柴西北区新近系湖相细粒沉积岩油气富集条件

张 旭1,2,3,刘成林1,郭泽清4,桂和荣2,田继先4,吴小平5,洪唯宇1,班东师1,徐思渊1,张 蔚1,张智辉1

(1.中国石油大学(北京) 地球科学学院,北京 102249; 2.宿州学院 国家煤矿水害防治工程技术研究中心,安徽 宿州 234000; 3.安徽省勘查技术院,安徽 合肥 230041; 4.中国石油勘探开发研究院廊坊分院,河北 廊坊 065007; 5.中国科学技术大学 地球和空间科学学院,安徽 合肥 230026)

摘 要:柴西北区是柴达木盆地重要的油气资源接替区,新近系湖相细粒沉积岩沉积厚度大,有较好的非常规油气资源潜力。然而,由于咸化沉积环境、粒度小、岩石致密等条件的限制,细粒沉积岩成烃与成储机制复杂,油气富集条件尚不明确,勘探程度较低。以柴西北区新近系主力烃源岩上干柴沟组为研究对象,采集19口代表性钻井岩芯样品,分析样品有机质丰度、类型和成熟度,结合X衍射、岩石薄片、高压压汞、低温液氮吸附实验和扫描电镜,系统分析了其烃源岩特征、矿物组成、储集条件及影响因素,并优选出有利勘探区。研究表明:柴西北区上干柴沟组细粒沉积岩厚度普遍>500 m;烃源岩有机碳含量不高,有57%样品TOC含量<0.4%,20%样品TOC含量高于0.6%,少量样品TOC含量超过1.0%;有机质以Ⅱ1型和Ⅲ型为主,少量Ⅰ型有机质;样品Ro为0.54%~1.24%,处于低成熟—成熟阶段。样品非均质性强,孔隙结构复杂,介孔较发育,介孔孔隙体积增量为0.001 0~0.015 2 cm3/g,平均为0.007 5 cm3/g。样品中氯仿沥青“A”含量与TOC含量、介孔体积均呈正相关性,与Tmax,BET比表面积及宏孔体积均无明显相关性。样品孔隙体积受到TOC含量、矿物含量及热成熟度控制。综合细粒沉积岩烃源岩及储集条件,初步优选干柴沟—油泉子—南翼山地区为柴西北区上干柴沟组致密油/页岩油勘探有利区。

关键词:致密油/页岩油;细粒沉积岩;柴西北区;孔隙结构;咸化湖盆

国内外对细粒沉积物一直有相关研究,但究其具体定义还比较模糊,一般是指岩石粒径<62.5 μm的黏土、粉砂级沉积物,主要包括黏土矿物、粉砂、碳酸盐、有机质等[1-3]。随着页岩油气、致密油气等非常规油气资源研究的迅速发展,中国学者针对细粒沉积岩油气的研究也在不断深入[4-10]。近年来,国内研究多集中在泥页岩的沉积演化、空间展布、生烃条件、储层物性特征、含气特征、吸附机理、成藏规律研究,并且在南方扬子地区古生代海相地层中已经取得了页岩气勘探突破,在海陆过渡相泥页岩中也逐步深入研究[10-16]。随着勘探的不断进行,中国陆相盆地泥页岩热成熟度相对海相泥页岩而言较低,也表现出了良好的油气资源潜力[17-21]。中国新生界咸化湖盆沉积研究认为具有高的初始生成率和良好的聚集、保存条件,烃源岩中的有机质含量中等—较高、干酪根类型好和有机质向烃转化率高,有利于油气生成,具有形成大型油田的条件[22]。朱德燕等[23]在济阳坳陷新生界沙河街组研究时发现在咸水阶段发育的纹层状(层状)泥质灰岩/灰质泥岩具有生烃能力强、储集性好、含油性高、可动性好的特征,是页岩油气富集的甜点岩性。张林晔等[24]在研究东营凹陷新生界沙河街组咸化湖相泥页岩中发现:在生烃与溶蚀叠合作用下形成的丰富的有机质-矿物混合体内储集空间对陆相页岩油气赋存具有重要意义。

