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  中国地质 2022, Vol. 49 Issue (2): 369-382  
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季天愚, 杨威, 武雪琼, 蒲仁海, 李德江, 刘满仓, 缪卫东, 苏楠, 叶颖. 2022. 塔里木盆地台盆区寒武系盖层评价及对油气盖层有利区的优选[J]. 中国地质, 49(2): 369-382.  
Ji Tianyu, Yang Wei, Wu Xueqiong, Pu Renhai, Li Dejiang, Liu Mancang, Miao Weidong, Su Nan, Ye Ying. 2022. Evaluation of Cambrian caprock in the platform-basin area of Tarim Basin and optimization of favorable area for oil and gas caprock[J]. Geology in China, 49(2): 369-382. (in Chinese with English abstract).  

塔里木盆地台盆区寒武系盖层评价及对油气盖层有利区的优选
季天愚1,2, 杨威1, 武雪琼1, 蒲仁海2, 李德江1, 刘满仓1, 缪卫东1, 苏楠1, 叶颖3    
1. 中国石油勘探开发研究院,北京 100083;
2. 西北大学大陆动力学国家重点实验室,地质学系,陕西 西安 710069;
3. 中国石油华北油田第四采油厂,河北 廊坊 065000
摘要[研究目的] 塔里木盆地台盆区中寒武统发育大面积的膏盐岩、膏质泥岩和膏质云岩地层,探讨该套地层的油气封盖能力和展布可为塔里木盆地寒武系盐下领域的勘探提供依据。[研究方法] 对4口井的44个岩心样品进行镜下薄片、物性参数和突破压力测试。通过建立各参数之间的关系,定量地对研究区中寒武统不同岩性岩石的封盖能力进行评价;结合二维地震和14口钻井资料对研究区中寒武统膏盐岩、膏质泥岩和膏质云岩地层的厚度进行研究;最终综合研究区中寒武统微观和宏观的评价结果,并以实际钻井的勘探情况作为约束,建立针对该研究区盖层封盖能力的综合评价方式。[研究结果] 研究结果认为膏盐岩相比膏质泥岩与膏质云岩具有更好的封盖能力,但断裂和石膏的埋藏地质条件会影响盖层的封盖能力。其中阿瓦提凹陷、塔中隆起北部、满西低凸起南部以及巴楚隆起中北部膏盐岩盖层厚度较大,以巴楚隆起北部为中心,膏盐岩厚度向四周呈不规则状递减,环绕状分布。[结论] 认为阿瓦提凹陷、满西低凸起西部和南部、塔中隆起北部和西部,以及塔北隆起中部为盖层发育的有利区。
关键词石油天然气    中寒武统    膏盐岩    盖层    封闭性    油气勘查工程    塔里木盆地    
中图分类号:P618.13            文献标志码:A             文章编号:1000-3657(2022)02-0369-14
Evaluation of Cambrian caprock in the platform-basin area of Tarim Basin and optimization of favorable area for oil and gas caprock
JI Tianyu1,2, YANG Wei1, WU Xueqiong1, PU Renhai2, LI Dejiang1, LIU Mancang1, MIAO Weidong1, SU Nan1, YE Ying3    
1. Research Institute of Petroleum Exploration Development, PetroChina, Beijing 100083, China;
2. State Key Laboratory of Continental Dynamics; Dept. of Geology, Northwest University, Xi'an 710069, Shaanxi, China;
3. 4th Production Plant Huabei Oilfield, Petrochina, Langfang 065000, Hebei, China
Abstract: This paper is the result of oil and gas exploration engineering.
[Objective] Gypsum-salt rocks, gypsum-bearing mudstone, and gypsum-bearing dolomite were extensively deposited in the platform-basin area of Tarim Basin during the Middle Cambrian. The study on the oil and gas sealing capacity and distribution of this set of strata can provide a basis for the exploration of the Cambrian subsalt field in the Tarim Basin. [Methods] 44 core samples from 4 wells were studied under microscopic thin sections and tested for physical parameters and breakthrough pressure. Based on the relationship among physical parameters, the sealing ability of the Middle Cambrian rocks in the study area was quantitatively evaluated. Meanwhile, we studied the thicknesses of the Middle Cambrian gypsum-salt rocks, gypsum-bearing mudstone, and gypsum-bearing dolomite using two-dimensional seismic data and drilling data from 14 wells. The microscopic and macroscopic appraisals of Middle Cambrian successions are constrained by drilling data obtained during exploration. Finally, we proposed a comprehensive method to evaluate the sealing ability of the caprock. [Results] The results suggest that gypsum-salt rocks have better sealing ability than gypsum-bearing mudstone and gypsum-bearing dolomite, but the faults and burial condition will affect the sealing ability of the caprock. It shows that the thicknesses of the gypsum caprocks are great in the Awati Depression, the north of the Tazhong Uplift, the south of the Manxi Low Uplift, and the middle and north of the Bachu Uplift. The thinness of gypsum is greatest in the Bachu Uplift and decreases unevenly towards the surrounding areas. [Conclusions] The research results show that the Awati Depression, the West and south of Manxi Low Uplift, the north and west of Tazhong Uplift, and the middle of Tabei Uplift are favorable areas for the development of the caprock.
Key words: oil and gas    Middle Cambrian    gypsum salt rock    caprock    sealing ability    oil and gas exploration engineering    Tarim Basin    

