Characteristics and origin of the tight grainstone reservoirs of the Yijianfang Formation, middle Ordovician in the Shunnan area, Tarim Basin

Tight carbonate reservoirs are widely distributed in the world and have great economic exploration potential. The Ordovician Yijianfang Formation grainstones in the Shunnan area, Tarim Basin are considered to be hydrocarbon reservoirs. However, there are few visible pores from thin sections and cores, resulting in poor understanding of the pore systems in these tight limestone reservoirs. Core samples collected from 8 wells were prepared to characterize pore networks by combining the optical microscope, scanning electron microscope, helium gas porosimeter, nuclear magnetic resonance and micro-CT analysis. Carbon and oxygen isotopes and fluid inclusion thermometry were adopted to determine the timing of pore-related dissolution. For Yijianfang grainstones, porosity mostly ranges from 0.2 to 3.9%; permeability mostly ranges from 0.01 to 1 mD. Petrographic observations revealed that both intragranular macropores and micropores within grains constitute effective reservoir space. Besides, micropores could account for 50-90% of the total porosity. The intragranular pores only appear in the upper part of the cycle, and the micropores developed in upper and middle of the cycle. There are no pores in the lower part of the cycle. During brief subaerial exposure shortly after deposition, meteoric dissolution created intragranular pores and vugs in grainstones in upper part of the sedimentary cycle. Meanwhile, influenced by the freshwater-involved mineralogical stabilization, micropores transformed from between micrites to between calcite microcrystals within grains in grainstones in the upper and middle part of the cycle. In the burial diagenetic environment, vugs and interparticle pores were destroyed by compaction and burial cements. For packstones in the lower part of the cycle, mineralogical stabilization occurred without freshwater during progressive burial, leading to tight fabric with no or minor micropores. In summary, whether tight limestones can be effective reservoirs depends on whether freshwater is involved in dissolution and mineralogical stabilization. In general, tight grainstone reservoirs of shallowing-upward cycle could provide abundant micropores with few macropores due to the freshwater-involved dissolution and stabilization during subaerial exposure.