Abstract
Mold pore cementation is the key factor constraining the reservoir property in the study area. The anhydrite dissolution pores in the Ordovician Majiagou Formation of southeastern Ordos Basin are commonly filled by minerals such as dolomite, calcite, pyrite, and quartz accounting for more than 90% of the total molds resulting in significant porosity volume reduction. The anhydrite dissolution pores in the Jingbian Gas Field in the middle east of the basin, however, are rarely filled by minerals with more than 30% molds, remaining open to become good reservoir space. Studies reveal that the calcite filling in anhydrite dissolution pores has a relatively negative δ18O value (-15.58‰~-8.96‰ VPDB) and negative δ13C value (-7.56‰~0.26‰ VPDB), which is interpreted to be caused by thermochemical sulfate reduction (TSR). The higher homogenization temperatures (140-234°C) and high salinity (19.13-23.18 wt.% NaCl equivalent) of the primary inclusions in calcite confirm the above interpretation. Dolomite is the second most abundant carbonate formed as by-product of TSR, which is promoted by the precipitation of calcite and resulted enriched in Mg2+/Ca2+ ratio in the pore water. Pyrite forms by the reaction of H2S released from TSR with the Fe2+ in the horizon, which is supported by its cubic habit and relatively high δ34S value (10.50‰~24.00‰VCDT). Quartz with relatively high homogenization temperature (113-154°C) is considered to precipitate in low-pH solution from calcite and pyrite precipitation after TSR. The southeastern Ordos Basin is much lower than the Jingbian Gas Field in paleogeographic location, which is submerged in the sea water of marine phreatic environments for a long time when sea water flooded from the southeastern direction. TSR occurs due to calcium sulfate enriched in pore water resulting in the minerals of dolomite, calcite, pyrite, and quartz filling in the molds leading to the low porosity and permeability of the study area.
Highlights
The anhydrite nodule-bearing dolomite is widely distributed in the Ordovician Majiagou Formation of southeastern Ordos Basin
The anhydrite dissolution pores are commonly filled by minerals, such as dolomite, calcite, pyrite, and quartz, resulting in significant porosity and volume reduction
The anhydrite dissolution pores are commonly filled by minerals such as dolomite, calcite, pyrite, and quartz, which resulted in significant porosity and volume reduction
Summary
The anhydrite nodule-bearing dolomite is widely distributed in the Ordovician Majiagou Formation of southeastern Ordos Basin. The filling of anhydrite dissolution pores by calcite and dolomite can be interpreted as the replacement of sulfate by carbonate driven by various hydrological processes, such as bacterial sulfate reduction [7,8,9]. Pierre and Rouchy [7] interpreted the low δ13C value from organic origin and the low δ18O value caused by the large quantities of energy released during the bacterial sulfate reduction It can be the result of an active volcanogenic system [2] or even associated with cycling of seawater through hydrothermal anhydrite in midocean ridges [10]. Based on the outcrops and well cores in the southeastern Ordos Basin, thin section observation, stable isotopic analyses, scanning electron microscope (SEM), and fluid inclusion analyses are undertaken to study the time, process, and conditions of evaporite replacement to provide valuable information on the pore fluid properties and predict the reservoir quality
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