塔中地区热化学硫酸盐还原作用对深埋白云岩储层的改造

2016-12-23贾连奇蔡春芳李红霞汪天凯孔令武

沉积学报 2016年6期

贾连奇蔡春芳,2 李红霞汪天凯张文孔令武

(1.中国科学院地质与地球物理研究所北京 100029；2.长江大学武汉 430100；3.中国石油塔里木油田分公司勘探开发研究院新疆库尔勒 841000；4.中海油研究总院北京 100028)

贾连奇1蔡春芳1,2李红霞1汪天凯1张文3孔令武4

塔里木盆地塔中地区深埋碳酸盐岩储层显示出极强的非均质性。如灰岩地层孔隙度极低，而含酸性气藏的白云岩储层最大孔隙度高达27%。然而，造成这些现象的原因仍然不清楚。通过岩芯、薄片、扫描电镜观察，结合流体包裹体均一温度、盐度分析，方解石、白云石的碳氧同位素测定，试图解决这一问题。前人研究认为，塔中地区碳酸盐岩优质储层分布主要受控于原始沉积条件(如高能的礁滩相)和表生溶蚀，热液活动和断裂活动也起到重要作用。然而，多期流体活动和成岩作用导致大量同生期和表生期形成的溶蚀孔洞被破坏。在某些井区(如ZG9井和TZ75井)，埋藏溶蚀作用可能对优质储层形成起到重要作用。在寒武系和奥陶系岩芯中发现了大量硬石膏、重晶石、黄铁矿、沥青、方解石等,方解石交代硫酸盐，方解石具有较高的均一温度及较低的碳同位素值说明其形成与热化学硫酸盐还原作用(TSR)有关。在发生TSR的白云岩井段，储层物性较好，说明TSR可能对深埋储层的改善具有促进作用。这些认识有助于指导深层寒武系碳酸盐岩储层的进一步勘探。

埋藏溶蚀热化学硫酸盐还原作用寒武系奥陶系塔中地区

0 引言

深埋碳酸盐岩储层(>4 500 m)是我国陆上油气勘探发展的重要接替领域，意义重大[1]。目前，在塔里木盆地埋深4 500～7 000 m的碳酸盐岩中发现了大量的油气，显示出巨大的资源潜力[1-2]。塔里木盆地最近的勘探资料显示，深逾6 000 m的井段还存在物性较好的储层。例如，塔深1井在8 402 m深的地层中仍发现直径8 cm左右的洞穴[3-4]，中深5井在深达6 200 m的中寒武统阿瓦塔格组粉晶白云岩地层中见到油气显示，中深1井在下寒武统肖尔布拉克组6 597.6～6 785 m井段的储层中仍然发现较好的储层，日产气30 322 m3。类似地，国外碳酸盐岩油气勘探过程中发现了众多埋深大、物性好的储层实例[5-7]。然而，这些深埋碳酸盐岩储层中高孔隙的来源仍然有着较大的争议。

一般来说，海相碳酸盐岩的储层物性主要受控于原始的沉积特征(如沉积相、沉积组构、原始矿物、古气候、海平面波动等)和后期的成岩改造(如表生期和埋藏期流体改造)[8-10]。多数情况下，深埋碳酸盐岩储层经历后期压实、胶结、充填等成岩作用后，原生孔隙大量消失[5,11-12]。Halleyetal.[11]研究了南佛罗里达地区灰岩孔隙度随埋深的变化，发现由于机械压实和胶结作用，灰岩埋深>5 000 m，孔隙度<3%，埋深>6 000 m，孔隙基本上消失殆尽。塔里木盆地经历多期改造，多期流体活动和复杂的成岩作用，形成于同生期和表生暴露期的孔隙可能只有少部分保存下来。如果保存下来的孔隙十分有限，那么在某些岩芯上观察到的丰富孔隙又是如何形成的？是否存在深埋藏环境下的强烈的溶蚀作用？

