MICROFACIES ANALYSIS OF REEFS AND THE CONTROLLING FACTORS IN MIDDLE ORDOVICIAN, NORTHWEST TARIM BASIN, CHINA

A large number of biological reefs, as one of the most important oil and gas reservoir types, are developed in middle Ordovician in Tarim basin. To better predict the distribution of reefs, it is necessary to know the sedimentary characteristics and the controlling factors.

A case study on the Nanyigou, Xikeer and Yijianfang profiles in northwest Tarim basin is presented. Through detailed field geological investigations and petrological characteristics analysis, it is concluded that the biological reefs are baffle-reefs. Reef-building organisms are Calathium sp., Bryozoa, echinoderms and calcium algae while reef-inserted organisms are ostracod, brachiopoda, gastropod, foraminifera and trilobites. Eight petrographic types are established, namely reef limestone, algal limestone, bioclastic limestone, bioclastic packstone, packstone, bioclastic mudstone, sparry bioclastic limestone, sparry intraclastic limestones. Individual reef can be divided into three sedimentary microfacies (reef base, reef core and reef cover) from bottom to top. Reefs in Yingshan formation developed in intra-platform, they are small and low in number. Reefs in Yijianfang Formation developed in platform margin; they are large and high in number, with obvious migration evaluation characteristics.

Through analysis of trace elements, carbon oxygen isotopes and strontium isotopes, the controlling factors of the distribution of reefs are studied. The carbon isotopes value of reef core is the largest, which reflects rise of sea level and the mass propagation of reef-forming organisms. Oxygen isotopes value reflects the change of temperature, and reefs are mainly formed within a temperature range of 18~33℃. The change of strontium isotope values has a negative correlation with the change of sea-level.The results of this present work show that the development of reefs is controlled by paleoenvironment, such as paleo-temperature, paleo-salinity and other factors. The sea level changes ultimately control the formation of the reef body, development and extinction.