Carbon and oxygen isotopic composition of carbonate cements of different phases in terrigenous siliciclastic reservoirs and significance for their origin: A case study from sandstones of the Triassic Yanchang Formation, southwestern Ordos Basin, China

Early carbonate cements in the Yanchang Formation sandstones are composed mainly of calcite with relatively heavier carbon isotope (their δ 18 O values range from −0.3‰–−0.1‰) and lighter oxygen isotope (their δ 18 O values range from −22.1‰–−19.5‰). Generally, they are closely related to the direct precipitation of oversaturated calcium carbonate from alkaline lake water. This kind of cementation plays an important role in enhancing the anti-compaction ability of sandstones, preserving intragranular volume and providing the mass basis for later dissolution caused by acidic fluid flow to produce secondary porosity. Ferriferous calcites are characterized by relatively light carbon isotope with δ 13 C values ranging from −8.02‰ to −3.23‰, and lighter oxygen isotope with δ 18 O values ranging from −22.9‰ to −19.7‰, which is obviously related to the decarboxylation of organic matter during the late period of early diagenesis to the early period of late diagenesis. As the mid-late diagenetic products, ferriferous calcites in the study area are considered as the characteristic authigenic minerals for indicating large-scaled hydrocarbon influx and migration within the clastic reservoir. The late ankerite is relatively heavy in carbon isotope with δ 13 C values ranging from −1.92‰ to −0.84‰, and shows a wide range of variations in oxygen isotopic composition, with δ 18 O values ranging from −20.5‰ to −12.6‰. They are believed to have nothing to do with decarboxylation, but the previously formed marine carbonate rock fragments may serve as the chief carbon source for their precipitation, and the alkaline diagenetic environment at the mid-late stage would promote this process.