Dolomitization Research of Ordovician Majiagou 5₄ Sub-member, Jingbian Platform

Key words: 
• penecontemporaneous dolomitization /
• reflux dolomitization /
• burial dolomization /
• Jingbian Platform /
• Majiagou 5₄ Sub-member

Abstract: In Jingbian platform, Ordovician Majiagou 5 member is an important natural gas reservoir in Changqing oilfield. But there exists a long argument about its dolomitization type. This issue has been discussed on the basis of Majiagou 5₄ sub-member. In this paper, after researching the sedimentary and geochemical characteristics of different rock types, the authors draw a conclusion that penecontemporaneous, reflux and burial dolomitization are main dolomitization types in this region. (1) Micritic and silt-sized crystal dolomite which is always associated with anhydrite, deposited in restricted and evaporative tidal flat with high salinity, and formed by penecontemporaneous dolomitization. (2) Coarser silt-sized crystal dolomite is distributed among the layers of micritic and silt-sized crystal dolomite with anhydrite, and its coarser crystal grain, lower degree of order and slightly richer calcium show that this type dolomite is formed by reflux dolomitization. (3) Much secondary calcite is spread in karst layer, and the metasomasis residual texture shows that the grain size of the dolomite before dedolomitization is mainly fine crystal, partially with euhedral-subhedral texture, zonal structure, and bright border with vague center. Their inclusion temperature span is from 52.30 ℃ to 78.05℃, and the characteristics of carbon and oxygen isotopes are more similar to typical burial dolomites. All these indicate that the dolomite before dedolomitization is formed in shallow and middle-depth burial diagenetic environment. Clarifying the dolomitization type of Majiagou 5₄ sub-member is very important to rebuild lithofacies palaeogeographic, research the reservoir formation mechanism, and improve the success possibility of exploration.