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Volume 41 Issue 2
Apr.  2023
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WEI TianYuan, CAI ChunFang, HU YongJie, LIU DaWei, JIANG ZiWen. Origin of Reservoirs in the Lower Cambrian Xiaoerbulak Formation, Tarim Basin[J]. Acta Sedimentologica Sinica, 2023, 41(2): 527-544. doi: 10.14027/j.issn.1000-0550.2021.116
Citation: WEI TianYuan, CAI ChunFang, HU YongJie, LIU DaWei, JIANG ZiWen. Origin of Reservoirs in the Lower Cambrian Xiaoerbulak Formation, Tarim Basin[J]. Acta Sedimentologica Sinica, 2023, 41(2): 527-544. doi: 10.14027/j.issn.1000-0550.2021.116

Origin of Reservoirs in the Lower Cambrian Xiaoerbulak Formation, Tarim Basin

doi: 10.14027/j.issn.1000-0550.2021.116
Funds:

National Science and Technology Major Project 2017ZX0508-003-040

  • Received Date: 2021-06-04
  • Accepted Date: 2021-09-14
  • Rev Recd Date: 2021-08-23
  • Available Online: 2021-09-14
  • Publish Date: 2023-04-10
  • Well-developed deep-time and deep-burial dolostone reservoirs are currently sedimentlogical research topics in petroleum exploration, for example in the Sichuan and Tarim Basins in China, and the South Oman Salt Basin. Due to the influence of sedimentary environments and constructive diageneses, ancient deep and ultra-deep dolostone (>4 500 m) have demonstrated high-quality reservoir properties in some case studies. The porosity of some intervals is up to 10%⁃20%, producing oil and gas of industrial standard, and are of great importance in hydrocarbon exploration. However, our understanding of the genesis of the pore structures of deeply buried dolostone reservoirs is still limited: are they generated from later diagenesis, or are they preserved sediments retaining early diagenetic pores? Distinguishing between these two types of pores and explaining their formation mechanisms is a controversial and much-debated topic in sedimentology. The Cambrian Xiaoerbulak Formation underwent long periods of deep burial settings, to a maximum burial depth of 6 000 m. This formation is characterized by the complexity of the diagenesis of dolostone and the heterogeneity of the reservoirs. In addition, they are associated with many different microbialite types.Overall, this suggested that the Xiaoerbulak Formation is a suitable case for solving the debated issue of late microbialitic dissolution. For this reason, the Cambrian Xiaoerbulak Formation in the Tarim Basin was selected as the research object, and the rocks from typical wells and field profiles were carefully selected and used in a variety of advanced experiments, comprising multi-scale geological and geochemical methods included microscopic observation of rocks and minerals, stable δ13C and δ18O isotope analysis, and in-situ trace element and rare earth element analyses. Combined with basic data from previous sedimentological studies, this paper attaches importance to the analysis of fluid⁃rock interactions and solves deep and ultra-deep issues such as the evolution of diagenetic fluids and dissolution transformation of dolomite reservoirs. Firstly, based on comprehensive and detailed petrological observations, it is concluded that there are four main types of rocks developed in the Xiaoerbulak Formation: microbialites (thrombolite dolostone, foam spongy dolostone, oncolite dolostone, stromatolite dolostone, bonded sandy dolostone, and algal framework dolostone), dolo-grainstone, crystalline dolomite (dolo-micrite and fine crystalline dolomite) and micritic limestone. The analyses of porosity and permeability suggest that dolo-grainstone and thrombolites contain the best reservoir properties. In contrast, stromatolite dolostone and fine crystalline dolostone have lower porosities and permeabilities. Cathodoluminescence studies, fluid inclusion data and carbon and oxygen isotope testing suggest that matrix dolomite and fine-crystalline dolomite were formed in sedimentary oxidized seawater with δ18O values of -6.16‰ ± 0.72‰. Subsequently, fine-crystalline dolomite cements (δ18O = -7.47‰ ± 1.2‰) were mainly formed in near-surface to shallow burial environments. In deep burial conditions, coarse-crystalline dolomite cements (δ18O =-11.35‰ ± 1.41‰) and saddle dolomites (δ18O = -10.44‰ ± 0.6‰) were developed. With gradually increasing depth, oxygen isotope values of carbonate minerals show a decreasing trend, indicating a rising formation temperature. However, these dolomites have relatively positive carbon isotopes, and there is no significant change among them. By comparison, due to the oxidation of hydrocarbons and their incorporation into calcite, the calcite has negative δ13C values (-4.07‰ ± 0.92‰), suggesting that it is typical of thermochemical sulfate reduction (TSR).The presence of hydrothermal minerals, including saddle dolomite and authigenic quartz, is evidence of homogenization temperatures above that of the formation. In-situ trace element and rare earth tests show that diagenetic fluids evolved from early to late stages: distribution curves indicate enriched heavy rare earth elements (HREE), to enrichment of medium rare earth elements (MREE), and finally to a mode of enrichment of light rare earth elements (LREE). In consequence, the following evolutionary sequence of diagenetic fluids was comprehensively considered: from seawater-dominated pore water, shallow-buried iron-rich pore water, medium-deep buried manganese-rich pore water, to lately crust-derived hydrothermal fluid-dominated pore water, and finally evolved into acidic fluids related to TSR dissolution. It is noteworthy that the properties of the fluids gradually evolved from low-temperature and low-salinity to high-temperature and high-salinity. Quantitative statistics show that hydrothermal dissolution increased the porosity of the reservoir by 3%; the maximum porosity is close to 10%.Thus it is concluded that the late dissolution re-formation of those formed in early high-energy mound-shoal facies contributed most to the development of high-quality reservoirs. Depending on the type of sedimentation- diagenetic re-formation, three categories of high-quality reservoirs are classified in the Xiaoerbulak Formation as follows: (i) hydrothermal/meteoric water-altered thrombolitic reservoir of high-energy reef and shoal facies; (ii) hydrothermal/TSR-altered shoal facies dolo-grainstone reservoirs; and (iii) TSR-altered gypsum-related dolostone reservoirs. This list provides favorable exploration directions for Xiaoerbulak Formation carbonate oil and gas exploration in the Tarim Basin, and also helps to better understand the genesis of deep and ultra-deep high-quality dolomite reservoirs.
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  • Received:  2021-06-04
  • Revised:  2021-08-23
  • Accepted:  2021-09-14
  • Published:  2023-04-10

