Characteristics and Formation Mechanism of Mesozoic Underpressured Reservoirs in Ordos Basin
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摘要: 首次对鄂尔多斯盆地中生界油藏压力进行了研究, 发现中生界油藏主要为超低压油藏, 并且不同地区和不同层位油藏的异常低压差别显著.研究结果表明, 随着地层剥蚀厚度增大和油藏抬升后温度降低值增加, 储层压力系数呈减少趋势; 延长组油藏抬升温度降低后使储层孔隙水的体积收缩量达0.82%~1.94%.这些指示了盆地在白垩纪末期长时间强烈抬升, 地层剥蚀和古地温降低作用是形成低压油藏的主要原因.认为鄂尔多斯盆地中生界这种低压封闭体系对油藏的保存有利, 同时对油藏调整、油气运移再富集和油水分布等成藏方面可能起到了重要作用.提出了陕北地区长6低压油藏由东南向西北方向富集和长7砂岩透镜体不含水低压油气藏形成, 均与这种低压封闭体系分布密切相关.Abstract: The pressure characteristics of Mesozoic oil reservoirs in Ordos Basin was studied for the first time. It is found that the Mesozoic oil reservoirs are mainly ultra-underpressured ones and the abnormal underpressure difference in oil reservoirs among different regions and different layers is distinct. The results show that oil reservoir pressure coefficients exhibit a decreasing trend with the increase of eroded strata thickness and temperature drop value of oil reservoirs after, and the reservoir porewater volume contraction amounts to 0.82%-1.94% as a result of temperature drop after uplift of Yanchang Formation oil reservoirs, which indicates that the function of strata erosion and paleotemperature decrease due to long time and strong uplift of the basin at the end of the Cretaceous result in the formation of underpressured oil reservoirs. The underpressured closed system of Mesozoic in the Ordos Basin is believed to be advantageous for oil reservoir preservation and may have played an important role in oil reservoir adjustment, re-enrichment of hydrocarbon by migration and oil and water distribution in oil reservoirs. It is concluded that Chang-6 underpressured reservoir enriched from the southeast to the northwest in the North Shaanxi area and presence of Chang-7 lenticular sandstone body reservoirs with absence of water are closely related to the distribution of such underpressured closed systems.
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Key words:
- underpressured oil reservoir /
- formation mechanism /
- oil bearing formation /
- lenses /
- Mesozoic /
- Ordos Basin
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表 1 鄂尔多斯盆地中生界油藏储层平均压力系数
Table 1. Mean pressure coefficients of Mesozoic reservoir rocks
层位 马岭 姬塬 华庆 西峰 吴旗 陕北 平均值 延10 0.93(1) 0.93(1) 延安组 0.80(4) 0.80(4) 长1 0.70(1) 0.86(1) 0.65(1) 0.74(3) 长2 0.75(7) 0.81(5) 0.83(3) 0.71(4) 0.77(14) 长3 0.71(16) 0.74(2) 0.71(18) 长4+5 0.67(7) 0.69(2) 0.84(2) 0.72(11) 长6 0.63(8) 0.69(5) 0.71(35) 0.70(48) 长7 0.83(2) 0.70(1) 长8 0.74(10) 0.76(16) 0.75(26) 长9 0.75(1) 0.75(1) 长10 0.79(1) 0.79(1) 注:括号内数字为样品数;压力系数资料来自长庆油田,为实测数据. 表 2 m*值分布
Table 2. Distribution of m* values
古峰庄 姬塬 白豹 西峰 洛川 最大埋深(m) 3 015 3 150 3 300 3 000 3 015 最大剥蚀厚度(m) 630 600 1 100 630 1 500 最大残余厚度(m) 2 385 2 550 2 200 2 370 1 515 最大剥蚀时m* 0.042 0.044 0.057 0.042 0.040 表 3 长7烃源岩最大古地温和延长组油藏二期成藏古地温
Table 3. The max paleogeothermal for Chang-7 source rocks and paleotempeature of the second stage of Yanchang Formation reservoir forming
古峰庄 姬塬 白豹 西峰 洛川 早白垩世长7烃源岩最大埋深(m) 2 800 2 850 2 950 2 300 2 000 长7烃源岩生烃最大古地温(℃) 130 125 130 110 100 二期成藏古温度(℃) 110 110 110 100 100 注:长7烃源岩生烃最大古地温根据地层埋藏史和热演化史研究所获得;二期成藏古温度为储层包裹体均一温度. 表 4 鄂尔多斯盆地延长组各油层组油藏现今平均温度(℃)
Table 4. Average current temperature of Yanchang Formation (℃)
层位 姬塬 华庆 西峰 吴旗 陕北 长1 61.6(1) 38.6(3) 长2 62.9(6) 51.5(6) 47.6(2) 31.5(14) 长3 57.0(12) 长4+5 69.9(5) 60.9(2) 44.8(4) 长6 62.7(5) 57.5(7) 45.6(56) 长7 59.6(1) 长8 63.5(8) 66.1(11) 长10 平均 64.8 58.3 66.1 52.6 40.1 △T 45.2 51.7 33.9 59.9 注:括号内数字为样品数;△T为成藏二期成藏温度与现今油藏平均温度之差,即油藏抬升后平均降低温度. 表 5 鄂尔多斯盆地延长组油藏储层孔隙水体积收缩百分含量(%)
Table 5. Pore volume shrinkage percentage contents for Yanchang Formation (%)
层位 姬塬 华庆 西峰 陕北 长2 1.49 1.94 1.47 长3 长4+5 1.13 1.59 长6 1.28 1.21 长8 1.33 0.82 -
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