1 引言

在油气勘探中,盖层对于油气成藏至关重要。膏盐岩和泥岩作为区域盖层得到了广泛的关注和研究(赵庆波等,1994吕延防等,2000吕修祥等,2014付晓飞等,2018),同时,近几年大量的研究表明致密碳酸盐岩也可作为盖层封堵碳酸盐岩储层中的油气(杨传忠等,1991吕延防等,2005鲁雪松等,2007付广等,2014)。物性封闭、超压封闭以及烃浓度封闭是盖层的三种重要的封闭机理,尤以物性封闭最为基础和关键(Watts, 1987蒋有录,1998周雁等,2012)。针对盖层的封闭性评价,前人主要通过宏观和微观两个方面进行研究(Downey, 1984):宏观评价包括盖层岩性、分布范围、厚度、盖地比等(吕延防等,2000王庭斌,2005张林晔等,2010);微观封闭能力可通过孔隙度、渗透率、突破压力等参数进行评价(黄志龙等,1994付广等,1995邓祖佑等,2000刘东鹰,2010付晓飞等,2018)。

塔里木盆地寒武系以中寒武统膏盐岩为界,可将寒武系的白云岩地层分为盐上白云岩和盐下白云岩(杨海军,2015)。中深1井和轮探1井在寒武系盐下白云岩地层中获得工业油气流是塔里木盆地寒武系盐下白云岩勘探的重大突破(杨海军等,2020),而中寒武统这一膏盐岩重要的发育层位,因其可作为良好的盖层而被学者重视(王招明等,2014沈安江等,2016张纪智等,2017易士威等,2019)。自1994年至今,钻至塔里木盆地寒武系盐下的钻井已超20余口,除中深1井和轮探1井外,其余钻井皆因不同原因而失利,保存条件不佳是导致盐下油气勘探失利的重要原因之一,如舒探1井、方1井等。虽然前人针对塔里木盆地寒武系盖层已做了较多工作:如研究中寒武统的岩相古地理特征(高志前等,2011田雷等,2018;朱永进等,2019),分析中寒武统膏盐岩的分布范围(金之钧等,2010),以及通过岩心和露头样品对中寒武统膏盐岩的封盖性进行定量分析等(吕修祥等, 2000, 2014余海波等,2015林潼等,2019Liao et al., 2019)。前人已对塔里木盆地下寒武统肖尔布拉克组储层以及玉尔吐斯组烃源岩进行了大量研究(白忠凯等,2018Bai et al., 2019),但较少对盆地内部中寒武统盖层进行系统研究。

