping Formation in northern Fuling of Chongqing and its surrounding areas
ω (TOC) /% Ro/% δ13C 划分范围 频率 平均 划分范围 频率 平均 划分范围/‰ 频率/% 平均/‰ <2.0 <1.3 -26.0~-23.0 17.64 2.0~4.0 9.6 6.05 1.3~2.0 2 2.3 -28.0~-26.0 50.12 -27.55 >4.0 90.4 >2.0 98 -31.0~-28.0 32.24 表 2 重庆涪陵北部钻井优质沉凝灰岩段厚度预测误差
Table 2. Prediction errors of high-quality sedimentary tuff section thickness in typical wells in northern Fuling, Chongqing
井名 X1 S1 L2 优质沉凝灰岩段预测厚度/m 14.8 17.1 17.3 优质沉凝灰岩段实际厚度/m 15.0 16.6 16.8 厚度误差/m 0.2 0.5 0.5 -
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