The origin and distribution of Ordovician Yingshan dolomite on the northern slope of Tazhong area in Tarim Basin

An important exploration area of Ordovician Yingshan formation in the north Tazhong region has been demonstrated to be rich in oil and gas, especially the dolomite reservoirs in Shunnan Low Uplift and Gulong Uplift. In the study area, the dolomite interbedded with limestone decrease upward and is obviously controlled by the sedimentary facies laterally. Dolomite in the study area can be divided into two main types: fine-to-medium-grained, subhedral-to-anhedral, planar dolomite that distributed along stylolites (A1) and fine-to-medium-grained, euhedral-to-subhedral, planar sucrosic dolomite (A2). The cathodoluminescence features are recorded by common dark red light and a spot of zonal structures. The content of the oxygen isotope values and the trace elements, such as Mg, Na, Fe, and Mn, is higher in the A2 dolomite than that in A1 dolomite. Besides, the oxygen isotope values of dolomite are higher than those in limestone, which indicated that the dolomites are origin from evaporating brine. Geochemical and petrographical characteristics of Ordovician Yingshan dolomite indicated that they might be products of multi-stage dolomitization. The distribution of the sucrosic dolomite (A2) has no relationship with the third-order sequence, but coincides with the second-order sequence. It almost merely distributes in the transgressive system tracts of second-order sequence. Based on characteristics and analyses of the typical examples of single inclined platforms, the dolomite distribution of Yingshan formation is controlled by the second-order sequence in vertical, and decided by the sedimentary in horizontal. Microscopically, the grain sizes of the original sediments also work in the distribution of dolomite. The depth of water deepens as the sea-level rising, which can block the process of dolomitization. In vertical, the content of dolomite reduces upward, and in lateral, the dolomite migrates from the margin of the platform to the inner platform, showing as a regressive bed. However, dolomite that distributed along stylolites seems to be formed at shallow-burial depths.