柴达木盆地是中国典型的中新生代陆相盆地,目前所发现的油气资源主要分布在柴北缘(侏罗系)、三湖坳陷(第四系)及柴西地区(古近系—新近系)[25-26]。柴西地区在古近纪—新近纪为咸化湖盆沉积,其烃源岩形成、生烃演化及油气藏形成均与咸化湖盆的演化有着紧密联系[27-29]。其新近纪沉积形成的细粒沉积岩独具特点:咸化湖盆沉积、有机质丰度不高、热演化程度不高、有机质转换率较高[30-31],而生烃转化率较高的烃源岩是中国陆相致密油优质烃源岩发育的一种主要类型[32]。柴西地区新近系细粒沉积岩(泥页岩、泥质粉砂岩等)主要发育在柴西北区,厚度大,分布范围广,具有较好的非常规油气资源潜力[33-35]。然而,由于岩石致密、粒度小及油气理论认识等因素的限制,在前人研究中多将细粒沉积岩作为常规油气藏的烃源岩或盖层来单独研究,针对细粒沉积岩中赋存的非常规油气资源勘探程度较低,缺乏系统性研究。随着非常规油气地质理论的不断深入,在研究区的南翼山、开特米里克、油泉子、小梁山等多个油气田和含油气构造不断被发现之后[35-36],柴西北区作为柴达木盆地油气资源的重要接替区,对研究区细粒沉积岩的油气地质条件研究越来越受关注[28]。已有研究表明上干柴沟组是柴西地区的主力生油岩,厚度大,有机质丰度总体不高,成熟度不高,部分已经进入生烃高峰,有机质生烃转化率高,并且油藏可能聚集在非构造圈闭中,具有较好的页岩(致密)油气资源潜力[31,35,37-40]。因此,急切需要对研究区上干柴沟组细粒沉积岩的油气富集条件进行系统研究,为后期勘探开发研究提供参考依据。

1 地质概况

柴达木盆地位于青海省青藏高原北部,盆内海拔为2 600~3 000 m,周边为阿尔金山、祁连山和昆仑山所环绕,大致呈北西西向不规则的三角形盆地,是青藏高原地区面积最大的陆相中新生代含油气盆地[25,36]。复杂的地质演化过程对柴达木盆地构造、沉积及油气成藏具有重要的控制作用[41],在新近纪—第四纪主要为挤压褶皱、坳陷阶段[42-44]。柴达木古湖盆在古近纪—新近纪逐渐由西往东迁移,经历了“发生—发展—消亡”三大演化阶段[30]。柴西北区分布于狮子沟—大乌斯(南乌斯和北乌斯)构造带以北[35],包括茫崖凹陷、大风山凸起2个二级构造单元(图1)。研究区在新近纪发育地层包括:上干柴沟组(N1)、下油砂山组上油砂山组及狮子沟组均发育有厚度较大的泥岩、粉砂岩、泥质粉砂岩、碳酸盐岩等细粒沉积岩,最大厚度可达2 000 m(图1)。

图1 柴达木盆地西部地区构造单元划分图及地层综合柱状[25,45-46]
Fig.1 Tectonic units and comprehensive stratigraphic column of the western Qaidam Basin[25,45-46]

2 样品测试分析

本次采集研究区柴7井、南8井、油6井等(图1)新近系上干柴沟组泥岩、粉砂质泥岩和泥质粉砂岩岩芯样品,分析了其有机质丰度、类型和成熟度、矿物组成等特征,并对12块样品进行高压压汞、低温液氮吸附、扫描电镜等系列分析测试。

