Geochemical, mineralogical, and petrological analyses for the interpretation of the sedimentary environment of the Middle-Late Ordovician Majiagou Formation (northern China) as a tool for more effective gas exploration

Abstract

Core samples from the deeply buried Ordovician Majiagou Formation below the Huainan Coalfield (E China) have been investigated for their carbonate types, major and trace elements (including rare earth elements) and C and O isotopes. The objective was to get a better insight into the possible occurrences of gas (and possibly oil) derived from Carboniferous coals. It was found that the carbonates are dolomites with strongly varying amounts of CaO and MgO. The low concentrations of SiO 2 , Al 2 O 3 , and Fe 2 O 3 indicate deposition in a normal marine environment with little terrigenous input. The Na 2 O/K 2 O, Fe/Mn and Sr/Ba ratios, as well as the Ga values indicate mainly a marine salinity and a hot and humid climate. The slight depletion of Ce and Eu, the depletion of heavy REE and the enrichment of light REE indicate deposition in a reducing environment. It thus appears that the Majiagou Formation below the Huainan Coalfield closely resembles that in the eastern part of the Ordos Basin, where several gas reservoirs are present, so that the Majiagou Formation under the Huainan Coalfield represents a promising target for hydrocarbon exploration.