Volcanic Hydrothermal Diagenesis and Its Implication for Reservoir Formation in the Ordovician Limestone, Tarim Basin, NW China

    Volcanic hydrothermal fluids in sedimentary basins continuously alter sedimentary strata and influence the development of hydrocarbon reservoirs. However, there has been ongoing debate regarding whether volcanic hydrothermal alteration degrades reservoir quality by metamorphism and filling or improves it by dissolution. Taking the Ordovician Yijianfang Formation limestone in the Tarim Basin for example, renowned for ultra-deep burial conditions, the development of strike-slip fault reservoirs and abundant hydrocarbon resources, this study investigated the alteration lithofacies and reservoir characteristics of limestone within the YAB ~YJF series of outcrops featuring diabase intrusions in the Bachu area. The result reveals that alterations in the limestone by volcanic hydrothermal fluids include marbleization, dissolution, silicification, and filling.

    Marbleization is identified as a destructive diagenesis, where the marble formed from limestone exhibits dense lithology, coarse calcite crystals in mutual interlocking contact. Dissolution displays selectivity, strongly dissolving reefal limestone, bioclastic limestone and grain limestone. Features such as moldic pores formed after the dissolution of nautiloids and their fragments, as well as needle-like dissolution pores, are commonly observed. Particularly in fluorite-rich outcrop (YJF-B), strata-bound dissolution caves formed by volcanic hydrothermal fluids are evident, with the largest cave measuring approximately 2.5 m in height, 5 m in width, and 15 m in length. These caves, varying in size, are distributed in a stepped pattern from top to bottom, interconnected by fractures, and contain fluorite, hydrothermal travertine, and gypsum. Caves and pores of various sizes are commonly filled with calcite. Analyses of ⁸⁷Sr/⁸⁶Sr ratios for calcite fillings yield values mostly between 0.710 and 0.711. Reservoirs quality tests of the dissolution layer show a porosity of 4.12% and a permeability of 0.052 × 10⁻³ μm². In some layers with well-developed dissolution pores, porosity and permeability can reach 11.74%, 7.803 × 10⁻³ μm² individually, significantly higher than the average porosity, being lower than 2%, for the unaltered host rocks. This indicates that deep-seated volcanic hydrothermal fluids associated with magma emplacement substantially improved the reservoirs quality of the limestone.

    Based on the types of precipitated hydrothermal minerals, the main fluid components are inferred to include CO₂, Si, F, S. Establishing the spatial relationships among dissolution pores, caves, and hydrothermal mineral reveals that during ascent, volcanic hydrothermal fluids preferentially cause dissolution, forming smaller strata-bound dissolution pores. When fractures are present, the fluids migrate upward along them, leading to continuous dissolution and the formation of large dissolution caves. As the dissolution diminishes, earlier dissolution products precipitate as silicification and filling, forming a sealing layer above the layers with dissolution pores and caves. Although silicification and filling accompany dissolution, with precipitation occurring immediately within newly formed dissolution pores, the two diagenesis is relatively weak where bottom dissolution is strong. However, when dissolution weakens, pore-filling and host silicification becomes the primary destructive diagenesis for reservoir formation.

    The research confirms that within the Ordovician limestone of the Tarim Basin, in areas characterized by ultra-deep burial, strike-slip fault and volcanic activity development such as the Fuman Oilfield, reservoirs formed by volcanic hydrothermal dissolution could do exist.