Oil & Gas Geology ›› 2020, Vol. 41 ›› Issue (5): 1073-1082.doi: 10.11743/ogg20200517

• Petroleum Geology • Previous Articles     Next Articles

Geological features and their participation in the formation of silicified clastic reservoirs in the Shahejie Formation of Laizhouwan Sag, Bohai Sea

Lixin Tian1(), Qingbin Wang2, Xiaojian Liu2, Yiwei Hao2   

  1. 1. CNOOC International Ltd., Beijing 100027, China
    2. Tianjin Branch of CNOOC, Tianjin 300459, China
  • Received:2018-04-28 Online:2020-10-28 Published:2020-10-22

Abstract:

Silicified clastic rocks in the Laizhouwan Sag, Bohai Bay, are characterized by intergranular pores among quartz crystals filled with siliceous materials and by secondary intensive quartz overgrowth.Some areas even exhibit low-level metamorphism of blastopsammitic texture, which is rarely seen in petroliferous basins.The geological features of the siliceous clastic rocks in the sag are revealed by means of seismic and log interpretations, petrological analyses and zircon U-Pb dating.It has been confirmed for the first time that volcanic activities did occur during the deposition of the Shahejie Formation in the sag.Siliceous thermal fluid activities are verified by well-developed siliceous veins of multi-stage filling and charging, intensive overgrowth of the secondary quartz, and the blastopsammitic texture of rocks.Almost all the clay minerals in intergranular pores were thermally altered to biotites and muscovites, and growing "broom-shaped" mica minerals are observed.A large number of pyrite are developed in agglomerates or random clusters.Pyrite with different occurrence is derived from multi-phase hydrothermal solution.The hydrothermal mineral assemblage of pyrite-barite-monazite-galena-rutile-anhydrite is identified under the field-emission scanning electron microscope.With siliceous hot fluids from deep entering reservoirs, the overgrown clastic quartz and siliceous precipitates worked together to fully stuff the intergranular pores, causing deterioration in physical properties of the reservoirs.However, the silicification saved the situation by greatly enhancing the reservoir brittleness, which generated numerous fracture and micro-fracture networks during later tectonic movements.In addition, the hydrothermal fluids dissolved the feldspar minerals in the reservoirs, resulting in a large amount of dissolution pores and moldic holes in feldspar grains.Thus high-quality reservoirs of pore-fissure type are formed with well-connected fracture networks.

Key words: hydrothermal fluid, diagenesis, reservoir genesis, silicified clastic rock, Laizhouwan Sag, Bohai Sea

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