Petrology, mineralogy and geochemistry of Ordovician rocks in the southwest of Tarim Basin, implications for genetic mechanism and evolution model of the hydrothermal reformed-paleokarst carbonate reservoir

Abstract

Paleokarst carbonate reservoir plays an important role in the exploration of oil and gas from shallow to ultra-deep strata. Multistage karstification is superimposed to form the present ultra-deep karst-hydrothermal reformed carbonate reservoirs in the Yubei area of the Tarim Basin. Based on petrology identification, stable carbon and oxygen isotope analysis, inductively coupled plasma mass spectrometry (ICP-MS), and Laser Ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), we investigated paleokarst periods, chemical property of hydrothermal fluids, and the evolution model and formation mechanism of paleokarst-hydrothermal reformed Ordovician carbonate reservoirs. The results show that the Ordovician strata experienced five periods of karstification from the middle Caledonian period to the late Himalayan period. The Caledonian quasi-contemporaneous karst and the Hercynian epigenetic karstification provided the most fundamental pore – fracture framework for carbonate reservoirs, and the later multistage deep burial karstification re-transformed the established reservoir space by hydrothermal dissolution under thermochemical sulfate reduction (TSR) reaction. It is proposed that the thermal convection between the hydrothermal fluids from the upper mantle and carbonate rocks formed two kinds of hydrothermal fluids in Ordovician strata. The first kind of hydrothermal fluid is performed with a SiO2-enriched fluid under the acidic environment based on TSR, forming siliceous cements and hydrothermal dissolution pores in the Lower Ordovician strata. The second kind of hydrothermal fluids appeared as an alkaline CO32—enriched fluid with the reversible reaction between HCO3− and CaCO3, forming hydrothermal calcite and dolomite near the migration channel in Upper Ordovician carbonate rocks. The periodic changes of in situ trace element concentrations of multistage quartz fillings indicated that paleokarst reservoirs are controlled by thermal convection under episodic tectonic activity. As a summary, we established the migration pattern of hydrothermal fluids in the thin section scale and the evolution model of the hydrothermal reformed paleokarst reservoirs in basin scale.