AAPG Geoscience Technology Workshop

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Tectonic Structure of the Karish Gas Field, Offshore Israel

Abstract

The Karish Gas Field was discovered offshore Israel in 2013 through the exploration well Karish-1. The well encountered the prolific Tamar sands finding hydrocarbons in the A, B, C and D sandstone intervals. The Karish structure was interpreted on good quality 3D seismic data. The observed structural complexity of the field dictated the necessity to investigate in depth the structural evolution of the former, in order to understand: (1) the deformation of each Karish reservoir interval, and (2) fault evolution since faults may affect lateral and vertical fluid migration. Since both processes may have considerable influence on the final volumes of accumulated gas, comprehending the magnitude of their influence may unlock further upside potential in areas adjacent to the main Karish Field. The seismic interpretation shows that the top 5 km of sediments are of Cenozoic age overlying the Mesozoic rift and syn-rift sediments. The investigation focuses on the time interval between Late Oligocene to Miocene. The examined interval is subdivided into three units: (1) a lower unit of Late Oligocene to Early Miocene age, (2) an intermediate unit of Middle-Late Miocene age, and (3) an upper unit of Latest Miocene age. The Karish Gas Field is a three way closure bounded to the north by the Karish Shear Zone (KSZ), an ENE-WSW zone of deformation exceeding 16 km length that extends to depths of more than 8 km into the Mesozoic sediments of the Levant Basin. The tectonic structure of the Karish Field is characterized by: (1) an NE-SW arcuate fold that progressively parallels the KSZ, (2) a series of NW-SE conjugate normal faults that propagate north and south of the KSZ, and (3) closely spaced ESE-WNW and ENE-WSW en echelon faults located along the KSZ. The Late Vourdigalian-Serravallian strata show thickness variations and onlapping seismic reflections indicating deposition-related deformation, while the post-Serravallian strata are nearly horizontal with almost constant bed thicknesses. The NW-SE normal faults represent different generations of conjugate faults with horizontal fault intersections. The older normal faults are observed on both sides of the KSZ often truncating the Middle Miocene unit and are onlapped by undeformed Late Miocene strata, while the younger normal faults are restricted north of the KSZ cutting the base of the Messinian salt. Along the KSZ, an anastomosing network of ESE-WNW en echelon faults cuts also the base of the Messinian salt. Our structural analysis implies that the tectonic structures observed within the Karish Field exhibit several changes in their development that are gradual but can be tentatively grouped into two periods of tectonic deformation: (1) an initial period of convergent deformation during Early Miocene, and (2) a second period of strike-slip faulting during Middle-Late Miocene. We propose that the ESE-WNW and ENE-WSW en echelon faults represent R- and P- shears respectively linking the NW conjugate normal faults with the KSZ. From the spatial distribution and the geometry of the faults, we suggest that the KSZ is a dextral transtensional zone. The complex structural evolution calls for the necessity for quantifying the observed deformation using innovative structural modeling methods (e.g., sequential kinematic modeling and restoration). Forward and backward modeling of critical geological events and the determination of deformation rates will allow for extrapolating interpretations at areas of poor seismic quality (near the KSZ shadow zone) adding confidence to the reservoir volumes, and eventually enhance optimisation of gas production.