New insights into the formation mechanism of deep-ultra-deep carbonate reservoirs and the direction of oil and gas exploration in extra-deep strata

doi:10.13745/j.esf.sf.2023.2.35

Abstract

Abstract:

In deep carbonate strata of the Sichuan and Tarim Basins a series of large scale oil and gas fields have been discovered successively, including the Jingbian, Tahe, Puguang and Yuanba fields, and a “ternary reservoir control” theory for reef-shoal carbonate reservoirs has been developed. In recent years, high-quality carbonate reservoir rocks are discovered in ultra-deep layers at depths of greater than 7000 m or even 8000 m. The deep reservoirs show diverse rock types and complex interactions between the reservoir controlling factors such as facies belt, fluid, pressure, and fault. Using the newly obtained ultra-deep drilling data we carried out a systematic investigation into the formation mechanism of the ultra-deep carbonate reservoir rocks. In-depth studies of the Ordovician strata of Tahe-Shunbei-Shunnan area, Tarim Basin and the Permian Qixia and Maokou Formations in the Sichuan Basin provided further insights into reservoir formation by mechanisms of fault-related fracturing and coupled multi-type fluid alteration. Results for the Upper Sinian Dengying Formation in the Sichuan Basin and the Upper Sinian Qigebulake Formation in the Tarim Basin suggested that the Neoproterozoic “dolomite sea” depositional environment, early dissolution, and early hydrocarbon charging are the key factors for reservoir formation and preservation. Physical simulation experiment on dolomite reservoir formation in gypsum-bearing facies revealed that dolomite recrystallization can significantly improve reservoir properties under gypsum-salt capping and overpressure and relatively closed environment. The new insights enriched the “ternary reservoir control” theory and provided guidance for oil and gas exploration in ultra-deep and even extra-deep strata. In superimposed basins in central and western China, fault-controlled and fault/dissolution dual-controlled reservoirs, ancient microbial mound-shoal dolomite reservoirs, especially those with Cambrian thick gypsum-salt caprocks, are widely distributed in marine carbonate rocks and can be considered the main exploration targets at > 10000 m depths.

Key words: deep-ultra-deep strata, fault-controlled fracture-pore reservoir, fault/fluid dual-controlled reservoir mechanism, microbialite reservoir, gypsum-bearing dolomite reservoir, formation mechanism of carbonate reservoirs

CLC Number:

TE122.2

MA Yongsheng, CAI Xunyu, LI Huili, ZHU Dongya, ZHANG Juntao, YANG Min, DUAN Jinbao, DENG Shang, YOU Donghua, WU Chongyang, CHEN Senran. New insights into the formation mechanism of deep-ultra-deep carbonate reservoirs and the direction of oil and gas exploration in extra-deep strata[J]. Earth Science Frontiers, 2023, 30(6): 1-13.

Figures/Tables 6

Fig.1 The diagram of diagenetic evolution and controlling reservoir mechanism of deep carbonate reservoirs

Fig.2 Cores, drill well, and seismic characteristics of fault-controlled fractured-cavity reservoir in well SHB84X

Fig.3 Characteristics of fault-hydrothermal silicified carbonate reservoirs in the Tarim Basin

Fig.4 Characteristic of typical fault-hydrothermal dolomite reservoirs in Permian, Sichuan Basin

Fig.5 Characteristics of typical microbial microbialite reservoirs in the Upper Sinian

Fig.6 Comparison of dolomite recrystallization experiment results under fluid overpressure and normal pressure conditions in the two-pressure reactor

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