Origin of Reservoirs in the Lower Cambrian Xiaoerbulak Formation, Tarim Basin

doi: 10.14027/j.issn.1000-0550.2021.116
Funds:

National Science and Technology Major Project 2017ZX0508-003-040

Abstract: Well-developed deep-time and deep-burial dolostone reservoirs are currently sedimentlogical research topics in petroleum exploration, for example in the Sichuan and Tarim Basins in China, and the South Oman Salt Basin. Due to the influence of sedimentary environments and constructive diageneses, ancient deep and ultra-deep dolostone (>4 500 m) have demonstrated high-quality reservoir properties in some case studies. The porosity of some intervals is up to 10%⁃20%, producing oil and gas of industrial standard, and are of great importance in hydrocarbon exploration. However, our understanding of the genesis of the pore structures of deeply buried dolostone reservoirs is still limited: are they generated from later diagenesis, or are they preserved sediments retaining early diagenetic pores? Distinguishing between these two types of pores and explaining their formation mechanisms is a controversial and much-debated topic in sedimentology. The Cambrian Xiaoerbulak Formation underwent long periods of deep burial settings, to a maximum burial depth of 6 000 m. This formation is characterized by the complexity of the diagenesis of dolostone and the heterogeneity of the reservoirs. In addition, they are associated with many different microbialite types.Overall, this suggested that the Xiaoerbulak Formation is a suitable case for solving the debated issue of late microbialitic dissolution. For this reason, the Cambrian Xiaoerbulak Formation in the Tarim Basin was selected as the research object, and the rocks from typical wells and field profiles were carefully selected and used in a variety of advanced experiments, comprising multi-scale geological and geochemical methods included microscopic observation of rocks and minerals, stable δ13C and δ18O isotope analysis, and in-situ trace element and rare earth element analyses. Combined with basic data from previous sedimentological studies, this paper attaches importance to the analysis of fluid⁃rock interactions and solves deep and ultra-deep issues such as the evolution of diagenetic fluids and dissolution transformation of dolomite reservoirs. Firstly, based on comprehensive and detailed petrological observations, it is concluded that there are four main types of rocks developed in the Xiaoerbulak Formation: microbialites (thrombolite dolostone, foam spongy dolostone, oncolite dolostone, stromatolite dolostone, bonded sandy dolostone, and algal framework dolostone), dolo-grainstone, crystalline dolomite (dolo-micrite and fine crystalline dolomite) and micritic limestone. The analyses of porosity and permeability suggest that dolo-grainstone and thrombolites contain the best reservoir properties. In contrast, stromatolite dolostone and fine crystalline dolostone have lower porosities and permeabilities. Cathodoluminescence studies, fluid inclusion data and carbon and oxygen isotope testing suggest that matrix dolomite and fine-crystalline dolomite were formed in sedimentary oxidized seawater with δ18O values of -6.16‰ ± 0.72‰. Subsequently, fine-crystalline dolomite cements (δ18O = -7.47‰ ± 1.2‰) were mainly formed in near-surface to shallow burial environments. In deep burial conditions, coarse-crystalline dolomite cements (δ18O =-11.35‰ ± 1.41‰) and saddle dolomites (δ18O = -10.44‰ ± 0.6‰) were developed. With gradually increasing depth, oxygen isotope values of carbonate minerals show a decreasing trend, indicating a rising formation temperature. However, these dolomites have relatively positive carbon isotopes, and there is no significant change among them. By comparison, due to the oxidation of hydrocarbons and their incorporation into calcite, the calcite has negative δ13C values (-4.07‰ ± 0.92‰), suggesting that it is typical of thermochemical sulfate reduction (TSR).The presence of hydrothermal minerals, including saddle dolomite and authigenic quartz, is evidence of homogenization temperatures above that of the formation. In-situ trace element and rare earth tests show that diagenetic fluids evolved from early to late stages: distribution curves indicate enriched heavy rare earth elements (HREE), to enrichment of medium rare earth elements (MREE), and finally to a mode of enrichment of light rare earth elements (LREE). In consequence, the following evolutionary sequence of diagenetic fluids was comprehensively considered: from seawater-dominated pore water, shallow-buried iron-rich pore water, medium-deep buried manganese-rich pore water, to lately crust-derived hydrothermal fluid-dominated pore water, and finally evolved into acidic fluids related to TSR dissolution. It is noteworthy that the properties of the fluids gradually evolved from low-temperature and low-salinity to high-temperature and high-salinity. Quantitative statistics show that hydrothermal dissolution increased the porosity of the reservoir by 3%; the maximum porosity is close to 10%.Thus it is concluded that the late dissolution re-formation of those formed in early high-energy mound-shoal facies contributed most to the development of high-quality reservoirs. Depending on the type of sedimentation- diagenetic re-formation, three categories of high-quality reservoirs are classified in the Xiaoerbulak Formation as follows: (i) hydrothermal/meteoric water-altered thrombolitic reservoir of high-energy reef and shoal facies; (ii) hydrothermal/TSR-altered shoal facies dolo-grainstone reservoirs; and (iii) TSR-altered gypsum-related dolostone reservoirs. This list provides favorable exploration directions for Xiaoerbulak Formation carbonate oil and gas exploration in the Tarim Basin, and also helps to better understand the genesis of deep and ultra-deep high-quality dolomite reservoirs.

