Secondary calcite in carbonate reservoirs of oil fields: a method for its quantitative determination

Background: Carbonate rocks form subsalt hydrocarbon reservoirs in the Northwestern Margin Zone of the Pre-Caspian Basin. In this context, identifying and analyzing the causes of epigenetic alterations affecting the reservoir properties of carbonate formations is of particular importance, as these alterations influence the optimization of the hydrocarbon exploration and production. These factors underscore the scientific and practical relevance of the present study. Aim: To identify the causes of alteration and to quantitatively assess the reservoir properties of carbonate formations affected by secondary epigenetic transformations. Materials and methods: This study investigates subsalt carbonate reservoirs from oil and gas fields in the Northwestern Margin Zone of the Pre-Caspian Basin. The analysis involved both macroscopic and microscopic examination techniques, supported by a range of laboratory tools, including thermal analyzers, thermogravimetric instruments, and X-ray diffractometry. The method proposed in this paper relies on DTA data obtained during dynamic heating of dolomite, calcite and magnesite, focusing on their thermal decomposition behavior. Mineralogical and compositional control of the reservoir formations was carried out using X-ray phase analysis. Results: A thermal analysis–based technique was developed to quantify secondary calcite in carbonate reservoir formations from subsalt oil and gas fields in the Northwestern Margin Zone of the Pre-Caspian petroleum province. Thermal parameters associated with epigenetic transformations of the dolomite–calcite assemblage was established using representative carbonate samples. These transformations are characterized by the infilling of pore spaces with secondary minerals, leading to reduced porosity and permeability in the affected reservoirs. Conclusion: Accounting for the structural properties of sedimentary rocks described in this study can significantly improve the quality of hydrocarbon exploration. The proposed method provides detailed information on the mineral composition of the reservoir, their filtration-capacity characteristics of carbonate minerals, the crystallinity of their components, lattice properties, and the physical behavior of magnesium, calcium, and other trace elements. Secondary calcite formed through epigenetic alteration of the host rocks has a negative impact on the porosity and permeability of the reservoirs.