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作者简介:

薛罗(1988—),男,陕西绥德人,工程师,硕士,从事盆地模拟及油气成藏研究工作。E-mail:xueluo@petrochina.com.cn。

中图分类号:TE121.2

文献标识码:A

文章编号:1009-9603(2022)02-0053-08

DOI:10.13673/j.cnki.cn37-1359/te.2022.02.006

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目录contents

    摘要

    Melut盆地Ruman潜山经历了多期构造演化,导致该区油气成藏规律复杂。为落实Ruman潜山构造演化对油气成藏的控制作用,进一步探索油气勘探潜力,综合利用地震、钻井及区域构造资料,开展基于平衡剖面技术的构造演化分析,并结合烃源岩生排烃史,对Ruman潜山周缘油藏动态成藏过程进行剖析。结果表明:Ruman潜山自白垩纪以来经历了2期规模较大的翘倾抬升;白垩纪末期的翘倾抬升有利于潜山周缘地层-岩性圈闭的形成,古近纪末期的翘倾抬升导致潜山暴露,先期成藏的原生油藏遭受破坏,形成次生型稠油油藏;新近纪以来构造活动弱,围绕Ruman潜山形成晚期成藏型白垩系Galhak组地层-岩性稠油油藏以及新近系Jimidi组构造-岩性稠油油藏,二者均具有较大的勘探潜力。

    Abstract

    The Ruman buried hill in the Melut Basin has experienced multi-stage tectonic evolution,resulting in complex hydrocarbon accumulation modes in this area. To reveal the role of the tectonic evolution of the Ruman buried hill in con- trolling hydrocarbon accumulation and further explore the exploration potential in this area,this paper combined seismic, drilling,and regional tectonic data for structural evolution analysis based on the balanced cross-section technique. More- over,based on the hydrocarbon generation and expulsion history of source rocks,the dynamic accumulation process of reser- voirs around the Ruman buried hill was analyzed. The results show that the Ruman buried hill has experienced two largescale tilting uplift activities since the Cretaceous. The tilting uplift in the late Cretaceous was beneficial to the stratigraphic-lithologic traps around the buried hill,and those in the late Paleogene led to the exposure of buried hills,which destroyed the primary reservoirs that formed earlier,leading to secondary heavy oil reservoirs. In addition,the tectonic activity has been weak since the Neogene. It results in the stratigraphic-lithologic heavy oil reservoirs of the Cretaceous Galhak Forma- tion and the structural-lithologic heavy oil reservoirs of the Neogene Jimidi Formation in the late accumulation around the Ruman buried hill,and both the reservoirs show great exploration potential.

  • 南苏丹 Melut 盆地是中非重要的含油气盆地, 中国石油于 2000 年进入并主导该区的勘探工作。 在盆地北部,针对 Yabus-Adar 组主力储盖组合,先后发现了 Palogue 和 Moleeta 等亿吨级油田[1-2],有效地支撑了中国石油“海外大庆”的建设。但经过十多年的勘探,主力产层剩余构造圈闭逐渐减少、面积变小,勘探工作须向新区、新层系、新类型转移。而北部地区唯一尚未开展规模勘探的 Ruman 地区则是潜在的增储上产区域,目前该区 Ruman潜山在多套层系发现稠油油藏,且油藏类型多样,与区域 “厚层砂岩(Yabus 组)+区域泥岩(Adar 组)”主力储盖组合不同[3],油气的形成与分布明显受 Ruman 潜山构造演化影响[4],成藏规律复杂,前人尚未针对该区的构造演化与油气成藏关系开展研究;而中外多个盆地的油气勘探也证实潜山的构造演化与其周缘的油气成藏密切相关[5-7]。因此,笔者利用平衡剖面技术,通过 Ruman 潜山构造演化恢复,结合烃源岩生排烃史,系统分析潜山构造演化对不同沉积时期油藏的控制作用,以期为研究区下步油气勘探提供地质理论依据,同时对于 Melut 盆地南部遭受多期构造抬升的广大低勘探程度区也具有一定的借鉴意义。