WEI TianYuan, CAI ChunFang, HU YongJie, LIU DaWei, JIANG ZiWen. Origin of Reservoirs in the Lower Cambrian Xiaoerbulak Formation, Tarim Basin[J]. Acta Sedimentologica Sinica, 2023, 41(2): 527-544. doi: 10.14027/j.issn.1000-0550.2021.116
Citation: WEI TianYuan, CAI ChunFang, HU YongJie, LIU DaWei, JIANG ZiWen. Origin of Reservoirs in the Lower Cambrian Xiaoerbulak Formation, Tarim Basin[J]. Acta Sedimentologica Sinica, 2023, 41(2): 527-544. doi: 10.14027/j.issn.1000-0550.2021.116
  • 白云岩富含大量的油气资源。资料表明80%以上的北美碳酸盐岩油气藏赋存于白云岩[1]。早于志留纪的46个碳酸盐岩油气田中,90%以上是以白云岩为储集层[2]。深层—超深层(>4 500 m)白云岩储层成因是近年来石油勘探领域的研究热点[3]。典型实例包括:塔里木盆地寒武系[4]、四川盆地上二叠统—下三叠统白云岩储层[57]、波斯湾下三叠统白云岩储层[8]和侏罗纪白云岩储层[9]。其中,塔里木盆地寒武系盐下勘探面积达150 000 km2,占全盆地面积四分之一。目前已发现塔里木盆地30多个碳酸盐岩规模油气田,探明石油地质储量超过21×108 t,探明天然气地质储量超过5 000×108 m3[10]。位于塔中隆起的ZS1井和ZS1C 井在中寒武统盐上(ZS1井6 426~6 487 m)和盐下(中深1井6 597~6 785 m,ZS1C井6 861~6 944 m)获得工业油气流[11],塔北地区轮探1井则在寒武系吾松格尔组8 203~8 260 m白云岩储层中获日产油134 m3、天然气4.59×104 m3[12],指示塔里木盆地盐下碳酸盐岩储层勘探潜力巨大。

    海相碳酸盐岩储层物性受控于沉积环境和成岩改造。深层储层由于受上覆沉积物的重压,经历强烈的压实胶结作用,原始孔隙被消耗殆尽,早期岩溶形成的孔洞也较难保存,但相关研究表明埋深大于5 000 m的白云岩储层仍发育大型溶蚀孔洞[1321]。孔隙的成因机制包括:地层水的冷却作用[2223],有机质热成熟产生的有机酸和CO2[2426],热液溶蚀[2729]和热化学硫酸盐还原作用(TSR)[6,1620,3033]。然而,考虑到埋藏条件下低的水岩比、饱和孔隙水、限制的孔隙水流动等特点,埋藏溶蚀对储层的改善作用存在一定的争议[3435]。对于热化学硫酸盐还原作用(TSR)而言,TSR产生的酸性流体对白云石和硬石膏进行溶解,但也会造成方解石的沉淀[3637],其对储层的作用有待进一步研究。

    针对国内勘探向深层进军的趋势,寻找深层规模储层成为重点和难点,而其成因研究成为指导油气勘探的理论基础。然而,塔里木盆地肖尔布拉克组白云岩经历多期构造运动、油气充注和复杂的成岩作用,储层形成是沉积环境、成岩作用和构造作用综合作用的结果,这种复杂性导致其有利的控制因素仍不明确。目前,普遍达成的共识是肖尔布拉克组储层具有明显的相控性,丘滩体系沉积为储层发育的基础(内缓坡浅滩[3839]、中缓坡丘滩及颗粒滩[4043])。前人已在研究层位识别出大气水[4344]、有机酸和埋藏白云石化[4547]、热液活动[44,4849]和热化学硫酸盐还原作用[5052]等多期溶蚀证据,证实了肖尔布拉克组经历多期次流体改造。但是关于各类溶蚀类型对储层的贡献,则尚未达成一致。比如,沈安江等[41]、郑剑锋等[42]、严威等[43]认为在此丘滩相基础上受高频海平面波动控制的准同生期溶蚀才是储层发育的关键性因素,热液活动对储层改造规模有限,仅在一定程度上起到了补充作用。赵文智等[4647]则认为相对于古岩溶作用,塔北地区构造—热液白云石化和热液溶蚀对储层贡献最大。程丽娟等[44]更支持准同生淡水溶蚀型和构造—热流体改造型储集体。黄擎宇等[40]、Jiang et al.[48]则认为相对于准同生期溶蚀,后期岩溶及热液溶蚀控制了储层的发育。Jiang et al.[50]、Cai et al.[52]更倾向于认为膏盐相关的热化学硫酸盐作用对储层形成贡献了主要力量。肖尔布拉克组不仅储层赋存机制和成因存在争议,而且在不同期次流体的性质和来源方面,缺乏深入探讨,各类溶蚀对储层的贡献也缺乏定量化的统计,制约深层碳酸盐岩的油气勘探。基于以上问题,对塔里木盆地肖尔布拉克组储层的形成机理做系统地研究。本文采取塔里木盆地典型露头及井下岩石样品,结合岩石学观察,并采用全岩地球化学以及原位分析实验方法,确立成岩序列,恢复成岩流体,定量估算成岩事件对储层的改造作用,并依据沉积—成岩类别划分储层。本研究不仅对油气勘探具有一定的指导意义,而且对深层优质碳酸盐岩储层成因具有理论意义。

  • 塔里木盆地是以塔克拉玛干沙漠为中心,北临天山山脉、西南为昆仑山脉、东南为阿尔金山脉的叠合型盆地[53],分为四个隆起区与五个拗陷区,分别为塔北、塔中、巴楚、东南隆起以及西南、塘古、东南、北部和库车拗陷。

