A disclosed effective porosity determination method for tight gas formations includes: obtaining a core sample sealed in a pressure-maintaining core vault during transport out of the borehole; coupling the core vault to a collection chamber; based at least in part on measured pressure, temperature, and fluid volumes in the collection chamber, deriving the number of moles of gas retrieved with the core sample; and combining the number of moles with a downhole pressure, a downhole temperature, and a downhole core sample volume to determine an effective porosity of the tight gas formation. A system embodiment includes: a coring tool having a core vault with a seal to provide pressure-preserved transport of a core sample from a tight gas formation; a collection chamber that attaches to the core vault to measure volumes of fluids and gas; and a processing unit that responsively determines an effective porosity of the tight gas formation.