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古水深恢复是岩相古地理恢复的重要内容之一,“将今论古”是判断古水深的一个重要原则,通常需要采用沉积学、地球化学和古生物学等多种手段和方法综合进行判断[1],地层厚度差值法、沉积物特征分析、微体古生物种群分析、Th/U比值、Co含量、原始有机质丰度等方法广泛运用于古水深恢复[2⁃15]。
陆相湖泊古水深的重建包括定性推测和定量计算两大类方法,通过对岩性组合、沉积构造、古生物组合及自生矿物等可以对沉积水体深度进行定性推测,但湖泊水体深度的定量计算主要采用地球化学方法和化石群分异度法[16]。不同水深的湖底通常具有不同的物理和化学条件,导致沉积物特征可能呈现出较大差异:杂色的泥—粉砂岩互层、波痕及泥裂等沉积构造组合通常指示间歇暴露的湖底和最大水深小于1 m的滨湖带[17];水深介于1~20 m的浅湖区亦以细粒沉积物为主,通常含有较多的植物碎片及双壳类化石,粉砂岩中沙纹层理及浪成交错层理较为发育;水深大于20 m的半深湖及深湖沉积区多以发育水平层理及韵律层理的泥岩为特征,植物化石及陆源碎屑稀少并缺乏底栖生物化石,可见槽模、沟模等与重力流相关的沉积构造。沉积速率和沉积物厚度在一定条件下能够反映沉积水体深度变化,浅水环境沉积速率通常远高于深水环境,在欠补偿盆地和补偿盆地中,单位时间内浅水区沉积地层厚度一般大于深水区沉积地层厚度,而在超补偿盆地中,沉积物厚度越大反映沉积水深越大[18]。泥岩中自生铁矿类型与水深通常有较好的对应关系[19],自生铁矿可以作为判别古水深的重要矿物,同时自生铁矿类型的不同也将导致不同深度带泥质沉积物颜色呈现规律性变化。大型湖泊水深一般与离岸距离存在正相关关系,因此反映离岸距离的Fe/Mn比值和(Al+Fe)/(Ca+Mg)[20]可以指示湖泊古水深的变化趋势[21⁃22],而常作为氧化/还原环境的重要指标的V/Ni比值、Ni/Co比值和Th/U比值[23]亦可作为古水深推测的辅助依据,Th/U比值的大小在一定程度上可以反映水体深度的变化(表1)。古水深的定量恢复主要采用古生物和地球化学分析的方法,其中,利用介形类化石群优势度对湖泊古水深进行计算结果较为可靠,浪基面附近化石分异度最高[16],鉴于介形类化石在中国陆相盆地中特为发育,该方法具有广泛的适用性;在沉积速率稳定的湖相泥岩中La和Co的丰度与水深存在密切联系,基于沉积物中La、Co含量的古水深恢复方适宜于碎屑岩沉积区水体深度的计算[8,24];根据沉积物中原始有机碳含量对古水深进行计算是一种新方法,其理论基础是咸水湖河口区沉积物中TOC含量与水深存在良好的相关性,该方法主要适用于烃源岩成熟度相对较低的陆相咸水湖盆沉积水深的恢复[13]。我国陆相含油气层系大部分与三角洲—湖泊沉积体系有关,古水深恢复对岩相古地理分析及烃源岩分布预测具有重要意义。
表 1 Th/U比值与古水深关系表(据庞军刚[1]修改)
Th/U比值 氧化—还原条件 古水深 >30 氧化 小于15 m,受河口影响可达20 m 30~10 弱氧化—弱还原 15~25 m,湖湾区可小于15 m 10~4 弱还原 大于25 m,湖湾区可小于15 m <4 强还原 根据岩石组合、沉积构造及古生物特征,结合元素地球化学及有机地球化学分析,对川东地区早侏罗世湖泊水体深度进行了恢复,确定了半深湖区主要分布范围,对四川盆地早侏罗世岩相古地理重建具有重要意义,同时对于研究区侏罗系页岩油、气的勘探部署具有一定的指导作用。
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沉积构造和古生物化石是反映沉积环境和沉积水体深度的重要标志。川东地区下侏罗统沉积构造及古生物化石的分布具有一定的规律性:凉高山组及大安寨段砂岩中发育大量平行层理及各类斜层理(图2a,b),主要分布于研究区北东部及南部(图3),反映近岸的浅水或岸上环境;浪成交错层理主要发育在粉砂岩或泥质粉砂岩中(图2c),为波浪作用的产物,理论上浪成交错层理可以形成于浅湖的任一区域,但浅水区波浪形成的纹层容易形成也容易被后期波浪所破坏,事实上邻近半深湖的浅湖区大浪形成的纹层才容易保存,即浪成交错层理所指示的位置已靠近半深湖;水平层理主要发育在暗色泥岩中(图2d,e),反映安静的较深水环境,主要分布于研究区中部;滑塌变形构造在粉砂岩、灰岩和膏质白云岩中均有发育