    塔里木盆地寒武系地层总厚度超过2 000 m,自下而上划分为六个组,为下寒武统玉尔吐斯组、肖尔布拉克组、吾松格尔组、中寒武统沙依里克组、阿瓦塔格组和上寒武统下丘里塔格组。玉尔吐斯组为塔里木盆地古生界油气的主要烃源岩[5455],在盆地东北缘库鲁克塔格地区为陆棚—深水盆地沉积,在西北缘则为斜坡相沉积[5657]。早寒武世为大规模海侵后伴随着海退,肖尔布拉克沉积时期塔北地区的台地结构样式由缓坡型台地转变为弱镶边型台地的过渡状态[58],发育中—厚层状粉晶白云岩、微生物白云岩[5961]图1)。肖尔布拉克组分为肖上段和肖下段,肖上段又可分为3个亚段(1、2和3段)。肖下段被划为外缓坡相,肖上1段和肖上2段属于中缓坡相,肖上3段属于内缓坡相。岩性从下到上依次为泥粉晶白云岩、粉细晶白云岩、细晶白云岩、泥粒白云岩和泥质白云岩,整体上为一套水体深度逐渐变浅、水动力由弱变强再变弱的沉积序列[49]。吾松格尔组主要发育亮晶砂屑白云岩、含膏粉—细晶白云岩,泥云坪相沉积。沙依里克组为云坪相,褐色盐岩、灰岩,红色泥岩与泥质白云岩。阿瓦塔格组上部以褐色白云岩以及灰质、膏质白云岩为主,中下部以褐色褐灰色盐岩、膏盐岩为主,夹白云岩、膏质泥岩。晚寒武世发生缓慢海侵运动,整个盆地范围内发育上寒武统下丘里塔格组,主要为局限台地相粉细晶—中粗晶白云岩,夹鲕粒白云岩、砂屑云岩、竹叶状砾屑白云岩、灰质白云岩和燧石条带。沉积环境逐渐由蒸发台地向半局限—开阔台地环境过渡,台地边缘相带变窄。具体来看,中西部台地区内广泛发育台内颗粒滩沉积体系;巴楚—塔中地区为局限台地相;柯坪地区则转变为局限台地的潟湖沉积环境。

    Figure 1.  Lithofacies paleogeographic map with locations of sampled wells and the Xiaoerbulak Formation outcrops in the Tarim Basin (modified from reference [61])

    塔里木盆地寒武系地层沉积后被快速埋深到5 000~8 000 m,侏罗纪开始被抬升至5 000 m,随后继续沉降形成现今地层所在深度[62]图2),而塔西北地区在早三叠世期间则被抬升至地表,随后缓慢沉降到现今深度。热历史受到多期次造山运动的影响与改造,其中二叠纪岩浆热运动影响最为强烈[60]

    Figure 2.  Burial histories of the Xiaoerbulak Formation, Tarim Basin (modified from reference [62])

  • 肖尔布拉克组发育多期次碳酸盐胶结物及其他类型的自生矿物。纤状白云石胶结物以泡沫绵层石白云岩腔体内的第一期胶结物产出,自形,长约65 μm,宽约20 μm,阴极发光下呈暗红色(图3a,b)。粉细晶白云石胶结物,生长于纤状白云石胶结物之上,充填于泡沫绵层腔体或溶蚀孔洞。白云石晶体具有它形的结构特征,粒径范围是80~130 μm。阴极发光下呈与围岩相近的暗红色(图3c,d)。细晶自形白云石胶结物呈菱形状分布于扩大溶蚀孔洞周边(图3i),粒径70~180 μm,阴极发光下呈暗红色,偶具亮红色环带。中粗晶白云石胶结物大多充填在溶蚀孔洞和裂缝内,沉淀于细晶白云石之后,菱形自形状,粒径250~600 μm,阴极发光下呈亮红色(图3e,f)。鞍形白云石大多充填在裂缝或热液溶蚀孔洞,生长在中粗晶白云石胶结物之上。粒径500~1 000 μm,具波状消光,阴极发光下大多为暗红色,偶具亮红色环带(图3g,h)。硬石膏广泛发育于肖尔布拉克组,以柱状发育于扩大的溶蚀孔附近(图3j),常见石膏被溶蚀形成铸模孔(图3k,l)。方解石以粒状或脉状充填在孔洞或裂缝中,充填在中粗晶白云石胶结物和鞍形白云石晶间孔中,粒径范围是500~800 μm,阴极发光下不发光(图3g)。自生石英主要发育于溶蚀孔洞和裂缝附近,充填鞍形白云石晶间孔,呈粒状(图3i)。

    Figure 3.  Thin⁃section and cathodoluminesence photomicroraphs of different kinds of minerals filled/cemented

  • 肖尔布拉克组主要发育微生物云岩、颗粒云岩、晶粒云岩和泥晶灰岩。塔里木盆地的白云岩的类型与原始沉积相带密切相关,具有明显的相控特征[3839]。在早寒武世碳酸盐缓坡背景下,肖上段,塔里木盆地柯坪—巴楚地区大面积微生物丘滩复合体亚相发育,柯坪露头形成了丰富的各类微生物白云岩,丘滩体受古地貌地形和洋流背景控制[42];巴中—塔中地区则更为发育内缓坡浅滩相颗粒云岩。肖下段,塔里木盆地主要发育深水潟湖相泥粉晶云岩和泥晶灰岩(图4,5)。本文共识别出以下几种类型微生物岩:凝块石白云岩、泡沫棉层云岩、核形石云岩、叠层石云岩、黏结砂屑云岩、藻格架云岩。

    Figure 4.  Section of microbial rocks in Keping outcrop, Tarim Basin

    Figure 5.  Sedimentary facies model of microbial rocks from the Keping area of the Xiaoerbulake Formation (modified from reference [43])

    凝块石白云岩主要发育于肖下段至肖上1段,外缓坡相顶部至中缓坡相下部,总体上水体较深,水动力较弱。凝块由暗色丝状藻纹层组成,具有互相缠绕联结且向一定方向延伸的特征(图6a)。颗粒凝块的粒径范围是0.1~0.3 mm,凝块石白云岩发育大量不规则顺层溶蚀孔洞,孔径1~2 mm。

