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Article Navigation >  Acta Sedimentologica Sinica  > 2024  >  42(5): 1753-1764

SUN LiDong, YANG Liang, LI XiaoMei, ZHOU Xiang, HU Bo, CAI Zhuang, DU Ying. Paleoenvironment and Main Controlling Factors of Source Rocks in the Shahezi Formation, Xujiaweizi Fault Depression[J]. Acta Sedimentologica Sinica, 2024, 42(5): 1753-1764. doi: 10.14027/j.issn.1000-0550.2022.117
Citation: SUN LiDong, YANG Liang, LI XiaoMei, ZHOU Xiang, HU Bo, CAI Zhuang, DU Ying. Paleoenvironment and Main Controlling Factors of Source Rocks in the Shahezi Formation, Xujiaweizi Fault Depression[J]. Acta Sedimentologica Sinica, 2024, 42(5): 1753-1764. doi: 10.14027/j.issn.1000-0550.2022.117
shu

Paleoenvironment and Main Controlling Factors of Source Rocks in the Shahezi Formation, Xujiaweizi Fault Depression

doi: 10.14027/j.issn.1000-0550.2022.117
  • 1.

    Exploration and Production Research Institute, Daqing Oilfield, CNPC, Daqing, Heilongjiang 163712, China

Funds:

National Science and Technology Major Project 2016ZX05001-002

Important National Science and Technology Project of CNPC 2021DJ0205

  • Received Date: 2022-07-08
  • Accepted Date: 2022-11-20
  • Rev Recd Date: 2022-09-16
    • Available Online: 2022-11-20
  • Publish Date: 2024-10-10
  • Objective The study make clear the main controlling factors of source rock in Shahezi Formation, and provides geologic basis for the optimal selection of gas exploration in deep layer of Songliao Basin. Methods We recovered the sedimentary facies and water property in the Shahazi Formation, based on core observation, organic geochemical analysis, and major and trace elements testing, determining fault activity, sedimentary facies, water properties, and their control on source rock quality. Results Two source rock, mudstone and coal, were identified in the Shahezi Formation. The faulting during the deposition of the first source rock in the Shahezi Formation was characterized by strong activity, generating a small lake with great water depth, high salinity, and strong reducing properties. These source rocks were mudstone in a semi-deep lake. The faulting activity during the deposition of the second source rock gradually weakened, with the lake size increasing while water depth decreased. Salinity and reducibility were also weakened, and the coal in shore lake was the primary source rock. Conclusions The source rock type was mainly controlled by fault activity, sedimentary facies, and water properties. The downthrown side become important storage for mudstone because of the fault activity, which determined the thickness of mudstone. The sedimentary facies controlled the source rock type through their impact on organic matter import. The difference between paleoenvironments, which was determined by paleosalins, paleowater depth, and redox, is the key factor for the enrichment and preservation of organic matter, and the main controlling factor for source rock quality distribution.
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  • Received:  2022-07-08
  • Revised:  2022-09-16
  • Accepted:  2022-11-20
  • Published:  2024-10-10

Paleoenvironment and Main Controlling Factors of Source Rocks in the Shahezi Formation, Xujiaweizi Fault Depression

doi: 10.14027/j.issn.1000-0550.2022.117
  • 1. Exploration and Production Research Institute, Daqing Oilfield, CNPC, Daqing, Heilongjiang 163712, China
Funds:

National Science and Technology Major Project 2016ZX05001-002

Important National Science and Technology Project of CNPC 2021DJ0205

  • Received Date: 2022-07-08
  • Accepted Date: 2022-11-20
  • Rev Recd Date: 2022-09-16
  • Available Online: 2022-11-20
  • Publish Date: 2024-10-10

