Paleo-Oil-Reservoirs Reconstruction and Oil Correlation of Changxing Formation in the Yuanba Gas Field, Sichuan Basin
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摘要: 目前虽认为元坝气田长兴组天然气主要为原油裂解气，但尚未进行该气田的古油藏的定量恢复并计算原油裂解气的资源量，并系统分析古油藏原油的来源.长兴组储层普遍可见固体沥青，是原油裂解的直接产物，且原油裂解在本区构造抬升变形前就已经完成，因此储层沥青可用来识别古油层的分布.根据储层沥青的纵向和平面分布，确定了7个可靠的古油藏和4个可能的古油藏，并运用容积法恢复出本区聚集的古原油为6.14×108 t，计算出相应的原油裂解气为3 807.08×108 m3，远大于现今气田的天然气探明储量，表明原油裂解气可以提供充足的气源，进一步证明了天然气主要为原油裂解气.通过长兴组储层沥青与不同层系烃源岩干酪根的碳同位素δ13C值对比，并结合烃源层分布和TOC等资料，确定古油藏原油主要来源于有机质类型以Ⅱ型为主的上二叠统吴家坪组烃源岩，其次为长兴组/大隆组烃源岩.后者主要分布在广元-南江-通江地区，该区的天然气勘探不能忽视该套烃源岩的生烃潜力与成藏贡献.Abstract: The natural gases from the Changxing Formation (P2ch) in the Yuanba gas field are mainly derived from oil cracking, but the volume of oil-cracking-gas has not been evaluated quantitatively and the source of the paleo-oil has not been studied systematically. Solid bitumen, commonly developed in the P2ch reservoirs, is a direct product of oil cracking which had finished before the regional uplift and structure deformation in the Yuanba gas field. As a result, the paleo-oil zone can be recognized by the solid bitumen. Basing on the lateral and vertical distribution of solid bitumen, seven reliable and four probable paleo-oil reservoirs have been found out. The paleo-oil accumulated in these reservoirs by volumetric method is about 6.14×108 t, and the corresponding oil-cracking-gas is about 3 807.08×108 m3. The volume of oil-cracking-gas is much larger than the natural gas found in the Yuanba gas field, which indicates that the oil-cracking-gas can supply sufficient gases for the present gas reservoirs, and further proves that the natural gases are mainly derived from oil cracking. The δ13C correlation of solid bitumen and kerogens from different source rocks, and total organic carbon (TOC) data show that the paleo-oil was derived from type-Ⅱ source rock mainly in the Upper Permian Wujiaping Formation, and then in the Changxing/Dalong Formation. The latter is mainly developed in Guangyuan-Nanjiang-Tongjiang area, and the hydrocarbon generated by this source rock cannot be neglected during the natural gas exploration in this area.
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Key words:
- paleo-oil reservoir /
- oil correlation /
- petroleum geology /
- Yuanba gas field /
- Sichuan basin
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图 1 川东北地区长兴期沉积相展布与主要气田的位置分布
Fig. 1. The sedimentary facies and locations of main gas fields of Changxing Formation in the northeastern Sichuan basin
图 2 元坝气田长兴组的典型固体沥青、沥青包裹体和油包裹体
a.Y27井，白云岩的基质孔隙中见不均匀分布的沥青；b.Y29井，白云岩中含有大量的固体沥青，使得整个岩心呈黑色；c.Y224井，白云岩的孔洞中见沥青不完全充填；d.Y29井，单偏光，岩石铸体薄片中见沥青充填在白云石的晶间孔中；e.Y102井，单偏光，铸体薄片中见大量沥青充填在晶间孔中；f.Y224井，单偏光，岩石铸体薄片见沥青充填在晶间孔及晶间扩大孔中；g~h.Y2井，可见较多的沥青包裹体分布(g为单偏光，h为相同视域的荧光)；i.Y2井，荧光，白云石的裂纹中见发蓝白色荧光的油包裹体
Fig. 2. The photographs of typical solid bitumen, bitumen inclusions and oil inclusions from the Changxing Formation reservoirs in the Yuanba gas field
图 3 元坝气田长兴组-飞仙关组固体沥青含量与古孔隙度的关系
Fig. 3. The solid bitumen content vs. paleo-porosity of the Changxing and Feixianguan Formations reservoirs in the Yuanba gas field
图 4 元坝气田长兴组Y2井(a)和Y123井(b)沥青与古油层分布
Fig. 4. The solid bitumen and paleo-oil zones distribution in the Y2 well and Y123 well from the Changxing Formation reservoirs in the Yuanba gas field
图 5 元坝气田长兴组古油藏的分布
Fig. 5. The paleo-oil reservoirs distribution of the Changxing Formation in the Yuanba gas field
图 6 元坝气田长兴组天然气乙烷、沥青和不同层位烃源岩的干酪根碳同位素对比
志留系和寒武系干酪根的碳同位素分布范围据朱扬明等(2008)，且样品为来源于川东北地区
Fig. 6. The δ13C correlation between ethane solid bitumen from Changxing Formation, and kerogens from different source rocks in the Yuanba gas field
图 7 元坝气田二叠系烃源岩的O/C原子比与H/C原子比的分布
Fig. 7. The atomic O/C and H/C ratios of Permian source rocks in the Yuanba gas field
图 8 川东北地区LB1-Y3-Y6-JX1井的二叠系总有机碳含量(TOC)剖面(钻井位置见图 1)
Fig. 8. The total organic carbon (TOC) sections of Permian source rocks in LB1-Y3-Y6-JX1 wells in the northeast Sichuan basin
图 9 川东北地区上二叠统大隆组烃源岩的厚度展布
Fig. 9. The source rocks distribution of Upper Permian Dalong Formation in the northeast Sichuan basin
表 1 元坝气田古原油规模及原油裂解气量估算
Table 1. The calculated volume of paleo-oil reservoirs and oil-cracking gases of the Changxing Formation in the Yuanba gas field
落实情况 油藏 Ao(km2) H(m) So(%) Φ(%) Qoil(108 t) Qgas(108 m3) 可靠的 Y9井 10.72 36.57 70 4.80 0.11 65.70 可靠的 Y123-Y16井 60.56 19.17 70 9.36 0.61 379.40 可靠的 Y101井 48.95 25.46 70 9.10 0.64 395.98 可靠的 Y12井 81.27 30.00 70 7.70 1.06 655.48 可靠的 Y27-Y204-Y273井 99.06 38.93 70 10.01 2.17 1 347.83 可靠的 Y28井 13.21 30.00 70 7.14 0.16 98.80 可靠的 Y224井 24.87 39.25 70 11.94 0.66 406.95 推测的 Y10井 15.04 20.00 70 4.80 0.08 50.41 推测的 Y10-1井 10.66 50.00 70 4.96 0.15 92.31 推测的 Y103H井 12.24 30.00 70 4.96 0.10 63.59 推测的 Y225H井 48.24 30.00 70 4.96 0.40 250.63 总计 6.14 3 807.08 
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