  • 1 区域地质特征

  • 南苏丹 Melut盆地是在中非剪切带右旋走滑应力背景下形成的中新生代陆内被动裂谷盆地,面积约为3.3×104 km2[8-9],是中非陆内裂谷盆地中的第二大沉积盆地[110-11]。其具有“五坳两隆”的构造格局,包括北部坳陷、东部坳陷、中部坳陷、西部坳陷、南部坳陷、Adar 隆起和中央隆起;同时北部坳陷又发育Jamous凹陷、Moleeta凹陷及Ruman凹陷共3个构造单元(图1a,1b)。Melut 盆地主要烃源岩为下白垩统 Renk组,有机质丰度为 0.62%~2.92%,生烃潜量(S 1+S 2)最高达 19.53 mg/g;有机质类型以Ⅱ型干酪根为主,少量为Ⅰ型干酪根[1];镜质组反射率(R o) 普遍大于 0.5%,坳陷中心处 R o值超过 2.0%,为一套高丰度、高成熟度的优质烃源岩。烃源岩生排烃史模拟结果表明,Ruman 凹陷从晚白垩世(距今 103 Ma)开始生烃,有机质生油强度最高可达 320 mg/g,在白垩纪末期(距今 86 Ma)开始排烃,而在始新世区域盖层Adar组沉积时期(距今50 Ma)进入大规模排烃阶段,并持续至今,为 Ruman 凹陷的油气成藏提供了丰富的资源基础[12]。同时,自白垩纪以来盆地沉积了 4 套储盖组合,分别为下白垩统 Gayger+ Renk组、上白垩统 Galhak组、古近系 Samma/Yabus+ Adar组、新近系Jimidi+Miadol组。

  • Ruman 凹陷位于 Melut 盆地北部坳陷最西端,面积约为 500 km2,为西断东超的箕状凹陷,包括西部陡坡带、中央洼陷带、东部斜坡带和 Ruman 潜山共 4 个区带(图1c),Ruman 潜山位于凹陷东部斜坡带东侧。目前油气发现分布在 Ruman 潜山及东部斜坡带;基岩、Gayger 组、Galhak 组、Yabus 组及 Jimidi 组均有稠油油藏发现,其沿不整合分布,与 Ruman潜山的构造演化密切相关。

  • 2 构造演化特征

  • 2.1 地震反射特征

  • Ruman 潜山发育超覆不整合和削截不整合(图2)。一般而言,不整合是区域地壳运动及海(湖)平面升降等局部构造作用的结果,对于研究区沉积层序划分和构造演化具有重要意义[13-14]

  • Ruman地区的超覆不整合发育于晚白垩纪Gal⁃hak组沉积时期。Galhak组在地震剖面上是一套典型的楔形地层,Galhak 组底面为一套较为连续的地震强反射轴,内部地震反射轴大部分上超于 Galhak 组底面,表现为一系列上倾尖灭现象,由此可见, Galhak 组底面不整合主要是由沉积作用造成的,也反映出白垩纪 Ruman 潜山发生了构造抬升。而该区的削截不整合主要表现为古近系与白垩系地层削截终止于新近系Jimidi组底面,有高角度不整合、微角度不整合,Jimidi组底面是大型不整合,其地震反射特征整体表现为不连续弱反射,局部发育地震强反射振幅。这种削截不整合是典型的构造不整合,反映出在Jimidi组沉积之前,该区发生了较大规模的抬升,导致白垩系与古近系遭受不同程度剥蚀。而对于地层剥蚀厚度恢复、不同沉积时期潜山坡度刻画,则需对 Ruman潜山的构造演化进行定量分析。

  • 图1 Melut盆地Ruman凹陷区域构造位置及区带划分

  • Fig.1 Location and play division of Ruman Sag in Melut Basin

  • 图2 Ruman潜山地震反射特征(剖面位置见图1c)

  • Fig.2 Seismic reflection characteristics of Ruman buried hill(See Fig.1c for profile position)

  • 2.2 地层剥蚀厚度恢复

  • 平衡剖面技术是目前较好的定量研究构造演化的方法[15-16],而该技术应用的核心参数之一为地层剥蚀厚度[17]。地层剥蚀厚度恢复方法众多,应用比较广的有泥岩声波时差法[18]、磷灰石裂变径迹法[19]、镜质组反射率法、地层厚度趋势法[20] 和波动分析法[21] 等。但这些方法都有各自的适用性且对基础资料的要求存在较大差异,其中泥岩声波时差法及地层厚度趋势法仅需要原始测井及地震数据即可,计算准确性也较高。