本次样品TOC含量、有机质类型和热解实验均在中国石油大学(北京)分别按照《沉积岩中总有机碳的测定》(GB/T 19145—2003)、《陆相烃源岩地球化学评价方法》(SY/T 5735—1995)和《岩石热解分析》(GB/T 18602—2012)进行测定。由于岩石颗粒细小,发育孔裂隙多为微纳米级,因此选择高压压汞法、液氮吸附法和扫描电镜来系统观测样品中孔隙结构及其分布特征。压汞实验和液氮吸附实验在北京市理化分析测试中心进行,按照国家标准GB/T 21650.1—2008和GB/T 21650.2—2008分别用POREMASTER GT60压汞仪和NOVA 4200e全自动比表面积及孔径分布分析仪进行测试。由于不同方法测试精度有差异,常通过压汞法和液氮吸附法对孔隙体积进行联合表征[47-48],文中孔径>50 nm的宏孔体积主要采用压汞法测试数据,2~50 nm孔径的介孔体积为液氮吸附法BJH模型测定结果,孔隙比表面积为液氮吸附法BET模型测定结果。扫描电镜观测在中国石油大学(北京)进行,观测仪器型号为Quanta 200F。

3 细粒沉积岩特征

3.1 矿物岩石学特征

柴西北区上干柴沟组发育的细粒沉积岩主要为浅灰色、灰色、深灰色泥岩、粉砂质泥岩、泥质粉砂岩、粉砂岩等,块状构造,岩石致密(图2(a),(d),(g),(i))。岩石中矿物颗粒细小,光学显微镜下观察主要为混合状和纹层状,矿物类型主要为石英、长石、黏土矿物等(图2(b),(c),(e),(f),(k),(l))。纹层状粉砂质泥岩为砂质纹层与泥质纹层互层,泥质纹层中见有机质分布,砂质纹层中见云母、石英等矿物定向排列(图2(e),(f)),泥岩中有机质分布较多,砂质颗粒相对含量少且粒度更小(图2(h),(i))。对研究区13口钻井共49块样品进行X衍射分析,结果表明上干柴沟组细粒沉积岩主要发育石英、长石、方解石、白云石、铁白云石、黄铁矿、黏土矿物及部分盐类等矿物。不同样品中矿物含量差别较大,表现出强非均质性。柴7、柴8、风3、风4、碱1、油8、油6、油南1井样品石英和长石含量较高,多大于40%;而咸8、梁3、油14及南8井样品石英含量均较低,黏土矿物含量较高,多大于20%(表1,图3)。

(1)油6井,2 401 m,泥质粉砂岩((a)为岩芯宏观特征;(b)为正交偏光,有机质颗粒分布较多,平均粒度30 μm,颗粒分选好,磨圆较差;
(c)为相应单偏光下特征);(2)南8井,2 620 m,纹层状粉砂质泥岩((d)为岩芯宏观特征;(e)为正交偏光,粉砂质与泥质互层,颗粒分选、磨
圆较好,可见有机质颗粒分布,粉砂质层中见云母碎片顺层分布;(f)为相应单偏光下特征);(3)南8井,2 609 m,泥岩((g)为岩芯宏观特征;
(h)为泥质均匀分布,可见较多有机质分布,见少量云母碎片;(i)为相应单偏光下特征);(4)咸8井,2 765 m,泥质粉砂岩((j)为岩芯宏观特征;
(k)为泥质与粉砂不均匀分布,颗粒分选较好,磨圆较差;(l)为相应单片光下特征)
图2 柴西北区上干柴沟组细粒沉积岩岩石及矿物特征
Fig.2 Petrology and mineral of the Shangganchaigou fine-grained sedimentary rocks in northwestern Qaidam basin

表1 柴西北区上干柴沟组细粒沉积岩综合参数
Table 1 Comprehensive parameters of the Shangganchaigou fine-grained sedimentary rocks in northwestern Qaidam Basin