为了系统研究塔里木盆地台盆区中寒武统膏盐岩盖层,本文主要通过盖层的宏观特征和微观封闭能力两方面进行研究:(1)对44个岩心样品进行镜下薄片、物性参数及突破压力测试,定量地对中寒武统膏盐岩的封盖能力进行评价;(2)利用地震和钻井资料对中寒武世地层的厚度和平面展布进行研究;(3)结合中寒武世地层的宏观特征和微观封闭能力,建立盖层封闭能力的综合评价表,以对台盆区中寒武世地层进行盖层的综合评价,并探讨其油气地质意义,以期为塔里木盆地寒武系盐下领域的勘探提供参考和依据。

2 地质背景

塔里木盆地是中国最大的内陆沉积盆地,面积达5.6×105 km2图 1a),同时也是中国最大的含油气盆地。塔里木盆地是一个由古生界克拉通盆地和中—新生界前陆盆地组成的大型叠合复合盆地(贾承造,1999),其在太古界、元古界结晶基底之上沉积了巨厚的古生界、中生界及新生界沉积地层(Hu et al., 2000; Long et al., 2011; 周肖贝等,2012)。根据现今构造面貌和地层埋深可将塔里木盆地划分为7个一级构造单元,分别为塔北隆起、中央隆起、东南隆起、库车坳陷、北部坳陷、西南坳陷和东南坳陷(杨海军等,2020)(图 1a)。

图 1 研究区位置及塔里木盆地构造单元图(a)、寒武系地层柱状图(b) Fig. 1 Location of study area and structural unit map of Tarim Basin(a) and stratigraphic histogram of the Cambrian(b)

塔里木盆地寒武系表现出东盆西台的特点,以塔里木盆地东部台缘带为界可将塔里木盆地划分为塔东碳酸盐岩盆地相区和塔西碳酸盐岩台地相区(杜金虎等,2016),其中塔西台地区为本文的研究重点。根据露头、钻井和古生物资料,可将塔西台地相区寒武系分为3统6组(蔡习尧等,2009熊剑飞等,2011Zhang et al., 2020),由下至上分别为下统的玉尔吐斯组、肖尔布拉克组和吾松格尔组,中统的沙依里克组和阿瓦塔格组以及上统的下丘里塔格群(图 1b)。其中玉尔吐斯组以黑色泥页岩为主,为良好的烃源岩(杜金虎等,2016);肖尔布拉克组在塔中、巴麦地区作为储层自下而上发育薄层泥晶白云岩、晶粒白云岩和藻白云岩,而轮探1井和新和1井在塔北地区肖尔布拉克组揭示的都是灰岩,为非储层;吾松格尔组以泥质白云岩为主,并局部发育薄—中层膏盐岩;中寒武统发育最厚可达400 m的蒸发岩,并以膏盐岩夹膏云岩或含泥云岩为主;上寒武统以砂屑云岩和灰质云岩为主。

塔里木盆地中寒武世虽继承了早寒武世南北分异的沉积格局,但由于当时炎热干旱的气候,水平面的下降以及古隆起幅度的降低,其与早寒武世的沉积格局已有明显差异(陈永权等,2015)。塔里木盆地台缘带西部台地相区中寒武统的岩相以膏盐岩、泥云岩、膏云岩以及膏质泥岩等为主,表现为一大型蒸发台地相沉积特征,同时又可将其细分为膏盐湖、膏云坪、泥云坪等亚相。塔里木盆地中寒武统沉积模式由西向东表现为蒸发台地、台地边缘、斜坡相及盆地相,其中蒸发台地主要发育于台缘带以西的台地相区(朱永进等,2020)。在蒸发台地内部,膏盐湖大面积分布,且以膏盐湖为中心,膏云坪与泥云坪沿膏盐湖外围依次成环带状分布(图 2)。

图 2 塔里木盆地中寒武统岩相古地理图(修改自朱永进等,2020 Fig. 2 Lithofacies paleogeographic map of the Middle Cambrian in Tarim Basin (modified from Zhu Yongjin et al., 2020)
3 盖层封闭能力微观评价

