Third EAGE Conference on Offshore Exploration and Development in Mexico

Summary is not available

The eastern edge of Campeche salt basin (aka Sureste basin in the shallow water) consists of a NW-SE trending Reforma-Akal fold and thrust belt along the flank of the Yucatan platform. This fold belt represents the eastern limit of the Chiapaneca orogenic belt. An area of the fold belt was studied which experienced three tectonic events: (1) NW-directed extension during the Middle-Late Mesozoic associated with rifting and later gravity tectonics during drifting of the Gulf of Mexico, (2) Tertiary NE-directed contraction which led to the development of a series of NW-SE trending folds, and (3) Middle Miocene to present NW-directed extensional and transtension associated with gravity sliding.

Interpretation of sub-regional seismic data in the eastern Sureste basin along with well data suggest that the Late Jurassic – Middle Cretaceous extension event happened in two phases. Two sets of extensional faults with different initiation ages both trend northwest-southeast with predominately opposite dip. The faults sole into the autochthonous salt layer or its equivalent weld and their footwalls typically have triangular shape salt roller that merge with the mother salt detachment layer. The first phase of faulting is Late Jurassic and is characterized by faults which principally dip basinward and have slightly curved fault surfaces. Kimmerdgian and Tithonian sections thickness growth are observed on the hanging walls of these faults. The second phase of faulting is Middle-Late Cretaceous. Phase two faults characteristically have landward dips, pronounced listric shapes, and offset the older phase of extensional faults. The second phase of faulting shows growth in the Middle-Upper Cretaceous sedimentary section on their hanging walls. The extreme extension of the second phase of faulting in the northern part of the eastern Sureste basin led to the development of Mesozoic rafts, where the adjacent footwall and hanging wall blocks are separated and are no longer in contact.

The first phase of extension is interpreted to be associated extensional faulting associated with rifting, but the second phase is thought to be have been occurred during and after the rift to drift transition of the Gulf of Mexico opening. Thermal subsidence related to the formation of oceanic crust to the west is considered the principal drivers of the Middle-Late Mesozoic extension.

A westerly dipping base of salt developed during the Mesozoic which triggered gravity sliding causing supra-salt strata to slide basin-ward on autochthonous salt. The rugosity along base salt probably increased friction along the salt detachment hindering movement along the basin-ward faults which caused landward dipping faults to form.

Raft development associated with the landward-dipping faults is mainly observed in the northern part of the study area. The absence of raft tectonics, and characteristic overlap of hanging wall cut-offs against their footwalls is typical in the south of the study area. This relationship might have been caused by variable original salt thicknesses. Thick salt in the north probably facilitated more extreme extension.

The observed structural relationships provide additional insights in understanding the geometry and development of the extensional system and helps in describing the risk associated with hydrocarbon exploration in this part of the basin.

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Summary not available.

Summary is not available.

It is well known that naturally fractured carbonates can make important reservoirs and conduits for hydrocarbons in many basins worldwide. The brittleness of low porosity carbonates when subjected to high stress causes the rock to fail and form associated microfractures close to the failure event, thus, enhancing its porosity and permeability near and along faults and sharp flexures. Although development of these reservoirs can be complex due to the heterogeneous mobility pattern, steep production decline, potentially premature water breakthrough, among other issues, still vast reserves continue to be drained from them in many fields around the world.

The deep water Salina Basin presents an excellent opportunity to explore for naturally fractured carbonate reservoirs in the Mesozoic. Regional paleogeographic studies supported by wells evidence the presence of shallow Upper Jurassic limestones and dolomites and basinal Cretaceous limestones. The dominant lithofacies is mudstone and wackestone followed by packstone and the porosity is low, concentrated in microfractures. Most wells targeting the Mesozoic were drilled at the top of structural and although the wells avoided penetrating faults, many of them exhibit microfractures but in general formation tests yielded low hydrocarbon production. Past and current experience exploring and developing naturally fractured carbonates dictate the need to drill wells, perpendicular to the fractures orientation and as close as possible to faults. The fracture orientation can be obtained from current regional stress state, fracture data from neighbor wells and anisotropic studies from 3C seismic if available.

The low production of Mesozoic targeted wells in the deep water Salina Basin can be explained by the fact that those wells were drilled away from faults, which are the structural events that concentrate the highest fracture density, thus the best reservoir. Barring the obvious differences, a raw analogy that help understand the concept is that while channels are the reservoir fairways in siliciclastics, faults are the reservoir fairways in naturally fractured carbonates.

Seismic data provides spatial information that supports a variety of decisions made in hydrocarbon exploration and production. Recent advances in inversion-based velocity model building and imaging algorithms improve the reliability of this data from velocity models to migrated gathers.

From the model-building side, FWI is routinely employed to improve the accuracy and resolution of velocity models in an automated fashion. While the maximum offset in the data limits the use of transmitted energy, reflection events can overcome this limitation to produce deep updates in complex areas.

We use an FWI velocity gradient that enables addressing the model-building challenge in a variety of complex salt regimes. To compensate for illumination, Least-Squares Migration (LSM) enhances the high-wavenumber content of the seismic images hampered by the complex overburden and imperfect acquisition geometries. Here, we apply a workflow combining FWI and LSM to solve the challenging pre-salt problems in offshore Brazil. Results from FWI show that it is possible to capture the variability in velocity in carbonate layers, define the clastic sedimentary mini basins and the heterogeneity of the salt. The FWI velocity model was then used for the application of high-end LSM imaging. The resulting images show a significant improvement in the definition of fault patterns and stratal geometries, particularly in the pre-salt sequences. Likewise, LSM corrects the uneven illumination of the pre-salt angle gathers.