导致埋藏溶蚀的流体包括：有机酸、H2S、CO2、热液流体以及热化学硫酸盐还原作用(TSR)形成的流体[3]。热液流体是指比地层温度高(>5℃)的外部流体[13]，由于化学性质与储层流体不同，破坏了原有的水—岩平衡，于是既可能导致矿物溶解，也可能导致沉淀作用。在碳酸盐岩储层内的埋藏溶蚀流体中，热液流体被认为是深埋优质储层分布的关键[14-15]，北美、西班牙及中东的某些优质白云岩储层被认为是深层热液流体改造的结果[15-19]。塔中地区存在热液流体活动已经成为了共识[20-22]，并认为热液流体活动对深埋碳酸盐储层具有明显的改造作用[23-28]。然而，TSR对深埋碳酸盐岩储层的改造研究较少，只有Lietal.[29]讨论了TSR对于奥陶系灰岩储层的影响。前人对于深埋碳酸盐岩储层的研究发现，在高H2S含量的深埋白云岩井段经常发现具有异常高的孔隙[3,30-33]。塔中地区也发现含有较高H2S含量的白云岩储层，有必要探讨TSR对孔隙的影响。为此本文从流体地球化学角度入手，通过59口井岩芯样品的岩芯观察、薄片观察、扫描电镜，包裹体均一温度、盐度，碳氧同位素等方法确定TSR发生及其对储层的改造作用。本项研究对于指导碳酸盐岩油气勘探具有理论和实际意义。

1 区域地质概况

塔中隆起位于塔里木盆地中部，西与巴楚隆起相接，东邻东南隆起，南为西南坳陷，北接北部坳陷(图1)。塔中隆起是一个在寒武纪—奥陶纪巨型褶皱背斜基础上形成并长期发育的继承性隆起，始于中晚奥陶世前，石炭纪前基本定型，海西期以后构造以小型改造为主[34]。塔中地区具有“东西分块，南北分带”的构造格局。将塔中隆起分为4个构造单元：I.北斜坡；II.中央断垒带；III.南斜坡；IV.东部潜山区(图1)。由于加里东中期强烈隆升，塔中地区经历了较长时间的剥蚀和沉积缺失，多数地区缺少一间房组、土木休克组沉积，鹰山组顶部发育准层状不整合面。奥陶系(O)为一套潮上至潮间带的局限台地至台地边缘相沉积，根据岩性、电性特征及区域对比，自上而下分为：上统桑塔木组、良里塔格组、中下统鹰山组、蓬莱坝组。上奥陶统为一套潮间带局限台地至台地边缘相沉积。桑塔木组(O3s)为暗色泥岩、泥灰岩。良里塔格组(O3l)为中厚层状灰色、浅灰色砂屑灰岩、含泥灰岩、泥灰岩、生屑灰岩。鹰山组(O1-2y)：中、上部为中厚—巨厚层状灰岩，夹中厚—厚层状含泥灰岩，局部见一层泥质灰岩；下部为中厚—巨厚层状含云灰岩，间夹中厚层状含泥灰岩。属藻坪—潮间带下部或潮下边缘滩的沉积。蓬莱坝组(O1p)：上部以中厚—巨厚层状含云灰岩、云质灰岩为主，夹中厚层状含泥灰岩、灰云岩；中部以中厚—巨厚层状云质灰岩、灰质云岩、中厚—厚层状灰云岩互层为主，夹中厚层状含泥云岩、含泥云质灰岩、泥质云岩；下部为厚—巨厚层状灰云岩、灰质云岩，夹一层厚的白云岩。属于一套潮间带—局限台地相沉积。寒武系(∈)为一套浅海碳酸盐台地潮坪沉积及局限澙湖的沉积组合。根据岩性、电性特征，及区域地层对比情况，自上而下分为上统：下丘里塔格群；中统：阿瓦塔格组、沙依里克组；下统：吾松格尔组、肖尔布拉克组，玉尔吐斯组。其中下丘里塔格群由巨厚层状白云岩组成，夹中厚—厚层状含泥云岩、灰质云岩、含泥云岩。阿瓦塔格组上部为中厚层状含膏泥质云岩、膏质云岩、泥质云岩、含泥云岩与中厚—厚层状含膏云岩、白云岩呈略等厚互层。沙依里克组为中厚—巨厚层状白云岩、含灰云岩，夹一厚层状含泥灰质云岩。吾松格尔组为中厚—巨厚层状含泥云岩、中厚—厚层状泥质云岩、泥云岩与中厚—巨厚层状白云岩呈略等厚互层，顶部含薄层的云质石膏岩。肖尔布拉克组上部为中厚—巨厚层状白云岩、中厚层状含灰云岩夹含泥云岩；中部为中厚—厚层状泥云岩夹一厚层状浅灰色白云岩；底部为巨厚层状深灰色白云岩。玉尔吐斯组下部为薄层硅质岩、黑色页岩，上部为厚层状深灰色白云岩。

