Dissolution of Calcareous Mudstone by Indigenous Organic Acids during Thermal Maturation in Qaidam Basin

<title>Abstract</title> <p>In order to investigate the dissolution of calcareous mudstone by native organic acids during thermal maturation, a series of hydrous pyrolysis experiments were initially performed. The results show that formic acid, acetic acid, oxalic acid, propionic acid, butyric acid, and succinic acid were identified in the hydrocarbon-generated thermal evolution products. Acetic acid exhibits the highest concentration, the other five are low-abundance but highly reactive in dissolution. Organic acid content evolves in three stages with thermal maturity (Ro): (I) generating at Ro ≤ 0.7%; (II) stabilizing in dynamic equilibrium at Ro = 0.7–1.8%; and (III) declining progressively at Ro &gt; 1.8%. Reservoir modification and hydrocarbon accumulation also occur in these three coupled stages. (I)The initial modification stage, characterized by a limited porosity gain and the formation of a nascent pore network, serves as a fundamental framework for in situ hydrocarbon preservation.(II) Pore-expansion stage: rapid increase in porosity and sustained high permeability. Hydrocarbons are retained internally until the expulsion threshold is reached and then migrate laterally or vertically. (III) Reservoir-space reorganization stage: overpressure-induced microfractures increase porosity but reduce permeability through pore-throat occlusion and flow-path tortuosity—favoring long-term hydrocarbon preservation.</p>