Magnetic, Geochemical and Mineralogical Structure and Significance in Complete Space of Oil and Gas Reservoir: Implications for Formation Mechanism of "Chimney Effect
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摘要: 位于松辽盆地南部某油气田边缘一个油气显示井的磁性测量结果表明, 岩石具有明显的磁性变异结构特征.以磁化率(κ) 为例, 在低缓的背景磁性上主要呈现高强度与中等强度的2类异常: 大于400× 10-5为强磁性, 100× 10-5~ 150× 10-5为中等强度磁性.结合录井资料与地球化学分析解释可知, 强磁性异常主要位于取样间隔的底部干层区, 少数样品位于浅部油气显示层; 中等强度异常主要位于2 70 0m附近的油气显示层.岩石磁性与地球化学成分对应分析表明, 油气显示层岩石的C1和C2成分与磁化率(κ)、饱和磁化强度(Js) 和饱和等温剩余磁化强度(SIRM) 之间呈明显的正相关, 与内禀矫顽力(Hc) 为逆相关, 而干层岩石相关性不明显.矿物成分分析显示, 岩石中主要含铁矿物为磁铁矿、磁赤铁矿、赤铁矿、黄铁矿和菱铁矿.其中磁赤铁矿主要分布在具有中等磁性强度的油气显示层, 而高含量(最高含量达12.5 %) 的磁铁矿主要位于底部干层.磁赤铁矿与赤铁矿、黄铁矿及菱铁矿含量之间相关特征初步表明, 自生磁赤铁矿可能主要来源于原地赤铁矿与黄铁矿.推测油气显示层岩石中磁性偏软的磁赤铁矿属于烃蚀变的产物, 为烃微渗漏效应(或烟筒效应) 的全空间形成机理提供了新证据Abstract: Magnetic measurements of rocks in an oil and gas bearing well located on the margin of an oil and gas field, southern Songliao basin, show that the rocks there are characterized by the conspicuous magnetic anomalous structure. The magnetic susceptibility anomalies (κ), for example, are classified as two categories: the high intensity (κ > 400×10-5) and the moderate intensity (κ : 100×10-5 - 150×10-5) on the low background magnetism. As shown in the interpretation of the logging references and the geochemical analysis, the strong magnetic anomaly is located mainly in the dry layer at the bottom of the sampling interstitial positions. Few samples are located in the oil and gas display layer in the shallow part of the field. The intermediate magnetic anomaly is located in the oil and gas display layer at the depth around 2 700 meters. The correlation between the magnetism and geochemical compositions shows a positive correlation between geochemical compositions C1, C2 and magnetic intensities parameters (i.e. susceptibility κ, saturation magnetization Js and saturation isothermal remnant magnetization SIRM), but a negative correlation between geochemical compositions C1, C2 and intrinsic coercivity in the rocks of the oil and gas bearing layers. However, these correlations are not found in dry layers. The analysis of the iron bearing mineral compositions indicates that the major iron bearing minerals in the rocks are magnetite, maghemite, hematite, pyrite and siderite. In particular, the maghemite distributes mainly in the oil and gas bearing layer with the intermediate magnetic intensity. But the magnetite with the iron contents of 12.5% distributes mainly in the dry layer at the bottom. The correlations in contents between maghemite and hematite and between pyrite and siderite indicate that the authigenous maghemite may have originated largely from the autochthonous hematite and pyrite. It can be inferred that the maghemite, whose magnetic intensity is weak inclined in the rocks of the oil and gas bearing layer, is a product of the hydrocarbon alteration, providing us with some new evidence for the complete space formation mechanism of the hydrocarbon micro percolation effect (or chimney effect).
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图 2 烃成分随深度变化曲线
a1, a2.顶空气烃成分; b1, b2.吸附烃成分
Fig. 2. Geochemical compositions of samples changing with depth
图 3 含铁矿物含量随深度的变化曲线
a.磁铁矿; b.磁赤铁矿; c.赤铁矿; d.菱铁矿; e.黄铁矿
Fig. 3. Scatter plots of iron-bearing minerals content changing with depth
图 4 磁性参量与烃成分之间的相互关系
a1—a8.全部油气显示层; b1—b8.全部干层
Fig. 4. Scatter plots of relations between magnetic parameters and hydrocarbon compositions
图 5 磁赤铁矿与赤铁矿(a)、菱铁矿(b) 及黄铁矿(c) 含量相互关系
Fig. 5. Relationship of contents between maghemite and hematite (a), siderite (b) and pyrite (c)
表 1 主要代表性层位样品磁性参量的统计值
Table 1. Magnetic parameters of rocks
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