图1 塔中地区构造图Fig.1 The structural map of the Tazhong area in Tarim Basin

2 埋藏溶蚀识别标志及溶蚀机理

Choquetteetal.[35]最早提出碳酸盐岩埋藏溶蚀成因孔隙的概念，之后得到了世界各地碳酸盐岩储层研究者的广泛支持[3,14,36-39]，并建立了识别埋藏成因孔隙的一些标志，如分布于晚期裂缝、缝合线附近的孔洞、高温成岩矿物或含油气包裹体的矿物被溶解等。这些研究成果为深埋碳酸盐岩油气勘探提供依据。确定溶蚀作用与TSR有关，首先需要确定溶蚀发生在深埋环境。结合前人的研究成果和我们的观察和测试，总结出塔中地区如下深埋溶蚀的标志：

(1) 高温沉淀的矿物(粗晶白云石、方解石等均一温度高的矿物)被溶蚀(图2 A)。粗晶白云石通常充填孔洞，或沿孔洞壁向内生长。

(2) 孔洞中充填了高温粗晶白云石和高硫同位素值的黄铁矿(图2 B)，溶蚀孔洞极有可能是高温、深埋条件下溶蚀的产物。因为早期形成的孔洞往往含有陆源黏土矿物、浅埋条件生物成因的富32S的黄铁矿。

(3) 被高温矿物交代而剩余的孔隙空间，如萤石交代方解石所残留的孔隙(图2 C，D)。从方解石和萤石的晶胞参数计算表明，1个方解石分子的体积为61.30 Å3，1个萤石分子的体积为40.76 Å3，因此，萤石交代方解石后体积减小33.5%，从而产生大量的晶间孔隙[40]。

(4) 溶蚀缝洞切穿埋藏期流体沉淀矿物，或这些矿物中发育溶蚀孔洞(图2 E)。

图2 深埋溶蚀识别标志1.粗晶白云石、方解石被溶蚀，T75井，∈3，4 841.1 m，单偏光；B.黄铁矿与粗晶白云石共生，粗晶白云石被溶蚀，TZ75井，∈3，4 830.4 m，反射光；C.方解石、萤石充填在孔洞中，发育溶孔，TZ45井，O3l，6 050.1 m，单偏光；D.灰岩围岩被热液溶蚀，洞穴中充填萤石，TZ45井，O3l，6 050.1 m；E.晚期高温重晶石被溶蚀，ZG51井，O3l，5 003.1 m，正交偏光；F.缝合线被溶蚀，缝合线中充填少量沥青，ZG203，O1-2y，6 570.6 m，单偏光；G.晚期高温方解石被溶蚀，TZ243，O1p，4 468.4 m，单偏光；H. G中方解石中包裹体照片，单偏光；I. H对应的荧光照片，烃类包裹体发蓝色荧光。Fig.2 Photomicrographs showing indicators of deep-burial dissolution

(5) 沿成岩晚期的缝合线或晚期裂缝产生溶蚀孔隙，这些缝合线或裂缝及其溶孔中常充填原油或沥青(图2 F)。

(6) 含油、气或沥青包裹体的矿物被溶解，说明溶解作用发生在油、气充注和沥青形成以后(图2 G，H，I)。

TSR可能是导致塔中地区发生埋藏溶蚀作用的重要的成岩作用。TSR期间碳酸盐岩发生溶解，尤其是白云石溶解作用更强，可以用水溶硬石膏、白云石与油气烃类反应的机制解释。基于川东北飞仙关组富H2S白云岩和川西贫H2S灰岩的对比研究，Caietal.[3]提出水溶硬石膏、白云石能与油气烃类反应，反应式如下：