Abstract: Objective The study make clear the main controlling factors of source rock in Shahezi Formation, and provides geologic basis for the optimal selection of gas exploration in deep layer of Songliao Basin. Methods We recovered the sedimentary facies and water property in the Shahazi Formation, based on core observation, organic geochemical analysis, and major and trace elements testing, determining fault activity, sedimentary facies, water properties, and their control on source rock quality. Results Two source rock, mudstone and coal, were identified in the Shahezi Formation. The faulting during the deposition of the first source rock in the Shahezi Formation was characterized by strong activity, generating a small lake with great water depth, high salinity, and strong reducing properties. These source rocks were mudstone in a semi-deep lake. The faulting activity during the deposition of the second source rock gradually weakened, with the lake size increasing while water depth decreased. Salinity and reducibility were also weakened, and the coal in shore lake was the primary source rock. Conclusions The source rock type was mainly controlled by fault activity, sedimentary facies, and water properties. The downthrown side become important storage for mudstone because of the fault activity, which determined the thickness of mudstone. The sedimentary facies controlled the source rock type through their impact on organic matter import. The difference between paleoenvironments, which was determined by paleosalins, paleowater depth, and redox, is the key factor for the enrichment and preservation of organic matter, and the main controlling factor for source rock quality distribution.

SUN LiDong, YANG Liang, LI XiaoMei, ZHOU Xiang, HU Bo, CAI Zhuang, DU Ying. Paleoenvironment and Main Controlling Factors of Source Rocks in the Shahezi Formation, Xujiaweizi Fault Depression[J]. Acta Sedimentologica Sinica, 2024, 42(5): 1753-1764. doi: 10.14027/j.issn.1000-0550.2022.117
Citation: SUN LiDong, YANG Liang, LI XiaoMei, ZHOU Xiang, HU Bo, CAI Zhuang, DU Ying. Paleoenvironment and Main Controlling Factors of Source Rocks in the Shahezi Formation, Xujiaweizi Fault Depression[J]. Acta Sedimentologica Sinica, 2024, 42(5): 1753-1764. doi: 10.14027/j.issn.1000-0550.2022.117
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0.   引言

  • 优质烃源岩是油气形成的基础,其成因及分布预测更是影响油气勘探决策的关键因素。通常,烃源岩的发育受构造特征、沉积相和沉积环境等因素影响[1⁃3],尤其是湖泊面积小、相变迅速的小型断陷湖盆,环境变化对烃源岩的控制更为明显。沉积岩中常、微量元素组成是沉积环境变迁在地层中留下的可追溯记录,利用泥岩中元素含量及相关元素比值可恢复沉积时古沉积环境特征[4]。大量优质烃源岩沉积古环境相关的研究表明,古盐度、古生产力和氧化还原条件等沉积环境因素通过控制有机质的富集和保存,直接决定了烃源岩的质量[5⁃8]。徐家围子断陷是松辽盆地深层最主要的含气断陷,具有沉积环境复杂、烃源岩类型多、非均质性强的特点,不同类型烃源岩的质量及分布影响了该断陷多类型气藏的富集。前人对徐家围子断陷的构造特征、沉积相和储层特征进行了大量研究,但对于沙河子组沉积特征及古沉积环境尚未进行深入研究[9]。本次研究在沙河子组烃源岩特征和沉积环境、水体环境恢复的基础上,讨论断裂、沉积相和古沉积环境对烃源岩形成的控制作用,以期为松辽盆地深层天然气勘探方向优选提供地质依据。

1.   区域地质概况

  • 徐家围子断陷构造上位于松辽盆地深层东南断陷带,西邻中央古隆起、东接朝阳沟阶地,面积约5 350 km2,天然气探明地质储量近3 000×1012 m3,是盆地内发现天然气资源最多的深部断陷[10]。徐家围子断陷具有“下断上坳”的二元结构,深层即断陷期地层依次为火石岭组、沙河子组、营城组、登娄库组和泉头组。沙河子组是盆地强烈断陷期沉积的一套含煤湖相碎屑岩,也是盆地深层沙河子组致密气、营城组火山岩气、登娄库组构造气等多类型气藏的气源岩,可分为下部沙I段、上部沙II段2个层段和4个亚段（图1）。

    Figure 1.  Tectonic location of the Xujiaweizi Fault Depression and stratigraphic column of the Shahezi Formation

