Experiment for the differential dissolution of dolomite of Sinian Dengying Formation in the Gaoshiti-Moxi area,the Sichuan basin

LUO Wenjun; JI Shaocong; LIU Xixiang; DAN Yong; LIANG Bin; NIE Guoquan

doi:10.11932/karst20230612

Abstract：In recent years,important discoveries have been made in natural gas exploration of Dengying Formation in Gaoshiti-Moxi area of the Sichuan basin.The gas-bearing reservoirs are mainly located in the fourth member of the Dengying Formation,and the reservoir rock types are mainly algal agglomerate dolomite,algal arenaceous dolomite,and algal-laminated dolomite.Though many previous studies on the dolomite of Dengying Formation in the Gaoshiti-Moxi area of the Sichuan basin have been conducted,they mainly focus on reservoir characteristics,paleogeomorphology characterization,gas reservoir productivity,etc.There are relatively few studies on simulation experiments of dolomite dissolution.The carbonate rock dissolution experiment is an important method to study the favorable conditions and distribution laws of carbonate rock dissolution.Since the 1970s,scholars at home and abroad have successively carried out simulation experiments of carbonate rock dissolution to explore the influence of composition,structure,temperature,pressure,fluid and other factors on dissolution.Early dissolution experiments mainly simulated surface environments,with experimental temperatures below 100℃.In the 1980s,scholars at home and abroad mainly studied the dissolution mechanism of carbonate rock in a deep burial environment.The experimental method was the surface reaction between fluid and rock particles or blocks. This study takes the algal dolomite of the Dengying Formation in the Gaoshiti-Moxi area as the research object.The dissolution rate,surface morphology,and microscopic characteristics of the dolomite of Dengying Formation are studied based on dissolution experiments with rock slices and thin sections.Meanwhile,the effects of lithology,structure,and reaction time on the dissolution degree of the dolomite are analyzed.The experimental results can not only display the quantitative indicator of dissolution—the dissolution rate,but also directly demonstrate dissolution characteristics and changes in pore structure after dissolution.The results of the dissolution experiment indicate as follows.(1)All samples indicate high initial dissolution rates in the early stage of the experiment,and as the dissolution time increases,the dissolution rate shows a significant attenuation and then tends to stabilize.(2)All samples undergo a certain degree of dissolution in a weak acid environment,and there are significant differences in the degree of dissolution among samples with different lithology and structure.There are significant differences in dissolution rates among different samples.Dissolution rates of algal-laminated dolomite and algal arenaceous dolomite are the highest,followed by that of algal agglomerate dolomite,and the dissolution rate of algal-layered siliceous dolomite is the lowest.(3)Observation and comparison of the microscopic dissolution characteristics of samples at different reaction times show that samples developed with intergranular and inter-crystalline pores exhibited a higher degree of dissolution along these pores.Samples developed with microcracks exhibit a higher degree of dissolution along these microcracks.(4)Regular recording of experimental data during the experiment can accurately illustrate the formation and evolution of dissolution pores and fractures.Conducting dissolution experiments with rock slices and thin sections can not only provide quantitative indicators of dissolution rates,but also show changes in dissolution structure,enabling a more comprehensive understanding of dissolution laws.(5)The development of algal dolomite reservoirs in the Dengying Formation may be related to the dissolution of algal dolomite,because a large number of dissolution pores developed by the dissolution of algal dolomite ultimately formed the appearance of the current karst reservoirs of Dengying Formation,which were mostly developed in algal dolomite with high algal content.Through dissolution experiments,the differences in dissolution of different dolomites in the study area have been analyzed,which is of great significance for predicting the distribution of high-quality reservoirs and guiding oil and gas exploration.