Megaseismic section across the northeastern slope of the Arabian Plate, Iraq

Abstract

ABSTRACTA composite seismic image (Megaseismic Line 7) was constructed across Iraq using 16 pre-existing seismic lines that were recorded between 1975 and 1983. The image is the first of 15 megaseismic lines that will eventually form a rectilinear grid that covers Iraq. It is oriented in a SW-NE direction, and extends approximately 500 kilometers from the Iraq-Saudi Arabia border to the Iraq-Iran border. The seismic lines were recorded using a 48-channel system with either a vibroseis or dynamite source. The maximum offset varies from 2,400 to 5,000 meters. The seismic data was reprocessed using a common datum of 300 meters above sea level. Data quality is good where the source was dynamite and the terrain consists of gravel and sand surfaces; it is poor where vibroseis was used or/and the outcrops were carbonates. The final stacked section and Hilbert attributes (reflection amplitude and instantaneous phase) were displayed at different scales to determine the best perspective for interpretation. A total of 14 reflections, corresponding to Miocene to Permian horizons, were identified using synthetic seismograms from five wells. The horizons generally dip towards the northeast, except at the location of the Tel Ghazal oil field where syndepositional growth is inferred. Various seismic stratigraphic geometries, such as sigmoidal features, onlap, toplap and downlap, were identified and used to define disconformities and angular unconformities.The oldest two horizons that could be picked are from the tops of the Triassic Kurra Chine and Permian Chia Zairi formations. Below the oldest Permian reflection, the middle Carboniferous “Hercynian unconformity” was tentatively picked. The Paleozoic pre-Permian succession is not adequately imaged in the seismic data, nor is the crystalline basement seen. The seismic interpretation was compared to the profiles of the Bouguer gravity anomaly and the total magnetic field, and good correlations were established. The regional line helped identify several previously unknown structural features including the Ma’aniya Depression in the Western Desert, and two anticlinal structures: the first being within that depression and the second directly to the southwest of Tel Ghazal oil field.