乌审旗—志丹地区奥陶系岩溶古地貌与马五_41气水分布关系

Effect of karst paleogeomorphology of Ordovician on gas-water distribution of Ma 541 in Wushenqi-Zhidan area

鄂尔多斯盆地乌审旗—志丹地区奥陶系岩溶古地貌的形态对区内气水的分布具有控制作用。运用补偿厚度印模法(毛儿沟灰岩至本溪组底地层厚度)恢复了研究区奥陶系岩溶古地貌,其形态包括岩溶高地、岩溶斜坡及岩溶盆地等二级古地貌,并进一步识别出台地、浅洼、残丘、沟槽及洼地等三级古地貌。整体上,乌审旗—吴起一带属于岩溶高地,乌审旗东侧—靖边—志丹一带属于岩溶斜坡,横山—安塞一带以东属于岩溶盆地。通过典型气藏剖面分析及古地貌与气水分布的叠合,发现研究区的水主要分布于岩溶高地,气主要分布于岩溶陡坡或残丘部位。研究认为:岩溶高地虽具有良好的储集空间,但后期因盆地构造演化,气水重新分异运移导致地层水较富集;岩溶斜坡处于岩溶高地与岩溶盆地的过渡区域,岩溶水排泄畅通,伴随强烈的膏溶作用,充填作用与胶结作用均不发育,从而形成良好的孔、洞、缝等储集空间,具有良好的天然气富集条件;岩溶高地与岩溶斜坡的过渡带及残丘部位也均具有良好的天然气富集条件。

Karst paleogeomorphology of Ordovician controls gas-water distribution in Wushenqi-Zhidan area of Ordos Basin. Based on the regional tectonic setting and stratigraphic distribution, the compensation thickness impression method （the stratum thickness from the Maoergou limestone to the bottom of Benxi Formation） was used to rebuild the karst paleogeomorphology of Ordovician which includes three types of second-order paleogeomorphology and five types of third-order paleogeomorphology in wushenqi-zhidan area. The three types of second-order paleogeomorphology include karst highland, karst slope and karst basin, and the five types of third-order paleogeomorphology include tableland, shallow depression, monadnock, groove and depression. 0verall, the karst highland mainly located in the Wushenqi-Wuqi area, most of the central part of the study area like the east of the Wushenqi-Jianbian-Zhidan belongs to karst slope, and the east of the Hengshan-ansai area pertains to karst basin. Through the analysis of gas reservoir profile and the superposition of karst paleogeomorphology and gas-water distribution, it is found that the water in the study area is mainly distributed in the karst highland, while the gas is mainly distributed in the karst slope or the karst monadnock. In conclusion, although karst highland has a good reservoir space, because of the late tectonic inversion, the re-differentiation and migration of the gas and water lead to relatively enriched groundwater. However, karst slope is located in the transitional zone between karst highland and karst basin and contains a large number of the soluble minerals. It formed well developed reservoir space like pores, vugs and fractures because it has experienced the strong gypsum-dissolving process and the weak cementation. Therefore, it is the most favorable gas accumulating paleogeo- morphology unit, followed by karst highland, karst basin worst. In addition, karst monadnock and the contact zone between karst highland and karst slope also have a good natural gas accumulation condition.