Oil & Gas Geology ›› 2024, Vol. 45 ›› Issue (6): 1736-1754.doi: 10.11743/ogg20240618

• Petroleum Geology • Previous Articles     Next Articles

Occurrence conditions and exploration targets of deep coal-rock gas in the Huhehu Sag, Hailar Basin

Xuefeng BAI1,2(), Geng GAO1,3(), Biao WANG1,3, Jingsheng LI1,3, Junhui LI1,3, Hui XU4, Wenjuan MA1,3, Lu LIU1,3   

  1. 1.State Key Laboratory of Continental Shale Oil,Daqing,Heilongjiang 163712,China
    2.Daqing Oilfield Limited Company,Daqing,Heilongjiang 163458,China
    3.Research Institute of Exploration and Development,Daqing Oilfield Limited Company,Daqing,Heilongjiang 163712,China
    4.Changzhou University,Changzhou,Jiangsu 213164,China
  • Received:2024-06-08 Revised:2024-10-26 Online:2024-12-30 Published:2024-12-31
  • Contact: Geng GAO E-mail:bxf@petrochina.com.cn;ggao_njucsu@petrochina.com.cn

Abstract:

Hydrocarbon reservoirs have been identified in the 2nd member of the Cretaceous Nantun Formation (also referred to as the Nan 2 Member) in the Huhehu Sag, Hailar Basin. However, limited studies have been conducted on the occurrence conditions and exploration targets of deep coal-rock gas therein. In this study, we analyze the fundamental conditions for hydrocarbon accumulation, including source rocks, deposits, and structures, in the Huhehu Sag and examine the developmental characteristics and gas-bearing features of deep coal reservoirs. Accordingly, we explore the occurrence conditions of deep coal-rock gas in the sag and propose potential exploration targets. The findings reveal that in Nantun Formation there are two suites of mature source rocks in the mudstones and coals, which provide a favorable material basis for the deep coal-rock gas generation and accumulation. During the deposition of the Nan 2 Member, Hailar Basin, a faulted lacustrine basin, exhibits swamps, which create a favorable coal-forming setting. The initial extension stage of the basin witnessed the formation of thick coals and coal-mudstone coexistence, while its stable extension and contraction stages featured thin coals and sandstone-coal coexistence. The ramp-faulted terrace zone exhibits thin coals and low gas content. In contrast, the trough and steep slope zones are characterized by thick coal rocks and well-developed cleats, with organic pores interconnected by microfractures, contributing to high connectivity and a well-developed pore-fracture system. So it is rich in free gas. The fault system formed in the early extension stage provides pathways for the vertical and lateral migration of deep coal-rock gas. The Huhehu Sag exhibits deep coal-rock gas resources predicted at 1.79×1012 m3, suggesting considerable exploration potential and establishing this sag as an important new risk exploration target in the faulted lacustrine basin. Additionally, the trough and steep-slope zones are identified as the most favorable exploration targets for deep coal-rock gas.

Key words: risk exploration, trough zone, steep-slope zone, deep coal-rock gas, the 2nd member of the Nantun Formation (Nan 2 Member), Huhehu Sag, Hailar Basin

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