巴西桑托斯盆地CO_(2)区域分布及主控因素

Distribution and Main Controlling Factors of CO_(2) in Santos Basin,Brazil

桑托斯盆地盐下油气田中发现了大量CO_(2),给油气勘探开发和生产都带来诸多困难和挑战.利用地层测试、样品分析及文献资料等,明确了CO_(2)成因及来源,统计分析了其区域分布特征,并基于区域重磁和深源地震等资料,剖析了控制CO_(2)分布的地质因素.盆内CO_(2)主要为幔源‒岩浆成因,且幔源CO_(2)贡献了至少92%的CO_(2)总量.区域上,CO_(2)自陆向海呈增加趋势,并相对集中在盆地东部隆起带上.地壳减薄和地幔局部隆升是控制CO_(2)宏观分布最重要的背景因素.极端的地壳伸展造成了圣保罗地台下部陆壳强烈拉伸减薄,形成了一个面积约5.1×10^(4) km^(2)的地壳减薄区,造成了富含CO_(2)的地幔物质上拱进入陆壳,宏观上决定了盆内CO_(2)区域分布.此区域之外,出现高含量CO_(2)的可能性大幅降低.岩浆侵入和活动断层都是沟通隆升地幔和浅部储层的重要路径,但以断裂沟通最常见.NW-SE向区域走滑断裂和NE-SW向I-II级正断层对CO_(2)在浅部地层中的分配起控制作用,两组断裂交汇部位或周缘是幔源岩浆或CO_(2)最集中发育区.

An extraordinarily high amount of CO_(2) found in pre-salt section in Santos basin poses great challenges to the oil and gas exploration and development. In this study, combining regional gravity and magnetic database, deep-seismic lines, well drilling, formation test and samples dataset of the basin, the origin of CO_(2) was clarified firstly, and its regional distribution patterns were statistically analyzed and their main controlling factors were explored as well. The widely distributed CO_(2) in presalt reservoirs in Santos basin is mainly sourced from mantle, and mantle-derived CO_(2) contributes at least 92% of the total volume of CO_(2). Regionally, CO_(2) abundance gradually increases from continent to ocean, and is relatively concentrated in the eastern uplift of the basin. On its margin usually occurs the current maximum CO_(2) content values. The extra-high CO_(2) abundance in Santos basin is the result of the combined action of various geological factors such as crust & mantle transition, regional tectonic evolution, magma events and fault activity. Among them, continental crust thinning and mantle uplifting are the most important background factors, controlling the regional distribution of CO_(2). The intensive extension of crust caused a strong stretching and thinning of the lower continental crust of Sao Paulo platform, forming a crustal thinning area,around 5.1×10^(4) km^(2), which caused the CO_(2)-rich mantle material to intrude upward into the continental crust. This uplifted mantle has directly controlled the regional distribution of CO_(2) in Santos basin. Outside this area, the risk of CO_(2) is significantly reduced. Magma injection or active faults are both important pathways for CO_(2) migration and accumulation, with most common cases involving active faults leading CO_(2) from"uplifted"mantle to shallower reservoirs. The NW-SE strike-slip faults and the NE-SW class Ⅰ-Ⅱ normal faults have an obvious control effect on distribution of CO_(2) in shallower formation: NW-SE strike-slip fault could extend into deep mantle, while the NE-SW normal faults distribute these CO_(2) in shallower layers. So that their intersection points or periphery areas are the most favorable areas for accumulation of magma and CO_(2).