Remote sensing based approach for mapping of CO₂ sequestered regions in Samail ophiolite massifs of the Sultanate of Oman

Abstract

Documentation of chemical weathering and CO₂ sequestration in the Samail ophiolite massifs of the Sultanate of Oman represents an important case study for Geological Carbon Capture and Storage System (GCCSS). The present research study demonstrates the capability of remote sensing technique for mapping of weathered zones and potential CO₂ sequestration area abundances at different scales within peridotites in the northern mountain region of the Samail ophiolite massifs. The carbonate mineral index (CI) applied with other mineral indices to the TIR wavelength region of Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) TIR spectral bands 13 and 14 mapped CO₂ sequestered minerals along the structural- and wadi-controlled CO₂ flowing regions. Peridotites, the source rocks of CO₂ sequestration in the study area, were mapped using an ASTER 8, 3 and 1 band combinations. The decorrelated Landsat TM image discriminated the rock types associated with peridotites of ophiolite sequences and delineated the region of weathered and altered serpentinized peridotites in the zone of CO₂ sequestration. CO₂ sequestration mapping was carried out using Landsat TM satellite data that span 20 years (1986, 1998, 2000, 2003 and 2006) to assess the present status of CO₂ sequestration in this region. The image interpretations are verified with existing geological maps and through field and laboratory studies.

The spectral measurements of carbonate minerals at 1300 to 2500 nm with the spectral resolution of ~ 7 nm using a PIMA SP infrared spectrometer in the field and laboratory show the presence of hydroxyl-bearing minerals and carbonates that have spectral absorption features around 1.4 μm, 1.9 μm and 2.35 μm. The strong absorptions around 2.35 μm are mainly due to CO bonds in carbonate minerals such as calcite (CaCO₃), dolomite (CaMg(CO₃)₂), magnesite (MgCO₃), aragonite (CaCO₃) and siderite (FeCO₃), which form 15 to 57%, 12 to 53%, 9 to 38%, 11 to 21% and 3 to 8% respectively in the samples. The absorptions around 1.4 μm and 1.9 μm are caused by hydration effects of hydroxyl minerals including antigorite and montmorillonite present at 10 to 21% and 37 to 81% respectively in the samples. The alterations of serpentinite are evidenced by the presence of antigorite and lizardite minerals. X-ray powder diffraction analyses further confirms the occurrence of CO₂ sequestered major carbonate minerals such as aragonite, calcite and dolomite in the samples. The study demonstrates that the ASTER and Landsat TM satellite multispectral sensors are useful to detect the carbonate minerals, to delineate the peridotites and to discriminate the areal abundance of potential CO₂ sequestration. This technique is a useful tool to map and monitor the region of CO₂ sequestration in well exposed arid and semi-arid regions and to analyze and understand this aspect of the world geological carbon capture and storage system.

Introduction

The continued increase in CO₂ emissions and the related changes in the concentration of the greenhouse gas in the atmosphere and ocean is a global issue, and much consideration is being given to how CO₂ can be removed from the atmosphere. Rutt Bridges (2011) stated that the Carbon Capture and Storage (CCS) system faces storage challenges in site selection, CO₂ transportation costs, pipeline and site permits, and uncertainties in monitoring CO₂ sequestration. But, in Earth history, nature is performing silicate weathering at the earth's surface which is estimated to remove CO₂ from the atmosphere equivalent to approximately 150 Mt C per year (Gaillardet et al., 1999), over an order of magnitude smaller than the 5.5 Gt C emitted by fossil fuel combustion (IPCC, 2007). If weathering rates can be increased over large areas even within the range observed today, the acceleration of weathering processes could play a meaningful role in an integrated strategy for atmospheric removal of CO₂.

CO₂ is a major weathering agent in silicate rocks such as peridotites by the conversion of nearly all of the CaO and part of the MgO to carbonate minerals. Over the past several years many papers have been published that discuss the advantages of CO₂ injection into mafic and ultramafic rock formations, including deep-sea basalts (Goldberg et al., 2008, Oelkers et al., 2008) and peridotites (Kelemen and Matter, 2008, Andreani et al., 2009, Matter and Kelemen, 2009, Kelemen et al., 2011). The two recent workshops report on geological carbon capture and storage in mafic and ultramafic rocks (Godard et al., 2011), and the scientific drilling in the Samail ophiolite of Oman (Kelemen et al., 2013) emphasizes the importance of research studies on the geological carbon capture and storage system and the need of scientific drilling in the Samail ophiolite of Oman. Thus, the study of CO₂ sequestration in ultramafic peridotites of ophiolite regions is of great worldwide importance. CO₂ sequestration due to weathering in peridotites in ophiolite sequences of the Samail massifs of the Sultanate of Oman is a major and typical example and needs to be studied in more detail. Due to the 1169 km² extent of the Samail ophiolite massifs and extremely rugged topography with elevations between 0 and 2500 m, exhaustive sampling and detailed mapping are impossible to assess the potential of CO₂ sequestration in this region. Remote sensing is able to provide more continuous mineralogical and lithological mapping in the almost continuous barren rock exposures in an arid climate like the region of the Samail massifs. The Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) and Landsat sensors cover the wavelengths of absorption regions of peridotites and CO₂ bearing minerals. Several works on mapping of ophiolite sequences using remote sensing have been carried out (Abrams et al., 1983, Abrams and Hook, 1995, Sabins, 1997, Abdeen et al., 2001, Kusky and Ramadan, 2002, Rowan and Mars, 2003, Rowan et al., 2003, Combe et al., 2006, Mars and Rowan, 2006, Gad and Kusky, 2007, Rajendran et al., 2012, Rajendran et al., 2013, Rajendran and Nasir, 2013, Rajendran and Nasir, 2014), but few studies have focused on the application of remote sensing to mapping of CO₂ sequestration worldwide. This study suggests that the detection of CO₂ sequestered carbonate minerals and mapping of peridotite (the source rock) occurrences are of great importance in the GCCSS and global monitoring of CO₂ which will increase the knowledge among scientists to improve and create a sustainable GCCSS in the future. Thus, the present study is undertaken to demonstrate the capability of remote sensing techniques to detect the occurrences of CO₂ sequestered carbonate minerals, to discriminate the peridotites (the source rock of CO₂ sequestration) and to map the region of CO₂ sequestration ongoing in the ophiolite sequences of northern Samail massifs of the Sultanate of Oman (Fig. 1) as a part of research work on the GCCSS.