  • 结合实际资料,采用泥岩声波时差法对 Ruman 潜山顶面古近系的地层剥蚀厚度进行恢复。在声波时差曲线上,不整合特征明显,Jimidi组底上覆及下伏地层的声波时差曲线趋势差异大,具有“两段性”特征。将声波时差取自然对数值,拟合不整合之下深度-声波时差关系曲线的回归方程,将该区地表声波时差650 μs/m取自然对数(6.47 μs/m),代入回归方程,计算剥蚀前古地表埋深,其与不整合的埋深差值即为剥蚀厚度。利用该方法对研究区的典型井进行古近系地层剥蚀厚度恢复,结果显示,Ruman 潜山顶部地层在古近纪末期剥蚀范围广,古近系地层剥蚀厚度均在百米以上,例如 RC-1 和 RN-2 井古近系地层剥蚀厚度分别为 157 和 179 m(图3)。

  • 图3 单井泥岩声波时差法恢复古近系地层剥蚀厚度

  • Fig.3 Recovery of stratum erosion thickness by single well mudstone acoustic transit time method

  • 对于白垩系利用泥岩声波时差法难以恢复地层剥蚀厚度。在泥岩声波时差曲线上,如 RN-2 井Jimidi 组底不整合之下的声波时差曲线连续,不存在明显的间断特征,而事实上 Galhak 组顶部为 Ya⁃ bus 组,中间缺失 Samma 组和 Melut 组,反映出在白垩系 Galhak 组沉积时期,该区地层剥蚀厚度较小,导致在声波时差曲线上特征不明显。相对泥岩声波时差法,地层厚度趋势法的应用范围较广,其原理为地层在一定产状下,由盆地中心向盆地边缘按一定趋势逐渐减薄直至尖灭为 0。因此,以 Ruman 潜山顶为剥蚀原点,利用距离 Ruman潜山剥蚀原点一定距离的未剥蚀地层的厚度变化趋势来确定剥蚀区白垩系沉积厚度是可行的。

  • 采用地层厚度趋势法定量恢复 Galhak组厚度,结果显示,当Galhak组地层厚度为0时,其与潜山顶面剥蚀原点的距离为 135 m,表明在 Galhak 组沉积时期潜山顶面曾发生小范围的暴露,没有沉积 Gal⁃ hak 组(图4a);而下白垩统 Gayger 组地层厚度恢复结果显示,在潜山顶面 Gayger组原始地层厚度约为 19 m,但现今无 Gayger 组,应为晚白垩世潜山抬升暴露造成的(图4b)。

  • 图4 地层厚度趋势法恢复古近系地层剥蚀厚度

  • Fig.4 Recovery of Paleogene stratum erosion thickness by stratigraphic thickness trend method

  • 2.3 构造演化阶段划分

  • 在对区域构造演化分析的基础上,结合地层剥蚀厚度参数,应用平衡剖面技术对 Ruman潜山所在的 Ruman 凹陷进行构造演化恢复。通过对 Ruman 凹陷的构造演化分析,进而明确 Ruman潜山的构造演化过程。从区域构造环境来看,Ruman凹陷与所处的北部坳陷一样,都经历了“三裂一拗”的构造演化阶段(图5)。

  • 裂陷Ⅰ幕为盆地初始裂陷期,形成了凹陷最重要的Renk组烃源岩,同时Ruman潜山位于水下未暴露,潜山顶部有 Gayger 组和 Renk 组厚度约为 40 m 的沉积。

  • 裂陷Ⅱ幕初期为 Galhak组沉积时期,凹陷裂陷强度大,潜山坡度由 11°快速变为 22°,最大沉降量为 1 300 m,伸展率达 4.2%(表1),使得 Ruman 潜山在 Galhak 组沉积时期发生翘倾抬升,潜山露出水面。通过古地貌恢复,在 Galhak 组沉积时期,潜山暴露范围仅有几个平方公里,局部物源供应微弱; 但西北长轴区域物源供应充足,平行于潜山走向沉积了多套滩坝砂,上倾尖灭岩性体非常发育,围绕潜山形成一系列地层-岩性圈闭[422]。Melut 组和 Samma 组沉积时期,潜山持续暴露,直到裂陷Ⅱ幕晚期,潜山再次位于水下,顶部沉积 Yabus 组,同时形成 Gayger组不整合圈闭、基岩潜山风化壳裂缝圈闭。

