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彭阳油田长3段成藏条件与成藏规律

2016-04-18兰朝利

兰朝利,王 奇,张 欣

(中国石油大学(北京) 石油工程教育部重点实验室,北京 102249)



彭阳油田长3段成藏条件与成藏规律

兰朝利,王 奇,张 欣

(中国石油大学(北京) 石油工程教育部重点实验室,北京 102249)

摘要:根据岩心分析、测井与试油资料,评价了彭阳油田长3段油藏成藏条件,厘定了关键成藏要素与成藏规律。长3段油藏烃源岩为延长组长6、长7段暗色泥岩,在早白垩世末期成熟,由断层、裂缝和延长组顶不整合疏导体系大量进入目的层。油藏经历了4次挤压抬升,三叠纪末期挤压奠定其构造形态,形成了鼻状和低幅度圈闭;后三期挤压使目的层整体抬升,但未破坏油藏圈闭。构造、储层与聚集条件是控制油气成藏的关键因素。长3段构造、构造-岩性圈闭形成早于或同步于油气大量充注时期。受小型三角洲平原分流河道控制,长3段砂体宽度窄、厚度薄,且后期遭受了较强的成岩作用,决定了该油藏规模普遍较小。

关键词:成藏条件;成藏规律;延长组长3段;彭阳油田;鄂尔多斯盆地

兰朝利,王奇,张欣.彭阳油田长3段成藏条件与成藏规律[J].西安石油大学学报(自然科学版),2016,31(2):1-10.

LAN Chaoli,WANG Qi,ZHANG Xin.Hydrocarbon accumulation conditions and rules in sandstone reservoir of the third member of Yanchang Formation in Pengyang Oilfield [J].Journal of Xi'an Shiyou University (Natural Science Edition),2016,31(2):1-10.

引言

油气藏成藏研究的进程伴随着不同类型油气藏的发现而不断向前迈进,例如,从最早期的专注于构造油气藏到后期发现并研究岩性油气藏、构造-岩性油气藏以及地层油气藏[1]。随着盆地模拟、数值模拟与测试技术的进步,油气成藏的研究开始借助某些关键技术而集中于个别成藏要素的研究,并且成果显著。例如:盆地模拟与数值模拟技术的使用让人们认识到油气二次运移是借助优势运移通道进行的[2-4];有机地球化学分析、流体包裹体分析以及成岩矿物同位素定年证实了油气充注是幕式的[5-8];有机地球化学与压力测试研究表明有机质成熟度不仅与温度有关,还与压力有关[9]。深盆气理论的提出拓宽了石油地质学家对油气成藏的认识[10],并直接导致了盆地中心型油气藏[11-12]、连续型油气藏概念的提出及对成藏机理的探讨[13-14],据此在我国鄂尔多斯盆地与四川盆地发现了储量丰富的连续型油气藏。

油气成藏规律的研究基于对油气成藏6个基本要素的评价,包括烃源岩、储层、盖层、运移通道、聚集条件与保存条件,即“生、储、盖、运、聚和保”[1]。对特定油气藏而言,上述要素中可能仅有部分要素起关键作用。例如,鄂尔多斯盆地,在烃源岩大面积分布且构造作用较弱的背景下,运移通道与储层物性对油气成藏起关键作用[13-14],鼻状与低幅度构造对局部油气聚集起关键作用[15]。

彭阳油田位于鄂尔多斯盆地西缘天环凹陷西南部,西侧毗邻西缘逆冲带[16-18](图1)。目前主要勘探开发侏罗系延安组延7、8、9段油藏。部分井在三叠系延长组长3段见较好油气显示。然而,彭阳油田发现较晚,研究程度较低,目前仅在侏罗系油藏成藏条件与成藏规律[19-21]与长3段原油地化特征[22]、储层特征[23]、储层质量控制因素[24-25]与有利区预测[26]方面进行过相关初步研究,相比之下对长3段油藏成藏规律研究薄弱。为此,本文通过对彭阳油田长3段油藏烃源岩、储层、盖层、运移通道、聚集条件与保存条件6个基本成藏要素的评价,筛选出控制长3段油气成藏的关键要素,在此基础上总结研究区长3段油气成藏规律,以指导长3段下步井位部署与区块优选,为彭阳油田增储稳产提供保障。