样号层位井号深度/m孔隙体积/(cm3·g-1)宏孔(压汞法)介孔(N2吸附法,BJH)宏孔+介孔比表面积(BET)/(m2·g-1)BJH平均孔径/nmTOC含量/%氯仿沥青“A”/%S1含量/(mg·g-1)Tmax/℃矿物含量/%石英+长石+黄铁矿黏土矿物碳酸盐岩1南83 066.500.001 00.008 10.009 12.649 612.979 20.340.029 0 0.111 8 44457.814.014.42南82 620.150.004 40.007 90.012 31.923 721.430 00.68 0.129 0 0.580 9 44443.35.933.93南82 609.300.001 30.012 20.013 53.475 315.792 10.390.026 5 0.103 2 43943.09.432.64油82 553.200.004 00.006 50.010 52.929 310.247 90.330.011 90.200 743476.17.612.85油82 548.850.001 60.015 20.016 85.137 910.481 40.87 0.125 8 0.601 2 44157.38.117.46上干油82 542.920.002 10.013 70.015 85.331 710.608 90.310.022 2 0.179 8 43742.924.316.57柴沟油82 128.970.000 30.006 60.006 92.428 512.183 40.510.037 0 0.101 7 44641.113.325.28组N1开24 311.310.001 00.001 00.002 00.452 311.678 40.13 0.006 1 0.020 0 45925.54.557.79风34 286.800.009 60.001 60.011 20.535 923.520 60.15 0.009 0 0.240 0 46572.04.54.110梁33 381.130.012 30.005 90.018 21.698 717.219 30.26 0.052 8 0.054 1 42060.68.120.511柴72 473.400.001 30.006 60.007 96.377 45.754 70.68 0.019 3 0.003 1 42666.04.919.212柴72 752.060.006 60.004 50.011 11.555 615.421 10.30 0.004 90.008 3 43972.04.914.2平均0.003 80.007 50.011 32.874 713.943 10.410.039 50.183 7————

图3 柴西北区上干柴沟组细粒沉积岩矿物组成
Fig.3 Mineral composition of the Shangganchaigou fine-grained sedimentary rocks in northwestern Qaidam Basin

3.2 有机地球化学特征

对咸7井等岩芯样品分析表明,有机碳含量总体不高,有57%样品TOC含量<0.4%,有20%样品(柴7、油6、南14等)TOC含量高于0.6%,少量样品TOC含量超过1.0%(图4)。有机质类型以Ⅱ1型和Ⅲ型为主,含有少量Ⅰ型有机质(图5)。镜质体反射率测定表明,Ro为0.54%~1.24%,样品多处于低成熟—成熟阶段,埋深超过3 500 m时基本达到成熟阶段。南翼山、油泉子、开特米里克地区上干柴沟组烃源岩成熟度相对较高,多处于成熟阶段;小梁山、咸水泉地区上干柴沟组烃源岩成熟度相对较低,处于低成熟阶段(图6)。

图4 柴西北区上干柴沟组细粒沉积岩有机碳(TOC)含量分布
Fig.4 Distribution histogram of TOC contents of the Shangganchaigou fine-grained sedimentary rocks in northwestern Qaidam Basin

图5 柴西北区上干柴沟组细粒沉积岩有机质类型分布
Fig.5 Organic matter types of the Shangganchaigou fine-grained sedimentary rocks in northwestern Qaidam Basin

图6 柴西北区上干柴沟组烃源岩有机质成熟度分布
Fig.6 Vitrinite reflectance of the Shangganchaigou fine-grained sedimentary rocks in northwestern Qaidam Basin

3.3 储集空间特征

良好的孔裂隙系统是影响细粒沉积岩非常规油气储集条件的主要因素,是细粒沉积岩中油气赋存富集的关键条件[4,8,13-14,18-20,40]

3.3.1 孔隙分布特征

选择柴7井等钻井共12块岩芯样品进行压汞实验,其压汞曲线表现3种类型:前峰型(>100 nm的孔隙优势发育)、后峰型(<100 nm的孔隙优势发育)和多峰型。风3、开2、柴7、南8井部分样品主要发育>100 nm的孔隙,<100 nm的孔隙发育较少(图7(a))。油8、梁3、南8井部分样品在<100 nm孔隙出现高峰,而>100 nm孔径孔隙贡献孔隙体积较少(图7(b))。油8、柴7井部分样品阶段进汞曲线出现多个峰值(图7(c))。

