Rock physics properties and influencing factors of subsalt carbonate reservoir of Ordovician Majiagou Formation in central Ordos Basin

Abstract

The carbonate reservoirs in the Ordovician Majiagou Formation of the Ordos Basin have undergone complex geological evolution, resulting in high-quality dolomite reservoirs that exhibit strong heterogeneity. Neglecting the fundamental factor of reservoir genetic mode, conventional rock physics experiments cannot accurately determine the seismic elastic responses of the target rock. Here, a set of carbonate samples from different sedimentary environments were selected elaborately based on geological and logging data. Subsequently, systematic petrological and rock physics measurements were conducted to investigate the variation of rock physics properties from both macro-geological and micro-structural perspectives. The measurement results illustrate that the microstructures in carbonate rocks are influenced by tectonic-sedimentary patterns and sea level fluctuation. Various rock types are observed: pore type dolomitic gypsum, argillaceous dolomite, and microcrystal dolomite in restricted-evaporative lagoon environments; dissolved pore type and crack-dissolved pore type dolomite in mound-shoal environments; and dissolved pore type gypsum dolomite in platform flat environments. Furthermore, the mineral components as the load-bearing frame and the pore structure jointly control the elastic properties. Samples with the same lithology exhibit similar load-bearing frames, leading to a strong statistical relationship between V_P and V_S. Concerning the pore structure, dissolved pores formed by atmospheric freshwater dissolution during the penecontemporaneous period have high stiffness, minimally affecting the elastic properties of reservoirs. Conversely, the lower stiffness of microcracks resulting from tectonic rupture significantly decreases the P-wave impedance and Poisson's ratio of dry samples, while increasing the Poisson's ratio of water-saturated samples. These findings enable the accurate recognition of the seismic elastic characteristics of high-quality dolomite reservoirs in mound-shoal environments, thus providing a rock physics experimental basis for improving the precision of seismic reservoir prediction in the study area.