    Figure 6.  Rock types and reservoir properties in Lower Cambrian Xiaoerbulake Formation, Tarim Basin

    泡沫绵层石白云岩发育于肖上2段中上部,中缓坡相,相对于肖下段,水体变浅,代表了动力稍强的浅水沉积环境,野外以灰白色滩状或丘状产出,具有明暗相间的条带状结构,并发育蜂窝状顺层溶孔。其由大小不一的泡沫腔体组成,腔体孔径约0.2 mm。部分体腔孔被栉状白云石和粉晶白云石充填物半充填或者完全充填,部分发育溶蚀扩大的似椭圆形顺层溶蚀孔(图6b),且孔径2~3 mm。

    核形石白云岩发育于肖上2段上部,中缓坡相,野外以颜色浅于基质的不规则颗粒状为特征。核形石颗粒呈不规则椭圆形—长条形,大小不一,散乱分布(图6c),粒径为0.5~1 mm。颗粒间未被白云石胶结物充填的部分构成现今残余孔隙。

    叠层石白云岩发育于肖上2段顶部以及肖上3段,中缓坡相至内缓坡相,野外以波状、弱波状以及层状明暗交替的纹层结构为特征。其具有明暗交替的亮暗纹层的结构特征,纹层厚0.2~0.5 mm,未被亮晶白云石及鞍形白云石充填或半充填的叠层石格架孔构成现今残余孔隙(图6d)。

    黏结砂屑云岩发育于肖上2段,中缓坡相。砂屑约0.2 mm,(菌)藻相互黏结在一起或藻屑相对均匀地黏结在一起所构成。局部会残留不规则状的微生物溶孔,孔径1~2 mm(图6e)。

    藻格架云岩主要发育在大型的藻丘建隆中,在苏盖特布拉克广泛发育[43]。舒探1井肖上段顶部发育部分格架结构,为柯坪露头区域微生物丘滩相向西南部分延伸(图6f),储层具有相控性。格架孔多为不规则状,孔径0.1~0.5 mm。

    颗粒云岩主要发育于肖上段顶部,内缓坡相。颗粒主要为藻砂屑,粒径0.1~0.25 mm,荧光下具有油气荧光显示,粒间半充填硬石膏、亮晶白云石等矿物(图6g~i),发育溶蚀扩大的粒间溶孔,孔径2~4 mm。

    泥粉晶、粉细晶云岩在研究区最为常见(图6j,k),泥粉晶云岩主要发育于肖下段,外缓坡相,低能水动力环境中,深灰色—黑色,晶粒小于30 μm,富含有机质,可见鞍形白云石全充填孔洞,致密。粉细晶云岩则发育于肖上段顶端,潮坪相,浅—深灰色,自形—半自形,晶粒大约50~250 μm,发育不规则状晶间孔及晶间溶孔,孔径0.1~1.5 mm。

    泥晶灰岩为深水潟湖相,处于水动力弱的深水环境。镜下,深灰色—黑色,夹泥质条带,偶见三叶虫碎片,反映整体水动力环境较弱。致密无孔(图6l)。

  • 早期沉积物以及原始沉积组构普遍经历白云石化,形成了粉细晶白云石基质,阴极发光下呈暗红色或不发光。纤状白云石胶结物以第一期胶结物分布在孔洞的内侧,并且具有暗红色阴极发光,指示其代表近地表的海相孔隙水胶结(图3a,b)。细晶白云石胶结物以第二期胶结物沉淀于溶蚀孔洞边缘,紧贴第一期胶结物,与基质具有相近的暗红色光,为准同生期白云石化产物。准同生期溶蚀孔洞外边缘可见基质为亮红色,为典型大气水改造特征(图3c,d)。随着埋深增加,脱离地表水体范围,逐渐进入封闭/半封闭流体环境,孔洞内发育细晶白云石(图3g),阴极发光下呈暗红色,偶具亮红色环带,指示浅埋藏白云石化。硬石膏结核大小不一,是同沉积及浅埋藏成因[50]。随着埋藏深度进一步增加,以压溶作用缝合线为标志,中深埋藏阶段,中粗晶白云石胶结物沉淀(图3e~g)。二叠纪发生区域性构造运动,造成深大断裂[63],为后期埋藏期热流体提供了通道,并对白云石和硬石膏结核造成溶蚀(图3j),并依次沉淀鞍形白云石和自生石英等热液矿物(图3h、i)。晚期TSR作用进一步对鞍形白云石和硬石膏改造,产生弯曲溶蚀边和膏模孔,并伴随方解石沉淀(图3g,k,l)。根据以上矿物充填/胶结顺序,得出如下的成岩序列(图7)。

    Figure 7.  Paragenetic sequence of the Xiaoerbulake Formation, Tarim Basin

  • 粉细晶白云岩、细晶白云石胶结物、中粗晶白云石胶结物和鞍形白云石具有相似的碳同位素组成,为0.9‰(av.)±1.08‰(σ)(n=26)(图8),而方解石δ13C值较其他充填物发生明显负偏(-4.07‰±0.92‰,n=20)。δ18O值自基质粉细晶白云石-6.16‰±0.72‰(n=10)、向细晶白云石胶结物-7.47‰±1.2‰(n=10)、中粗晶白云石-11.35‰±1.41‰(n=2)、鞍形白云石-10.44‰±0.6‰(n=6)和方解石依次递减-10.59‰±1.09‰(n=20)。

    Figure 8.  Carbon and oxygen isotope compositions of fillings and cements at different times

