PORE-FRACTURE FEATURES OF THE MARINE-CONTINENTAL TRANSITION SHALES FROM TAIYUAN FORMATION IN NORTHERN ORDOS BASIN AND PETROLEUM GEOLOGICAL IMPLICATION

The marine-continental transition shale of Taiyuan Formation in Ordos Basin serves as an unconventional gas reservoir. The micropores and microfissures of shale samples are observed with argon ion polishing-SEM technique to analyze their genesis. It is thought that sedimentation, diagenesis, metamorphism of organic matter, hydrocarbon generation of kerogen, dissolution and structural stress are the main controlling geological factors for the development of pores and fractures. By reference to the classification schemes at home and abroad for pore-fracture types of marine and continental shale, the pores and fractures of the marine-continental transition shale in Taiyuan Formation fall into three types, namely organic pore, inorganic pore and microfracture, each of which can be further subdivided into several styles. The micropores and microfractures can provide favorable reservoir space and seepage channels for shale gas, especially free gas, which is the precondition for high and stable yield by fracturing development.