对塔里木盆地台盆区寒武系盖层的微观封闭能力进行评价,突破压力测试是最直接且成熟的盖层封闭能力评价方法,突破压力越大,盖层的封闭能力越强,因此多将突破压力参数作为评价盖层封闭性的主要参数。其测试方法包括压汞法、分步法、驱替法以及连续法(黄志龙等,1994; Nygard et al., 2006)等。本文选取研究区4口钻遇中寒武统钻井的44个岩心样品,利用驱替法对岩样进行突破压力测试。为了消除岩样中黏土矿物吸水后膨胀导致的测试结果不准确,以及消除水对膏岩性质的影响,本次突破压力测试的岩样饱和液以煤油替换传统的盐水,结果如表 1所示。

表 1 台盆区寒武系盖层渗透率及突破压力统计 Table 1 Permeability and breakthrough pressure statistics of Cambrian caprock in the Tarim Basin

实验数据结果显示:4口井中寒武统的岩性为泥岩、膏盐岩和白云岩。其中舒探1井中寒武统阿瓦塔格组的取样深度范围为1673.10~1675.40 m,镜下薄片鉴定的岩性为膏质粉晶云岩,其饱和煤油突破压力最小0.7 MPa,最大4 MPa;在沙依里克组取样深度范围为1674.90~1675.40 m,岩性为浅灰色石膏岩,其饱和煤油突破压力最小为2.1 MPa,最大为14 MPa,且48 h未突破。中深5井阿瓦塔格组的取样深度范围为6189.0~6223.4 m,镜下薄片鉴定以膏岩为主,夹杂膏粉晶云岩,其饱和煤油突破压力最小为0.5 MPa,最大为14 MPa且48 h未突破,但大多数岩样的突破压力未超过4 MPa;在沙依里克组的取样深度为6545.4~6553.5 m,岩性以白云岩为主,夹有云质粉晶膏岩,其突破压力最小为0.5 MPa,最大为14 MPa且48 h未突破;相对于中深5井在阿瓦塔格组以突破压力低于4 MPa为主,其岩样在沙依里克组的突破压力大多高于9 MPa。牙哈10井的取样全部位于阿瓦塔格组,其取样深度范围为6173.9~6448.6 m,深度范围较大,岩性全部为白云岩,岩样突破压力从0.1 MPa到14 MPa且48 h未突破,突破压力跨度较大且无规律。英买36井取样较少,仅在阿瓦塔格组5597.7~5598.0 m取样3个,突破压力最小为0.7 MPa,最大为12.5 MPa,岩性为粉晶云岩。通过对上述数据进行整理分析,岩石渗透率与突破压力之间具有较为明显的负幂指数关系(图 3),即岩石的渗透率越大,其突破压力越小,反之则越大。

图 3 塔里木盆地台盆区中寒武统盖层岩石突破压力与渗透率关系图 Fig. 3 Relationship between breakthrough pressure and permeability of the Middle Cambrian caprock in Tarim Basin

表 1中,4口井大多数岩样的突破压力大于5 MPa,根据石油天然气盖层评价方法(SY/T 6942-2013)中突破压力的分级评价标准,当突破压力大于5 MPa,具有较好的封闭性。因此普遍认为当突破压力大于5 MPa时,样品可视为盖层。但仍有较多样品的突破压力低于5 MPa,有些仅为0.1 MPa,这样的样品不具有良好的封闭能力。

