Preliminary study on exploration prospect of carbonate gas in coal measures: a case study of Taiyuan Formation in North China

The exploration potential of carbonate gas in coal measures is still rarely studied.A case study of Taiyuan Formation is presented here.The Taiyuan Formation is one of the most favorable hydrocarbon-generating deposits in North China，and several limestone layers above the coals are well-developed in this formation.In recent years，Taiyuan Formation limestone gas is discovered in Huoxi Coalfield in the North China Block.This discovery can provide us in the future with significant inspiration in the exploration of carbonate gas in the coal measures.In this study，the case of Taiyuan Formation limestone gas was initially analyzed.Then，the distribution of limestone layers of Taiyuan Formation，source rocks and their thermal evolution，reservoir space of the limestone，cap rocks，trap development，migration and preservation conditions are summarized.The preliminary understandings are as follows.The coals，limestones in Taiyuan Formation and overlying mudstones can make up source-reservoir-cap assemblages.As the limestones are close to the coals，gas in the coal measures can be easily accumulated in the limestone layers.As for the thickness of coals and limestones in Taiyuan Formation，the northern part of North China is characterized by thick coals and overlying thin limestones，while the south part by thick limestone and underlying thin coals.The coals and dark mudstones in Taiyuan Formation are premium source rocks，and coals of moderate metamorphism are in favor of the formation of free gas.Karst and fissures acting as the reservoir space are well developed in the limestone layers in Taiyuan Formation.The overlying mudstones can work as good cap rocks.Thus，the Taiyuan Formation has the material fundament for the formation of carbonate gas reservoir in coal measures.Besides，the formation of the carbonate gas pool in coal measures needs good matching of geological factors.The development of reservoir space and trap widely occurring in the coal-bearing basins in North China is the prerequisite，while the late-stage gas accumulations are the key.The local uplift in a regional extensional background in the basins，as well as the mining activities accompanied by decreasing reservoir pressure，are helpful to the migration and accumulation of the coal measures gas to the carbonate reservoir.