CHARACTERIZATION OF THE LATE APTIAN TOP-DARIYAN DISCONFORMITY SURFACE OFFSHORE SW IRAN: A MULTI-PROXY APPROACH

M. Naderi-Khujin¹, A. Seyrafian^{1 *}, H. Vaziri-Moghaddam¹ and V. Tavakoli²

¹ Department of Geology, University of Isfahan, Hezar Jerib St, Isfahan, Iran 81746-73441.

² School of Geology, University of Tehran, Tehran, Iran.

^* Corresponding author, email: seyrafia@sci.ui.ac.ir

The late Aptian disconformity at the top of the carbonate-dominated Dariyan Formation is a regional-scale surface recognised throughout SW Iran and adjacent areas to the south. This study investigates the disconformity using wireline logs, petrographic analyses, SEM observations, and CL and stable isotope data from an oilfield offshore SW Iran. The top-Dariyan surface in the studied field is characterised by erosional incisions and clastic-filled fissures which, together with secondary porosity in the underlying carbonates, uranium enrichment and meteoric infill cements, are interpreted as subaerial exposure-related features. The clastic-filled fissures were identified from gamma-ray logs in horizontal wells, and infill material is composed of argillaceous carbonates, clays, quartz and iron-bearing minerals. The correlation of GR logs in 19 vertical wells resulted in the recognition of erosional incisions (up to about 16 m deep) on the top-Dariyan surface, together with thickening of siliciclastic sediments in the overlying basal part of the Kazhdumi Formation. Back-filled sediments in erosional incisions may form potential exploration targets. Secondary porosity in the uppermost 20 m of the Dariyan Formation was created by meteoric processes during subaerial exposure. Cathodoluminescence microscopy of diagenetic cements, carbon and oxygen isotope data and elemental analyses were used to reconstruct the diagenetic history of the carbonates in the Upper Dariyan interval.

Keywords: Iran, Dariyan Formation, Aptian, disconformity surface, carbonates, subaerial exposure, stable isotopes, karst, gamma logs, erosional incision, cement.

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