图1 彭阳油田构造位置Fig.1 Tectonic location of Pengyang Oilfield

1基本地质特征

彭阳地区延长组长3段为一套炭质泥岩、粉砂质泥岩、泥岩与粉砂岩、细砂岩互层,自下而上可划分为长33、长32和、长31亚段(或砂层组)。在各砂层组内部,根据沉积层序特征可进一步分成三分,包括长333、长332、长331、长323、长322、长321、长313、长312和和311共9个小层(图2、图3)。长3段含油层在各个亚段均有分布。由于三叠纪末期的构造抬升剥蚀,长3段顶部地层被不同程度剥蚀,西部剥蚀较严重,东部、北部地区长3段保存相对完整,与上覆侏罗系延安组含煤碎屑岩系呈不整合接触(图3)。

受鄂尔多斯盆地西缘构造演化史与天环坳陷总体构造格局控制[27-33],彭阳地区长3段构造形态为一东倾单斜,斜坡上发育鼻状构造,局部见低幅度背斜。在长3段内部,构造形态垂向上继承性相对较好,但受长3段顶剥蚀程度、构造作用与岩性等方面差异的影响,各亚段斜坡上鼻状构造或低幅度背斜数量不一,在平面上的位置也发生侧向迁移。

区域古地理格局[28,34-37]与沉积相标志反映长3段为相对潮湿气候条件下的小型砂质河控三角洲沉积,砂体分布主要受三角洲平原分流河道砂体控制,由于三角洲规模较小,砂体普遍较薄(图4),侧向尖灭快,局部具有迷宫状储层结构。垂向上,分流河道在长323、长322、长321和长332(图5)最发育,宽度较大,沉积砂体较厚,小层砂体最厚可达18 m(图4)。

碎屑成分石英含量较低,岩性以岩屑长石砂岩、长石岩屑砂岩和岩屑砂岩为主,成岩作用较强,孔隙类型以残余粒间孔为主,长石、岩屑溶孔占有一定比例,物性较延安组差,孔隙度以10%~15%为主,渗透率主要在(0.1~10.0)×10-3μm2。

图2 彭阳油田延长组长3段地层柱状图Fig.2 Lithological column of the third member of Triassic Yanchang Formation in Pengyang Oilfield

图3 彭阳油田y42-z84长3段地层划分对比Fig.3 Stratigraphic subdivision correlation of the Chang 3 member from well y42 to well z84 in Pengyang Oilfield

图4 彭阳油田长332和长323 小层砂体厚度Fig.4 Isopachous maps of Chang332 and Chang323 sandstones in Pengyang Oilfield

图5 彭阳油田长332和长323 小层沉积相平面图Fig.5 Sedimentary facies maps of Chang332 and Chang323 in Pengyang Oilfield

2成藏地质条件

2.1烃源岩

彭阳地区延长组烃源岩包括侏罗系尤其是延安组煤层、炭质泥岩与延长组长4+5、长6段炭质泥岩及长7段炭质泥岩和(或)油页岩。根据长3段原油与延安组、延长组烃源岩煤层、炭质泥岩或油页岩生物标志参数(如伽马蜡烷、C30藿烷等)的对比,结合研究区延长组区域烃源岩分析[38-46],认为彭阳地区长3段有效烃源岩主要为长6、长7段炭质泥岩或油页岩。平面上,彭阳地区距离长6、长7段生烃中心较远,位于有效烃源岩分布区边缘[38,41,43],油气成藏需要运移较长距离。从烃源岩与储层的关系来看,长3段砂岩储层位于延安组烃源岩之下、延长组烃源岩之上,垂向上处于有利成藏位置,但与长6、长7段主力烃源岩相隔较厚的长4+5段致密层。

2.2储盖层

彭阳地区长3段储层在其内部各亚段均有分布,具有相对较高的钻遇率。然而,长3段储层为细砂岩、粉砂质细砂岩,属于小型三角洲平原分流河道沉积,砂体宽度窄、空间分布孤立、连通性差。成岩作用导致长3段砂体物性进一步变差,储层纵横向分布均较为孤立[4]。长3段内与油层互层接触的炭质泥岩、粉砂质泥岩甚至是致密砂岩构成了储层的直接盖层,延安组延6段上部炭质泥岩夹煤线厚度较大,横向分布稳定,封盖能力相对较强,构成了延长组、延安组油藏的区域性盖层,但其在研究区西南部被剥蚀厚度较大[24,26],封盖能力下降。另外,延7—延9段炭质泥岩也构成了长3段的局部盖层,尤其是局部煤层厚达12 m且具有生烃能力,因此具有较强的封盖能力。

