Application of stable isotopes, trace elements and spectral gamma-ray log in resolving high-frequency stratigraphic sequences of a mixed carbonate-siliciclastic reservoirs

Abstract

Integrated methods have been applied for the high resolution sequence stratigraphic analysis of a mixed petroliferous carbonate-siliciclastic succession of the Asmari Formation (Oligocene-Miocene) in the Zagros Basin. Detailed sedimentological (core description) and petrographical (one thousand one hundred fifty thin sections) analysis led to identification of more than 38 carbonate and siliciclastic facies (basin wide) that have been deposited in four types of depositional environments, including inner ramp, mid ramp, outer ramp and basin, in homoclinal ramp type carbonate platform. An integrated multidisciplinary approach including core descriptions, petrography, biostratigraphy, gamma-ray spectrometry (e.g., uranium (U), thorium (Th) and Potassium (K)), oxygen and carbon isotopes (Chemo-stratigraphy) and trace element analysis have been used for stratigraphic interpretations of this succession. Based on petrography, biostratigraphy, lateral and vertical facies changes, and GR log, 6 third-order cycles distinguished, whose boundaries correspond to distinct δ13C and δ18O negative peaks. Trace elements (Fe and Mn) contents along with oxygen and carbon isotopes variations are compared with distribution of sedimentary facies and depositional sequences to investigate and correlate stratigraphic boundaries, which are determined by sequence stratigraphy (GR log and facies analysis) and stable isotope stratigraphy (Chemo-stratigraphic) in shallow marine carbonate platform of the Asmari. Negative isotope and elemental peaks coinciding with the sequence boundaries while the positive isotope and elemental peaks are correlated with maximum flooding surfaces. Changes in the frequency of uranium (U), thorium (Th), and Potassium (K) has been used to investigate the stratal surface and sequence boundaries as well as oxidation and reduction states. Fourth-order sequences that formed during the formation of the third-order sequence could be distinguished and separated by uranium log.