Origin of diagenetic calcite cements in the continental Qaidam Basin, NW China: Implication for fluid flow and hydrocarbon migration

Diagenetic carbonate cements have been widely investigated to reveal fluid information in marine basins. However, they have not been well studied in continental basins. This paper addresses this issue using a case study in the northern Qaidam Basin, northwest China. The calcite cements in the reservoir sandstones were investigated by an integrated analysis including petrology, mineralogy, carbon and oxygen isotopes and fluid inclusion. Calcite is the dominant cement and main cause of poor porosity in the sandstones. The δ13CPDB and δ18OPDB values of calcite cements display a wide isotopic range, varying from −13.5 to 2.5‰ and −16.7 to −4.7‰, respectively. However, the δ13CPDB and δ18OPDB values from the central basin are within a limited range (−4.2 to −2.0‰ and −11.2 to −9.4‰, respectively). Dull- and bright-luminescence zones and two populations of fluid inclusion indicate two generations of calcite (Ca 1 and Ca 2) in N1 well. Ca 1 and Ca 2 have similar δ13CPDB values (−8.7 to −7.0‰), whereas the δ18OPDB values (−13.7 to −13.1‰) of Ca 2 are more negative than those (−10.8 to −9.8‰) of Ca 1.