2.3运移通道

断层和裂缝是延长组、延安组圈闭能否聚集油气的关键因素之一[40,42]。垂向上,长3段与有效烃源岩层长6、长7段中间相隔长4+5段致密且生烃层,必须借助断层或裂缝穿透长4+5段致密厚层,才有可能使得长6、长7段烃源岩排出的油气达到长3段圈闭成藏。横向上,彭阳地区距离长6、长7段主力生烃层的生烃中心较远,烃源岩厚度较薄,近源成藏的几率较小,大量油气聚集必须依靠穿过长4+5致密层之后的水平运移才可实现,因此,与鄂尔多斯盆地延长组更靠近烃源岩生烃中心的岩性油藏相比[39,41,44-45],延长组顶部不整合面对长3段油气聚集起着更为关键的作用。岩心观察发现长3段砂层发育层理缝(图6),有助于油气在砂体层内运移,但长3段砂体成岩作用较强,侧向连通性差,因此,其砂体内油气运移能力有限。

图6 层理缝(Z133,2 013.99 m)Fig.6 Bedding fractures(well Z133,2 013.99 m)

2.4聚集条件

彭阳地区长3段沉积之后立即遭受了三叠纪末期的强烈构造挤压[28,30-33],奠定了东倾单斜形态,形成一系列东西向、北东向鼻状构造以及部分低幅度圈闭,这些鼻状构造与低幅度圈闭为后期油气聚集创造了有利条件。早侏罗世末期以来,研究区目的层相继经受了3期构造挤压、抬升,但未改变其构造形态。单井埋藏史模拟反映,长6—长7段烃源岩于侏罗纪末期开始成熟,在早白垩世末达到高峰,与盆地主生烃期相同[47-50]。长3段砂岩在早白垩世末期埋藏最深,压实作用达到最强,储层变得相对较致密,形成一系列岩性圈闭或构造岩性圈闭。因此,彭阳地区长3段构造圈闭形成早于生烃期,构造-岩性或岩性圈闭的形成与生烃期同步,有利于油气聚集。

2.5保存条件

晚白垩世以来,研究区长3段持续挤压与抬升[3,30,33],形成裂缝或断层,在一定程度上垂向疏导了油气,造成早期油藏不同程度的破坏。但是,由于长3段埋藏相对较深,而构造作用可能主要作用于浅层,所以这些断层或裂缝活动未造成浅层地层水的侵入,地层水成岩环境仍然相对封闭[51],水型以CaCl2型为主,仅西南部构造高部位局部井点见Na2SO4型,但其矿化度普遍高于50 g/L(表1),反映长3段油藏保存条件较好,后期改造破坏程度较小。

表1 彭阳油田长3段地层水矿化度

3成藏规律

长3段内部9个小层中,目前已在长311、长313、长321、长323、长331与长332小层发现工业油层,反映这些小层储层均具有成藏潜力,油层分布与层位不具有明显相关性。彭阳地区长3段油藏横向上距离主力烃源岩层长6、长7段发育区较远,垂向上与长4+5、长6、长7段致密岩层相隔,长3段储层必须依靠裂缝、断层、不整合疏导体系连通才能使油气运移聚集到目的层,表明构造(例如挤压可形成裂缝、断层和不整合面)对长3段油藏聚集的重要影响。与长3段油藏具有相似源岩的延安组油藏的发现表明这些油气运移通道在研究区足够发育,而延安组上部盖层也足以封盖延安组与延长组油气[20-21],部分已发现的出油井点与长3段良好的封闭环境也表明长3段具有较好的保存条件。因此,构造、储层与聚集条件是控制长3段油藏分布的重要因素。

(1)构造是控制长3段油藏成藏的重要因素。长6、长7段烃源岩生成的油气必须依靠断层、裂缝才能向上运移到长3段成藏,足见断层、裂缝分布对长3段油藏成藏的关键作用,同时也表明与裂缝、断层连通的构造圈闭或构造-岩性圈闭具有最有利的成藏条件。长3段已发现工业油流井点多位于鼻状构造上倾方向或低幅度背斜上,如Z307、Z354、Y39、Y43与Y76井(图7),也反映构造是控制长3段油藏成藏的重要因素。结合各期砂体分布,预测长3段油藏分布有利区见图7。

(2)较窄的宽度与迷宫状砂体结构决定了长3段油藏规模较小。研究区长3段砂体为小型三角洲平原分流河道砂体,砂体宽度窄,侧向尖灭快,局部侧向上叠置形成迷宫状砂体结构,如y43井西侧仅相距一个井距(约1 000 m),含油砂体即尖灭(图8)。两期砂体接触处因属三角洲平原沉积而泥砾发育,泥砾易压实形成致密层,相应叠合砂体连通性差,最终导致长3段油藏普遍规模较小,开发部署风险较大。