  • 中粗晶白云石胶结物流体包裹体均一温度94.3 ℃~121.2 ℃,低于方解石(128.2 ℃~152.3 ℃)和鞍形白云石充填物(136.3 ℃~182.5 ℃)(图9a)。盐度分布在9.98~18.96 wt.% NaCleqv,低于鞍形白云石(12.62~25.27 wt.% NaCleqv)和方解石(17.87~23.44 wt.% NaCleqv)(图9b)。石英晶体则具有相对低盐度高温的特征,流体包裹体均一温度117.4 ℃~146.5 ℃,盐度6.74~12.28 wt.% NaCleqv。

    Figure 9.  Fluid inclusion data of different diagenetic minerals

  • 将稀土元素划分为三部分:1)轻稀土(LREE)为La,Ce,Pr和Nd;2)中稀土(MREE)为Sm,Eu,Gd,Tb,Dy和Ho;3)重稀土(HREE)为Er,Tm,Yb和Lu。将Eu和Ce异常分别表达为Eu/Eu*=EuSN/(0.67SmSN+0.33TbSN)和Ce/Ce*=CeSN/(0.5LaSN+0.5PrSN[64]。基质白云石具有高Fe(502.5±160.3 μg/g,n=4),低Mn(168.39±52.12 μg/g,n=4)特征(图10)。其LREE亏损(NdSN/YbSN=0.51±0.22,n=8)、HREE富集(LaSN/SmSN=0.93,n=1;GdSN/YbSN=0.89,n=1),且具有轻微Ce负异常(CeSN/CeSN*=0.71±0.25,n=2),无Eu异常,与现代海水稀土配分模式相似(图11)。细晶白云石胶结物则显示总REE低、轻微MREE富集(GdSN/YbSN=1.54,n=1;LaSN/SmSN=0.51,n=1),无Ce、Eu异常。整体具有较高的Fe(1 468.27±1 630.13 μg/g,n=18)、Mn(279.74±134.06 μg/g,n=18)含量,且分布范围大。中粗晶白云石胶结物稀土配分曲线显示房顶状的MREE富集特点(GdSN/YbSN=2.76±1.8,n=2;LaSN/SmSN=0.31±0.2,n=2),无Ce、Eu异常。且Fe、Mn含量较高,分别为257.64±117.2 μg/g(n=8),305.14±138.57 μg/g(n=8)。

    Figure 10.  Fe and Mn contents of different kinds of filling in the Xiaoerbulak Formation, Tarim Basin

    Figure 11.  Average rare earth element and yttrium profiles for different kinds of filling

    鞍形白云石MREE富集(GdSN/YbSN=1.64,n=1;LaSN/SmSN=0.23,n=1),并显示出较高的Eu正异常(EuSN/EuSN*=1.86±0.74,n=3),无Ce异常。高Mn(623.15±219.58 μg/g,n=4),较高Fe(1 165.91± 285.39 μg/g,n=4)含量。方解石LREE富集(GdSN/YbSN=2.39±0.06,n=2;LaSN/SmSN=1.73±0.03,n=2),与基质白云石相差很大,且稀土总量高达55.06 μg/g,具有Eu正异常(EuSN/EuSN*=1.32±0.19,n=3)。Ce负异常(CeSN/CeSN*=0.1±0.03,n=3),其具有最低的Fe(109.49±92.72 μg/g,n=9)、Mn(7.68±8.20 μg/g,n=9)含量。

  • 根据成岩序列,结合多种地球化学测试,得出研究区如下的流体演化特征:沉积流体为海水,表现在基质δ13C值在1.02‰±1.21‰(n=10),δ18O值-6.16‰±0.72‰(n=10),落在早期海源灰岩白云石化范围内(寒武纪同时期海水δ13C介于2.5‰~0.5‰,δ18O介于9‰~-4‰[65]),反映海源流体成因。基质粉细晶白云石具有的与现代海水特征相近的LREE亏损、HREE富集特征,记录了海水信号。基质粉细晶白云石的总REE低,轻微Ce负异常(CeSN/CeSN*=0.71±0.25),高Fe(502.5±160.3 μg/g),低Mn(168.39±52.12 μg/g)含量记录了弱氧化的海水性质[65]

    随着沉积物逐渐埋藏至半脱离地表水体范围,进入半封闭的流体环境,流体逐渐演化为接近海水的孔隙水,反映在碳氧同位素值(δ13C集中在0.78‰±0.24‰、δ18O值-6.4‰±0.57‰)与基质相近的细晶白云石胶结物,且未观察到流体包裹体,阴极发光为暗红色,说明孔隙水仍然继承早成岩海水特点,为准同生期流体产物。此外,与基质相似的稀土配分特征进一步支持其继承了以古海水为主的孔隙水信号。

    随着埋藏加深进入浅埋藏成岩流体环境,沉积物脱离海水而逐渐演化为还原性孔隙水环境,与基质有显著差异。反映在δ18O值(-8.54‰±0.1‰,n=5)稍微负偏的细晶白云石胶结物,说明受到埋藏重结晶和地温升高影响。具有高Fe(1 468.27±1 630.13 μg/g,n=18)、高Mn(279.74±134.06 μg/g,n=18)含量和MREE富集特征。现今形成于铁化环境下的碳酸盐胶结物常具有典型的MREE富集特征[66]。此外,现今孔隙水研究表明锰化带和铁化带分别具有最高的Mn和Fe含量[6768]。这是因为在这两个还原带中,锰(氧)化物和铁(氧)化物的还原程度最大。因此主微量和稀土特征共同指示了在还原条件下孔隙水中铁锰氧化物被还原。

    中深埋藏阶段,孔隙水具有高温高盐度特征。中粗晶白云石胶结物流体包裹体具有高的均一温度(94.3 ℃~121.2 ℃),与进一步负偏δ18O值(-11.35‰±1.41‰)一致,均指示了较高的沉淀温度。铁锰氧化物进一步被还原,白云石具有更高的锰含量和MREE富集模式。