2.   沙河子组烃源岩形成的沉积环境与特征

    2.1.   沙河子组沉积相类型及分布

  • 沙河子组沉积期为松辽盆地断陷湖盆发育鼎盛期,研究区发育辫状河三角洲、扇三角洲、滨浅湖—半深湖和近岸水下扇等四种沉积相,断陷西侧以扇三角洲—滨浅湖、半深湖为主,局部见近岸水下扇；东侧则为辫状河三角洲,向前过渡为滨浅湖—半深湖相（图2）。沙I下亚段为初始断陷期,受古地貌影响沉积充填范围较小,沿徐西断裂下降盘发育多个小型扇三角洲朵叶体,徐中地区强烈下陷成半深湖,徐东斜坡发育辫状河三角洲。沙I上亚段时断陷西侧扇三角洲进一步扩大,以扇三角洲前缘最发育,徐东斜坡仍为辫状河三角洲,徐中地区湖盆范围扩大,发育滨浅湖、半深湖—深湖。沙II下亚段时东、西两侧发育三角洲平原迅速扩大。沙II上亚段时断裂活动减弱、湖泊面积减少,仅在三角洲沉积间隙见滨浅湖相。

    Figure 2.  Sedimentary profile of the Shahezi Formation in the Xujiaweizi Fault Depression

    • 2.2.   沙河子组烃源岩类型与特征

  • 岩心观察表明,沙河子组烃源岩分为暗色泥岩和煤岩两种类型。暗色泥岩包括辫状河（扇）三角洲前缘分流间湾灰色、灰绿色泥岩和半深湖相灰黑色泥岩三种类型。煤岩分为辫状河（扇）三角洲平原沼泽煤岩和滨浅湖煤岩两种类型,是分流河道两侧湿地泥炭沼泽化或湖泊大面积淤浅沼泽化形成的。不同类型烃源岩特征差异明显,暗色泥岩集中在下部沙I段,沙I下和沙I上亚段半深湖相泥岩TOC普遍大于2%,平均为4.06%和4.36%（表1）,为好—很好烃源岩,有机质类型以II型为主,部分为III型（图3）,沙II上和沙II下亚段三角洲前缘分流间湾暗色泥岩有机质丰度较差,TOC平均为2.34%和1.94%,为中等—好烃源岩。煤岩主要在沙II段,沙II上和沙II下亚段滨浅湖相煤岩TOC平均为29.65%和48.86%,全部为III型有机质,沙I下亚段三角洲平原沼泽煤岩厚度薄、分布零散,有机质丰度也相对较低。

    烃源岩类型有机质丰度有机质类型有机质成熟度
    TOC/%S1+S2/（mg/g）H/CO/CRo/%Tmax/℃
    沙I下泥岩0.26~45.964.03(44)0.01~122.842.86(44)0.42~1.140.79(14)0.02~1.160.22(14)2.18~3.912.40(52)480~598532(45)
    沙I上泥岩0.15~54.194.36(56)0.01~105.994.03(56)0.35~2.180.85(15)0.03~1.210.16(15)1.69~3.442.28(33)452~590512(36)
    煤岩1.31~65.1117.66(46)0.21~4.442.09(46)0.12~0.520.28(8)0.02~0.060.04(8)
    沙II下泥岩0.08~30.452.34(42)0.02~33.291.75(42)0.32~2.180.82(18)0.03~0.910.16(18)1.35~3.402.17(42)447~587493(42)
    煤岩2.86~75.5729.65(26)0.09~39.237.12(26)0.22~0.520.32(17)0.01~0.050.03(17)
    沙II上泥岩0.51~6.361.94(33)0.02~2.410.85(33)0.32~0.940.61(16)0.03~0.240.12(16)1.07~2.911.82(33)437~527457(33)
    煤岩0.63~83.0648.86(26)0.06~115.5216.83(26)0.22~0.680.38(12)0.02~0.060.04(12)

    Table 1.  Geochemical characteristics of source rock from the Shahezi Formation in the Xujiaweizi Fault Depression

    Figure 3.  Classification of elemental in kerogen, and pyrolysis parameters of mudstone in the Xujiaweizi Fault Depression

    沙河子组热演化程度高,沙I下亚段埋深最大,Ro和Tmax分别介于2.18%~3.91%和480 ℃~598 ℃,平均约2.40%和532 ℃,处于过成熟阶段。沙I上亚段Ro和Tmax平均为2.28%和480 ℃,达到高—过成熟。沙II下亚段Ro和Tmax分别介于1.35%~3.40%和306 ℃~587 ℃,平均为2.17%和437 ℃,为高成熟阶段。沙II上亚段Ro和Tmax平均为1.82 ℃和390 ℃,处于成熟阶段。