(图2f~h),主要分布在研究区中部(图3a),反映具有一定坡度的浅湖环境;渠模主要发育在大安寨段及东岳庙段发育水平层理的灰黑色泥岩中(图2i),成因与重力流的侵蚀作用有关,主要分布于研究区东部(图3a),反映水体较深的半深湖环境;植物根迹化石及完整的植物叶片化石在南部和北部有少量分布(图2j,k),主要指示陆生植物分布的岸上环境;双壳化石在大安寨段及东岳庙段分布最广,其中破碎的双壳化石堆积形成介屑灰岩指示高能的浅湖环境,而泥岩中毯状介壳层壳体保存相对完整(图2l,m),并且凹面朝下的介壳数量明显高于凹面朝上的介壳数量(图4),显然为死亡后的壳体被搬运至异地堆积的结果,结合其通常与具水平层理的暗色泥岩伴生的特征,主要指示水体较深的半深湖环境。沉积构造及古生物组合分布特征表明川东地区北东部及南部为浅水区、中部沉积水体深度相对较大(图3a)。
陆相沉积中Fe、Al、Mg、Ca等元素主要来自母岩风化物,由于其化学性质存在较大差异在搬运过程中将产生明显的化学分异作用[40],Fe和Al在近岸浅水区容易与氧离子结合而发生沉淀,而Mn、Mg、Ca通常在远岸的湖盆区富集,因此近岸、浅水环境沉积物中的Fe/ Mn比值及(Al+Fe)/(Ca+Mg)比值明显高于远岸、深水环境[21]。川东地区东北部及南部均具有较高的Fe/Mn比值和(Al+Fe)/(Ca+Mg)比值,反映近岸、浅水沉积环境;而中部及西北部地区Fe/Mn比值和(Al+Fe)/(Ca+Mg)比值相对较低,反映远岸较深水沉积环境(图3b)。
川东地区下侏罗统烃源岩有机质主要为Ⅱ1型、Ⅱ2型和Ⅲ型干酪根(表2),其中Ⅱ1型干酪根主要分布于广安—梁平一带,Ⅲ型干酪根主要分布于研究区南部及北东部,Ⅱ1型和Ⅲ型干酪根分布区之间主要为Ⅱ2型干酪根(图3c)。Ⅱ1型、Ⅱ2型和Ⅲ型干酪根的主要分区特征总体与据沉积构造组合、Fe/Mn比值及(Al+Fe)/(Ca+Mg)比值等地球化学指标推测的古水深变化趋势一致,即:Ⅱ1型干酪根主要分布在远岸的湖盆中央位置,而Ⅲ型干酪根主要分布于近岸和靠近物源区域,离岸距离控制了有机质类型(图3)。
表 2 川东地区下侏罗统古水深恢复
井名 层位 岩性 微相 Ro/% 干酪根类型 TOCr/% TOCo/% 古水深/mToc法 Co/(mg·kg-1) La/(mg·kg-1) 古水深/mLa-Co法 Th/U比值 古水深/mTh/U法 FT1 J1dn 灰黑色泥岩 半深湖 1.30 Ⅱ2 1.4 1.5 13.8 16.7 33.0 29.8 4.79 >25 FT1 J1dn 深灰色泥岩 半深湖 1.33 Ⅱ2 0.9 1.0 10.3 13.2 33.5 18.3 4.80 >25 FT1 J1dn 灰黑色泥岩 半深湖 1.45 Ⅱ2 1.5 1.7 15.7 13.1 28.8 19.7 4.38 >25 FT1 J1dn 深灰色泥岩 半深湖 1.42 Ⅱ2 1.3 1.4 13.8 10.9 22.5 15.4 4.73 >25 FT1 J1dn 灰黑色泥岩 半深湖 1.39 Ⅱ2 1.1 1.2 12.1 18.3 31.6 36.3 5.33 >25 FT1 J1d 灰黑色泥岩 半深湖 1.57 Ⅱ2 1.3 1.4 13.6 24.5 25.1 65.4 5.85 >25 FT1 J1d 灰黑色泥岩 半深湖 1.60 Ⅱ2 1.8 1.9 18.5 19.1 45.7 33.0 5.52 >25 TD021-X8 J1l 深灰色泥岩 前三角洲 1.35 Ⅱ2 0.2 0.2 6.6 26.3 38.4 66.3 5.50 >25 TD021-X8 J1l 灰黑色泥岩 前三角洲 1.31 Ⅱ2 2.2 2.4 24.8 17.1 41.0 27.9 4.86 >25 MX001-H8 J1d 灰黑色泥岩 半深湖 1.19 Ⅱ2 3.6 3.8 59.9 19.7 40.2 37.6 4.57 >25 MX001-H8 J1d 灰黑色含灰泥岩 半深湖 1.02 Ⅱ2 2.3 2.4 24.6 15.0 30.2 25.2 4.38 >25 MX001-H8 J1d 深灰色含介壳泥岩 半深湖 1.11 Ⅱ2 0.7 0.8 9.2 14.9 35.9 22.6 4.75 >25 MX001-H7 J1dn 灰色泥岩 浅湖(滩缘) 1.10 Ⅱ2 0.9 1.0 10.5 10.1 25.6 12.2 4.76 <15 MX001-H7 J1dn 灰色泥岩 浅湖(滩缘) 1.10 Ⅱ1 0.6 0.6 8.1 12.1 22.3 19.0 4.95 <15 MX001-H7 J1d 深灰色含介壳泥岩 半深湖 1.10 Ⅱ2 1.6 1.8 16.7 19.4 45.1 34.3 5.11 >25 MX001-H7 J1d 深灰色含介壳泥岩 半深湖 1.10 Ⅱ2 1.8 1.9 18.2 16.8 39.8 27.4 5.59 >25 YA012-X8 J1dn 深灰色含灰泥岩 半深湖 1.21 Ⅱ2 1.7 1.8 17.3 18.6 22.8 41.5 5.57 >25 YA012-X8 J1dn 深灰色泥岩 半深湖 1.20 Ⅱ2 1.1 1.2 11.5 19.4 31.5 40.5 4.05 >25 YA012-X8 J1dn 深灰色泥岩 半深湖 1.20 Ⅱ2 1.2 1.3 12.4 14.7 30.4 24.1 4.83 >25 YA012-X8 J1m 灰色含灰泥岩 浅湖 1.34 Ⅱ2 1.3 1.4 13.4 17.2 38.9 29.1 4.74 >25 YA012-X8 J1m 灰色含灰泥岩 浅湖 1.20 Ⅱ2 0.9 1.0 10.5 19.0 39.6 35.3 5.16 >25 YD003-H2 J1l 灰黑色泥岩 前三角洲 1.20 Ⅱ2 1.2 1.3 12.5 16.9 47.7 24.5 4.39 >25 YD003-H2 J1l 灰黑色泥岩 前三角洲 0.85 Ⅱ2 1.5 1.6 14.9 19.4 42.4 35.5 4.60 >25 LG83 J1dn 深灰色泥岩 浅湖(滩间) 1.20 Ⅱ1 0.6 0.6 8.4 14.4 15.6 29.1 4.47 <15 LG83 J1dn 深灰色泥岩 浅湖(滩间) 1.20 Ⅱ1 0.7 0.8 9.3 13.5 35.6 18.4 5.03 <15 TD007-H5 J1dn 深灰色泥岩 浅湖(滩缘) 1.20 Ⅱ1 0.8 0.9 9.8 14.2 40.6 18.7 5.35 <15 TD007-H5 J1dn 深灰色泥岩 浅湖(滩缘) 1.20 Ⅱ1 0.8 0.8 9.4 19.6 37.6 38.5 5.22 <15 TD021-X7 J1dn 深灰色泥岩 浅湖(滩缘) 1.20 Ⅱ1 1.7 1.8 17.2 15.2 30.0 25.9 4.57 <15 TD002-12 J1dn 灰色泥岩 浅湖(滩间) 1.20 Ⅱ2 0.5 0.5 7.7 17.8 41.1 30.3 4.25 <15 W081-H1 J1dn 含灰泥岩 浅湖(滩间) 1.10 Ⅱ2 0.1 0.1 6.0 13.5 23.9 22.7 2.79 <15 W081-H1 J1dn 含灰泥岩 浅湖(滩间) 1.10 Ⅱ2 0.1 0.1 6.2 13.8 39.0 18.1 5.46 <15 X018-H1 J1z 杂色泥岩 滨湖 1.10 Ⅲ 0.7 0.7 8.8 19.3 47.8 32.8 5.45 >25 -
四川盆地早侏罗世呈不对称状的“箕状”特征[41],主要发育陆相湖泊—三角洲沉积体系,不同地区湖泊开放度的不同导致湖泊水体盐度存在较大差异导致[42],川东地区早侏罗世湖泊水体具有一定的盐度[43],满足La-Co法[24]和TOC法[13]等古水深计算或判别的基本条件。
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利用湖相泥岩中La和Co的含量可以对古水深进行定量计算[24],其公式为:
h =C/V s3/2(1) V s=V o×N Co/(S Co-t ×T Co)(2) t =S La/N La(3) 式(1)中:h为古水深(单位m);C为常数(3.05×105);Vs可由式(2)计算出。
式(2)中:Vs为样品沉积时的沉积速率(单位m/Ma);Vo为正常环境的沉积速率(m/Ma),湖相泥岩沉积速率一般介于0.2×103~0.3×103 m/Ma(本次取值0.3×103 m/Ma);NCo为正常湖泊沉积物中Co的平均值(20 mg/kg);SCo为实测样品Co丰度(mg/kg);TCo为陆源碎屑岩中Co的平均值(4.68 mg/kg);t表示陆源Co对样品的影响,可由式(3)计算出。
