Well Yunningdi 1 in the Ninglang area of Yunnan Province (SW China) produced a stable natural gas flow for the first time from Devonian strata in the west of the Kangdian ancient continent, which can be considered as a major progress and a great discovery in oil and gas investigation targeting new areas and sedimentary series. On the basis of integrated analysis of drill core observations, well logging interpretation, and seismic and geochemical data concerning the Well Yunningdi 1 in the Ninglang-Yanyuan Basin, comprehensive research on geochemical characteristics of source rocks, petrophysical properties, and hydrocarbon accumulation and preservation conditions has been conducted in this current work. This, provides a systemic evaluation of the geological conditions for hydrocarbon accumulation in the Devonian strata system in the Ninglang-Yanyuan basin (SW China). The results show that: (1) The bioclastic organic-rich shales developed in the Middle Devonian Dacaozi Formation in Well Yunningdi 1 have a cumulative thickness of over 130m, and also a moderate degree of thermal evolution, suggesting an ideal material base for shale gas generation and accumulation. (2) The shale reservoirs of the Dacaozi Formation display a high content of brittle minerals with an average of 63%, which is favorable for reservoir stimulation. Besides, a relatively high porosity of 0.66%~3.3% suggests a widespread development of micro-nanoscale pores, as confirmed by FE-SEM images showing organic pores, dissolved pores, inter-particle pores, inter-crystalline pores, interlayer pores, and microfractures, which constitute important storage spaces and migration pathways for shale gas. (3) Reef-beach facies reservoirs are developed in the overlying Middle Devonian Tanshanping Formation. The lower segment is characterized by high porosity (up to approximately 4.9%) reef limestones indicating good physical reservoir properties, and also well-developed primary pores, dissolved pores and stylolites. It is noteworthy that the primary pore systems are dominated by biogenic skeleton-supported pores mainly consisting of abundant stromatoporoids and corals. The upper segment has a displacement pressure, indicating effective source-reservoir-cap assemblages in the shale reservoirs in the Dacaozi Formation. To sum up, the Devonian depositional system in the Ninglang-Yanyuan basin is suggested as a target for future exploration and investigation directions due to its good and realistic geological conditions for the development of potential conventional and unconventional natural gas reservoirs. This investigation also lends some additional and valuable insight into potential mechanisms of natural gas accumulation in complex structural belts surrounding the Sichuan Basin, and can be used as a key reference for shale reservoir evaluation in the Upper Yangtze Platform.
ZHANG Ziya, SHI Dishi, LAN Zhiqin, ZHANG Cong, QIN Yinglun, CHEN Rong, KANG Haixia. Geological characteristics and exploration directions of Devonian strata in the Ninglang-Yanyuan Basin. Petroleum Science Bulletin, 2020, 02: 161-171.
