Mesogenetic diagenesis of the Ordovician limestone in Yubei area, Tarim Basin, NW China

The Lower Ordovician succession, consisting of the Yingshan and Penglaiba formations, encompasses significant petroleum reservoirs in Yubei area, southwest Tarim Basin. Mesogenetic dissolution and dolomitization strongly contributed to the increase in rock porosities here. Wells YB 7, YB 5, and YB 1-2X drilled in eastern Yubei area met the deposits of Yingshan and Penglaiba formations, providing abundant data for the mesogenetic diagenesis analysis. Among them, Well YB 5 encountered the thickest Penglaiba Formation and was chosen for detailed analysis. Eight diagenetic elements (i.e., micrite encrust, radial fibrous calcite cement, stylolites, poikilotopic calcite cement, dolomite, fracture, bladed quartz, and equant quartz) are identified and ascribed to seven paragenetic events. Their Paragenetic sequence evidences Yingshan and Penglaiba Formations experienced severe deep burial diagenesis. The presence of green luminescent fluorite, non-luminescent and zoned quartz, and violet to orange luminescent saddle dolomite with sweeping extinctions, together with the positive Eu2+ anomalies shown in the rare earth element (REE) patterns all help to ascribe the buried dissolution and dolomitization to the incorporation of hydrothermal fluids. The inconsistency between the REE patterns of the Yingshan and Penglaiba formations and that of the organic-rich source rocks of Lianglitage and Bachu Formations exclude the assumption that hydrocarbon influx caused the mesogenetic dissolution. High-angle fractures, generated by the Middle Caledonian–Early Hercynian tectonic movements, acted as conduits for hydrothermal fluid migration and promoted the mesogenetic dissolution to form dolomite reservoirs. The coincidence that samples with positive Eu2+ anomalies all fall in the basalt province, while samples lacking positive Eu2+ anomalies from Tahe fall out, backs up that the main source of the hydrothermal fluid promoting the mesogenetic dissolution was probably derived from the Permian magma. Strata possessing similar geochemical and petrological characteristics as Yingshan and Penglaiba formations in eastern Yubei area, and experiencing subsequent tectonic and magmatic activities could be potential exploration targets in further exploration practice in Tarim Basin and in the other basins in the world.