式(3)中:SLa(单位mg/kg)表示实测样品中La的丰度;NLa为陆源碎屑岩中La的平均值(38.99 mg/kg)。
La-Co法计算结果表明川东地区下侏罗统湖相泥质岩样品形成于12.2~66.3 m,其中自流井组东岳庙段及马鞍山段沉积水体深度可能较大安寨段更大,大部分湖相泥质岩形成于半深湖—深湖环境(表2)。
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现代咸水湖河口区沉积物中TOC含量与水深存在良好的相关性,利用沉积物中原始有机碳含量成为一种沉积水体深度恢复的新方法 [13],其计算公式为:
H =5.637 5e0.618*TOCr* Kc’(4) 式中:H为古水深(单位m);TOCr为残余有机碳,Kc’为有机碳恢复系数,根据Ro及干酪根类型进行确定[44](图5)。
图 5 有机碳恢复系数(Kc,Kc’)与成熟度、排烃效率和有机质类型的关系(据卢双舫等[44]修改)
TOC法计算结果表明川东地区下侏罗统湖相泥质岩样品形成于6~59.9 m的水深,绝大部分泥质岩沉积水深不超过20 m,仅少量样品形成于较大水深环境,自流井组东岳庙段、马鞍山段及凉高山组水深相对更大(表2)。
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湖底氧化还原环境主要受水深和水动力条件控制:河流入湖口及滨、浅湖区水体动荡,水深一般小于15 m,湖底处于氧化环境;开放湖域水深介于15~25 m的湖底或水深小于15 m的局限湖湾区,一般处于弱氧化—弱还原环境;水深大于25 m的湖底多位于远离河口的低能带,处于还原—强还原环境[45]。因此,对沉积时期的氧化还原背景进行分析,结合沉积相分析可以大致估算古水深范围。石陨石和岩浆岩中钍/铀(Th/U)比值一般稳定在3~4[46⁃47],由于钍元素化学性质较为稳定,基本不受氧化还原条件影响[48],而铀是一种变价元素,对成岩环境极为敏感,极易发生迁移和沉淀[49],因此,泥质沉积物中Th/U比值常作为判别氧化—还原条件的重要指标[50],并间接反映沉积水体深度[1,48](表1)。绝大部分实测的泥质岩Th/U比值介于4~10,少量样品Th/U比值小于4(表2),反映主要形成于还原环境,考虑到浅湖(滩间、滩缘)多为局限环境、而半深湖及前三角洲为开放环境(图6),推测前三角洲及半深湖样品古水深大于25 m,浅湖(滩间、滩缘)古水深小于15 m(表2)。
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古水深恢复的方法较多,但每种方法都存在一定的局限性:由于沉积物特征并非完全受控于水体深度,同时不少沉积构造受湖面开阔程度控制[1],如波痕可以出现在不同水深环境,因此根据沉积物特征和沉积构造所确定的古水深精度有限;地层厚度同时受湖平面变化、基底沉降速率及沉积物通量等多因素控制,因此只有在确定沉积补偿点、基准面和差异沉降量可以忽略不计的条件下才能根据地层厚度对古水深进行恢复[18];鉴于自生铁矿的类型主要与氧化—还原条件有关,而沉积环境的氧化—还原背景同时受到水深、有机质丰度等多种因素控制,利用自生铁矿类型恢复沉积水体深度的方法在煤系地层中未必适用,该方法的适用性同时受到自生铁矿含量的制约;根据古生物及古生态对古水深进行恢复,需要沉积物中含有大量介形虫或遗迹化石,在生物遗体和遗迹化石匮乏的沉积环境中并不适用[1];Fe/Mn比值、(Al+Fe)/(Ca+Mg)比值、V/Ni比值、Ni/Co比值、Th/U比值和La-Co含量等方法均属于元素地球化学方法,沉积物中元素的丰度同时受到母岩类型和沉积—成岩环境控制,对样品均有较高的要求,通常需要较纯的泥岩样品,而大部分浅水环境的沉积物都含有较多的石英、长石及岩屑,继承自母岩的元素含量所占权重较大可能影响判别结果,同时La-Co法对古水深的恢复结果还受到沉积速率赋值的影响;TOC法对古水深进行恢复最重要的是需要对沉积物中原始有机碳含量进行恢复,在有机质成熟度较高的条件下难以准确恢复原始有机碳含量,也就无法根据TOC含量对古水深进行定量计算;同时该方法建立于现代青海湖布哈河口区,理论上也只能适用于发育三角洲的咸水湖泊沉积[13]。在研究工作中需要根据湖泊沉积背景、沉积物及古生物组合特征,尽量选择多种较为适用的方法进行古水深的恢复,相互印证、判断恢复结果的可靠性。