图7 柴西北区上干柴沟组细粒沉积岩孔隙分布特征
(高压压汞法)
Fig.7 Pore size distribution of the Shangganchaigou fine-grained sedimentary rocks in northwestern Qaidam Basin using high pressure mercury intrusion

液氮吸附实验所测定样品中孔隙分布特征与压汞法测定结果具有很好的对应,南8、油8井样品中在100 nm处出现高峰且孔隙体积较大(图8(a),(c))。南8井样品吸附-脱附曲线滞后环均很小,表现为曲线平缓,到饱和蒸汽压附近变陡(图8(b)),表明孔隙多为开放孔,主要为两端均开口的平行板状孔。油8、柴7(2 473.40 m)井样品吸附-脱附曲线出现滞后环,表明孔隙多为半开放型孔隙,并且吸附曲线在相对压力接近0.5时有明显拐点,表明样品中存在一定量的细颈瓶孔(图8(d),(f))。而开2、风3、柴7(2 752.06 m)样品中<100 nm孔径的孔隙较不发育(图8(e)),吸附-脱附曲线滞后环均很小,主要为两端均开口的平行板状孔(图8(f))。样品的吸附曲线上升速率与其开放孔的开放程度有着密切关系,吸附曲线越陡说明孔隙的开放程度越大,在相对压力接近1时,均未达到饱和吸附,表明发生毛细凝结。

图8 柴西北区上干柴沟组细粒沉积岩孔隙分布(BJH)和吸附—脱附曲线(N2吸附法)
Fig.8 Pore size distribution and adsorption-desorption curves of the Shangganchaigou fine-grained sedimentary rocks in northwestern Qaidam Basin obtained from low temperature liquid nitrogen adsorption using BJH model

总体上,研究区上干柴沟组细粒沉积岩宏孔体积增量为0.000 3~0.012 3 cm3/g,平均为0.003 8 cm3/g;介孔孔隙体积增量为0.001 0~0.015 2 cm3/g,平均为0.007 5 cm3/g,介孔较发育。孔隙BET比表面积为0.452 3~6.377 4 m2/g,平均为2.874 7 m2/g。在BJH模型中样品孔隙平均孔径分布范围为5.754 7~23.520 6 nm,平均孔径为13.943 1 nm(表1)。

3.3.2 孔隙形貌特征

扫描电镜下观测孔隙形貌特征与压汞、液氮吸附测定结果表现出较为一致规律。柴7井(2 752.06 m)样品中发育大量粒间孔、粒内孔和黏土矿物晶间孔,多为纳米级平行板状孔,宏孔、介孔均较为发育,并且孔隙开放性好(图9(a)~(c))。南8井样品中粒间孔和粒内孔均有发育,粒内孔隙复杂,开放性好(图9(d)~(f))。风3井(4 286.80 m)样品表面黏土矿物、石英、碳酸盐矿物堆积,发育大量粒间孔,黏土矿物晶间孔发育,多为微纳米孔隙(图9(g)~(i))。油8井(2 548.85 m)样品表面可见大量片状盐类矿物,部分孔隙被盐类矿物覆盖,宏孔体积较小,但在盐类矿物间隙矿物堆积发育大量纳米孔隙,可以提供较大比表面积和介孔体积(图9(j)~(l))。

总体上,柴7(2 752.06 m)、南8井、油8井、梁3井均有样品孔隙体积较大,开放性好;风3井样品孔隙孔径大,比表面积较小,开2井样品孔隙体积和比表面积较小。