3.   沙河子组沉积期古环境特征

    3.1.   沙河子组沉积期古气候条件

  • 沉积岩中常、微量元素组成及相对含量记录了水体环境的变化,可提供古气候、古盐度、古生产力、氧化还原条件等环境变迁的信息[11]。潮湿气候条件下,沉积岩中Fe、Mn、Cr、V、Ni和Co等喜湿元素富集,气候干燥、水分蒸发时喜干型元素Ca、Mg、K、Na、Sr和Ba析出,利用二者之比即气候指数C和Sr/Cu比可反映沉积时古气候。通常温暖、潮湿气候下0.6<C<1、1<Sr/Cu<10,而0.2<C<0.6和Sr/Cu>10指示半潮湿—半干燥气候,干燥气候条件下C<0.2[12]。沙河子组不同层段泥岩的气候指数C和Sr/Cu变化较小,沙I段气候指数C介于0.05~1.02,平均为0.29,Sr/Cu介于5.36~24.88,平均为14.82,指示其为半潮湿—半干旱气候（图4）。沙II段气候指数C和Sr/Cu分别介于0.02~0.64和5.48~30.72,平均为0.24和18.02,与沙I段相似,表明沙河子组时气候稳定,为半潮湿—半干旱气候（图5a）。

    Figure 4.  Change modes of mudstone elemental contents and total organic carbon (TOC) of Da21 in the Xujiaweizi Fault Depression

    Figure 5.  Composition of mudstones in the Shahezi Formation, Xujiaweizi Fault Depression

    • 3.2.   沙河子组沉积期古盐度特征

  • 古盐度通过影响生物种类、繁盛程度控制有机质富集,沉积物中Sr含量、Sr/Ba和Mg/Ca与古盐度正相关,是水体盐度的指示[13]。通常Mg/Ca≤0.25时为微咸水,0.25<Mg/Ca≤0.50时为半咸水,0.50<Mg/Ca≤1.00时指示咸水,Mg/Ca大于1.00则为盐湖沉积[14]。下部沙I段Sr/Ba和Mg/Ca相对较大,介于0.04~0.66和0.56~4.74,平均为0.42和1.16,为咸水—盐湖环境（图5b）,沙II段沉积时盐度降低,Sr/Ba介于0.02~0.46,平均为0.16,Mg/Ca介于0.08~2.19,平均0.25,为半咸水—咸水环境。不同层段Sr含量也具有类似特征,沙I段Sr含量较高,从329 μg/g到594 μg/g均有分布,为咸水—盐湖,而沙II段沉积时Sr含量减小,为半咸水—咸水环境（图5c）。

    • 3.3.   沙河子组沉积期氧化还原条件

  • 贫氧强还原的深水环境有利于有机质保存,是形成优质烃源岩的必要条件。富氧的环境中V/（V+Ni）≤0.45、V/Cr≥4.25,厌氧的强还原环境中V/（V+Ni）≥0.60、V/Cr≤2.0,0.45<V/（V+Ni）<0.60和2.00<V/Cr<4.25则为贫氧的弱还原环境[15]。沙I段V/（V+Ni）普遍大于0.5,介于0.53~0.95,平均为0.82,V/Cr介于0.23~2.39,平均为1.58,为厌氧的还原环境；沙II段V/（V+Ni）和V/Cr分别介于0.44~0.78和0.88~3.64,平均为0.55和2.63,相对于沙I段水体还原性减弱,为还原—弱还原环境（图5d）。沙河子组Pr/Ph也具类型特征,Pr/Ph介于0.12~2.00,平均为0.84,大部分样品处于强还原—还原环境（图5e）；Pr/nC17和Ph/nC18关系也表明,沙河子组烃源岩均形成于还原环境,下部沙I段还原性更强（图5f）。

    • 3.4.   沙河子组沉积期古水深特征

  • 不同水体深度下元素的富集呈规律性变化,Fe、K、Al易与黏土矿物结合在滨岸带富集,而Mn、Ca和Mg则吸附于黏土矿物中,经长距离搬运到湖盆中部沉淀,因此随水体加深Fe/Mn、（Al+Fe）/（Ca+Mg）迅速减小。通常30<Fe/Mn<50为半深湖沉积,Fe/Mn>50为浅湖[16]。沙I段Fe/Mn和（Al+Fe）/（Ca+Mg）分别介于18.74~78.51和1.37~7.31,平均为33.18和5.63,以半深湖—深湖为主；上部沙II段Fe/Mn和（Al+Fe）/（Ca+Mg）明显偏高,分别介于32.39~292.62和3.92~19.82,平均为74.72和7.83,表明沙II段水体变浅（图5g）,随水深减小,V/Cr增大、还原性减弱（图5h）。

