Method for Restoring the Initial Porosity of Bioclastic Mixed Rocks with Body Cavities: A Case Study of Mixed Rock Reservoirs in the Es12 Formation Around Bozhong Sag, Bohai Bay Basin

Key words: 
• mixed rock /
• gastropoda /
• initial porosity /
• porosity evolution /
• body cavity pore

Abstract: [Objective] The Paleogene Shahejie Formation around the Bozhong sag, Bohai Bay Basin is characterized by the development of mixed rock reservoirs, which are rich in abundant oil and gas resources. The unique biomass cavity pores in mixed rock reservoirs make the initial porosity of mixed rocks cannot be obtained by the conventional initial porosity recovery formula for mixed rocks, and there is a lack of initial porosity recovery methods for mixed rocks both at home and abroad, and the accurate recovery of the initial porosity is a key element in the study of the evolution of the reservoir. [Methods] In this study, modern snail samples were selected to determine the volume of snail body cavities using experimental and formulaic methods. Subsequently, a physical simulation experiment was conducted to simulate the filling conditions of snail body cavities in a real depositional environment. Finally, based on commonly used formulas for calculating initial porosity in reservoirs, a formula suitable for mixed rock reservoirs was derived. [Results and Discussions] Studies have shown that the species of snails is the biggest factor affecting the percentage of body cavity pore volume of snails. Different species of snails have different sizes of effective storage space, i.e., body cavity pore porosity; the main reason affecting the difference in body cavity pore porosity of the same species of snails is the size of the body cavity pore lumen. Taking the mixed rock sample from well QHD36-3-A at a depth of 3765.03m with a high content of fragmented bioclasts as an example, the initial porosity of this mixed rock at that depth was determined to be 51.68%. [Conclusions] In this paper, a new calculation method to find the initial porosity of mixed rocks is established based on the previous formula and combined with physical simulation experiments, which is of great significance for the study of mixed reservoir evolution.