鄂尔多斯盆地南缘寒武纪铜川裂陷的发现及油气地质意义

随着鄂尔多斯盆地海相碳酸盐岩勘探不断深入,新的认识认为盆地南缘寒武系存在构造分异,并发现一个NE向裂陷,即铜川裂陷。综合利用地震、测井、钻井岩心以及露头等资料,结合区域构造背景,对铜川裂陷边界断裂、沉积充填和演化特征开展的研究表明:①铜川裂陷受寒武纪伸展断裂控制,总体呈NE向;裂陷呈南宽北窄、西陡东缓,裂陷内地层厚度由NE向SW不断增加,具有SW厚、NE薄的特征。②裂陷内部与裂陷周缘寒武系的沉积差异明显。裂陷周缘下寒武统发育不全,以高能沉积为主;而裂陷内部下寒武统发育齐全,以低能沉积为主;裂陷内部中—下寒武统厚度显著增加,其中以徐庄组和毛庄组增厚最明显。③铜川裂陷的演化分为蓟县纪—震旦纪的裂陷隐伏阶段,辛集组—朱砂洞组沉积期的裂陷雏形形成阶段,馒头组—徐庄组沉积期的裂陷充填阶段以及张夏组—三山子组沉积期的裂陷萎缩消亡4个阶段。铜川裂陷控制了鄂尔多斯盆地南缘寒武系烃源岩、礁滩相带和有利储层的分布。在紧邻裂陷边缘的微古地貌高部位,其成藏条件优越,是鄂尔多斯盆地南缘寒武系的有利勘探区。

Geological characteristics and petroleum geological significance of Cambrian Tongchuan rift in the southern margin of Ordos Basin

With the deepening exploration of marine carbonate rocks in Ordos Basin,a new understanding holds that there is tectonic differentiation in Cambrian in the southern margin of the basin,and a NE-trending rift,i.e.,Tongchuan rift,has been discovered.Comprehensively using seismic,logging,drilling core and outcrop data,based on the regional tectonic background,the study on the boundary faults,sedimentary filling and evolution characteristics of Tongchuan rift show as follows.(1) Controlled by Cambrian extensional faults,Tongchuan rift is generally NE-trending.The rift is wide in the south and narrow in the north,steep in the west and gentle in the east.The thickness of strata in the rift increases from northeast to southwest,being thick in the southwest and thin in the northeast.(2) There are significant differences in the Cambrian sediments between the interior and the surrounding area of the rift.The Lower Cambrian in the surrounding area of the rift is underdeveloped and dominated by high-energy sediments;the Lower Cambrian in the interior of the rift is well-developed and dominated by low-energy sediments;the thickness of the Middle-Lower Cambrian in the interior of the rift is significantly increased,especially in Xuzhuang Formation and Maozhuang Formation.(3) The evolution of Tongchuan rift can be divided into four stages,i.e.,the concealed rifting stage from Jixian Period to Sinian Period,the initial rifting stage in the sedimentary period from Xinji Formation to Zhushadong Formation,the rift filling stage in the sedimentary period form Mantou Formation to Xuzhuang Formation,as well as the rift shrinkage and extinction stage in the sedimentary period form Zhangxia Formation to Sanshanzi Formation.Tongchuan rift controls the distribution of Cambrian source rocks,reef beach facies and favorable reservoirs in the southern margin of Ordos Basin.The top part of micropaleogeomorphology adjacent to the margin of the rift has superior accumulation conditions and is a favorable Cambrian exploration area in the southern margin of Ordos Basin.