Diagenesis and Petroleum Significance of Veins in Marine Carbonate: A Case Study from the Subsalt Ordovician Fourth Member of the Majiagou Formation, Ordos Basin, China.

Fluid activity in marine carbonate strata is commonly recorded by calcite veins, which are important archives of diagenesis and hydrocarbon charging. However, the origin and evolution of veins in subsalt carbonate reservoirs remain poorly constrained. This study investigates calcite veins from the subsalt Fourth Member of the Ordovician Majiagou Formation (O2m4) in the central-eastern Ordos Basin, China. Petrography, carbon and oxygen isotopes, in-situ trace elements, and fluid inclusion studies were integrated to constrain vein-forming fluids, vein evolution, and their relationship with hydrocarbon charging. The results show that the veins were predominantly formed during a single precipitation stage and that their parent fluids were mainly host-rock-buffered burial fluids, with limited contribution from adjacent strata. These veins were subsequently modified to a limited extent by later hydrocarbon-bearing fluids. Fluid inclusion assemblages and homogenization temperatures indicate that hydrocarbon activity recorded by the O2m4 vein system can be divided into three stages: (a) limited early hydrocarbon charging during shallow burial, (b) widespread oil charging during the Late Triassic to Early Jurassic, and (c) later cracking of previously accumulated oil to gas during the Early Cretaceous. The hydrocarbon records preserved in O2m4 therefore indicate mature-oil charging followed by later oil cracking. This study provides new constraints on the coupling among vein formation, fluid evolution, and hydrocarbon accumulation in subsalt marine carbonate reservoirs and offers implications for natural gas exploration in the Majiagou Formation.