    埋藏期热液流体充注,具有高温高盐度特征,热液矿物鞍形白云石、石英流体包裹体均一温度峰值分别为140 ℃~160 ℃、130 ℃~150 ℃。鞍形白云石被限定在早二叠世热液白云石化生成[69],流体包裹体温度高于肖尔布拉克组在早二叠世古地温100 ℃[7072]。且鞍形白云石氧同位素进一步负偏,分布在-10.44‰±0.6‰,为典型高温热液特征,这支持上述成岩矿物代表热液沉淀[28]。Eu正异常,被认为是热液流体和塔里木盆地富含Eu的前寒武纪斜长岩基底水岩相互作用的结果[73],且酸性和高温环境下均有利于流体Eu以正二价还原态存在,导致Eu正异常[7475],因此推测成岩流体为还原酸性热液[76]。鞍形白云石MREE富集可能指示其从酸性壳源流体中沉淀[75],结合高Mn含量(623.15±219.58 μg/g,n=4),较高Fe(1 165.91±285.39 μg/g,n=4),无Ce负异常,支持铁锰氧化物被还原[66,77]

    方解石稀土配分曲线与基质白云石相差很大,且稀土总量高达55.06 μg/g,远高于其他充填物,结合盐度特征,共同指示了最高盐度流体。此外,最低的Fe(109.49±92.72 μg/g,n=9)、Mn(7.68±8.20 μg/g,n=9)含量可能说明铁参与了TSR反应并生成黄铁矿[78]图3j),该过程会释放H+,产生酸性流体。虽然TSR作用会生成低盐度的水[79],但是方解石仍具有高盐度特征,被认为是产生的低盐度水加入下寒武统地层水,而非方解石[48]。LREE富集可能是流体和富含LREE的火成岩基底反应的结果,轻微Eu正异常(1.43±0.1),被认为是高温还原流体[74,80]。结合塔里木盆地火山岩浆活动在二叠纪很活跃,且广泛发育酸性碱性岩浆喷发和辉绿岩侵入体[72,81],认为酸性还原热流体可能来自二叠纪岩浆热流体。

    综上,流体依次演化为:沉积弱氧化海水、接近海水的孔隙水、浅埋藏富铁孔隙水、中深埋藏高温高盐度孔隙水、埋藏期高盐度还原酸性壳源热流体、晚期最高盐度高温TSR流体(表1)。

    沉积/成岩流体均一温度与盐度稀土配分模式微量元素含量性质产物
    沉积海水40 ℃,3.5 wt%[48]LREE亏损 HREE富集中—高Fe 低Mn弱氧化性粉细晶白云石基质
    接近海水的孔隙水小于50 ℃[48]LREE亏损 HREE富集中—高Fe 低Mn低盐度氧同位素接近基质的细晶白云石胶结物
    浅埋藏条件孔隙水70 ℃~100 ℃[44],6~14wt.%[48]MREE富集极高Fe 高Mn稍高温,还原性氧同位素偏负的细晶白云石胶结物
    中深埋藏条件孔隙水94 ℃~121 ℃,10~29 wt.%MREE富集高Fe 高Mn高温高盐度中粗晶白云石胶结物
    壳源热流体136 ℃~182 ℃,13~25 wt.%MREE富集极高Fe 极高Mn高温高盐度还原酸性鞍形白云石,石英
    TSR相关流体128 ℃~152 ℃,18~23 wt.%LREE富集低Fe 低Mn高温高盐度酸性方解石
  • 根据沉积相和成岩流体类型我们将优质储层类型划分为三类。

  • 该类储层发育针状溶孔,阴极发光下溶孔周围呈亮红色,反映富含锰的大气水改造(图3d)。根据肖尔布拉克组微生物白云岩样品孔渗测试结果,凝块石白云岩则兼具高的平均孔隙度4.63%和最高的平均渗透率3.25×10-3 μm2;泡沫绵层石白云岩平均渗透率却最低,只有0.02×10-3 μm2;叠层石白云岩平均孔隙度和平均渗透率渗均较低;作为对照组的泥粉晶白云岩则具有相对低的孔隙度(图12)。

    Figure 12.  Porosity⁃permeability histogram for different kinds of dolostone

    凝块石因杂乱的结构和抗压实,而具有较高的孔隙度。相反,叠层石因定向的生长结构和较弱的抗压实,而具有较差的物性[82]。泡沫绵层石由于发育空腔结构,尽管受到后期矿物充填,仍有较多的孔隙残余,但空腔之间相互不连接,渗透率很低(图3b)。因此,本文认为最有利的原岩结构为凝块石结构,不同于宋金民等[8384]提出的叠层石结构。

    孔隙在野外以凝块石建造的四级向上变浅的旋回上部更发育,说明受到了层序界面控制,反映流体倾向于顺高孔渗带流动特征,即更倾向于沿着具有高的初始孔隙度部分微生物岩进行改造。面孔率统计显示单独的大气水溶蚀可造成平均孔隙度增加5%(n=3)。此类储层主要分布于塔里木盆地柯坪地区肖下段凝块石丘至肖上2段的藻屑滩和泡沫绵层石滩、方1井等(图3)。

    在灰岩被白云石化不久,处于半固结—固结状态时,由于海平面下降或构造抬升引起的微生物礁滩相沉积物暴露地表,会接受大气淡水淋滤改造,该时期形成的孔隙多为选择性溶蚀孔,如粒内溶孔及粒间孔(图3a,c)。塔里木盆地下寒武统微生物岩显示泡沫绵层石和凝块石白云岩由于受大气水淋滤和热液改造,发育大量微生物骨架间溶孔和骨架内溶孔[8587],而具有较好的储集物性,显示后期改造对储层发育仍具有重要控制作用。

