Origin of porphyry dolomite in the 5th sub-member of the 5th member of Ordovician Majiagou Formation (M55sub-member), Ordos Basin

Abstract

Large number of heterogeneous porphyry dolomites have been found in the 5th sub-member of the 5th member of Ordovician Majiagou Formation (M55 sub-member) in the central and eastern parts of Ordos Basin, and many drilled wells have shown good oil and gas potential, which has become the focus for oil and gas exploration, but the genetic mechanism is still undetermined. Based on petrological characteristics and geochemical analysis, the origin of porphyry dolomite in this area is studied. Results show that porphyry dolomite can be classified into three types: unevenly, obviously and slightly porphyry dolomite. According to the morphology characteristics of cloud spots, the structure of cloud spots is supposed to be caused by biological disturbance, and the cathodoluminescence of spots and matrix is the same. Porphyry dolomite has relatively higher contents of Fe, Mn and Na and lower content of Sr, but there is no significant difference between the contents of Fe, Mn and Sr in spots and matrix. The δ13C values of porphyry dolomites are highly comparable to those of seawater at the same time, and the δ18O values are all negative with no significant difference among the three types of dolomites. The ratio of 87Sr/86Sr is higher than that of seawater in the same period. Petrological and geochemical characteristics show that biodisturbance causes the heterogeneity of rocks, and the degree of disturbance affects porphyry rate. The biodisturbed limestone of porphyry dolomite series is formed by the dolomitization of the osmotic reflux of high salinity seawater. The dolomite underwent recrystallization after re-burial.

Keywords

• porphyry dolomite
• geochemical characteristics
• osmotic reflux dolomitization
• 5th sub-member of 5th member of majiagou formation
• ordos basin