Anisotropy evolution during early maturation of organic-rich carbonates

Abstract

The organic-rich Late Cretaceous carbonates of the Ghareb and Mishash formations are potentially both self-sourced unconventional reservoirs and high quality source rocks for conventional petroleum systems in the eastern Mediterranean region. In this study we use cores from depths ranging from 340 to 1290 m in the Shefela and Golan Heights basins in Israel, which records the diagenesis and transformation phases of organic-rich chalk into limestone. Understanding such source rocks is complicated by the fact that each formation is unique and heterogeneity is common within each formation. Moreover, in situ samples at various stages of maturation are rarely available. We apply the three-plug measuring technique for a vertical transversely isotropic (VTI) medium to investigate the ultrasonic wave velocity and permeability anisotropies of organic-rich chalks (ORC) within the tested sequence.We propose that in ORC diagenesis, the characteristics and distribution of the organic matter in the rock matrix change significantly throughout burial and thermal maturation, strongly affecting the velocities. Measuring the velocity hysteresis under varying confining pressures revealed that development of soft porosity with preferred orientation plays a crucial role in the early maturation stage (TMAX of 428–434°C), particularly under low effective stresses, while it is absent in the immature stage (TMAX of 403–415°C). The sensitivity of the wave velocities at low effective stresses results in variations in velocity anisotropy, which may potentially be utilized in seismic exploration to delineate the highly-pressured ‘sweet spots’ in unconventional organic-rich carbonates.