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

摘要

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 delta C-13(pDB) and delta O-18(pDB) values of calcite cements display a wide isotopic range, varying from - 13.5 to 2.5%o and - 16.7 to - 4.7 parts per thousand, respectively. However, the delta C-13(PDB) and delta O-18(pDB) values from the central basin are within a limited range (- 4.2 to - 2.0 parts per thousand and -11.2 to - 9.4 parts per thousand, 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 delta C-13(pDB) values (- 8.7 to - 7.0 parts per thousand), whereas the delta O-18(pDB) values (- 13.7 to - 13.1 parts per thousand) of Ca 2 are more negative than those (-10.8 to - 9.8 parts per thousand) of Ca 1.