(a)~(c)柴7井,2 752.06 m,泥质粉砂岩((a)放大2 000倍,发育粒间孔、粒内孔;(b)放大15 000倍,大量黏土矿物晶间孔,孔隙为平行板状孔;
(c)图(b)相应放大特征,放大30 000倍);(d)~(f)南8井,2 620.15 m,泥岩((d)放大2 000倍,发育有粒间孔;(e)放大3 000倍,发育有较多粒
间孔;(f)放大10 000倍,粒内孔发育,孔隙复杂);(g)~(i)风3井,4 286.80 m,粉砂岩((g)放大2 500倍,黏土矿物晶间孔发育;(h)放大
5 000倍,石英颗粒大量粒间孔隙;(i)放大4 000倍,石英颗粒堆积,大量粒间孔发育);(j)~(l)油8井,2 548.85 m,粉砂质泥岩((j)放大
2 000倍,大量片状盐类覆盖,粒间孔隙发育;(k)放大3 000倍,盐类与石英颗粒等粒间孔发育;(l)k相应放大特征,放大8 000倍)
图9 柴西北区上干柴沟组细粒沉积岩扫描电镜观测
Fig.9 Morphology of the Shangganchaigou fine-grained sedimentary rocks in northwestern Qaidam Basin by using scanning electronic microscope

4 讨 论

4.1 含油量影响因素分析

前人研究认为氯仿沥青“A”含量和热解烃量(S1)均可以反映陆相页岩含油量大小,并且与烃源岩有机质含量、成熟度及储集空间等有关[49-50]。综合考虑以上影响因素,对研究区上干柴沟组细粒沉积岩中氯仿沥青“A”含量分析发现,样品TOC含量、宏孔+介孔总体积以及介孔体积均与其氯仿沥青“A”含量具有一定的线性正相关性,而表征样品热成熟度的Tmax值、孔隙BET比表面积和宏孔体积均与氯仿沥青“A”含量无明显相关性(图10),样品的氯仿沥青“A”含量主要与其TOC含量和介孔体积有关。

图10 柴西北区上干柴沟组细粒沉积岩含油性影响因素分析
Fig.10 Analysis of the influence factors of oil bearing of the Shangganchaigou fine-grained sedimentary rocks in northwestern Qaidam Basin

结合表1和图10进一步分析,2号和5号样品具有较高的TOC含量,并且达到成熟阶段,有较好的生烃物质基础,并且宏孔+介孔总体积较大,介孔为孔隙体积主要贡献者,具有较高的氯仿沥青“A”含量。虽然11号样品也具有相对高含量TOC含量,但处于未成熟阶段,孔隙发育较差,孔隙体积小,可以提供烃类储集的空间有限,因此氯仿沥青“A”含量不高。尽管3号和6号样品介孔体积大,能够提供较多的储集空间,但有机碳含量较低,处于低成熟阶段,生烃物质相对较少,氯仿沥青“A”含量也不高。8号、9号和12号样品虽然达到高成熟阶段,但TOC含量低,介孔体积小,因此氯仿沥青“A”含量很低。因此,含油量较高的样品既要有较高的TOC含量,达到低—成熟阶段,能够较好的提供生烃基础,也要有较好的储集空间发育,介孔体积贡献相对较大。

对样品孔隙影响因素研究发现其TOC含量与宏孔体积有一定负相关性,而与介孔体积有一定正相关性(图11(a))。样品的成熟度(Tmax)与其宏孔、介孔体积均没有明显线性相关性,但在低—成熟阶段介孔体积出现高值(图11(b))。石英+长石+黄铁矿含量与样品宏孔体积有一定正相关性,该含量大于60%的样品具有较大宏孔体积,介孔体积也较发育(图11(c)),这与石英、长石等矿物颗粒聚集易形成大量的粒间孔、粒内孔隙有关(图9(h),(i))。黏土矿物含量与样品宏孔体积有一定线性负相关性,与介孔体积有线性正相关性(图11(d)),这与黏土矿物层间易形成大量微纳米级孔隙有关(图9(g),高黏土矿物含量样品中介孔较发育。碳酸盐岩矿物含量与其宏孔、介孔体积均没有明显相关性(图11(e))。因此,研究区样品孔隙体积发育与TOC含量、矿物含量和热成熟度均有关系。