4.   沙河子组烃源岩形成控制因素

    4.1.   断裂活动形成可容纳空间,控制了泥岩的厚度

  • 湖相烃源岩中有机质富集受多种因素控制,其中构造活动从宏观上形成可容纳空间并决定沉积体发育,进而影响断陷湖盆烃源岩的形成[17]。沙河子组沉积期徐西、徐中和徐东断裂活动在下降盘形成巨大可容纳空间,是暗色泥岩堆积的主要场所。尤其是沙I下亚段时,徐西断裂活动强度最大,此时湖盆面积较小,徐家围子断陷迅速下陷,在徐西断裂下降盘处形成深凹区,堆积巨厚的非补偿性沉积的暗色泥岩（图6）,最大厚度达120 m。沙I上亚段时徐西、徐中断裂仍保持较高的活动强度,沿徐西、徐中断裂下降盘形成Da2井、S3井、Xu28井、Xu904井等串珠状分布的深凹区,堆积巨厚的滨浅湖、半深湖相泥岩。

    Figure 6.  Cross⁃section of different source rocks in the Shahezi Formation, Xujiaweizi Fault Depression

    沙II下亚段,徐西断裂活动性减弱,对暗色泥岩的控制也随之减弱,徐中地区暗色泥岩最大厚度仅为80 m（图7）。沙II上亚段时随徐西、徐中断裂活动进一步减弱,水体持续变浅,暗色泥岩厚度进一步减小,同时沙河子沉积末期徐东断裂开始活动,造成沙河子地层抬升,造成断陷西部和南部沙II上和沙II下亚段大范围抬升剥蚀,仅在Da2、Xu28、Xu401等局部深凹残留沙II段泥岩。

    Figure 7.  Distribution of mudstones in the Shahezi Formation, Xujiaweizi Fault Depression

    总体上,徐西、徐中断裂活动控制了沙I段暗色泥岩的分布,由断裂活动形成的安达凹陷、徐中凹陷等局部深凹是泥岩富集有利区,远离断裂的徐东地区泥岩不发育；沙II段时随徐西、徐东断裂活动降低、对泥岩控制减弱,泥岩厚度明显减小；沙河子沉积期末徐东断裂活动造成地层抬升、沙II段大范围剥蚀,泥岩局限在北部安达凹陷和徐中凹陷。

    • 4.2.   沉积相影响有机质输入,决定了烃源岩类型

  • 陆相断陷湖盆面积小、沉积相变快,不同沉积环境中机质输入存在差异,决定了烃源岩类型,是影响烃源岩发育的重要因素。沙I下亚段扇三角洲前缘分流间湾泥岩CPI和OEP分别介于0.62~1.03和0.76~1.16,奇偶优势不明显,C21+22/C28+29介于0.85~6.42,∑C21-/∑C21+介于0.45~1.34,是以nC21为主峰的前峰型（图8a）,母质为低等水生生物和陆源高等植物的混合型母质。沙I上亚段辫状河三角洲前缘分流间湾泥岩也有类似特征,CPI和OEP平均为0.73和0.85,C21+22/C28+29介于0.86~5.82,∑C21-/∑C21+介于0.49~1.26,是以nC21、nC23为主峰的前峰型（图8b）,反映低等水生生物是烃源岩母质的重要来源,同时有部分陆源高等植物混入。沙I上亚段滨浅湖泥岩C21+22/C28+29介于1.24~23.66,∑C21-/∑C21+介于0.69~2.12,正构烷烃是以nC19为主峰的前峰型（图8c）,母质以水生生物为主,有机质类型以II型为主,含少量I型。沙II上亚段半深湖相泥岩正构烷烃是以nC19为主峰的前峰型（图8d）,C21+22/C28+29介于1.29~24.05,∑C21-/∑C21+介于0.89~2.29,表明母质以水生生物为主,同时含部分陆源高等植物,有机质类型以II型为主,含少量I型。

