Coal-bearing source rocks formed in the transitional stage from faulting to depression nearshore China——A case from the Pinghu Formation in the Xihu Sag
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摘要:
西湖凹陷是东海陆架盆地最重要的含油气凹陷，古近系晚始新统平湖组烃源岩是西湖凹陷主力烃源岩，晚始新世平湖组沉积时期西湖凹陷处于断―坳转换时期，平湖组为一套海陆过渡相煤系地层，形成于半封闭海湾沉积环境。本文对断坳转换平湖组煤系烃源岩的分布、母质来源和生烃特征进行系统分析。结果表明：断—坳转换下的煤系烃源岩广覆式分布，平湖组煤系烃源岩富含反映干燥针叶类裸子植物来源的异海松烷和潮湿蕨类植物来源的β-扁植烷等二萜类化合物，从斜坡带向凹陷中央具有β-扁植烷增加的特征。从显微结构上表现为富含树脂体的煤系烃源岩，其具有“早期利于生油、晚期持续生气”的生烃模式。正是这种分布格局、母质来源和生烃特征决定了西湖凹陷油气藏具有“东气西油”的分布格局。
Abstract:The Xihu Sag is the most important oil-bearing sag in the East China Sea Continental Shelf Basin, in which prevailed the Late Eocene source rocks of the Pinghu Formation, which was formed while the sag was situated in a transitional stage from faulting to depression. The Pinghu Formation is a set of marine-continental transitional coal-bearing sequence deposited in a semi-enclosed depositional environment of a bay. The authors systematically studied the distribution pattern, source of parent material and hydrocarbon generation characteristics of the coal-bearing source rocks. The results show that the coal-series source rocks were distributed widely in the sag and formed in a faulting-depression transitional stage. They are rich in iso-seasones and wet fern-derived β-bentane, reflecting a source from dried coniferous gymnosperms, in which diterpenoids are characterized by an increase in β-flatane from the slope zone to the center of the depression. From the microstructure, it is a kind of resin-rich coal-based source rock, which generated oil in early stage but gas in late-stage. The distribution pattern of “Gas in the East and Oil in the West” in the Xihu Sag is also determined by the distribution pattern, parent material source and hydrocarbon generation characteristics of the source rocks.
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Key words:
- source rock /
- resinite /
- diterpanes /
- the Xihu Sag
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表 1 西湖凹陷烃源岩丰度统计表
Table 1. Organic matter abundance in source rocks of Pinghu Formation in Xihu Sag
岩性 层段 TOC/% （S1+S2）/（mg/g） HI/（mgHC/gTOC） 氯仿沥青A/% HC/10−6 泥岩 一、二段 0.5～9.7
1.68（336）0.22～27.0
3.97（336）31.4～656.3
222.5（311）0.0033～0.624
0.108（44）157.6～5 884
633.84（43）三、四段 0.5～9.8
1.96（389）0.21～44.4
4.69（387）21.7～840.2
193.2（383）0.0142～0.619
0.16（26）110.7～2 834
866.73（21）五段 0.5～6.59
1.26（98）0.34～9.79
2.45（98）30.8～453.3
11.7（98）0.0276～0.5074
0.127（4）351.1～1 298.2
432.7（3）煤 一、二段 13.99～72.9
46.01（36）18.3～219.2
123.16（36）114～868.9
260.7（22）0.2261～3.3928
1.53（18）755.4～10 677.1
474.2（15）三、四段 20.8～57.5
38.95（21）51.4～215.8
123.6（21）192.9～403.3
261.8（17）1.81～1.93
1.87（2）8 203～10 998
9 600.5（2）五段 12.65～61.9
41.72（5）37.3～193.8
91.43（5）92.3～326.0
226.3（5）0.313（1） 1 447.62（1） 注： 