  • 裂陷Ⅲ幕初期为Adar组沉积时期,凹陷再次发生强裂陷作用,Ruman 潜山翘倾抬升,凹陷沉积区域盖层Adar组厚层泥岩,Yabus组形成构造-地层圈闭。而在Lau组沉积时期,凹陷继续裂陷,潜山再次翘倾抬升,同时由于区域构造运动,Melut盆地整体发生抬升[10],造成 Ruman 潜山的大规模暴露,先期在潜山顶部沉积的Yabus组和Adar组等地层均遭受剥蚀。

  • 拗陷阶段,Ruman 潜山构造稳定,整体位于水下,顶部沉积了盆地广泛发育的 Jimidi 组河流相砂岩和 Miadol 组厚层泥岩,同时在 Jimidi 组底不整合之上,形成受不整合影响的 Jimidi 组构造-岩性圈闭。

  • 因此,从构造演化来看,Ruman 潜山在晚白垩纪和晚古近纪存在 2 期规模较大的翘倾抬升,潜山顶部白垩系 Gager 组、Renk 组、Galhak 组和古近系 Yabus 组、Adar 组均遭受剥蚀,在新近纪拗陷阶段,潜山顶部沉积了Jimidi组和Miadol组储盖组合。

  • 3 构造演化对油气成藏的控制作用

  • 在潜山构造演化史研究的基础上,结合前人对研究区油气生排烃史的研究[12],可重建潜山及周缘油气成藏过程。从目前潜山及周缘的油气发现情况来看,基岩、Gayger 组、Galhak 组、Yabus 组及Jimidi组均为潜在的勘探对象。但其发现的油藏类型各不相同,基岩为裂缝油藏,Gayger组和Yabus组为受不整合控制形成的地层不整合油藏,Galhak 组为受岩性控制形成的上倾尖灭岩性油藏,而 Jimidi 组为构造-岩性油藏。研究表明,各类油藏的形成受构造演化影响,是一个动态的成藏过程,油气成藏关键时期分别为区域盖层Adar组沉积之后(区域盖层形成及大规模排烃阶段)和Lau组沉积之后(潜山顶部暴露剥蚀,油藏受到调整改造)。

  • 图5 Ruman凹陷构造演化史(剖面位置见图1b)

  • Fig.5 Tectonic evolution history of Ruman Sag(see Fig.1b for profile position)

  • 表1 Ruman凹陷构造演化参数

  • Table1 Tectonic evolution parameters of Ruman Sag

  • 3.1 原生油藏形成阶段

  • 裂陷Ⅲ幕初期区域盖层Adar组沉积之后,潜山高部位Yabus组垂向及侧向封堵条件好,形成构造-地层圈闭,同时烃源岩已处于大规模排烃阶段,凹陷中心 Renk 组生成的油气向上运移至 Galhak 组和 Melut组等富砂地层,进而侧向运移至潜山高部位。因此在 Adar 组沉积之后,潜山高部位 Yabus 组构造-地层圈闭油气充注程度高,在潜山高部位发育大型构造-地层油藏。而对于基岩,由于 Galhak 组沉积时期的翘倾抬升作用,曾发生局部暴露,易形成风化壳与基岩裂缝,同时其侧向一边与 Gayger组相邻,一边对接 Adar 组,上覆与 Yabus 组不整合接触,因此 Adar 组沉积之后,基岩与西侧 Gayger 组可整体成藏,基岩形成潜山风化壳/裂缝油藏,而 Gay⁃ ger组为不整合油藏(图6a)。