图11 柴西北区上干柴沟组细粒沉积岩孔隙体积与影响因素关系
Fig.11 Analysis of the influence factors of pore volume of the Shangganchaigou fine-grained sedimentary rocks in northwestern Qaidam Basin

图12 柴西北区上干柴沟组细粒沉积岩油气条件综合评价
Fig.12 Comprehensive evaluation of hydrocarbon accumulation conditions of the Shangganchaigou fine-grained sedimentary rocks in northwestern Qaidam Basin

4.2 有利勘探区初步优选

在渐新世至中新世中期(E32—N12),柴达木古湖盆水体变深,湖面扩大,广泛发育湖相地层[30],柴西北区上干柴沟组沉积中心分布在茫崖凹陷。细粒沉积岩形成的油气藏受到烃源岩厚度、有机碳含量、成熟度、储集空间等众多因素的影响[2-4,8-9,17-21,47-48]。金强等(2001)对柴西地区上、下干柴沟组生油岩进行分析,确定出有效生油岩的有机碳含量下限为0.4%[51]。根据钻井资料统计,柴西北区上干柴沟组细粒沉积岩厚度普遍>500 m,在油泉子、开特米里克地区可达900余米,由此往外厚度逐渐减小。有机碳含量由西往东逐渐减少,在油泉子、干柴沟、南翼山等地区TOC含量较高,平均含量大于0.6%。茫崖凹陷内细粒沉积岩总体处于低成熟—成熟阶段,在开特米里克和鄂博山附近成熟度可达到1.0%~1.3%。南8、油8、柴7、梁3井测定孔隙体积及比表面积较大,开2和风3井样品孔隙体积及比表面积较小。因此,综合细粒沉积岩厚度、TOC含量、成熟度及储集空间特征,干柴沟-油泉子-南翼山地区上干柴沟组细粒沉积岩厚度大、有机质演化成熟、TOC含量较高、储集空间发育(图12),初步优选为下一步非常规油气有利勘探区。

5 结 论

(1)柴西北区上干柴沟组细粒沉积岩厚度普遍大于500 m,在油泉子、开特米里克地区可达900余m,由此往外厚度逐渐减小;有机碳含量不高,TOC含量>0.6%的烃源岩主要分布在干柴沟、油泉子和南翼山等地区;有机质以Ⅱ1-Ⅲ型为主,少量Ⅰ型有机质;烃源岩多处于低成熟—成熟阶段。

(2)柴西北区上干柴沟组细粒沉积岩非均质性强,介孔较为发育。样品氯仿沥青“A”含量与TOC含量、介孔体积均有正相关性,与Tmax,BET比表面积及宏孔体积均无明显相关性;孔隙体积受到TOC含量、矿物含量及热成熟度控制。

(3)干柴沟—油泉子—南翼山地区上干柴沟组细粒沉积岩发育厚度大、有机质演化成熟、TOC含量较高、储集空间发育,可以作为柴西北区致密油/页岩油下一步勘探有利区。

致谢 本文得到了中国石油勘探开发研究院廊坊分院和青海油田勘探开发研究院的领导、专家的支持和帮助,在此一并表示感谢。

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Hydrocarbon accumulation conditions of neogene lacustrine fine-grained sedimentary rocks in northwestern Qaidam Basin

ZHANG Xu1,2,3,LIU Chenglin1,GUO Zeqing4,GUI Herong2,TIAN Jixian4,WU Xiaoping5,HONG Weiyu1,BAN Dongshi1,XU Siyuan1,ZHANG Wei1,ZHANG Zhihui1

