鄂尔多斯盆地西缘马家滩地区延长组烃源岩研究

鄂尔多斯盆地延长组深湖相页岩在盆地内部均有分布,但在其西缘冲断带地区是否仍然存在、如何分布、是否具有生烃潜力,这些问题尚待进一步研究。本文根据近期在石沟驿向斜所钻的驿探1及其周边钻井、露头和地震资料对延长组暗色页岩厚度、层位、展布和生烃条件进行了研究。烃源岩测井响应、地震层位追定、地震正演模型及烃源岩热解等分析结果表明:(1)青铜峡-固原断裂以东的西缘冲断带中部马家滩地区仍然发育2-4层延长组黑色烃源岩,呈高伽马、高时差和低电阻率测井响应特征,但其单层厚度较薄,一般只有3-5m,累计厚度最大20m左右,一般分布在长10、长9、长8和长7等层位,在地震剖面上以强振幅响应;(2)与盆地内部不同,马家滩地区在延长组底部长10出现薄层黑色页岩,并成为区分上、中三叠统的标志;(3)马家滩地区深浅湖呈"十字"分布,与盆地主体湖盆并不连通,单独存在。主体生油岩为长9-长10段烃源岩;(4)驿探1井热解样品均达到较好-好的烃源岩条件,具备较好生烃潜力。

An approach to the source rocks from the Upper Triassic Yanchang Formation in the Majiatan region on the western margin of the Ordos Basin

The deep lacustrine shales occur mostly in the interior of the Ordos Basin. However, there remains uncertain about how and whether they occur or have the hydrocarbon potential in the thrust zones on the western margin of the ordos Basin. The present paper gives a detailed study of thickness, horizon, distribution and hydrocarbon potential of the black shales from the Yanchang Formation on the basis of outcrops and seismic data from the Yitan-1 well and other wells in the Shigouyi geosyncline. The results of well logs, seismic horizon tracing, seismic forward modeling and pyrolytic analysis of the source rocks led us to draw the following conclusions. (1) There are still 2 to 4 beds of the black source rocks in the Yanchang Formation, Majiatan region in the central part of the thrust zone east of Qingtongxia-Guyuan fault. These black source rocks are characterized by high gamma, high time difference and low-resistivity well logs, 3 to 5 m in thickness for a single bed with a total thickness of about 20 m in the Chang-10, Chang-9, Chang-8 and Chang-7 horizons, and also with high amplitude responses. (2) The thin-bedded black shales in the Chang-10 horizon at the bottom of the Yanchang Formation in the Majiatan region are interpreted as the indicator for the distinction from the Upper and Middle Triassic strata. (3) The deep and shallow parts of the lake exhibit the "cross" distribution, isolated and discontinous with the main body of the lake basin. The Chang-10 and Chang-9 horizons are believed to be major source rocks. (4) The samples collected from the Yitan-1 well for the pyrolytic analysis indicate better to good source rocks with good hydrocarbon potential.