3 热化学硫酸盐还原作用的地球化学标志

在TSR过程中，溶于水中的烃类与水溶相硫酸盐在大于120C的条件下反应[42-46]。在储层温度条件下，硫酸盐是极为稳定的，必须通过活化作用才能反应。这种活化作用可能通过硫酸盐与H2S，元素硫，有机含硫化合物的反应或硫酸镁离子对的扰动作用实现[47-50]。TSR能够导致烃类部分或完全破坏，并产生还原形式的硫(元素硫和H2S)、氧化形式的碳(碳酸盐岩矿物和CO2)、水和有机含硫化合物[42,45,51-57]。

塔中地区的H2S主要是TSR成因[21,45,55,58-59]。已经在塔中地区寒武系—奥陶系中发现了大量发生TSR的岩矿证据，包括TZ54井下奥陶统硬石膏的方解石化[45]及TZ12井上奥陶统方解石交代重晶石[21]，TZ408和TZ75等井上寒武统白云岩地层中方解石部分交代硬石膏的现象或硬石膏的方解石假晶[59]，与TSR成因方解石共生的柱状硬石膏和元素硫球状体[60]。相似的硬石膏和黄铁矿硫同位素和高温的富集12C的方解石也证实发生TSR[21,45,59]。并在原油中检测出2-硫代金刚烷、烷基—四氢噻吩、硫醇[55,58,61]等TSR相关的含硫化合物，奥陶系与石炭系原油中异常高的二苯并噻吩也被认为是与TSR有关[62]。

本次研究也观察到大量硫磺，TSR成因方解石，黄铁矿及硫酸盐共生，并发现TSR成因方解石交代硫酸盐的现象(图3)，认为奥陶系鹰山组碳酸盐岩地层内可能发生较强的TSR，例如ZG9井附近。ZG9最初测定的H2S含量甚至超过40%。然而，ZG9为水井，H2S在水中的溶解度明显高于甲烷、氮气、二氧化碳等，导致H2S含量被高估。经过模拟计算后，H2S含量估值为5%～20%[61]。ZG9附近的多口井H2S含量大于5%，如ZG6(7.8%)、ZG501(6.98%)，ZG432(7.45%)[61]。一般认为H2S含量大于5%只能由TSR产生[42,47]。Caietal.[61]认为ZG9井储层沥青为TSR蚀变的产物。白云石和方解石均一温度(Th)指示矿物形成于高温环境(图4)。白云岩孔洞中充填的这些方解石碳同位素接近-8‰，具有明显的有机碳来源(图5)。同时具有高均一温度(Th)和低碳同位素值表明这些方解石由TSR形成的[3,52,61,63]。方解石的盐度较高，均大于16%，说明形成于深埋藏环境。因此，可以排除土壤有机碳源造成的碳同位素低值的可能性。

4 热化学硫酸盐还原作用对储层的改造作用

塔中地区碳酸盐岩有效储层的分布主要受控于高能沉积环境(如礁滩相)、表生溶蚀，热液活动和断裂活动也起到非常重要作用[21,28-29,60,64-65]。虽然与TSR相关的溶蚀作用不可能是塔中地区大规模储层形成的主要机制，但是在局部地区，TSR可能导致孔隙增加或重新分配。如ZG9井鹰山组白云岩段，其储层物性比邻井(如ZG203、ZG7井)同层位的灰岩段好得多。下文将深入剖析ZG9井白云岩溶蚀机理。

图3 TSR反应中包含的各种成岩矿物(引自Jia et al.[63])A.粗晶TSR方解石与黄铁矿共生，TZ3井，O1-2y，3 907.7 m，反射光；B.白云岩溶孔中充填元素硫，TZ243井，O1p，5 718.3 m；C.灰岩围岩孔洞中充填晶形很好的硬石膏，TZ162井，O1-2y，5 252.4 m，单偏光；D.TSR方解石交代硬石膏，TZ3井，O1-2y，3 907.7 m，正交偏光。Fig.3 Photos showing different types of diagenetic minerals involved in TSR reaction

图4 ZG9井鹰山组白云石围岩、孔洞充填方解石均一温度、盐度Fig.4 Histograms showing homogenization temperatures and salinity measured from two-phase aqueous inclusions of calcite and dolomite from the Ordovician Yingshan Formation in Well ZG9

图5 ZG9井鹰山组白云岩围岩、孔洞充填方解石碳氧同位素值对比图Fig.5 Cross-plot of stable carbon and oxygen isotopic compositions for dolomite and calcite from the Ordovician Yingshan Formation in Well ZG9

