Shale brittleness and ductility always attract much attention in shale gas exploration and development.The determination of the depth interval of brittle-ductile transition of shale is key to the evaluation of shale gas preservation and hydraulic fracturing.Based on rock mechanics tests

we established a set of methods for determining the brittle-ductile transition zone of shale.The pre-consolidation pressure was determined through uniaxial strain test

and the threshold of over-consolidation ratio(OCR) and the critical confining pressure of brittle-ductile transition of shales were defined via triaxial compression test.And in turn

the bottom boundary of brittle zone was determined according to the maximum palaeo-depth combined with the OCR threshold

while the top boundary of ductile zone was determined by the critical confining pressure of brittle-ductile transition.The brittle-ductile transition zone is the depth interval between the bottom boundary of brittle zone and the top boundary of ductile zone.The shale brittle zone

ductile zone and brittle-ductile transition zone defined by the above methods can be used for evaluations of shale gas preservation conditions and hydraulic fracturing.The results show that(1) in the brittle zone

the shale gas is not well preserved as brittle fractures are easy to occur under tectonic events;(2) in the ductile zone

the hydraulic fracturing does not work well because the hydraulic fractures are easy to close; but(3) in the brittle-ductile transition zone

the shales are not only more favorable for shale gas preservation but also better in fracturability.Therefore

the brittle-ductile transition zone is the best interval for the exploration and development of marine shale gas in southern China.The exploration practice indicates that the burial depth of the shale gas play zones with high and stable yield in the Silurian Longmaxi Formation is just within the brittle-ductile transition zone determined by the methods proposed in this study.