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Pre-salt in the Deepwater Gulf of Mexico: Evolution and Play Potential in the Syn-rift and Sag Fill in Offshore Mexico

Abstract

Pre-salt syn-rift and sag plays of the deepwater GoM are poorly understood and untested. Regional 2D seismic data reveals elements of the crustal structure in the basin and a complex syn-rift history along Mexico's Eastern and Yucatan margins. This pre-salt sequence in offshore Mexico reaches a thickness of up to 8km. The rift grabens accommodating the syn-rift fill are presumed to have stretched from the Georgia Embayment to the Tamaulipas Arch in this basin. Modern-day examples of graben systems illustrate the abundance of fluvio-lacustrine systems that can be envisioned in the paleo-reconstruction. As there is no apparent incision of the base-salt unconformity nor indication of interbedded sag and salt sequences, the limit of autochthonous salt can be used a proxy for mapping the extent of pre-salt units throughout the GoM. This observation is not consistent with the idea of catastrophic or gradual infilling of a subaerial depression (air-filled hole) well below sea level. Strata within the pre-salt package are parallel to sub-parallel seismic reflectors that progressively fill grabens and onlap the underlying rifted basement. The uppermost pre-salt sequences are topped by a well-defined, generally unfaulted, base-salt unconformable surface. Log and core data from available onshore wells (and offshore US GoM) show these strata as having terrestrial clastic, red-bed origin. Clusters of high amplitude sub-parallel reflectors allude to interbedded volcanics, but the Outer Marginal Trough (Pindell et al, 2014) itself is devoid of magmatic material. A comparison with the Tupi field in Santos Basin – one of the most successful examples of pre-salt plays – gives us important information in terms of a possible analog for source and trap characteristics. We observe several examples in the pre-salt Gulf of Mexico that suggest a similar type of depositional system. Enhanced geophysical techniques can be utilized to constrain the lithology in unexplored areas. AVO analysis and the overall Vp to Vs Ratio suggests a largely clastic lithology with minor amounts of interbedded volcanics in the pre-salt package. There is a possibility for preserved porosity in this clastic section, which increase reservoir potential.