ZG9井第一筒次和第三筒次取芯段均为致密灰岩，物性很差(图6A，C)；第二筒次6 260.66～6 268.21 m段白云岩岩芯上发育白云石被溶蚀而形成的不规则孔洞，部分孔洞直径到达2～4 cm(图6B)，最大孔隙度高达27%(图7)。镜下观察晶间溶孔发育，白云石溶解导致晶体边缘不规则呈港湾状(图8A，B)。从扫描电镜的背散射图上能够清晰看到，固体沥青孤立分布于孔隙中央，晶形极好的白云石被溶解，说明白云石溶蚀发生在油气充注后(图8C，D)。溶孔内经常充填晚期方解石(图8B)，却未见晚期方解石溶解。在灰岩中并未见到溶孔。ZG9井的白云岩被认为是热液白云岩化形成的[66]，本次研究发现白云石均一温度为96.1℃～121.6℃(图4)，也证实白云石形成于深埋藏环境。灰岩段与白云岩段储层物性差异明显，显然不能用抗压实作用的差异来解释，倒退溶解也解释不了。我们认为这一差异是与沉积时期水动力条件、原始孔隙大小有关。无论是白云岩段、还是灰岩段，在白云岩化之前都是灰岩，但是，两者晶粒大小不同、原始孔隙度差异，导致后期深部富Mg2+的热卤水优先流入相对多孔的灰岩，发生了热液白云岩化作用。而致密的灰岩则因热流体无法进入，没有发生白云岩化作用。热液白云岩化发生，表明溶液对白云石是超饱和的，对方解石是不饱和的，于是，发生方解石的溶解作用、白云石对方解石的交代作用，而不可能同时发生白云石的溶解作用。白云岩段白云石溶解作用，可能的机理包括，①中奥陶世—晚奥陶世期间地层抬升，发生风化壳岩溶作用；②热化学硫酸盐还原作用。前人研究认为，热液白云岩化作用与二叠纪热液活动有关[66]。因此，在中奥陶世—晚奥陶世期间地层抬升之时，鹰山组尚未发生热液白云岩化作用，表明白云石溶解不可能是风化壳岩溶的产物，相反，应该发生在深埋环境中。

在寒武系地层中也发现了TSR改造白云岩储层的现象。例如TZ75井。之前的工作已经讨论过TZ75等井上寒武统白云岩储层中发生TSR[59]。发生TSR的井段岩芯上发现大量白云岩的溶孔，薄片观察表明储层中的部分孔隙由晚期白云石和晚期方解石溶蚀形成的(图2A)。晚期高温方解石和白云石溶蚀孔隙的存在提供了埋藏溶蚀作用的溶蚀效应的证据。岩石学观察表明，TZ75井多孔白云岩储层常与丰富的黄铁矿、硬石膏和TSR方解石共生[59]。深埋溶蚀孔隙与TSR反应物和生成物的密切共生关系，表明溶蚀作用很可能与TSR有关。如图9所示，塔中75井岩芯孔隙度结果显示，发生TSR的上寒武统下部的孔隙度明显高于之上的各个地层，而未发生TSR的上寒武统上部、蓬莱坝组与良里塔格组孔隙度相近。类似地，ZS1C井6 912～6 932 m白云岩储层为气层，硫化氢含量高达11.4%，孔隙度为6.5%，储层段上下的地层孔隙度都在1%以下。这些现象说明TSR作用可能一定程度上增加了白云岩的孔隙度。

图6 ZG9井岩芯照片A.第一筒次，灰色泥晶灰岩，取芯井段6 241～6 249.95 m，十分致密；B.第二筒次，浅灰色白云岩，取芯井段6 260.66～6 268.21 m，孔洞极为发育；C. 灰色泥晶灰岩，取芯井段6 424～6 431 m，十分致密。Fig.6 Photos of core samples in Well ZG9

图7 ZG9井取芯井段孔隙度变化Fig.7 Porosity variations in the cored interval in Well ZG9

图8 白云石埋藏溶蚀(ZG9井，6 265 m，O1-2y; B和D引自Jia et al.[63])A.白云石被大量溶蚀；B.白云石溶孔中充填方解石(被茜素红染为红色)C.沥青充填在孔隙中心，说明油气充注后，白云石还发生了一次溶蚀；D.晶形极好的白云石被溶解。Fig.8 Photomicrographs showing burial dolomite dissolutions in Well ZG9

