Abstract:

The shale in the second member of Kongdian Formation(Ek₂) in Cangdong Sag with large thickness, high values and excellent types of organic abundance, is one of the important intervals for dense petroleum exploration. Based on the detail observation and description of cores, this paper studied lithologic characteristics, fracture types and development characteristics(including length, opening,angle and filling)of shales in Ek₂ in Cangdong Sag of Huanghua Depression by the observation of polarizing microscope and fluorescence microscope and laser scanning confocal microscope together with analysis of X-ray diffraction and rock pyrolysis. Furthermore, it analyzed the main controlling factors of fracture development. The shales of Ek₂ in Cangdong Sag are mainly composed of felsic, carbonate and clay minerals; the main lithologies are dolomitic shale, silty mudstone, argillaceous dolostone, sandy dolomite and dolomite, etc; the main types of sedimentary structures include massive structure, lamellar structure, laminated structure and lenticular structure, etc.Four sorts of fractures develop in Ek₂, i.e. structural fracture, interlayer lamellation crack, vertical-difference load fracture and overpressure-breaking fracture, among which the structural fracture is the most, followed by the interlayer lamellation crack and vertical-difference load fracture, and the overpressure-breaking fracture is the least. The main fillings in fractures are pyrite, analcime, calcite and asphalt. The results show that tectonism, lithology, mineral composition, sedimentary structure, diagenesis and organic matter abundance(TOC) are the main controlling factors for the development of fractures in shales of Ek₂ in Cangdong Sag.

Key words: shale, fracture, main controlling factor, Cangdong Sag, Ek₂