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

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.