湖泊—三角洲沉积体系亚相的划分主要以湖岸线及浪基面为划分依据,滨湖亚相主要处于枯水期湖岸线与洪水期湖岸线之间,正常浪基面与枯水期湖岸线之间为浅湖亚相,正常浪基面与风暴浪基面之间为半深湖亚相,风暴浪基面之下为深湖亚相。湖浪的最大波长受到湖域面积、风速及水深等多种因素控制,一般说来湖域面积越大、湖浪波长越大,亦即浪基面深度越大(表3),根据岩相古地理恢复结果估算川东地区早侏罗世最大湖域面积超过87 500 km2(图6),以密执安湖为参照,推测川东地区早侏罗世正常浪基面深度也在15 m左右,即15 m水深为大致为浅湖/半深湖分界。
湖泊 有效面积s/km2 最大波长λ/m 波基面深度0.5λ/m 密执安湖 57 757 30 15 青海湖 4 635 15 7.5 鄱阳湖 4 600 15 7.5 太湖 2 428 16 8 抚仙湖 216 18 9 长荡湖 81.9 1.6 0.8 根据Th/U比值确定的古水深与浅湖沉积较为吻合,半深湖沉积的恢复古水深可能偏大,而滨湖沉积则存在较大矛盾(表2,样品对应图2k)。从La-Co法的恢复结果来看,相当一部分样品恢复古水深与根据沉积构造确定的沉积亚相存在较大矛盾,特别是多个滨、浅湖样品计算古水深大于29 m显然不合常理,而TOC法古水深恢复结果除1件滨湖样品外、其余样品与沉积相较为吻合(表2、图6)。鉴于湖相沉积样品的沉积速率Vs难以精确确定,采用La-Co法对古水深进行恢复仍需完善[54],而川东地区早侏罗世沉积环境与现代青海湖布哈河口区较为相似且有机质成熟度相对较低(原始有机碳TOCo恢复数据较为可靠),因此采用TOC法恢复的川东地区早侏罗世古水深结果更为可靠,但该公式只适用于浅湖及以下深度环境沉积水体深度的恢复[13],不适合滨湖等暴露环境古水深的恢复。
Paleo-water-depth Reconstruction of Early Jurassic Lakes in the Eastern Sichuan Basin
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摘要: 目的 川东地区下侏罗统湖泊—三角洲沉积具有较大的勘探潜力,但基础地质研究的滞后制约了勘探进程,古水深恢复对川东地区下侏罗统岩相古地理恢复及页岩油气勘探具有重要指导意义。 方法 根据岩石类型、沉积构造和古生物化石分布特征,结合Fe/Mn比值、(Al+Fe)/(Ca+Mg)比值及干酪根类型对川东地区自流井组和凉高山组古水深变化趋势进行分析,并采用La-Co法、TOC法及Th/U比值法对早侏罗世古水深进行定量恢复。 结果与结论 沉积构造及古生物分布特征表明早侏罗世川东地区中部长期处于湖盆低洼位置,Fe/Mn比值和(Al+Fe)/(Ca+Mg)比值的变化趋势反映研究区东北部和南部为近岸浅水区。La-Co法、TOC法及Th/U比值法古水深恢复结果存在一定差异,La-Co法恢复的古水深与沉积相存在较大矛盾,Th/U比值法对浅湖沉积区古水深恢复较为可靠、但在滨湖及半深湖沉积区可靠度较低,而基于TOC法的古水深恢复结果最为可靠、与沉积相吻合度最高。早侏罗世川东地区湖泊浪基面约15 m,TOC法古水深恢复结果表明川东地区下侏罗统半深湖广泛发育,早侏罗世湖泊最大水深约60 m,广安—万州一带广泛发育半深湖—深湖沉积,有利于有机质的富集和保存,是页岩油气勘探的有利位置。Abstract: Objective The Lower Jurassic lacustrine and delta deposits in the eastern Sichuan Basin have significant hydrocarbon exploration potential,but the lag of basic geological research restricts the exploration process. Paleo-water-depth reconstruction of the Lower Jurassic helps guide lithofacies paleogeography and shale oil -gas exploration in the eastern Sichuan Basin. Methods Based on the rock types,sedimentary structures,and