Discussion on preservation conditions of shale gas with"Sandwich" structure in South Yellow Sea basin

In order to look for Mesozoic-Paleozoic shale gas in the South Yellow Sea basin, a comparative evaluation of preservation conditions is performed among the Barnett shale gas reservoirs in the Fortworth basin of USA, Jiaoshiba shale gas reservoirs in the Sichuan basin and potential shale gas reservoirs in the Guizhou Province. Results show that the so-called "Sandwich" structure is of great importance for shale gas accumulation and preservation. The "Sandwich" structure is a kind of special reservoir-cap rock assemblage which consists of a limestone or dolomite cap and a limestone or dolomite base with a sequence of mudstone or shale layer in the middle. It is observed that the Mesozoic-Paleozoic strata in the Lower Yangtze, in particular the Laoshan uplift, are characterized by weakly deformed Paleozoic with thrust fault sealing on both flanks of the uplift. "Sandwich" structure may exist in the lower Cambrian and Permian strata for shale gas preservation and accumulation. Breakthrough of shale gas exploration is expectable in the South Yellow Sea. The margins of both flanks of the Laoshan uplift will be the preferred targets for shale gas drilling.