(1.College of Geosciences,China University of Petroleum,Beijing 102249,China; 2.National Engineering Research Center of Coal Mine Water Hazard Controlling,Suzhou University,Suzhou 234000,China; 3.Geological Exploration Technology Institute of Anhui Province,Hefei 230041,China; 4.Research Institute of Petroleum Exploration and Development,PetroChina,Langfang 065007,China; 5.School of Earth and Space Sciences,University of Science and Technology of China,Hefei 230026,China)

Abstract:Thick Neogene fine-grained sedimentary rocks were developed in the northwestern Qaidam Basin,where is an important replacement area for oil and gas resource exploration in the Qaidam Basin and has a tremendous unconventional oil and gas resource potential.However,limited by the salinized depositional environment,poor reservoir properties,and the largely unknown enrichment mechanisms of unconventional oil resource,the prospections on the fine-grained sedimentary rocks in northwestern Qaidam Basin are still situated at the initial stage.This research focused on the Shangganchaigou Formation of Neogene,that is a primary hydrocarbon source rock in the northwestern Qaidam Basin.Core samples from 19 representative drilling wells were collected to analyze the abundance,type,and maturity of organic matter.Based on X-ray diffraction,rock thin section,high-pressure mercury intrusion,low-temperature liquid nitrogen adsorption,and SEM (scanning electron microscopy) data,the hydrocarbon source rock characteristics,mineral composition,accumulation conditions and their controlling factors in the fine-grained rocks of the Shangganchagou Formation was systematically investigated.Moreover,this research also identified the favorable horizons for hydrocarbon exploration.The result shows the thickness of the fine-grained rocks of the Shangganchaigou Formation in the study area is generally more than 500 m.Relatively low TOC contents present in the all studied samples,with 57% samples are less than 0.4%,20% samples are more than 0.6%,and a few more than 1.0%.Organic matter in these rocks samples is mainly type Ⅱ1,Ⅲ and few type Ⅰ.The vitrinite reflectance (Ro) is ranging from 0.54%-1.24% exhibiting thermally low mature to mature stage.Pores with complex structure and strong heterogeneity are developed in the samples.The incremental pore volume of mesopores is 0.001 0-0.015 2 cm3/g,and the mean mesopore volume is 0.007 5 cm3/g.The chloroform bitumen “A” content of the samples has a positive relationship with TOC content and mesopore volume,while it has no obvious correlation with Tmax,BET surface area,and macropore volume.The pore volume of the samples is controlled by TOC content,mineral composition,and the thermal maturity.Based on the comprehensive evaluation of hydrocarbon generation and accumulation conditions of the Shanggaichaigou fine-grained rocks,it is found that there is great tight oil/shale oil potential in the Ganchaigou-Youquanzi-Nanyishan area in the northwestern Qaidam Basin.

Key words:tight oil/shale oil;fine-grained sedimentary rocks;Northwestern Qaidam Basin;pore structure;saline lacustrine facies

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张旭,刘成林,郭泽清,等.柴西北区新近系湖相细粒沉积岩油气富集条件[J].煤炭学报,2020,45(8):2824-2837.

ZHANG Xu,LIU Chenglin,GUO Zeqing,et al.Hydrocarbon accumulation conditions of neogene lacustrine fi-ne-grained sedimentary rocks in northwestern Qaidam Basin[J].Journal of China Coal Society,2020,45(8):2824-2837.

中图分类号:P618.11;P618.13

文献标志码:A

文章编号:0253-9993(2020)08-2824-14

收稿日期:2019-05-05

修回日期:2019-08-31

责任编辑:韩晋平

DOI:10.13225/j.cnki.jccs.2019.0579

基金项目:国家自然科学基金资助项目(41872127);安徽省自然科学基金资助项目(1908085MD105);中国博士后科学基金资助项目(2019M662200)

作者简介:张 旭(1990—),男,安徽巢湖人,博士。E-mail:xuzhangcags@163.com

通讯作者:桂和荣(1963—),男,安徽舒城人,教授,博士生导师。E-mail:hrgui@163.com