Relationship between heterogeneity and seismic velocities of the Yudongzi Triassic oolitic reservoirs in the Erlangmiao area, northwest Sichuan Basin, China

In this paper, we use a range of parameters to quantitatively analyze the heterogeneity of oolitic reservoirs aiming to establish a predictive relationship between the reservoir heterogeneity and seismic velocities. The Yudongzi Oolitic shoals on the Triassic carbonate platform margin in the Erlangmiao area, NW Sichuan Basin, contain two fades elements: a stabilized sand flat and a mobile fringe. The stabilized sand flat consists mainly of oolitic limestones with moderately sorted grains of 0.1-6 mm diameter that account for 40-65% of the rock components. The mobile fringe is characterized by residual oolitic dolostones in which the grains are extremely sorted with a diameter of 0.1-0.5 mm and account for 70-80% of the rock components. These oolitic shoals occurred in the lower part of the Feixianguan Formation in each of the four sequence sets (i.e., Pss1-Pss4), and they were continuously progradational from Pss1 to Pss4. Diagenesis and reservoir properties of the oolitic shoals are closely related to their depositional environments, with the mobile fringe tending to contain potential reservoirs. Predictive relationships between the geological properties and seismic velocities of the oolitic reservoirs show that the content, mean diameter, mean radius ratio, and standard deviation of grains and dolomite content are linearly related to seismic velocities, while the porosity has a logistical correlation with seismic velocities. The seismic velocities decrease either with an increase of grain content or dolomite content, or with a decrease of mean grain diameter, mean radius ratio or standard deviation, and they decrease rapidly at first, and then slowly with an increase of porosity. With these correlations, impedance data from seismic exploration could be used to predict depositional environments, mineral composition, and porosity distribution of the oolitic reservoirs. (C) 2012 Elsevier B.V. All rights reserved.