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

Abstract

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.Three origins of calcite cement were clearly identified: (i) hydrocarbon source rocks in thermal degradation stage, (ii) lacustrine carbonates in deep strata, and (iii) paleo-lakewater. Two episodes of organic fluids derived from the Jurassic source rocks flowed towards the basin margin, resulting in two rounds of acidic→hydrocarbon→alkaline fluid evolution identified in the reservoir sandstones. Calcite (Ca 1) precipitation generated highly calcite-cemented sandstones, which severely downgraded the reservoir quality. However, the second episode of acidic fluid created new porous zones for subsequent hydrocarbon accumulation. Although the cementation history in the central basin did not occlude all porosity, the presence of fluids derived from buried lacustrine carbonates suggests a poor prospect for hydrocarbon migration. The good spatial correlation between the occurrence of the 13C-depleted calcite cement and the presence of hydrocarbons suggests that the 13C-depleted calcite (relative to lacustrine carbonates) may be an indicator of hydrocarbon migration in the northern Qaidam Basin.