Characterization of microseepage-induced diagenetic changes in the Upper-Red Formation, Qom region, Iran. Part II: A new insight using reflectance spectroscopic analysis

Author(s)

Asadzadeh, Saeid ^[1] ; Souza Filho, Carlos Roberto ^[1]

Affiliation

^[1] Univ Campinas UNICAMP, Rua Carlos Gomes 250, POB 6152, BR-13083855 Campinas, SP - Brazil

• Document type: Journal article
• Source: MARINE AND PETROLEUM GEOLOGY; v. 117, JUL 2020.
• Web of Science Citations: 1

Abstract

The classic microseepage model supports the notion that hydrocarbon pools leak to the surface over time and trigger an array of diagenetic physicochemical and mineralogical changes in the overlying sediments. Mapping the induced alteration mineralogy for microseepage targeting has been an appealing yet ambiguous practice, mostly because of our incomplete understanding of the diversity of changes occurring in the environment. To close this gap, an integrated spectroscopic approach is adopted here to investigate the variability, quantity, and physicochemistry of diagenetic minerals induced by active microseepage systems over the Alborz oil reservoir in the Qom region, Iran. The area includes an effectively sealed carbonate reservoir overlain by thick Oligocene sediments of the Upper Red Formation. A large suite of samples collected from microseepage-affected zones was spectrally characterized in the 350-2500 nm wavelength region and then statistically contrasted against unaffected samples gathered from a nearby control area. The analysis reveals a novel array of mineralogic indicators for microseepage systems. It indicates that smectites (montmorillonite, nontronite), Al-rich mixed-layer illite/smectite, Fe-rich chlorite and traces of palygorskite constitute the clay alterations. This array is associated with the enrichment of K2O and loss of Na2O and MgO from the system, as indicated by X-ray fluorescence measurements. In the arid climate of the region, the iron-rich red-beds (Fe2O3 > 6%) are only moderately bleached and, thus, the subsequent chemical weathering of the relict irons has triggered goethite (Al-poor), and likely ferrihydrite and maghemite precipitation. Consequently, besides the typical ferric iron bleaching, the modification in the mineralogy of iron oxides/oxyhydroxides (marked by a shift in the absorption minimum at similar to 900 nm) is suggested as an additional indicator of microseepage systems. Fe-chlorites constitute the dominant ferrous mineral present in the environment. The affected strata are also associated with prevalent calcite and gypsum cements, and occasionally jarosite occurrences. Based on the silica contents of the host-rock, the microseepage-induced alterations are classified into two categories, namely `felsic' and `mafic', wherein the latter is typified by the assemblages described here. This study provides new insights into microseepage-induced alterations and highlights the significance of reflectance spectroscopic techniques in facies determination and the alteration footprints mapping of onshore microseepage systems. (AU)

• FAPESP's process: 15/06663-7 - Characterization and Comparative Study of Onshore Hydrocarbon Seepages and Petroleum Plays in Central Iran and Southeastern Brazil using Spectral Remote Sensing
• Grantee: Saeid Asadzadeh
• Support type: Scholarships in Brazil - Doctorate
• FAPESP's process: 17/25002-7 - Evaluation of Longwave Infrared (LWIR) and surface temperature data for terrestrial hydrocarbon microseepage characterization
• Grantee: Saeid Asadzadeh
• Support type: Scholarships in Brazil - Post-Doctorate