Development of effective reservoir space

genetic mechanism and reservoir quality of deep to ultra-deep limestone are key concerns of deep to ultra-deep oil and gas exploration.Detailed core observations

computerized tomography

microscopy

cathodoluminescence and scanning electron microscopy analyses were carried out to characterize the petrology and pore spaces of limestone samples from the Yijianfang Formation in Shunnan-7 and Shuntuo-1 wells in Tarim Basin.Pores in the Lower Yijianfang Formation limestones (Well Shunnan-7) are dominated by fenestrae and shelter pores

which survived the cementation in the deposition to early diagenetic stages.Micro-pores are calcite intercrystalline pores in algae particles

and their formation might be jointly controlled by recrystallization and maturation of original organic matters.The pores of boundstones in the Upper Yijianfang Formation (Well Shuntuo-1) are dominated by moldic pores in biological shells and inter-crystalline pores within aggregates of silica spheres and euhedral calcites;their micro-pores are mainly biomembrane micro-pores

inter-crystalline micro-pores resulted from recrystallization of calcites and micro-pores in euhedral quartz in limestone matrix.Hydrothermal alteration in the early diagenetic stage may be the major genetic mechanism of pores and micro-pores in Well Shuntuo-1.The early hydrocarbon charging may play an important role in the preservation of pores and micro-pores during burial.Our results show that the characteristics and genesis of pores and micro-pores of limestones with different structural features in different wells are different.Thus

the controlling factors for the development and distribution of ultra-deep limestone reservoirs are complicated.