"Sweet spot" identification and prediction is the key to develop tight gas reservoirs with low permeability.In order to explore the types and genesis of "sweet spots" in deep low permeability tight gas reservoirs with fewer wells in the sea

and to guide the exploration and development of oil and gas fields

the study investigated the "sweet spots" in X gas field in the Xihu Sag

East China Sea Shelf Basin

through multiple methods

such as core observation

cast thin section

cathodoluminescence and scanning electron microscopy thin section identification

grain size analysis

nuclear magnetic resonance

high pressure and constant pressure mercury intrusion porosimetry(MIP)

porosity and permeability test and elemental analysis.First of all

the definition of "sweet spot" was explored

and three classes of "sweet spots" were identified based on the theoretical production capacity

physical property

pore structure and bound water content:"Sweet spot" Class Ⅰ is high yield gas reservoirs with Type Ⅰ pore structure being dominant and the irreducible water saturation mostly in the range of 22% to 57%; "Sweet spot" Class Ⅱ is moderate yield gas reservoirs with Type Ⅱ pore structure being dominant

followed by Type Ⅰ pore structure and Type Ⅲ pore structure being rare

and irreducible water saturation varies mostly between 45% and 55%; "Sweet spot" Class Ⅲ is low-yield gas reservoirs

with Type Ⅲ pore structure being dominant and a small number of IV pore structures developed

and irreducible water saturation mostly between 45% and 55%.It is believed that Class Ⅰ and Ⅱ "sweet spots" can be efficiently and economically developed at present

but Class Ⅲ "sweet spot" has "potentials" for future development.According to the research

we propose that (1) burial depth is the decisive factor in controlling the development of "sweet spot"

thus no Class Ⅰ and Ⅱ "sweet spots" occur in areas with burial depth larger than 4 000 m in general; (2) sedimentary environment is the root cause controlling the development of "sweet spot"; (3) event sedimentation and diagenesis greatly impact the formation of Class Ⅰ and Ⅱ "sweet spot"; (4) dissolution is the main controlling factor for the development of Class Ⅲ "sweet spot"; (5) tectonic movement is a necessary condition for the formation and preservation of "sweet spots" in the study area.