(3)长3段油藏主要为构造-岩性油藏。研究区长3段构造形态以东倾单斜为主,斜坡上发育东

图7 彭阳油田长3段油藏分布Fig.7 Oil reservoir distribution of the Chang 3 Member in Pengyang Oilfield

图8 彭阳油田长3段典型构造-岩性油藏剖面Fig.8 Typical structural-lithological reservoir profile of the Chang 3 Member in Pengyang Oilfield

西向鼻状构造或低幅度背斜,而砂体主要呈北东走向,这样物性较好的砂体在上倾方向以长3段三角洲平原分流河道间泥岩或成岩致密砂岩为遮挡,形成上倾方向尖灭构造-岩性油藏(图8)。该类油藏是彭阳地区长3段油藏的主要类型。构造油藏分布受低幅度圈闭分布控制,而低幅度圈闭本身并不十分发育,因此构造油藏分布有限。另外,研究区西南部长3段地层被侵蚀,导致部分砂岩暴露,后期被上覆延安组细粒含煤层系覆盖,形成部分地层不整合油藏,由于长3段砂体不发育且剥蚀区域偏于西南部,因此这类油藏也不发育。

4结论

(1)长3段油藏油源充足,断层、裂缝与延长组顶不整合组成的油气疏导体系具有良好的疏导性能,盖层封盖条件好。

(2)三叠纪末期挤压奠定了长3段东倾单斜构造形态,形成了东西向、北东向鼻状构造和低幅度圈闭;早侏罗世末期、晚侏罗世末期与晚白垩世挤压使目的层整体抬升,但未破坏油藏圈闭,具有较好的保存条件。

(3)构造、储层与聚集条件是控制长3段油藏分布的关键因素。长3段构造圈闭形成早于油气大量充注时期,构造-岩性圈闭形成时期与油气大量充注时期同步,有利于油气成藏。长3段砂体宽度窄、厚度薄、成岩作用较强,导致长3段油藏规模普遍较小,勘探风险较大。长3段油藏主要发育构造-岩性油藏。

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责任编辑:王辉

Hydrocarbon Accumulation Conditions and Rules in Sandstone Reservoir of the Third Member of Yanchang Formation in Pengyang Oilfield

LAN Chaoli,WANG Qi,ZHANG Xin

(MOE Key Laboratory of Petroleum Engineering,China University of Petroleum (Beijing),Beijing 102249,China)

Abstract:The hydrocarbon accumulation conditions of the sandstone reservoirs of the third member of the Yanchang Formation (Chang 3) in Pengyang Oilfield were evaluated based on core analysis,logging and oil testing data,and the key hydrocarbon accumulation factors and rules were determined.The hydrocarbon in Chang 3 reservoir is mainly from the dark mudstones of Chang 6 and Chang 7 member,it matured in the end of the early Cretaceous and largely entered the Chang 3 reservoir through faults,fractures and the unconformity on the top of the Yanchang Formation.The Chang 3 reservoir had undergone compression and uplift 4 times in the end of Triassic,the early Jurassic,the middle Jurassic and the late Cretaceous respectively.The compression and uplift in the end of the Triassic established a basic monocline with east tendency and E-W striking nose structures and low-amplitude anticlines for Chang 3 member;the later three compressions made the Chang 3 member uplift as a whole,but did not damage the structural traps.Structure,reservoir distribution and hydrocarbon accumulation conditions are the key factors to the hydrocarbon accumulation in Chang 3 member.The structural traps and the structural-lithological traps in the Chang 3 member formed before or at the same time a great deal of hydrocarbon charged the reservoir.Under the control of the small delta plain distributary channel,the width and thickness of the Chang 3 sandbody are small,and the sandbody was subjected to strong diagenesis,which determines that the scale of the Chang 3 reservoir is generally small.

Key words:hydrocarbon accumulation condition;hydrocarbon accumulation rule;the third member of Yanchang Formation;Pengyang Oilfield;Ordos Basin

文章编号:1673-064X(2016)02-0001-10

文献标识码:A

DOI:10.3969/j.issn.1673-064X.2016.02.001

中图分类号:TE112.31

作者简介:兰朝利(1972-),男,博士,讲师,主要从事油气藏描述和开发研究。E-mail:lanchaoli@163.com

基金项目:国家自然基金项目(编号:41172127);中国石油天然气集团公司石油科技中青年创新基金项目(编号:04E7023)

收稿日期:2015-10-18