GNGTS 2019 - Atti del 38° Convegno Nazionale

540 GNGTS 2019 S essione 3.1 at the end of the Miocene, a thick evaporitic layer deposited. A scientific drilling proposal within the International Ocean Discovery Program (IODP) intends to drill two new boreholes in the Ionian basin (IODP-857C) through relatively undeformed remnants of the sedimentary succession, in the Ionian Abyssal Plain (IAP), and at the foot of the Malta Escarpment. The aim is to sample the formations of the upper evaporitic sequence and overlying lower Pliocene hemipelagic drape to understand the processes that led to the demise of the MSC. Drilling aims at recovery of the inferred Zanclean megaflood deposit (CU, Complex Unit) the Upper Unit (UU, gypsum, anhydrite, marls) and the uppermost part (approximately 150 m) of the so-called Mobile Unit (MU, halite). The stratigraphic nomenclature follows Lofi et al. (2011; 2018). The thickness of the Messinian evaporates, exceeding one kilometer, deposited, geologically- speaking, very rapidly implies some drilling risks as: - Over-pressure below salt, - Seepages through evaporates, - Hydrocarbons spills. This study intends to model and simulate fluid generation and migration paths beneath the Messinian salt deposits at the Malta Escarpment foot and in the center of the IAP in order to allow mitigation measures and safe drilling operations. Methods. The main problem of this modeling study is represented by the scarcity of lithological and petrophysical information below the Messinian evaporites. Many hypotheses and ambiguities are present in the model set-up. Different scenarios have been hypothesized and different modeling approach have been used, with the aim to consider the large range of different possible situations. Seismic interpretation. We used a dataset composed by several multichannel seismic reflection lines and the borehole DSDP Site 374 (Fig.1). The seismic lines were acquired by different projects. Therefore, they have different depths of investigation (DOI), resolution and z-axis domains: - Meteor seismic lines – M144 survey: 10 modern high-resolution post-stack time migrated (PoSTM) seismic lines (tot record: 6400 ms TWT), centered on DSDP Site 374. - CROP M2B & M3 seismic lines: crustal regional lines (high DOI but low resolution) partially re-processed through the pre-stack time migration (tot record: 9400 ms TWT). - MS27 & MS112 seismic lines: vintage lines partially re-processed and pre-stack depth migrated (PSDM), reaching a maximum depth of 19 km. - CUMECS seismic lines: 4 modern high-resolution multi-channel PoSTM seismic lines (tot record: 6000 ms TWT) along the Malta Escarpment. Information from the borehole DSDP Site 374 allowed calibrating the geological target despite a sparse sampling in the well column. During drilling operations some logs were acquired covering just some cores, providing Vp, salinity, chlorinity, pH, alkalinity, Ca++ vs Mg++, wet bulk density, thermal conductivity parameters. We interpreted a few horizons in the Plio-Quaternary hemipelagic succession (PQ), the Upper Unit top, the Mobile Unit top, the base of the Mobile Unit and a deeper reflector inner the pre-MSC succession. The two selected study areas have similar stratigraphic features, except for a chaotic body at the base of the Malta Escarpment. The Complex Unit (CU) has been ascribed by Micallef et al. (2018) as a mass transport deposit originated from the Atlantic water masses reentering the Mediterranean Sea after the MSC. PQ is characterized by high frequencies and continuous strata that cover the Messinian units on-lapping and pinching-out a positive structure of the Medina Mounts. In the Malta Escarpment area, there are evidences of some turbidites in the Plio-Quaternary succession. CU is a chaotic and transparent body at the base of the Malta Escarpment. The top of the UU is characterized by high amplitude reflection and an irregular corrugated shape. Tay et al. (2002) hypothesized that paleo-valleys filled by onlapping strata could be related to gypsum erosion and karstification.