distribution characteristics of paleontological fossils,combined with the Fe/Mn ratio,(Al+Fe) / (Ca+Mg) ratio,and kerogen type,the paleo-water-depth variation trend of the Ziliujing and Liangshan Formations in the eastern Sichuan Basin is studied. The Early Jurassic paleo-water-depth was reconstructed quantitatively using the La-Co,total organic carbon (TOC),and Th/U ratio methods. [ Results and Conclusions ] The characteristics of sedimentary structure and paleontological distribution indicate that the central part of the eastern Sichuan Basin was in a low-lying lake basin for an extended period during the Early Jurassic,and the variation trend of the Fe/Mn and (Al+Fe) / (Ca+Mg) ratios indicates that the northeast and south of the study area are derived from nearshore shallow water. The results of paleo-water-depth reconstruction for the three methods are quite different. The paleo-water-depth reconstructed based on the La-Co method contradicts that of the sedimentary facies. The Th/U ratio method is more reliable for paleo-water-depth reconstruction in shallow lake sedimentary areas,but less reliable in lakeside and semi-deep lake sedimentary areas. The paleo-water-depth reconstruction result based on the TOC method is the most reliable and has the highest coincidence with sediments. The base surface of the lake-wave in the eastern Sichuan Basin was approximately 15 m in the Early Jurassic. Paleo-water-depth reconstruction results based on the TOC method indicate that the Lower Jurassic semi-deep lakes were widely developed in the eastern Sichuan Basin,and the maximum water depth of the Early Jurassic lakes was approximately 60 m. Semi-deep lake to deep lake deposits were widely developed in the Guang'an-Wanzhou area,which benefits the enrichment and preservation of organic matter,creating a favorable position for shale oil and gas exploration.