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表 2 不同构造带平湖组泥岩丰度统计表
Table 2. Organic matter abundance in mudstones of Pinghu Formation from different tectonic units in Xihu Sag
构造带 TOC/% （S1+S2）/（mg/g） HI/（mgHC/gTOC） 氯仿沥青A/% HC/10−6 杭州斜坡带 0.51～3.9
1.4（39）0.28～14.9
2.75（39）51.6～382.8
155.9（35）0.003 3～0.687
0.032（12）126.3～2 400
672.2（5）平湖斜坡带 0.5～9.84
1.79（593）0.22～44.4
4.32（590）21.9～840.2
199.1（587）0.034 5～0.624
0.168（47）110.7～5 884
771.9（47）天台斜坡带 0.59～9.18
2.02（20）0.21～12.6
1.72（20）21.65～130.5
67.6（20）— — 黄岩构造带 0.52～3.3
1.38（109）0.51～12.3
4.39（109）63.4～656.3
334（88）0.028 3～0.288 6
0.082（15）167～1 653
474.2（15）天台构造带 0.51～8.37
2.34（62）0.3～16.5
3.44（62）44.9～191.3
112.1（62）— — 注： 
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表 3 不同构造带平湖组煤丰度统计表
Table 3. Organic matter abundance in coals of Pinghu Formation from different tectonic units in Xihu Sag
构造带 TOC/% （S1+S2）/（mg/g） HI/（mgHC/gTOC） 氯仿沥青A/% 平湖斜坡带 20.8～61.9
41.01（29）51.36～215.75
122.55（29）92.27～403.32
255.52（24）0.248～1.93
1.35（4）天台斜坡带 53.29（1） 159.7（1） 277（1） 黄岩构造带 12.65～72.95
42.76（26）18.29～219.15
115.32（26）114～869
278.1（13）0.226 1～3.392 8
1.542（17）天台构造带 42.4～67.8
54.78（6）104.49～164.56
129.22（6）180.3～239.5
215.3（6）注： 
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表 4 平湖组煤显微组分统计表
Table 4. Distribution of maceral composition in coals in the members of Pinghu Formation
组段 C1/% C2/% V/% I/% Al/% Cu/% Sp/% Re/% E/% 样品数 一段 5.0～21.6
13.53.0～84.4
48.377.5～93.5
86.50～7.7
2.80～1.9
0.80～3.3
1.30～6.6
1.62.6～9.9
6.86.2～16.5
10.76 二段 2.8～30.8
14.514.2～78
57.873.1～92
85.90～24.6
1.70～2.5
0.80.2～5.6
2.10.2～5.4
2.41.2～14
6.62.3～20.3
12.423 三段 0～40.9
15.612.8～95.8
53.977.7～99.6
870～14
1.40～8.8
0.40～13.8
2.10～8.8
2.10～19.7
5.90.4～21.7
11.634 四段 1.6～57.6
15.014.7～87.9
58.873.9～98.4
88.80～6.8
1.30～7.7
0.70～8.9
1.90～9.7
2.10～17.3
3.80.8～23.2
9.848 五段 1.7～21.2
9.937.2～80.9
63.374.6～97.7
87.20～8.5
2.10～15.1
1.90.3 ～5.4
20～4.7
1.30～17.7
4.51.5～23
10.619 注：C1为均质镜质体；C2为基质镜质体；V为镜质组；I为惰质组；Al为藻类体；Cu为角质体；Sp为孢子体；Re为树脂体；E为壳质组；各有机显微组分总和为100%。
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表 5 不同构造带煤显微组分统计表
Table 5. Maceral composition of coals from different tectonic units
构造带 C1/% C2/% V/% I/% Al/% Cu/% Sp/% Re/% E/% 数量（件） 杭州斜坡带 0.5～82.4
18.62.5～83.6
61.079.4～90.9
86.40～3.1
0.70～0.9
0.20.4～4.3
1.50.3～9.1
3.30～11
7.18.7～19.2
13.014 平湖斜坡带 0～57.6
14.63～95.8
54.464.3～99.6
87.10～14
1.70～15.1
0.90～13.8
2.10～9.7
2.10～19.7
4.90.4～33.1
11.1115 黄岩构造带 0.7～38.9
1635.2～84.4
63.973.1～93.5
87.60～24.6
1.40～11
1.20～6.6
1.60～5.4
1.20.4～13.8
6.62.3～16.6
10.943 嘉兴构造带 5.5～36.8
15.335.1～86.3
70.786～95.5
910～3.2
0.80～2.
70.90～1
0.40～2.2
1.12.6～10.9
5.83.8～13.7
8.28 天台构造带 0～67
22.69.9～98.3
63.177.6～98.7
90.90～6.4
2.00～6.1
1.30～6.3
1.00～0.3
0.10～15.1
3.21.3～16.4
7.19 注：中央反转构造带北部的为花港组煤样
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