内蒙古西部额济纳旗祥探9井石炭系—二叠系烃源岩地球化学特征

摘要

额济纳旗地区祥探9井揭示的石炭系—二叠系为火山岩+碎屑岩组合,火山岩锆石U-Pb谐和年龄分布在(302.2±7.8)～(319.9±4.5 )Ma之间,对应的地层时代相当于晚石炭世,岩性组合特征与大狐狸山剖面干泉组(C2P1g)可以对比,确定为干泉组.该井揭示石炭系—二叠系1027.4m(未穿),其中4m和4m以上的暗色泥岩(或粉砂质泥岩)20层,累计厚度288.5m,占地层厚度的28.1％.少量钻井岩心样品进行的烃源岩地球化学特征分析表明,有机质丰度中等—好,干酪根类型为Ⅰ—Ⅱ1型,有机质演化进入成熟—高成熟阶段,显示了良好的生烃条件.与邻近的大狐狸山剖面干泉组烃源岩样品分析结果对比,祥探9井取得的样品有机质丰度显著高于地表剖面样品,TOC为地表剖面样品的1.8倍,热解产油潜量(S1+S2)为地表剖面样品的295.3倍,氯仿沥青“A”为地表剖面样品的32.8倍,总烃(HC)为地表剖面样品的42倍.干酪根类型评价指标分析,井下岩心样品H/C-O/C能较好地反映干酪根类型,地表剖面样品受风化作用的影响H元素显著降低,H/C-O/C无法进行干酪根类型评价,说明风化作用对烃源岩丰度和类型评价指标的影响较大.%The Carboniferous-Permian Hthologic association of Xiangtan 9 well in Ejin Banner is composed of volcanic rocks and clastic rocks, the concordia ages of volcanic zircon are from 302.2Ma±7.8Ma to 319.9Ma±4.5Ma, which shows that the stratigraphic age is late Carboniferous. The lithologic association is consistent with that of Ganquan Formation along Dahulishan section, so the formation of Xiangtan 9 well should be Ganquan Formation. The Carboniferous-Permian strata in the drill hole is 1027.4m (nonpene-trated) in thickness. In this huge thickness, there are 20 layers of dark mudstones (silty mudstone) with the thickness of each layer more than 4m, and the total thickness of these layers is over 288.5m, accounting for 28.1% of the total thickness. The analytical results of geochemical characteristics of some drill hole cores indicate that the organic matter abundance is middle-higher, the kerogen type is I - II,, and the organic matter has developed into the mature-high mature period. All these characteristics show that the hydrocarbon source rocks have good hydrocarbon-generating conditions. Compared with the analytical results of Ganquan Formation hydrocarbon source rocks in adjacent Dhulishan section, the organic matter abundance of Xiangtan 9 well is obviously higher than that of the surface samples, 1.8 times that of surface samples for TOC, 295.3 times that of surface samples for potential hydrocarbon generation amount (S1+S2), 32.8 times that of surface samples for chloroform asphalt "A" and 42 times that of surface samples for total hydrocarbon. The kerogen type evaluation shows that H/C-O/C of the core samples can better reflect kerogen type, but the kerogen type is not effectively evaluated by H/C-O/C of the surface samples due to the decrease of H resulting from the weathering effect. All of the above data show that weathering has exerted great influence on the abundance and type of hydrocarbon source rocks.