Abstract: In Linhe Depression, Hetao Basin, high-yield oil flows with a daily rate of 100–1000 t/d have been achieved in deep to ultra-deep sandstone reservoirs in the Paleogene Linhe Formation in multiple exploration wells. Specially, the open flow oil and gas rates of 1285.77 m³/d and 1.07×10⁴ m³/d were tested at a depth of 6500 m in Well Hetan 101, which was the first high-yield well with an oil rate level of 1000 t/d in the ultra-deep clastic rocks in faulted lake basin in China, and also the highest oil flow well in the ultra-deep clastic rocks, with the main reservoir space of primary intergranular pores, greatly breaking through the lower depth limit of oil and gas exploration in conventional primary porosity type sandstone reservoirs. Based on thin sections, core samples, well drilling, logging, and a large amount of rock mineral laboratory test data, a comprehensive study of reservoir formation mode and lower exploration depth limit of effective primary porosity type sandstone reservoirs in the deep to ultra-deep formations has been conducted. The study results show that the high-quality reservoirs in Linhe Formation are mainly composed of high composition and high structural maturity quartz rich clastic sandstones. The reservoir space is dominated by primary intergranular pores with characteristics of large pores and coarse throats. The reservoir formation and evolution were controlled by four fields, including static rock compaction dynamic field, thermal compaction dynamic field, fluid salinity field, and fluid pressure field. The evolution of high-quality reservoirs experienced four diagenetic stages, namely early constant burial depth increase–rapid pore reduction, early–middle stage long-term slow burial depth increase–slow pore reduction, middle–late stage rapid burial depth increase–weak compaction pore reduction, and late stage continuous burial depth increase–hydrocarbon charging for pressurization and pore preservation, and the porosity of ultra-deep high-quality reservoirs before hydrocarbon charging reached up to 20%–30%. Furthermore, a power function relationship model between time temperature index TTI and porosity has been established, which indicates that thick sandstone (greater than 1 m) with low interstitial material content in the trough area is effective reservoir, the lower porosity limit of effective conventional reservoir is 8.5%, and the lower depth limit of effective reservoir reaches 9200 m, greatly expanding the lower depth limit of oil and gas exploration in deep to ultra-deep primary porosity type sandstone reservoirs.

Key words: Hetao Basin, Linhe Depression, Paleogene Linhe Formation, deep to ultra-deep formation, reservoir characteristics, reservoir evolution, lower exploration depth limit