Sztano O.,Eötvös Loránd University |
Sztano O.,TXM Oil and Gas Exploration Ltd. |
Szafian P.,TXM Oil and Gas Exploration Ltd. |
Magyar I.,MOL Hungarian Oil and Gas Plc. |
And 7 more authors.
Global and Planetary Change | Year: 2013
In the Late Miocene-Early Pliocene Lake Pannon, regression went on for about 6 Ma. Sediments arriving from the Alpine-Carpathian source area were partly accumulated on the flat-lying morphological shelf of the lake, whereas other portions of the sediment were passing through to the slope and deposited on the deep basin floor. The height of the slope exceeded 400-500mbased on correlatedwell and seismic data. An extended 3D seismic volume covering the Makó Trough, one of the largest and deepest depressionswithin the Pannonian Basin, provided an opportunity to study sequences and shelf-margin trajectories generated as a result of continuous slope advancement. The lithology of these shelf, slope and basin centre deposits was inferred fromseven well logs and 220 mcorematerial. In the Makó Trough the southeastward migrating shelf-margin was formed by alternating aggradational and progradational clinothems. Aggradational clinothems, i.e. aggradation accompanied by subordinate progradation, are characterised by rising shelf-margin trajectories. The shelf built up from inner-shelf to shelf-edge deltaic lobes which compose a few dozen metre thick coarsening-up units. The majority of the sand, however, was transported by effective turbidity currents through leveed channels into the basin, and deposited as thick, extended slopedetached turbidite lobes up to a distance of 30 km from the shelf edge. In aggradational clinothems both the shelf and the basin floor accreted vertically. Development of progradational clinothems resulted in horizontal (flat) shelf-margin trajectories. Corresponding reflections toplap at the shelf edge and downlap within a distance of few kilometres from the toe of the slope. The shelf was bypassed, sediments accumulated on the slope and directly at the slope-toe region as small simple lobes. Short-distance transportwas the result of clay-poor, non-effective turbidity currents. Consequently, the thickness of coeval basin-centre sediments remained negligible in progradational clinothems. Alternations of rising and horizontal shelf margin trajectories indicate that the climate- and subsidence-controlled lacustrine base-level rose continuously, though at varying rates. Descending trajectories were not observed. It means that base-level drops larger in amplitude than the seismic resolution (20-30 m), did not occur during the studied time interval, i.e. at 7-5 Ma ago, approximately corresponding to the Messinian age. As a result, major forced-regressive wedges or lowstand fans did not develop. This is in contrast with former stratigraphic models claiming that several 3rd-order sequences, including the intra-Messinian unconformity supposedly induced by hundred metres large lake-level drop, developed in Lake Pannon with significant volume of lowstand deposits as turbidites. Instead, aggradational and progradational clinothems are interpreted as fourth-order transgressive, early and late highstand systems tracts. These incomplete sequences represent less than 100 kyr time intervals. Due to climate control both on high rate of sediment supply and thewater budget of Lake Pannon, conditions were more favourable for deposition of large volumes of well-developed turbidite systems during base-level rise than during stagnation or minor base-level fall. Therefore, sand delivery to the basin centre was at maximum during the early highstand aggradational stage and atminimum during the late highstand progradational stage. The timing and position of sand accumulation in the Makó Trough of Lake Pannon is different from those predicted by "traditional" sequence stratigraphic considerations. © 2012 Elsevier B.V.
Drijkoningen G.,Technical University of Delft |
El Allouche N.,Technical University of Delft |
Thorbecke J.,Technical University of Delft |
Bada G.,TXM Oil and Gas Exploration Ltd.
Geophysics | Year: 2012
Under certain circumstances, marine streamer data contain nongeometrical shear body wave arrivals that can be used for imaging. These shear waves are generated via an evanescent compressional wave in the water and convert to propagating shear waves at the water bottom. They are called "nongeometrical" because the evanescent part in the water does not satisfy Snell's law for real angles, but only for complex angles. The propagating shear waves then undergo reflection and refraction in the subsurface, and arrive at the receivers via an evanescent compressional wave. The required circumstances are that sources and receivers are near the water bottom, irrespective of the total water depth, and that the shear-wave velocity of the water bottom is smaller than the P-wave velocity in the water, most often the normal situation. This claim has been tested during a seismic experiment in the river Danube, south of Budapest, Hungary. To show that the shear-related arrivals are body rather than surface waves, a borehole was drilled and used for multicomponent recordings. The streamer data indeed show evidence of shear waves propagating as body waves, and the borehole data confirm that these arrivals are refracted shear waves. To illustrate the effect, finite-difference modeling has been performed and it confirmed the presence of such shear waves. The streamer data were subsequently processed to obtain a shear-wave refraction section; this was obtained by removing the Scholte wave arrival, separating the wavefield into different refracted arrivals, stacking and depth-converting each refracted arrival before adding the different depth sections together. The obtained section can be compared directly with the standard P-wave reflection section. The comparison shows that this approach can deliver refracted-shear-wave sections from streamer data in an efficient manner, because neither the source nor receivers need to be situated on the water bottom. © 2012 Society of Exploration Geophysicists.