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图 1 四川盆地早侏罗世古地理略图及川东地区地理位置(据中石油西南油气田勘探开发研究院资料及郑荣才等[27]修改)
图 2 川东下侏罗统典型沉积构造及主要古生物特征
(a) grey medium sandstone with parallel bedding, well TD021⁃X8, 1 794.5 m, J1l; (b) gray fine sandstone with oblique bedding, well YT1, 2 157.4 m, J1l; (c) gray siltstone interbedded with argillaceous bedding, developed wave cross bedding, well YJ1, 2 048.1 m, J1l; (d) gray⁃black mudstone, with a few shells and developed horizontal bedding, well YT1, 2 433.7 m, J1d; (e) grayish black mudstone with horizontal bedding, well FT1, 1 723.8 m, J1l; (f) grey siltstone and dark gray mudstone, developed slump deformation structure, well YJ1H, 2 049.5 m, J1l; (g) grey gypsum dolostone, developed slump deformation structure, well BX2, 2 624.2 m, J1dn; (h) thin interbeds of gray bioclastic limestone and dark gray mudstone, horizontal bedding developed in mudstone, slump deformation structure developed in bioclastic limestone, well W081⁃H1, 754.4 m, J1dn; (i) gray⁃black mudstone interbedded with thin layer of mesoclastic limestone, horizontal bedding developed in mudstone, gutter cast developed, well FT1, 1 841.4 m, J1d; (j) light gray silty mudstone, plant root trace fossils can be seen. well YT1, 2 165.8 m, J1l; (k) gray silty mudstone, containing intact plant leaf impression fossils, well X018⁃H1, 1 836.7 m, J1z; (l) grayish black mudstone, horizontal bedding, tapestry⁃shaped shell layer, intact shell, well FT1, 1 850.5 m, J1d; (m) thin interlayer between dark gray mudstone and blanket shell layer, intact shell preservation, well MX001⁃H8, 1 862.4 m, J1d
图 3 川东地区下侏罗统古水深变化趋势及主要干酪根类型分布特征
(a) the trend of paleo⁃water⁃depth variation inferred from sedimentary structure; (b) the trend of paleo⁃water⁃depth variation inferred from geochemical indicators; (c) distribution characteristics of main kerogen types of Lower Jurassic source rocks (revised according to the data from the Institute of Exploration and Development, PetroChina Southwest Oil and Gas Field Company)
图 5 有机碳恢复系数(Kc,Kc’)与成熟度、排烃效率和有机质类型的关系(据卢双舫等[44]修改)
表 1 Th/U比值与古水深关系表(据庞军刚[1]修改)
Th/U比值 氧化—还原条件 古水深 >30 氧化 小于15 m,受河口影响可达20 m 30~10 弱氧化—弱还原 15~25 m,湖湾区可小于15 m 10~4 弱还原 大于25 m,湖湾区可小于15 m <4 强还原 表 2 川东地区下侏罗统古水深恢复