为弄清样品突破压力相差较大的原因,本文选取舒探1井、中深5井和牙哈10井不同突破压力对应的岩心薄片进行研究,发现除渗透率因素,裂缝的发育对于岩石突破压力具有较大的影响,如图 4所示。图 4a位于舒探1井1675.4 m深的沙依里克组,岩性为破碎角砾状粉泥晶膏质云岩,突破压力为5.5 MPa,渗透率0.0029 mD。图 4b位于舒探1井1674.30 m深的阿瓦塔格组,岩性为含云膏岩,渗透率为0.0077 mD,突破压力为3.5 MPa。图 4c位于舒探1井1674.90 m深的阿瓦塔格组,岩性为破碎角砾状膏质泥晶云岩,渗透率为0.0088 mD,突破压力为4.0 MPa。舒探1井岩样的物性较差,且易产生破裂,因此突破压力值较低。图 4def为中深5井中突破压力递增的3张镜下岩心薄片,岩性为膏质云岩、含膏泥晶云岩及含硬石膏粉晶云岩。由图可知,图 4d显示一条未充填的构造缝,裂缝的宽度约为0.05 mm,其所对应的突破压力为2.52 MPa;图 4e可见一条宽约0.01 mm且被硬石膏充填的构造缝,以及一条未充填的构造缝,其突破压力为5.37 MPa;图 4f可见被硬石膏充填的构造缝,其突破压力为12.22 MPa。由此可见,当岩石中存在未被充填的构造缝时,会造成该岩石具有较高的渗透率以及较低的突破压力,反之则具有较低的渗透率和较高的突破压力。牙哈10井中的泥晶云岩同样具有相同的特征。从图 4g中可见该岩样发育多条构造裂缝,其突破压力仅为0.7 MPa,图 4h中裂缝被方解石和白云石所充填,其突破压力为4.5 MPa,而图 4i中未见裂缝,其突破压力为11.5 MPa。因此,裂缝会降低岩石的突破压力并影响其封盖性。

图 4 研究区中寒武统样品典型显微特征(单偏光) a—舒探1井,1675.40 m,沙依里克组,破碎角砾状粉泥晶膏质云岩;b—舒探1井,1674.30 m,阿瓦塔格组,含云膏盐;c—舒探1井,1674.90 m,阿瓦塔格组,破碎角砾状膏质泥晶云岩;d—中深5井,6553.10 m,沙依里克组,膏质云岩;e—中深5井,6548 m,沙依里克组,含膏泥晶云岩;f—中深5井,6193.50 m,阿瓦塔格组,含硬石膏粉晶云岩;g—牙哈10井,阿瓦塔格组,6175.20 m,泥晶云岩;h—牙哈10井,阿瓦塔格组,6176.20 m,泥晶云岩;i—牙哈10井,阿瓦塔格组,6212.1 m,泥晶云岩 Fig. 4 Typical microscopic characteristics of the Middle Cambrian samples in the study area (monopolar light) a-Well Shutan 1, 1675.40 m, Shayilike Formation, micrite gypsum dolomite; b-Well Shutan 1, 1674.3m, Awatage Formation, dolomitic gypsum; c-Well Shutan 1, 1674.90m, Awatage Formation, gypsum micrite dolomite; d-Well Zhongshen 5, 6553.10m, Shayilike Formation, gypsum dolomite; e-Well Zhongshen 5, 6548m, Shayilike Formation, gypsum bearing micrite dolomite; f-Well Zhongshen 5, 6193.50m, Awatage Formation, anhydrite bearing dolomite; g-Well Yaha 10, Awatage Formation, 6175.20 m, micrite dolomite; h-Well Yaha 10, Awatage Formation, 6176.20m, micrite dolomite; i-Well Yaha 10, Awatage Formation, 6212.10m, micrite dolomite

通过对以上中寒武统岩石样品进行封盖性微观分析,发现相比于碳酸盐岩样品,膏岩的突破压力变化较大,且较多具有较大的孔隙度和渗透率。这与膏岩可作为优质盖层的认识不符,针对这一问题,林潼等(2021)通过对塔里木盆地台盆区中寒武统石膏样品进行三轴应力实验,认为石膏在650~700 m以浅时容易产生裂缝和断裂而导致封盖能力散失;700 m后,石膏由脆性转化为塑性,此时的封闭能力最强;随后当地层温度大于52℃时,石膏开始脱水向硬石膏转化,形成大量孔隙,此时封闭能力下降;当硬石膏埋深达3300 m以深,此深度对应脆-塑转化的边界压力,硬石膏重新具有塑性,同时压实作用也会减少石膏内部的孔隙,使其重新具备封闭能力。这为定量评价塔里木盆地台盆区中寒武统膏岩的封闭性提供了依据(林潼等,2021)。