    Figure 8.  Gas chromalogram of different source rocks in the Shahezi Formation, Xujiaweizi Fault Depression

    沙II段煤岩正构烷烃是以nC27为主峰的后峰型,指示其为III型干酪根（图8e,f）。煤岩发育受沉积环境控制明显,断裂活动对其影响较小。沙I段时湖盆范围小,断陷边部碎屑物质在两侧三角洲平原堆积,促进植物生长、沼泽化,在巨厚的砂砾岩中形成夹层型薄层煤岩,单层厚度小于2 m、分布范围窄（图9）。沙II沉积时,湖盆范围大、水体浅,早期沉积淤浅,植物大量生长并逐渐沼泽化,形成与暗色泥岩频繁互层的滨浅湖相煤岩,厚3~5 m,最大达60 m,也是沙河子组煤岩发育的主要时期。

    Figure 9.  Distribution of coals in the Shahezi Formation, Xujiaweizi Fault Depression

    • 4.3.   古沉积环境条件差异对烃源岩质量的控制作用

  • 优质烃源岩的形成是古气候、古盐度、古生产力等有机质输入和氧化—还原条件、沉积—沉降速率等保存条件综合作用的结果[18]。沙河子组沉积时为半潮湿—半干旱环境、气候波动较小,各层段间气候指数C和Sr/Cu与TOC相关性较差（图10a）,暗示沙河子组沉积时气候波动较小、对有机质富集的影响有限。

    Figure 10.  Relationship between TOC and paleoenvironment index in the Shahezi Formation, Xujiaweizi Fault Depression

    Sr含量和Mg/Ca等古盐度参数与TOC正相关（图10b）,表明古盐度有利于有机质富集。较高的盐度促进Ba、P等营养元素的输入,其中P作为藻类繁盛必须的限制性元素,是引发低等水生生物勃发、形成较高的古生产力的必要条件[19]。沙I段泥岩母质中藻类等低等水生生物含量较高,较高古生产力指标P/Al也与TOC正相关（图10c）,而沙II段煤岩母质为陆源高等植物,TOC与P/Al关系不明显。同时,较高的盐度造成水体中盐度分层、限制水体对流,形成有利于沙I段有机质保存的强还原环境。

    沙I段时强烈的断裂活动形成的半深湖—深湖既是烃源岩堆积的有利场所,同时深水、咸水条件下形成的强还原环境也是有机质保存的有利条件[20]。TOC与Fe/Mn、(Al+Fe)/(Ca+Mg)等古水深指标关系表明,由下部沙I段到上部沙II段,随水体变浅、盐度降低、还原性减弱,沙II段煤岩TOC也迅速降低（图10d）。V/Cr和Pr/Ph等氧化还原指标也具有类似的变化特征,进一步证实沙河子组下部沙I段沉积时强烈断陷形成的“深水窄盆”盐度高、还原性强、有机质保存条件好（图10e,f）,上部沙II段断裂活动弱,形成的“浅水广盆”盐度低、还原性较差,泥岩中有机质部分氧化、质量变差,而以滨浅湖相煤岩为主。由古盐度、古水深和氧化还原组成的古环境差异影响有机质富集和保存,是导致下部沙I段泥岩有机质丰度较高,而上部沙II段泥岩有机质丰度变差、煤岩发育的主要原因。

5.   结论

  • （1） 沙河子组暗色泥岩集中在下部沙I段,有机质丰度高,以II型干酪根为主,普遍达到过成熟阶段；煤岩主要分布在上部沙II段,为III型干酪根,处于成熟—过成熟阶段。

    （2） 沙I段沉积期断裂活动强,形成的“深水窄盆”水体深、盐度高、还原性好,有利于半深湖相泥岩有机质的富集和保存；沙II段沉积期断裂活动弱,形成“浅水广盆”水体浅、盐度低、还原性较差,不利泥岩发育,而以湖泊大面积淤浅沼泽化形成的煤岩为主。

    （3） 断裂下降盘可容纳空间大,是泥岩堆积的主要场所,控制了烃源岩厚度；沙I段时深水的高盐、强还原环境有利于泥岩中有机质富集,沙II段时水体变浅、盐度和还原性降低,泥岩质量随之变差,而以煤岩为主。

Reference (20)

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