Potential of subtle hydrocarbon reservoirs in paleo-valleys in the Pingxi area of Xihu Sag
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摘要:
西湖凹陷平湖油气田经多年的勘探开发，取得了丰硕的成果，但也面临着后备储量不足的严峻现状。油气田西部的平西地区，占地面积大、研究程度低，是发现新增资源量的潜在地区。利用三维地震资料，从古地貌着手，在平西地区自南向北识别出多个古沟谷，其限制型地貌为隐蔽油气藏提供了有利背景。结合古生物、微量元素、测录井、岩芯、地震等资料综合分析认为，始新统平湖组早期古沟谷内发育多期低位域扇三角洲沉积，自下而上扇体逐步将基底填平补齐，可作为有利的储集相带。随着海平面上升，扇三角洲之上发育海侵潮坪沉积，以稳定泥岩为主，形成一套良好的储盖组合。且平西地区地貌相对高，位于流体运移的有利指向区，扇三角洲低部位油源断裂发育，油气供给充足。古岸线之上扇三角洲根部为近源沉积，分选差、泥质填隙物含量高、压实作用强，与扇三角洲前缘存在明显物性差异，可以形成良好的封堵条件。“沟-扇-断”耦合背景下利于扇体型隐蔽油气藏的发育，对平湖油气田的可持续发展具有重要的现实意义。
Abstract:The Pinghu Oil and Gas Field in the Xihu Sag, after many years of development, has made great contributions to the oil industry in offshore China. However, it now faces severe challenges of insufficient reserves. Pingxi area, the west part of the oil and gas field, as a new territory with large area and low exploration degree, is expected to be a potential area for discovery of new resources. Based on 3D seismic data and paleogeomorphological studies, several paleo-valleys have been found from south to north in the area, which have favorable background for the formation of subtle oil and gas reservoirs. Comprehensive analysis of paleontology, trace elements, logging and cores in addition to seismic survey data suggest that there developed multi-stage lowstand fan deltaic systems in the early Eocene Pinghu Formation, which may gradually fill up the basement from bottom to top to become favorable reservoir facies belts for oil and gas accumulation. With the rise of sea level, transgressive tidal flat mainly composed of stable mudstone deposits settled down on the fan delta to form good reservoir-seal assemblages. The Pingxi area is fortunately located on the path of fluid migration due to its relatively high landform. The lower part of the fan delta is characterized by well-developed oil source faults and sufficient oil and gas supply, while the root of the fan delta above the ancient shoreline is close to source rocks which is high in argillaceous materials, poor in sorting, and strong in compaction. It is obviously different from that of the fan delta front, and may provide excellent sealing conditions. The valley-fan-fault coupling is favorable for the formation of fan type of subtle reservoirs, which are of significance to the sustainable development of Pinghu Oil and Gas Field in the future.
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Key words:
- Xihu Sag /
- Pingxi area /
- ancient gully /
- fan delta /
- subtle reservoir
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表 1 平湖油气田A井孢粉统计
Table 1. Pollen statistics from Well A of Pinghu Oil-Gas Field
个 样品
井深/m蕨类
孢子光面海金沙孢 粗网孢 具环水龙骨孢 卷柏 光面水龙骨单缝孢 裸子植物花粉 松粉 油杉粉 被子植物花粉 苗榆粉 黄杞粉 栎粉 紫树粉 大戟粉 椴粉 木犀粉 鉴定统计用盖玻片数 3 439.03 45 36 7 7 1 4 6 3 3 471.72 1 3 1 3 3 472.48 57 44 1 34 1 3 3 472.78 6 3 1 30 1 3 3 474.08 12 63 1 48 3 3 474.73 6 2 9 3 3 476.73 2 36 12 1 3 3 476.93 18 9 1 1 1 1 3 3 477.52 2 4 3 1 3 3 477.57 1 3 1 3 
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表 2 平湖油气田A井微量元素分析
Table 2. Trace element analysis of Well A of Pinghu Oil-Gas Field
层位 井深/m 岩性 锶/钡 硼/(μg/g) 平湖组下段 3 279.00 粉砂质泥岩 0.404 94 88.69 3 291.00 粉砂质泥岩 0.386 97 86.43 3 337.00 深灰色泥岩 0.228 94 57.63 3 393.50 深灰色泥岩 0.379 49 62.13 3 469.03 深灰色泥岩 0.212 45 50.64 3 471.72 深灰色泥岩 0.562 00 62.35 3 472.48 褐灰色泥岩 0.957 55 42.50 3 474.73 深灰色泥岩 0.697 46 38.39 3 476.93 褐灰色泥岩 0.661 65 33.69
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