4 盖层宏观发育特征 4.1 盖层厚度

盖层的厚度及平面展布特征对盖层的封闭能力具有重要影响,盖层厚度越大,平面展布面积越大且越稳定,盖层的封闭性能越好,反之则盖层的封闭能力越差(付晓飞等,2015)。以巴探5井为例,其中寒武统岩性可分为4大类:膏岩、盐岩、膏质云岩以及膏质泥岩,其中膏岩又可分为白云质膏岩、泥质膏岩等(图 5)。每一种岩性的测井响应特征明显,盐岩具有高声波、低密度和高电阻率的特征,纯净的膏岩自然伽马值低且密度高。这四种岩性中,盐岩的封盖能力最强,因为其具有极低的孔隙度和渗透率,同时具有很强的塑性;巴探5井中寒武统的埋深超过3300 m,其膏岩处于塑性状态且内部孔隙较少,同样具有较好的封盖性。又由于这4种岩性为互层的状态,因此该地层具有非常好的油气封盖性。

图 5 巴探5井中寒武统单井剖面 Fig. 5 Single well profile of the Middle Cambrian in well Batan 5

由膏盐岩沉积所形成的膏盐湖位于盆地的中西部,最大深度位于阿瓦提凹陷中部,厚度可达300多米。通过对钻揭寒武系各井盖层厚度以及盖地比的统计分析可知(表 2),可作为盖层的岩层厚度最厚可达376.5 m,盖层单层厚度最大可达47.2 m,同时盖层占地层厚度的比值为21.2%~86.2%。

表 2 塔里木盆地台盆区中寒武统盖层厚度统计 Table 2 Thickness statistics of the Middle Cambrian caprock in Tarim Basin
4.2 盖层平面展布

膏盐岩在地震剖面上表现为强振幅、中—高连续性以及平行—亚平行的地震相特征,通过拉平中寒武统底界后的地震剖面进行分析,可将膏盐湖等蒸发台地相沉积进行有效识别(图 6)。塔里木盆地台缘带西部台地相区中寒武统盖层展布受沉积期蒸发台地相控制,因此对蒸发台地地震相的追踪有助于研究盖层的平面展布。

图 6 研究区地震剖面:(a)原始地震剖面;(b) 局部放大的地震剖面;(c)将Є2底拉平后的地震剖面;(d) 解释后的地震剖面(位置见图 2) Fig. 6 Seismic section in the study area: (a) original seismic section, (b) locally amplified seismic section, (c) seismic section after leveling at the end of Є2, (d) interpreted seismic section (see Fig. 2 for location)

依据膏盐岩反射具高连续、强振幅的特征来判断其边界,再利用研究区内的钻井资料作为约束(图 7),并结合中寒武统岩相古地理特征,得到研究区膏盐岩的厚度分布(图 8)。可见研究区内膏盐岩分布范围较广,主要集中在阿瓦提凹陷、塔中隆起北部、满西低凸起南部以及巴楚隆起中北部。最大厚度的膏盐岩位于巴楚隆起中北部,可达300余米。以厚度最大的位置为中心,膏盐岩厚度向四周呈不规则状递减,呈环绕状分布。同时由图 8可知中寒武统膏盐岩具有良好的连续性和稳定性。膏盐岩具有较高的突破压力和低渗透率,且与膏质云岩和膏质泥岩互层分布,使这种大面积分布的膏盐岩盖层能为盐下油气的聚集提供了良好的封盖条件,是优质的区域性盖层。

图 7 舒探1-方1-和4-和6-巴东4-塔参1-中深5-中4井中寒武统连井对比剖面(位置见图 1 Fig. 7 Correlation profiles of connected wells in Middle Cambrian of Shutan 1-Fang1-He 4-He 6-Badong 4-Tacan 1-Zhongshen 5-Zhong4 (location in Fig. 1)
图 8 塔里木盆地中寒武统膏盐岩厚度图 Fig. 8 Isopach map of the Middle Cambrian gypsum salt rock in Tarim Basin
5 盖层封闭能力综合评价

