Controls of sedimentary-diagenetic evolution on rock physics properties in Precambrian-Lower Paleozoic ancient carbonate reservoirs

The Precambrian-Lower Paleozoic ancient carbonate reservoirs are pivotal for onshore hydrocarbon exploration in China. Nevertheless, their intricate diagenetic history leads to apparent inter- and intra-reservoir heterogeneity, strongly impacting the physical responses. Deciphering the rock physical characteristics compatible with geological processes is imperative for interpreting the geophysical responses of these reservoirs. To address this, rock physical experiments under reservoir conditions were conducted for carbonate rocks formed in different sedimentary systems from a geological perspective. The results illustrated that sedimentary-diagenetic evolution determines the material composition, dolomite genesis, and pore evolution, ultimately defining the two most critical parameters, load-bearing frame, and pore structure, controlling the physical properties of ancient carbonate rocks. Reservoir properties (porosity and permeability) are enhanced by strong hydrodynamic conditions that facilitate primary pore development, penecontemporaneous dissolution leading to secondary dissolved pores, early-stage dolomitization preserving pre-existing pores, and multi-stage tectonic movement forming microcracks (at the plug scale) with superior transport capacity. Concerning seismic properties, the slight decrease in P-wave velocity reflects that dissolution shapes the pores into a larger but stiffer shape. The significant decrease in P-wave velocity could be attributed to two factors, including weaker crystal contact boundary types formed due to different provenance, strong evaporation and silicification, and microcracks developed in regions with intense tectonic activities. Furthermore, these crystal contact boundary and pore structure types can be effectively identified by the difference in VP/VS ratio. This study enhances the understanding of rock physics in ancient carbonate reservoirs, which is relevant to the exploration of such reservoirs.