Ordovician reservoirs in Fuxian area: Gas accumulation patterns and their implications for the exploration of lower Paleozoic carbonates in the southern Ordos Basin

In recent years, the Fuxian area in the southeastern Ordos Basin has undergone significant exploration, with industrial gas flow tested in wells drilled into the Ordovician marine carbonates. Despite this, the gas accumulation patterns of this area are not fully understood, posing challenges for further exploration. Our analysis of geological conditions indicates that the Ordovician Majiagou Formation in this area hosts two gas plays: one found in weathering crusts and the other found in interior of the formation. We investigated various typical gas reservoirs in the area, focusing on differentiating the geological conditions and factors controlling gas accumulation in the weathering-crust and interior gas reservoirs. The results suggest three primary gas accumulation patterns in the Majiagou Formation in the Fuxian area: (1) upper gas accumulation in weathering crusts, present in the high parts of landforms such as residual paleo-hills or buried paleo-platform (Pattern I); (2) the stereoscopic pattern with gas accumulation in both weathering crusts and strata interior, arising in high parts of landforms such as residual paleo-hills or buried paleo-platforms (Pattern II); (3) lower gas accumulation in strata interior, occurring in the upper reaches and on both sides of paleo-trenches (Pattern III). This study will serve as a geological basis for future exploration deployment in the Fuxian area.