Abstract
Accurate recognition of the types of alteration fluid and the development mechanisms are important concerns in studying deep marine carbonate reservoirs. Major fluid types, such as seawater, meteoric water, deep burial formation water, hydrothermal fluid, and thermochemical sulfate reduction- (TSR-) derived fluid, were identified based on carbon, oxygen, and strontium isotope compositions of many samples from the Tarim, Sichuan, and Ordos basins in China. Compared with normal marine limestones, seawater calcite cement has similar isotopic compositions. Calcite cement precipitated from meteoric water has extremely light oxygen isotope compositions, and its δ18OV-PDB reaches -18.8‰. Due to the fractionation of oxygen isotopes at high temperatures (101.2~145.6°C), calcite precipitated from deep burial formation water and deep hydrothermal fluid has moderately light oxygen isotope compositions. The TSR process consumes organic matter to produce CO2/CO32-, and the calcite from TSR-derived fluid has very light carbon isotopes (δ18OV-PDB, -18.9‰) due to the incorporation of organic CO2/CO32-. Formation water and TSR-derived fluid generally originate and are confined within the carbonates and are consequently termed endogenous fluids. The 87Sr/86Sr ratios of calcite cements from endogenous fluids are basically the same as those of surrounding carbonates. Meteoric water and hydrothermal fluid originate outside the carbonate strata and are exogenous fluids. The 87Sr/86Sr ratios of calcite cements from exogenous fluids are higher than those of surrounding carbonates, up to 0.710558. For karst carbonate reservoirs developed in tectonic uplift-meteoric water environments, the reservoir spaces of karst caves and fractures occur principally under and near unconformity surfaces and megacrystalline calcite cements occur below the karst zone. In deep fault-hydrothermal fluid environments, high-quality carbonate reservoirs develop downward into ultradeep strata. In deep burial-TSR-derived fluid environments, dissolution porosity can be well preserved for a long geological time due to high CO2 and H2S concentrations.
Highlights
A large number of exploration examples worldwide have revealed abundant oil and gas resources in deep/ultradeep layers, mainly due to the occurrence of high-quality carbonate reservoirs
In the deep Ordovician carbonate reservoirs in the Ordos Basin, erosive residue was found to accumulate at the bottom of a dissolution vug in the Lian 5 well to form a bottom-identifying structure and the vug was filled with granular calcite, which did not emit cathodoluminescence under cathode ray irradiation (Figures 3(a) and 3(b))
The compositions of carbon, oxygen, and strontium isotopes in normal marine limestones may vary with the age of sedimentation; the influence of fluid dissolution and alteration on carbonates after sedimentation might be far greater than that of age
Summary
A large number of exploration examples worldwide have revealed abundant oil and gas resources in deep/ultradeep layers, mainly due to the occurrence of high-quality carbonate reservoirs. The development mechanisms of deep carbonate reservoirs in different tectonic-fluid environments should be studied in detail Minor elements, such as Fe and Mn, were used to identify hydrothermal fluid alteration [16, 17], and rare earth elements were used to identify alterations related to meteoric water and hydrothermal fluids [18]. In this study, based on a large number of samples from the lower Paleozoic deep/ultradeep carbonate reservoirs in the Tarim, Sichuan, and Ordos basins in China, we (1) statistically analyzed strontium, carbon, and oxygen isotopes and homogenization temperatures of fluid inclusions to identify the fluid types; (2) determined the tectonic background and tectonic-fluid environments for the development of different fluid alterations; and (3) investigated the characteristics and development mechanisms of high-quality reservoirs in different tectonic-fluid environments
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