井名 层位 岩性 微相 Ro/% 干酪根类型 TOCr/% TOCo/% 古水深/mToc法 Co/(mg·kg-1) La/(mg·kg-1) 古水深/mLa-Co法 Th/U比值 古水深/mTh/U法 FT1 J1dn 灰黑色泥岩 半深湖 1.30 Ⅱ2 1.4 1.5 13.8 16.7 33.0 29.8 4.79 >25 FT1 J1dn 深灰色泥岩 半深湖 1.33 Ⅱ2 0.9 1.0 10.3 13.2 33.5 18.3 4.80 >25 FT1 J1dn 灰黑色泥岩 半深湖 1.45 Ⅱ2 1.5 1.7 15.7 13.1 28.8 19.7 4.38 >25 FT1 J1dn 深灰色泥岩 半深湖 1.42 Ⅱ2 1.3 1.4 13.8 10.9 22.5 15.4 4.73 >25 FT1 J1dn 灰黑色泥岩 半深湖 1.39 Ⅱ2 1.1 1.2 12.1 18.3 31.6 36.3 5.33 >25 FT1 J1d 灰黑色泥岩 半深湖 1.57 Ⅱ2 1.3 1.4 13.6 24.5 25.1 65.4 5.85 >25 FT1 J1d 灰黑色泥岩 半深湖 1.60 Ⅱ2 1.8 1.9 18.5 19.1 45.7 33.0 5.52 >25 TD021-X8 J1l 深灰色泥岩 前三角洲 1.35 Ⅱ2 0.2 0.2 6.6 26.3 38.4 66.3 5.50 >25 TD021-X8 J1l 灰黑色泥岩 前三角洲 1.31 Ⅱ2 2.2 2.4 24.8 17.1 41.0 27.9 4.86 >25 MX001-H8 J1d 灰黑色泥岩 半深湖 1.19 Ⅱ2 3.6 3.8 59.9 19.7 40.2 37.6 4.57 >25 MX001-H8 J1d 灰黑色含灰泥岩 半深湖 1.02 Ⅱ2 2.3 2.4 24.6 15.0 30.2 25.2 4.38 >25 MX001-H8 J1d 深灰色含介壳泥岩 半深湖 1.11 Ⅱ2 0.7 0.8 9.2 14.9 35.9 22.6 4.75 >25 MX001-H7 J1dn 灰色泥岩 浅湖(滩缘) 1.10 Ⅱ2 0.9 1.0 10.5 10.1 25.6 12.2 4.76 <15 MX001-H7 J1dn 灰色泥岩 浅湖(滩缘) 1.10 Ⅱ1 0.6 0.6 8.1 12.1 22.3 19.0 4.95 <15 MX001-H7 J1d 深灰色含介壳泥岩 半深湖 1.10 Ⅱ2 1.6 1.8 16.7 19.4 45.1 34.3 5.11 >25 MX001-H7 J1d 深灰色含介壳泥岩 半深湖 1.10 Ⅱ2 1.8 1.9 18.2 16.8 39.8 27.4 5.59 >25 YA012-X8 J1dn 深灰色含灰泥岩 半深湖 1.21 Ⅱ2 1.7 1.8 17.3 18.6 22.8 41.5 5.57 >25 YA012-X8 J1dn 深灰色泥岩 半深湖 1.20 Ⅱ2 1.1 1.2 11.5 19.4 31.5 40.5 4.05 >25 YA012-X8 J1dn 深灰色泥岩 半深湖 1.20 Ⅱ2 1.2 1.3 12.4 14.7 30.4 24.1 4.83 >25 YA012-X8 J1m 灰色含灰泥岩 浅湖 1.34 Ⅱ2 1.3 1.4 13.4 17.2 38.9 29.1 4.74 >25 YA012-X8 J1m 灰色含灰泥岩 浅湖 1.20 Ⅱ2 0.9 1.0 10.5 19.0 39.6 35.3 5.16 >25 YD003-H2 J1l 灰黑色泥岩 前三角洲 1.20 Ⅱ2 1.2 1.3 12.5 16.9 47.7 24.5 4.39 >25 YD003-H2 J1l 灰黑色泥岩 前三角洲 0.85 Ⅱ2 1.5 1.6 14.9 19.4 42.4 35.5 4.60 >25 LG83 J1dn 深灰色泥岩 浅湖(滩间) 1.20 Ⅱ1 0.6 0.6 8.4 14.4 15.6 29.1 4.47 <15 LG83 J1dn 深灰色泥岩 浅湖(滩间) 1.20 Ⅱ1 0.7 0.8 9.3 13.5 35.6 18.4 5.03 <15 TD007-H5 J1dn 深灰色泥岩 浅湖(滩缘) 1.20 Ⅱ1 0.8 0.9 9.8 14.2 40.6 18.7 5.35 <15 TD007-H5 J1dn 深灰色泥岩 浅湖(滩缘) 1.20 Ⅱ1 0.8 0.8 9.4 19.6 37.6 38.5 5.22 <15 TD021-X7 J1dn 深灰色泥岩 浅湖(滩缘) 1.20 Ⅱ1 1.7 1.8 17.2 15.2 30.0 25.9 4.57 <15 TD002-12 J1dn 灰色泥岩 浅湖(滩间) 1.20 Ⅱ2 0.5 0.5 7.7 17.8 41.1 30.3 4.25 <15 W081-H1 J1dn 含灰泥岩 浅湖(滩间) 1.10 Ⅱ2 0.1 0.1 6.0 13.5 23.9 22.7 2.79 <15 W081-H1 J1dn 含灰泥岩 浅湖(滩间) 1.10 Ⅱ2 0.1 0.1 6.2 13.8 39.0 18.1 5.46 <15 X018-H1 J1z 杂色泥岩 滨湖 1.10 Ⅲ 0.7 0.7 8.8 19.3 47.8 32.8 5.45 >25 -
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