虽然膏盐岩具有良好的油气封闭性,但并不是膏盐岩存在的地区就是油气封闭的有利区。如上所述,其封闭性受其发育规模、厚度、连续性、构造作用以及成岩演化阶段等因素的影响(李永豪等,2016)。为更好地评价塔里木盆地台盆区中寒武统盖层的封盖性,建立了一套盖层封盖性综合定量评价方式(表 3)。

表 3 塔里木盆地台盆区中寒武统盖层封盖性综合定量评价 Table 3 Thickness statistics of the Middle Cambrian caprock in Tarim Basin

通过公式E=∑ai×bi可计算出各井中寒武统盖层的综合评价值,其中E为盖层综合评价权值,ai为第i项的评价参数,bi为第i项评价参数的权重。当盖层综合评价权值为3~4、2~3、1~2、0~1时,分别对应盖层品质为好、较好、一般、差。表 4为14口井的盖层综合评价。但在具体评价时,当断裂破坏程度为断穿地层时,即使其余各项指标良好,其盖层综合评价也应该差。

表 4 盖层封闭能力综合评价结果 Table 4 Thickness statistics of the Middle Cambrian caprock in Tarim Basin

表 4可知,舒探1井和方1井虽然多数指标较好,但由于有断穿盖层断裂的存在,导致两口井的盖层不具备封盖能力。同时,舒探1井中寒武统以膏岩为主,在较浅的埋深下石膏向硬石膏转化而导致内部孔隙度变大,封盖能力变差(林潼,2021)。塔参1井综合评价为一般,轮探1井、中4井、同1井、和4井以及巴东4井的盖层综合评价为好,其余井的盖层综合评价为较好。

6 盖层发育有利区

通过以上对研究区中寒武统盖层的微观、宏观以及综合评价,并以实际钻井的勘探情况作为约束,对研究区内主要构造单元盖层的封盖性进行分析,并指出盖层封盖的有利区。由于满西低凸起中西部及阿瓦提凹陷暂无太多钻井揭示中寒武统膏盐岩,但通过膏盐岩厚度图可知该区域膏盐岩厚度大、埋深大、分布广且连续,同时该区域的构造运动以整体升降为主,断层不发育,因此判断该地区为盖层发育的有利区。巴楚隆起膏盐岩发育,尤以北部地区的厚度最大。但位于巴楚隆起北部的舒探1井和方1井由于多期断层发育,且断穿盖层,因此该区域保存条件较差。塔中隆起中寒武统膏盐岩多发育于北部和南部地区,同时中深1井的油气突破证实该区域盖层具有良好的封闭性。塔北隆起中寒武统膏盐岩不发育,轮探1井揭示其主要以膏质云岩和灰岩为主,膏质云岩在一定深度的条件下同样具有较好的封盖性,且其单层最厚可达47.2 m,因此该区域为盖层发育的有利区。

7 结论

(1)塔里木盆地台盆区中寒武世地层岩性以膏盐岩、膏质泥岩和膏质云岩为主,其中膏盐岩具有很强的的油气封盖性。但岩石内的构造裂缝和石膏的埋藏地质条件对其封盖性具有较大影响。

(2)阿瓦提凹陷、塔中隆起北部、满西低凸起南部以及巴楚隆起中北部膏盐岩盖层厚度较大,以巴楚隆起北部为中心,膏盐岩厚度向四周呈不规则状递减,环绕状分布。且膏盐岩的分布具有良好的连续性和稳定性。膏盐岩、膏质云岩和膏质泥岩的互层式分布,使这种大面积分布的膏盐岩盖层能为盐下油气的聚集提供良好的封盖条件。

(3)通过对研究区盖层封闭能力的综合评价,认为阿瓦提凹陷、满西低凸起西部和南部、塔中隆起北部和西部,以及塔北隆起中部为盖层发育的有利区。

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