  • 颗粒白云岩中,常见顺纹层展布的鞍形白云石、方解石和石英部分充填孔洞。鞍形白云石和石英的包裹体和稀土微量元素指示了热液白云石和石英从热液中沉淀,且存在弯曲溶蚀边,说明后期发生了溶解作用(图13)。方解石同样存在港湾状溶蚀现象,且碳同位素很负,低至-6‰,显示来自以乙烷为主的烃类氧化作用[17,20]。此外,流体包裹体温度(140 ℃~150 ℃)显示其远远满足TSR反应的100 ℃温度条件。其具有最低的铁(109.49±92.72 μg/g,n=9)、锰(7.68±8.20 μg/g,n=9)含量,说明铁参与了热化学硫酸盐还原作用(TSR)并生成黄铁矿,故推测晚期方解石极有可能为TSR的产物。根据孔渗测试,颗粒云岩有较高的孔隙度约为4.51%,超过除凝块石外的大部分微生物岩(图12)。该类储层分布于舒探1井、楚探1井以及野外露头肖上段浅滩相,水体能量强。薄片点数统计,热液导致面孔率高达10%(n=4,图13)。面孔率统计显示单独的热液溶蚀可造成平均孔隙度增加3%(n=12)。通常,热液溶蚀和TSR溶蚀伴生出现,共同造成6%的孔隙度(n=3)。

    Figure 13.  Imaging logging characteristics of rock core in well Shutan1

    塔里木盆地在漫长的地质历史时期,共经历了四期热事件。分别为震旦纪—寒武纪、早奥陶世、二叠纪以及白垩纪。其中第三次热事件影响最强烈,伴随着大规模岩浆活动和辉绿岩侵入[72,81],导致高的水岩比,利于碳酸盐岩的溶解。同时巴楚地区构造运动形成深大断裂[62],使得热液沿着断裂运移。而且,热液本身在运移过程中也会通过水力压裂作用产生许多构造微裂缝,在提高岩石物性的同时利于热液流入[88],并凭借其高温还原酸性的特性对岩石进行溶蚀。Wang et al.[89]通过原位拉曼分析,认为在高温条件下,SO42-与Mg2+大量络合并发生两种液相分离,证实了高温条件下SO42-与Mg2+形成络合物。该过程会破坏Mg2+-H2O络合物,促进白云石溶解。在原始物性较好的层段,热液活动才具有较大的通量,更容易形成后期的溶蚀作用,从而对储层有促进作用[90]。高温还可使有机质加热,产生有机酸,进而溶蚀碳酸盐岩[91],酸性有利于碱性碳酸盐岩溶解。少数情况下,伴随鞍形白云石热液方解石、自生石英充填,半充填后的残留孔隙形成现今的储集空间。

  • 常见硬石膏、鞍形白云石被部分溶蚀,晚期膏模溶孔中残留有沥青质以及方解石交代白云石现象。方解石δ13C值低至-6‰,是由于有机质并入,TSR反应会优先利用12C,导致生成的方解石δ13C负偏,故推测其为TSR成因。该类储层主要分布在塔里木盆地肖尔布拉克组康2井、楚探1井、中深5井肖上段含膏白云岩层(图3k),石膏提供了TSR反应中的良好的SO42-来源。薄片点数显示,面孔率高达10%(n=2)。面孔率统计显示单独的TSR溶蚀可增大孔隙度约3%(n=5),而在周边或相邻较致密储层,则降低孔隙度0.2%~0.5%(n=4)。

    该类储层形成主要靠热硫酸盐还原作用(TSR)溶解石膏来增加孔隙度,也发生鞍形白云石的溶解作用。若只发生硬石膏溶蚀和方解石沉淀的现象,TSR会提高储层孔隙度1.6%,渗透率也有相应提高[92]。TSR过程中产生的H2S浓度是现今储层H2S浓度的5倍[93],可以造成晚期非选择性溶孔[94]。Hao et al.[37]认为只发生方解石的沉淀而无白云石的溶解。但是Fu et al.[79]通过三维PHAST实现了硬石膏溶解,方解石沉淀的过程,同时伴随白云石的溶解。在TSR过程中,硬石膏的溶解作用导致孔隙流体具有高的Ca/Mg比值,使白云石变得不稳定。同时,在高温和还原性流体的作用下,早期的白云石可以与硬石膏一起发生溶解作用和被方解石的交代现象,于是,导致现今方解石和天然气中CO2比烃类富含12C,同时增大了孔隙空间。该TSR反应所释放出来的Ca2+可以迁移到周边储层,导致相对致密储层变得更加致密。

    基于上述考虑,我们认为发育这类优质储层的条件包括:1)TSR反应前具有一定的储集空间,晶粒或颗粒白云岩比较有利,而泥晶、微晶白云岩则不利;2)储层含有硫酸盐矿物,特别是结核状石膏、硬石膏,而非柱状石膏、硬石膏;3)要有油气的充注;4)储层温度高于120 ℃。基于这一判别标准,我们认为寒武系盐下肖尔布拉克组上段及吾松格尔组中与膏盐互层的白云岩是有利的储层。这一预测与现今的勘探结果是吻合的。

  • (1) 塔里木盆地下寒武统肖尔布拉克组主要发育四类岩石类型,分别为颗粒云岩、微生物岩(凝块石、泡沫棉层白云石、核形石、叠层石、黏结砂屑云岩、藻格架云岩)、晶粒云岩(泥粉晶和粉细晶云岩)和泥晶灰岩。其中颗粒云岩和凝块石云岩物性最好。

    (2) 原位微量稀土、碳氧同位素以及流体包裹体温度数据指示的流体演化过程依次为沉积弱氧化海水、准同生期接近海水的孔隙水、浅埋藏富铁孔隙水、中深埋藏高温高盐度孔隙水、埋藏期较高盐度还原酸性壳源热流体、晚期极高盐度高温TSR热流体。

    (3) 划分出肖尔布拉克组三类优质储层类型,分别为微生物礁滩相大气水/热液改造凝块石白云岩、浅滩相热液/TSR溶蚀颗粒白云岩以及TSR有关的膏盐白云岩储层,为今后的塔里木盆地盐下肖尔布拉克碳酸盐岩油气勘探提供了有利的证据。

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