Innovative exploration and post-drilling insights from H3X well in Haizhong Sag of Beibuwan Basin
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摘要: 北部湾盆地海中凹陷前期历经几十年油气勘探，一直未取得商业发现，分析认为受“优质烃源岩不落实、规模储层发育区不清以及有利成藏区带不明”3个方面问题的制约。为了实现该地区的勘探突破，按照盆地分析的思路，重新开展了关键成藏要素的落实，基于原型盆地恢复以及“三古”特征明确了海中凹陷发育始新统流沙港组优质烃源岩；重新厘定了源—汇体系，发现在涠西南低凸起倾末端的海中陡坡带地貌平缓地区发育三角洲—滩坝有利沉积体系；按照“近大型导油气断裂带和储层发育区”的勘探思路，优选了海中陡坡带作为下步勘探的最有利区带。按照该思路部署的H3X井取得了高产突破，证实了海中凹陷为富生烃凹陷，油气主要来自于流沙港组，早油晚气，两期成藏；证实了海中陡坡带发育三角洲—滩坝砂沉积体系，滩坝砂虽单层薄但累厚大，深层储层受裂缝的有效改造物性变好；进一步明确了海中凹陷陡坡带是涠西探区规模增储的重要方向，划分为“上部常压构造圈闭”和“下部超压构造—岩性圈闭”2套成藏系统，断砂输导的匹配和有效储层的发育是成藏的关键要素，海三构造高部位是重点扩储方向，海四构造是首要的突破方向。Abstract: The Haizhong Sag of the Beibuwan Basin has undergone decades of oil and gas exploration, but no commercial achievements have been made. Previous analysis suggested that this is primarily constrained by three key issues: indefiniteness of high-quality source rocks, unclear large-scale reservoir developing zone, and unknown profitable hydrocarbon accumulation. To achieve exploration breakthroughs in the region, key factors controlling reservoir formation were reexamined and confirmed based on basin analysis methodologies. By reconstructing the prototype basin and analyzing the characteristics of paleobathymetric, paleogeomorphology, and paleo-provenance, the study identified that the Haizhong Sag developed high-quality source rocks of the Eocene Liushagang Formation. The source-to-sink system was redefined, revealing the presence of a favorable delta-beach bar sedimentary system in the relatively flat areas of the steep slope zone of the Haizhong Sag at the distal end of the Weixinan low uplift. Guided by the exploration strategy of exploring reservoirs near major fault zones of hydrocarbon transport and reservoir development areas, the steep slope zone in the Haizhong Sag was selected as the most favorable area for future exploration. The H3X well, deployed based on this strategy, achieved production breakthrough, confirming the Haizhong Sag as a hydrocarbon-rich sag with favorable conditions for dual-source hydrocarbon supply. The oil and gas mainly originated from the deeply buried Liushagang Formation of the Haizhong Sag. Oil formed in early stage and gas formed in late stage, with two periods of accumulation. The study also confirmed that the Haizhong steep zone developed a delta-beach bar sand sedimentary system. The single layer of beach-bar sand is thin, but the accumulated thickness is large. The deep reservoirs have been significantly improved by the crack modification with better physical property. Furthermore, it clarified that the steep slope zone in the Haizhong Sag is a critical target for large-scale reserve increase in Weixi area. Two hydrocarbon accumulation systems were identified, including the upper normal-pressure structural traps as well as the lower overpressure structural and lithologic traps. The connections between sand body and fault and the development of effective reservoirs are key elements for accumulation. The high position of the Hai-3 structure is the key target for reserve expansion, while the Hai-4 structure is the priority for new breakthroughs.
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图 2 北部湾盆地北部坳陷地层综合柱状图
Figure 2. Comprehensive stratigraphic column of northern depression of Beibuwan Basin
图 3 北部湾盆地北部坳陷流沙港组沉积期盆地原型
Figure 3. Basin prototype of Liushagang Formation during sedimentary periods in northern depression of Beibuwan Basin
图 4 北部湾盆地北部坳陷涠二段SQ8-TST(a)、涠三段SQ6-LST(b)沉积相
Figure 4. Sedimentary facies of Wei-2 member SQ8-TST (a) and Wei-3 member SQ6-LST (b) in northern depression of Beibuwan Basin
图 5 北部湾盆地海中凹陷地震反射特征
剖面位置见图 4。
Figure 5. Seismic reflection features of Haizhong Sag in Beibuwan Basin
图 6 北部湾盆地北部坳陷涠洲组油气成藏有利区带划分
①D洼西部陡坡带Ⅱ1类；②D洼北部断阶带Ⅱ2类；③D洼北部张扭带Ⅱ2类；④D洼东部斜坡带Ⅰ类；⑤海中凹陷北部陡坡带Ⅱ1类；⑥海中凹陷中部洼槽带Ⅱ2类；⑦海中凹陷南部斜坡内带Ⅱ2类；⑧海中凹陷南部斜坡外带Ⅲ类。
Figure 6. Favorable oil and gas accumulation zones in Weizhou Formation in northern depression of Beibuwan Basin
图 7 北部湾盆地H3X井、涠洲油田原油全烃色谱对比
Figure 7. Comparison of total hydrocarbon chromatograms of crude oil from H3X well and Weizhou Oilfield, Beibuwan Basin
图 8 北部湾盆地H3X井埋藏史恢复及流体包裹体照片
Figure 8. Burial history reconstruction and fluid inclusion photo of H3X well, Beibuwan Basin
图 9 北部湾盆地H3X井典型沉积现象岩心照片
a.3 453.9 m，涠三段，风暴沉积，泥岩碎屑；b.3 455.0 m，涠三段，砾质滩坝，富含生物化石；c.3 460.31 m，涠三段，砂纹层理；d.3 462.70 m，涠三段，生物化石；e.3 465.23 m，涠三段，波状层理；f.4 225.54 m，涠四段，生物扰动。
Figure 9. Core photos of typical sedimentary features from H3X well, Beibuwan Basin
图 10 北部湾盆地H3X井储集空间类型及成岩作用照片
a.3 454.12 m，涠三段，粒间孔、粒内溶孔、铸膜孔、微裂缝；b.3 455.42 m，涠三段，构造裂缝及成岩收缩缝；c.3 455.42 m，涠三段，构造裂缝；d.4 225.12 m，涠四段，构造裂缝；e.3 454.20 m，涠三段，长石溶蚀，高岭石充填式胶结；f.4 227.90 m，涠四段，高岭石充填式胶结。
Figure 10. Photos of storage space types and diagenetic processes in H3X well, Beibuwan Basin
图 11 北部湾盆地海中凹陷油气动态成藏模式
Figure 11. Dynamic oil and gas accumulation model of Haizhong Sag, Beibuwan Basin
表 1 北部湾盆地边缘富生烃凹陷定量判识标准
Table 1. Quantitative identification criteria for hydrocarbon-rich sags around Beibuwan Basin margins
古环境 指标参数 古水深 指标参数 有机质发育条件 指标参数 V/(V+Ni) Cu/Zn (Zr/Al)/10-4 古沉降速率/(m/Ma) 氧化环境 ＜0.46 ＞0.35 浅水 ＜20 有机质保存不利 ＜70 弱还原环境 0.46~0.60 0.21~0.35 还原环境 ＞0.60 ＜0.21 深水 ＞20 有机质保存有利 ＞70 
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