MINERAL CONTENT AND GEOCHEMISTRY CHARACTERISTICS OF CARBONATE ROCK IN WELL NO. XICHEN-1 AND GEOLOGICAL SIGNIFICANCE

Abstract

Cenozoic carbonate rocks in Xisha islands are the products of the South China Sea formation and evolution, as a result, their mineral component, chemistry composition, the contents of total Sr, and the 87Sr/86Sr preferably reflect marine paleoenvironment in which the rock formed. As the deepest cored well, Xichen 1 opens out 3 layers of dolostone and 4 layers of limestone. The dolostone terrane bears high MgO content, low total Sr content, and contains some facies minerals, such as rock salt, hematite and dialogite, which represent a high palaeosalinity and oxygenation environment, corresponding with 3 glacial events. The limestone terrane bears low MgO content, high total Sr content, and also contains some facies minerals, such as siderite and pyrite, which represent a low palaeosalinity and reducing environment, corresponding with 4 interglacial events. As a whole, the contents of total Sr, and the 87Sr/86Sr in late Cenozoic carbonate rock increase by degrees from bottom to top, which were controlled by Qinghai-Tibet uplift tectonic setting aroused by collision between Eurasian and Indian plates. While the contents of total Sr and the 87Sr/86Sr in late Cenozoic carbonate rock still present an obviously subsection characteristics. In middle-upper Miocene dolostone, the total Sr content is low, but the 87Sr/86 Sr shows an evidently increasing trend, which are related with strong dolomitization and regression caused by the biggest glacier event; In upper-upper Pliocene to Holocene limestone, the total Sr content is high and the 87Sr/86Sr shows an evidently decreasing trend with a large variety range, which are controlled by both dolomitization and violent mantle magma eruption.