  • 图6 Ruman潜山及周缘油气成藏演化过程

  • Fig.6 Evolution process of hydrocarbon accumulation around Ruman buried hill

  • 3.2 油藏调整改造阶段

  • 裂陷Ⅲ幕末期 Lau 组沉积时期,Ruman 潜山大规模暴露。潜山顶部沉积的 Gayger,Renk,Galhak, Yabus和Adar组均遭受不同程度剥蚀;Lau组顶面形成区域不整合,导致先期在潜山高部位形成的 Ya⁃bus组、Gayger 组和基岩油藏均遭受破坏,油气向上逸散,经构造破坏和调整改造,以及水洗、氧化等作用,原生油藏逐渐稠化,形成地层不整合稠油油藏。而此时对于 Tean 断裂带东斜坡潜山周缘 Galhak 组地层-岩性圈闭,Ruman 凹陷中心油气侧向运移至 Tean断裂带西侧Galhak组及上覆富砂地层,然后向潜山高部位运移,难以运移至Tean断裂带东斜坡潜山低部位的 Galhak 组地层-岩性圈闭,而 Tean 断裂带附近Renk组烃源岩尚未成熟,因此在Lau组沉积时期,Galhak 组地层-岩性圈闭依靠原地烃源岩成藏的概率很低(图6b)。

  • 3.3 现今多类型稠油油藏并存

  • 在区域拗陷阶段,新近纪之后,Ruman 潜山构造稳定,整体位于水下,潜山顶部沉积了盆地广泛发育的 Jimidi 组河流相砂岩和 Miadol 组厚层泥岩,形成 Jimidi-Miadol 组储盖组合,而 Jimidi 组底为区域不整合面,东侧为 Moleeta 凹陷边界断裂,均为 Jimidi组油气成藏提供了良好的油气运移通道。因此,一方面东侧Moleeta凹陷生成的油气可沿大断裂向上运移至Jimidi组;另一方面Jimidi组下伏稠油油藏向上逸散,沿不整合运移至Jimidi组成藏,但由于 Jimidi组整体埋深为600~700 m,埋藏浅,受水洗作用及生物降解作用影响,油藏也易变得稠化,因此 Jimidi组油藏成因为次生型晚期成藏,为受不整合、边界断裂及砂体展布多因素影响的构造-岩性油藏。目前勘探发现 Jimidi 组构造-岩性油藏单井产量较高,最高日产原油量达数十吨,且油藏展布面积广、勘探潜力大。

  • 而对于Galhak组沉积末期,已钻井揭示Tean断裂带东斜坡围绕潜山形成地层-岩性圈闭,其储层的泥岩顶板与底板厚度均在10 m以上,封堵条件较好;且在拗陷阶段,构造稳定,油气成藏后不易受生物、氧化等作用影响,已发现的稠油油藏应为低成熟原油运移成藏。利用 EASY%R o模型模拟烃源岩热演化史发现,对 Tean 断裂带东斜坡地层-岩性圈闭有贡献的烃源岩在距今 10 Ma 进入成熟门限,现今烃源岩R o基本在 0.7% 之下,其生成的低成熟-未成熟原油经垂向及侧向短距离运移至东斜坡 Gal⁃ hak 组地层-岩性圈闭,形成上倾尖灭地层-岩性油藏,因此 Galhak 组稠油成因为原生型,且为晚期成藏。研究发现围绕 Ruman潜山,在低成熟烃源岩生油窗内 Galhak 组发育多个地层-岩性圈闭,勘探面积大,是潜在的有利勘探领域。同时在拗陷阶段,经Lau组沉积时期调整改造的Yabus组、Gayger组和基岩油藏,现今均变为残留型稠油油藏,其勘探潜力有限(图6c)。

  • 4 结论

  • Ruman潜山构造演化过程中经历了2期规模较大的翘倾抬升。白垩纪末期 Galhak 组沉积时期的翘倾抬升导致潜山暴露,但剥蚀范围有限,围绕Gal⁃ hak组底面不整合形成一系列地层-岩性圈闭;古近纪末期 Adar-Lau 组沉积时期的翘倾抬升造成潜山暴露面积大、剥蚀范围广,潜山周缘高部位在 Adar 组沉积时期形成的 Yabus 组、Gayger 组和基岩原生油藏被破坏,现今为地层不整合残留稠油油藏;在拗陷阶段形成了Galhak组晚期成藏原生型地层-岩性稠油油藏及 Jimidi 组晚期成藏次生型构造-岩性稠油油藏,这2类油藏勘探潜力较大,是该区下一步主要的勘探方向。

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    《油气地质与采收率》
    《油气地质与采收率》入选北大《中文核心期刊要目总览》2020年版