鄂尔多斯盆地东部地区奥陶系马家沟组沉积环境与岩相古地理研究

Lithofacies Paleogeography and Sedimentary Environment in Ordovician Majiagou Formation,Eastern Ordos Basin

鄂尔多斯盆地是我国重要的含油气盆地,以奥陶系碳酸盐岩为主的层位赋存有相当丰富的天然气资源。马家沟组在鄂尔多斯盆地有广泛分布,其中马一段、马三段、马五段以白云岩、硬石膏岩和盐岩为主,夹少量石灰岩;马二段、马四段、马六段为石灰岩和白云岩。运用沉积学与地球化学方法,分析了研究区马家沟组的沉积环境,认为:马一段、马三段和马五段沉积时期气候干热,海平面下降,盆地东部龙探1井区一带主要为台地蒸发岩相沉积,发育了膏盐岩和盐岩沉积,其周边则发育了一套白云岩夹薄层硬石膏岩的含膏云坪相沉积,在盆地东部周边地区主要沉积了泥质白云岩、白云质灰岩和细晶白云岩的云灰坪和局限海相沉积。马二段和马四段沉积期气候湿热,海平面上升,研究区主要发育了石灰岩夹少量白云岩的开阔海相沉积,周缘地区发育了白云岩和灰岩互层的云灰坪或白云岩为主的局限海相沉积。并建立了马家沟组的沉积模式,绘制了马家沟组各个岩性段的岩相古地理平面图。

Ordos Basin is a major hydrocarbon-bearing basin in China and the Ordovician carbonate rock preserved plenty ofnatural gas resources. Majiagou Formation occurs widely around the whole basin, in which Member 1, Member 3 and Member 5 are composed dominantly of dolomite, anhydrite and salt rock, to a lesser extent, limestone; while Member 2, Member 4 and Member 6 are mainly limestone and dolomite. Using methods of sedimentary and geochemistry, this paper analyzed the sedimentary environment in the Majiagou Formation. It suggests that Member 1, Member 3 and Member 5 are characterized by a dry and hot climate as well as fall of the sea level. The area of Well Longtan 1 in the eastern basin is abundant of platform evaporites with the depositional anhydrite and salt rock, surrounded by a suite of dolomite intercalated by the thin layers of anhydrite from the anhydrite-dolomite platform sediment. It deposits muddy dolomite, dolomitic limestone and fine-grained dolomite in the limestone-dolomite platform and restricted sea. During the stage of Member 2 and Member 4 in Majiagou Formation, the climate is wet and hot with rising sea level. The study region presents limestone with little dolomite in the open sea environment; but the margin area is the restricted sea settings with interbeding dolomite and limestone. The sedimentary model has been built in Majiagou Formation and the lithofacies paleogeography map has been plotted for each Member in Majiagou Formation.