Gas Hydrate Mapping using 3D CSEM

Abstract

Gas hydrates have long been considered a viable energy source and the mapping of hydrates and associated free gas, using seismic properties such as impedance and velocity has improved considerably in the recent years. The “ice” like behavior of hydrates generate a seismic response which can be further modelled and imaged using advanced tools such as full waveform inversion (FWI) to get an accurate assessment of their gross rock volume. However, determining saturation of hydrates is still less understood since acoustic impedance responds to a wide range of saturations, which makes it challenging to predict the true volumes of free gas and/or gas hydrates using seismic data alone. Electrical resistivity has a better sensitivity to fluid variations within the pore space as described by Archie’s law. By combining acoustic and resistivity properties, one can discern low saturated hydrates from the saturated ones and make a more accurate estimation of true hydrate volumes. Controlled Source Electromagnetic (CSEM) geophysical tool measures electrical resistivity of the subsurface and has been proven to be very effective tool in mapping and quantifying both deep and shallow (400 m BML) conventional hydrocarbon accumulations (Morten et al, 2017). Hydrates being shallow, are well suited for the CSEM method. The lower operational frequency range (0.05 to 50 Hz), limits the vertical resolution of CSEM and its ability to differentiate the resistive response of a saturated gas hydrate from underlying saturated 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 data has been acquired and inverted in various gas hydrate provinces around the world. The results show a clear correlation between resistive anomalies seen in CSEM resistivity volume and strong reflective events identified on seismic. The results also show that these resistive anomalies do not always follow the seismic bottom simulating reflector (BSR), indicating that the BSR is not an accurate indicator of presence of gas hydrates. Average resistivity maps produced from CSEM resistivity volumes provide an overview of resistivity variation (and hence saturation variation) within the hydrates. The areal extent of resistivity anomalies derived from these maps, combined with the thickness information derived from seismic can give a more accurate estimation of saturated hydrate in-place volumes. In this paper we would like to share our experiences of mapping hydrates with case examples from around the world where CSEM data has been acquired. A 3D CSEM dataset from one such case example (from offshore Indonesia) is integrated and interpreted with 3D seismic and a qualitative and quantitative assessment of hydrates has been presented.

AAPG Datapages/Search and Discovery Article #90348 © 2019 AAPG Asia Pacific Region Geosciences Technology Workshop, Gas Hydrates – From Potential Geohazard to Carbon-Efficient Fuel?, Auckland, New Zealand, April 15-17, 2019