# Experimental Study on the Impact of CO2 Treatment on Different Lithofacies in Shale Oil Reservoirs
###### Abstract  
CO2 technology has been progressively used in the development of shale oil. After injection, CO2 can react with formation water to form carbonic acid, which then reacts with carbonate and silicate minerals, resulting in changes in porosity and permeability. However, there are some debates as to whether the effect of CO2 improves or damages porosity and permeability. So, in this paper, systematic experiments were carried out to clarify the interaction between CO2 and shale in different lithofacies and to draw a pertinent conclusion. The results showed that the shale in Qingshankou Formation could be divided into three main lithofacial types: foliaceous shale, laminated feisic shale and laminated diamatic shale. There were relatively more pores, some natural microfractures and small mineral particles in foliaceous shale, a few micropores and large mineral particles in laminated feisic shale, some biogenic calcium carbonate minerals and hardly any micropores in laminated diamatic shale. Due to the diversity of micromorphology and mineral composition, the effects of CO2 treatment had significant differences. For foliaceous shale, CO2 treatment had both improving and damaging effects on porosity and permeability; for laminated shale, both porosity and permeability improved significantly. So, it is necessary to identify the main lithofacies of target formation before the application of CO2 technology in shale oil reservoirs.  
shale oil; lithofacies; CO2 treatment; micromorphology; porosity and permeability  
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in changes in reservoir porosity and permeability. However, CO\({}_{2}\) has both positive and negative effects on reservoir porosity and permeability after complex reactions [20; 21; 22; 23; 24]. On the one hand, these reactions can dissolve part of the matrix minerals, increasing the matrix porosity effectively. Additionally, this dissolution can increase the size of the seepage channel and improve the pore connectivity, thereby increasing the permeability of shale oil reservoirs. On the other hand, new mineral precipitation will be produced after the interactions between CO\({}_{2}\) and minerals, such as potassium feldspar, albite and chlorite. Additionally, the dissolution of cement causes the migration of mineral particles. Both the precipitation and migration of minerals may block pores and throats and reduce matrix porosity. Furthermore, these cause a decrease in pore connectivity and damage the permeability of shale oil reservoirs.  
There are strong vertical heterogeneity and mineral composition differences in the continental shale oil reservoirs of China [25; 26; 27; 28]. So, the changes in porosity and permeability in different formations are significantly different after CO\({}_{2}\) injection. Therefore, it is necessary to carry out systematic experiments to clarify the interaction between CO\({}_{2}\) and shale in different lithofacies, and its effect on the porosity and permeability of shale oil reservoirs. In this paper, the main lithofacial types of the target formation are identified through core observation and X-ray fluorescence (XRF) spectrum analysis. Then, shale samples in different lithofacies are selected for CO\({}_{2}\) soaking experiments. Finally, the influence of CO\({}_{2}\) treatment on shale is explored by X-ray diffraction (XRD) mineral composition analysis, scanning electron microscope (SEM) micromorphology analysis and characterizations of porosity and permeability.