Variability in carbonate cementation induced by multistage fluid evolution in coal measures: Insights from the tight sandstones of the lower Jurassic Yangxia Formation, Kuqa Depression, Tarim Basin, China

Abstract

Carbonate cementation is a key factor in the densification of sandstone reservoirs in the Yangxia Formation of the Kuqa Depression; however, the formation mechanisms are not yet fully understood. The carbonate cements in the tight sandstones of the Yangxia Formation are primarily composed of siderite, ferroan calcite, ferroan dolomite, and ankerite. The δ¹⁸O, δ¹³C, and ⁸⁷Sr/⁸⁶Sr data indicate that the formation of these carbonate cements was controlled by the thermal evolution of the coal-bearing source rocks of the Yangxia Formation. Early-phase ferroan siderite (S1) and ferroan dolomite (D1) formed between 199.5 Ma and 170.54 Ma, with carbon derived from the reduction of CO₂ by methanogenic bacteria and calcium from the original formation water. Mid-phase ferroan calcite (C1) and ankerite (D2) formed between 72 Ma and 50 Ma, with carbon sourced from a mixture of inorganic CO₂ and CO₂ released during the thermal degradation of organic matter in the coal-bearing source rocks of the Yangxia Formation. Late-phase ferroan calcite (C2) formed between 18.6 Ma and 13.6 Ma, with carbon derived from a mixture of inorganic CO₂ and CO₂ released by the thermal decarboxylation of organic matter in the coal-bearing source rocks of the Yangxia Formation. The calcium in both mid-phase and late-phase carbonate cements was sourced from the smectite-to-illite transformation in the coal-bearing source rocks of the Yangxia Formation. Clumped isotope and fluid inclusion thermometry reveal that, prior to hydrocarbon charging, carbonate cementation significantly reduced the porosity and permeability of the sandstones in the Yangxia Formation, resulting in reservoir densification.