图9 TZ75井岩芯孔隙度与深度关系图Fig.9 Core porosity changes in TSR and non-TSR interval in Well TZ75

5 结论

(1) 塔中地区寒武系—奥陶系发生埋藏溶蚀作用，识别标志包括：高温或含烃类包裹体的晚期成岩矿物被溶蚀或切穿，沿成岩晚期的缝合线或晚期裂缝产生溶蚀孔隙。

(2) TSR溶蚀的机制主要为水溶硬石膏、白云石与油气烃类反应。这种机制在塔中深部的寒武系储层中可能广泛存在，有利于深埋优质白云岩储层的形成。

(3) 塔中地区寒武系—奥陶系发现大量硬石膏、重晶石、黄铁矿、方解石、沥青和硫化氢，岩矿、均一温度及同位素证据说明发生TSR。

(4) TSR可以促进深埋碳酸盐岩的溶解，导致孔隙增加或重新分配。在深埋条件下，TSR对白云岩的改造作用强于灰岩，因此更易成为优质储层。

致谢在论文撰写过程中受到了陈代钊等专家的亲切指导，在此一并感谢。

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Thermochemical Sulfate Reduction-related Mesogenetic Dissolution of Deeply Buried Dolostone Reservoirs in the Tazhong Area

JIA LianQi1CAI ChunFang1,2LI HongXia1WANG TianKai1ZHANG Wen3KONG LingWu4

(1. Key Laboratory of Petroleum Resources Research, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China; 2. Department of Geochemistry, Yangtze University, Wuhan 430100, China; 3. Research Institute of Petroleum Exploration and Development, Tarim Oilfield Company, PetroChina, Korla, Xinjaing 841000, China; 4. CNOOC Research Institute, Beijing 100028, China)

Deeply buried (4 500 to 7 000 m) carbonate reservoirs in the Tazhong area, Tarim basin, NW China show obvious heterogeneity with porosity from zero in limestones to 27.8% in sour dolostones. However, origin of the porosity remains puzzled. Petrographic features, C, O isotopes were determined, and fluid inclusions were analyzed on diagenetic calcite and dolomite from Ordovician reservoirs to solve the puzzle. Ordovician carbonate reservoirs in the Tazhong area are controlled mainly by initial sedimentary environments and primary and near-surface diagenetic processes. However, vugs and pores generated from eogenetic and telogenetic meteoric dissolution were observed to have partially been destroyed due to subsequent compaction, filling and cementation. In some localities or wells (e.g. well ZG9 and TZ75), mesogenetic dissolution (e.g. thermochemical sulfate reduction, TSR) probably played an important role in pore production and reservoir quality improvement. The occurrence of TSR within Ordovician carbonate reservoirs are supported by TSR-originated calcite replacement of sulphate, and the association of sulfur species including acid gases (H2S), pyrite, anhydrite or barite and elemental sulfur with hydrocarbon and12C-rich calcite with elevated homogenization temperatures. The TSR is observed to company with intensive dolomite dissolution, suggesting that TSR may have induced burial dissolution and thus probably increased the porosity of the sour dolostones reservoirs at least in some places. In contrast, no significant mesogenetic dissolution occurred in limestone reservoirs. The deeply buried sour dolostone reservoirs may therefore be potential exploration targets in Tarim basin.

mesogenetic dissolution; thermochemical sulfate reduction; Cambrian; Ordovician; Tazhong area

1000-0550(2016)06-1057-11

10.14027/j.cnki.cjxb.2016.06.005

2016-05-20；收修改稿日期： 2016-08-17

国家科技重大专项课题(2011ZX05008-003)；国家自然基金项目(41125009)[Foundation: National Science and Technology Major Project, No. 2011ZX05008-003; National Natural Science Foundation of China, No. 41125009]

贾连奇男 1985年出生博士后油气储层地质 E-mail: jlq@gmail.iggcas.ac.cn

蔡春芳男研究员 E-mail:cai_cf@mail.iggcas.ac.cn

TE122.2