Study on pore structure characterization of strong diagenesis sandstones and the logging response characteristics in Tazhong area, Tarim Basin

Abstract

The original framework of pore structure is determined by sedimentation; however, reservoirs with different rock components undergo different diagenetic processes with different intensities, so they have different pore structure characteristics. The logging response characteristics of strong diagenetic sandstone show a series of geophysical responses, such as acoustic, nuclear, and electrical, of the final pore structure style formed by diagenetic transformation after primary sedimentary structures. In low permeability reservoirs, the response characteristics are more reflected as the signal coupling of different geological factors instead of a linear correlation with porosity or permeability like in conventional reservoirs. The formation studied, the Silurian system in the Tazhong area, is mainly characterized by a large area and low abundance of lithological reservoirs with complex geological characteristics such as variable lithology, large physical property differences, and strong heterogeneity. Due to the early formation of the paleo-uplift in the Tazhong area, the Indosinian and Yanshan orogeny had negligible effects and formed a stable tectonic background. Furthermore, the evolution of pore structure is reflected in the transformation from primary pores to secondary or the remaining pores by cementation and dissolution in different diagenetic stages of different sand bodies in tidal flat faces. Therefore, the analysis and characterization of the pore structure system are particularly important. Using data based on 314 MICP samples, continuous changes of the pore structure, and the logging response in single layer and correlated layers, we conducted observations and analytics in the study. Based on the continuous MICP data, the relationship between the changes in pore structure, the geological principles, and the logging response characteristics is analysed. Our results show that the pore system heterogeneity is mainly caused by dissolution and cementation and differs in distribution. The main controlling factors of pore structure changes are transformed from sedimentation controls mainly to sedimentation–diagenesis or mainly diagenesis effects from the bottom-up case. Six types of pore structures are characterized, and we analysed the coupling effect of sedimentation and diagenesis factors on the logging response in the inconsistency features of curves. The classification of pore structure can help to analyse the law of pore structure change in sand body, and quantitatively characterize it from the perspective of control factors and logging response characteristics, which lays a foundation for the subsequent pore structure prediction.