鄂尔多斯盆地东部奥陶系风化壳古地貌特征嬗变及地质意义

Paleo-geomorphologic characteristic evolution and geological significance of the Ordovician weathering crust in eastern Ordos Basin

鄂尔多斯盆地奥陶系风化壳气藏的发育与前石炭纪古地貌格局密切相关,目前主要认为发育西高东低的古地貌格局,自西向东依次发育岩溶高地、岩溶斜坡及岩溶盆地的古地貌单元。但随着勘探的深入,在盆地东部地区奥陶系早期认为的岩溶盆地区已经发现了大量代表了较强岩溶作用的岩溶角砾岩等地质现象,因此有必要从古地貌动态演化角度重新认识影响岩溶古地貌发育的内外因素,研究地质历史中奥陶系风化壳古地貌的性质嬗变,指导对区域储集空间发育规律的进一步研究与预测。以盆地东部奥陶系顶部风化壳古地貌恢复为目的,综合研究了古地理背景、残留地层厚度、古岩溶特征等因素。认为早期在盆地中东部存在一个大范围的岩溶古高地,形成了岩溶型储集空间,但后期盆地东部逐渐演变为古岩溶盆地,形成了现今的前石炭纪古地貌格局,并造成早期形成的溶蚀类储集空间被大量充填,整体储层较致密,但由于溶洞塌陷形成的裂缝网络沟通了白云岩晶间孔储集空间,局部仍然可以保留有较好的储层,是值得重视的勘探目标。

The development of Ordovician weathering crust gas reservoir in Ordos Basin is closely related with pre-Carboniferous paleogeomorphology.Currently,it is believed that the paleo-geomorphology is developed high in the west and low in the east,including the paleo-geomorphologic units of karst plateau,karst slope and karst basin from west to east successively.With the further exploration in recent years,a large number of geological phenomena,such as karst breccia with strong karstification,have been found in the identified Early Ordovician karst basin of the eastern basin.Therefore,it is necessary to re-understand the internal and external factors that affect the development of karst paleo-geomorphology through dynamic evolution of paleo-geomorphology,study the paleo-geomorphologic evolution of Ordovician weathering crust in geological history,and guide further research and prediction of the development laws of regional reserving space.To recover the paleo-geomorphology of the Ordovician top weathering crust in the eastern basin,the paleo-geographic background,the thickness of residual strata,the paleo-karst characteristics and other factors are comprehensively considered in this study.It is believed that a large range of karst paleo-highs existed in the central-eastern basin during the early period to form karst reserving space.However,the eastern basin gradually evolved into the paleo-karst basin during the late period to form the current pre-Carboniferous paleo-geomorphology.As a result,the early formed dissolution reserving space was filled massively,thus forming the whole tight reservoir.However,due to the fracture network connection formed by the collapse of karst caves and the reserving space of dolomite inter-crystalline pores,favorable reservoirs can also be preserved locally and become valuable exploration targets.