--> Gas Hydrate Mapping using 3D CSEM

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Gas Hydrate Mapping using 3D CSEM

Abstract

The presence of gas hydrates in deep water Rakhine Basin, offshore Myanmar is widely acknowledged. Their identification is based on seismic attributes and relevant gas hydrate indicators. Saturation within the gas hydrates and the underlying associated free gas at a regional scale is however still poorly understood. In this paper, we would like to show how Controlled Source Electro-Magnetic (CSEM) derived resistivity can provide a better guidance to mapping saturated hydrates and free gas at a regional scale. Natural gas hydrates form in deep waters (> 500 m) under certain pressure-temperature conditions where gas molecules (usually methane) are trapped within the crystal structure of water to form a solid, crystalline compound. The zone that provides favorable conditions for the hydrates to form is termed as the gas hydrate stability zone (GHSZ). Solid hydrates are considered as a potential natural resource as well as a drilling hazard. In both cases, it is crucial for oil companies to accurately locate and map these shallow accumulations, either to estimate their volumes or to avoid them while drilling. Hydrate mapping usually relies on the seismic amplitude variations that occur within the GHSZ and the free gas beneath. The base of GHSZ is termed Bottom Simulating Reflector or “BSR”, a seismic event that occurs where low-velocity gas underlies higher-velocity hydrate-bearing strata, giving it a characteristic soft kick. Present dogma naturally equates a pronounced BSR with the presence of free gas with an overlying gas hydrate. However, given that acoustic impedance responds to a wide range of saturation, it is not always possible to detect variation in saturation of free gas and/or gas hydrates using seismic data alone. Electrical resistivity is more dependent on gas saturation as described by Archie’s law. Significant changes in resistivity are not achieved until majority of the conductive fluid (i.e. brine) is replaced by non-conductive fluids such as gas (Constable, 2010). CSEM measures electrical resistivity at a regional scale. CSEM has been proven to be very effective tool in mapping and quantifying shallow (400 m BML) conventional hydrocarbon accumulations (Morten et al, 2017) and hydrates being shallow, are ideal targets for CSEM method. The lower operational frequency range (0.05 to 50 Hz), limits the vertical resolution of CSEM and its ability to differentiate resistive response of a saturated gas hydrate from underlying saturated free gas. Hence one might need to rely on BSR from a seismic and CSEM co-rendered image in order to differentiate the hydrate from underlying free gas. The lateral resolution of the resistive geobody however, is well constrained due to 3D receiver grid coverage and the available azimuthal information. This makes it possible to map the areal extent of saturated hydrate/free gas accumulations more accurately. 3D CSEM has been acquired and inverted in various gas hydrate provinces around the world. The results show a clear correlation between resistive anomalies seen on CSEM resistivity volume and strong reflective events identified on seismic. The results also show that these resistive anomalies do not always follow the seismic BSR. Average resistivity map produced from CSEM resistivity volume provides an overview of resistivity variation (and hence the saturation variation) within the hydrates. Information derived from these maps can be used to identify locations that could be a potential drilling hazard. The areal extent of resistivity anomaly derived from these maps, combined with the thickness information derived from seismic can give a more accurate estimation of saturated hydrate volume in place. In this paper, we would like to share our experience of mapping hydrates with case examples from around the world where 3D CSEM data has been acquired. Through CSEM synthetic modeling and inversion studies we try to demonstrate how CSEM signal would image some of the more common gas hydrate-free gas scenarios.