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Varying free gas zone and bottom-simulating reflection (BSR) response across a deep graben off Trujillo, central Peru

Gery Herbozo and Juan Diaz-Naveas
Varying free gas zone and bottom-simulating reflection (BSR) response across a deep graben off Trujillo, central Peru
Petroleum Geoscience (August 2024) Pre-Issue Publication

Abstract

The complexity of marine gas hydrate systems at the Peruvian convergent margin has been linked to the post-Miocene history of vertical tectonics and subduction erosion that affected the forearc. Here, multichannel seismic data and published findings reveal that such a complexity has been further extended by the occurrence of the pre-Miocene deep Morsa Norte Graben (MNG) off Trujillo (8-9 degrees S) in the Central Peru margin. At 650-750 m water depth in the upper slope, directly above the MNG depocenter (which has a 4-6 km overburden thickness), continuous bottom-simulating reflections (BSRs) and concentrated sub-BSR high-amplitude reflections are confined beneath a layered basin with low-to-moderate near-seafloor heat flow (7-33 mW m (super -2) ). A deeper BSR modeled with a thermogenic gas composition is associated with the enhanced reflections. At 900 m water depth, sub-BSR reflections become less frequent in an area with a layered sediment cover defined by a moderate near-seafloor heat flow (15-33 mW m (super -2) ). At 1200 m water depth, where the MNG is relatively thick (3-4 km overburden thickness), faults connect patchy BSRs with a moderate-to-high near-seafloor heat flow (52-110 mW m (super -2) ). There, sub-BSR enhanced reflections are scarce. Immediately above the top of the gas hydrate stability zone (GHSZ), near-seafloor heat flow reaches 81 mW m (super -2) . Modeling suggests a water depth dependent transport of heat toward the seafloor with respect to the top of the GHSZ, implying that the closer the seafloor to the top of the GHSZ, the lower the advection of heat and vice versa. Recent, seafloor-related depositional and structural features amplify such relations in agreement with near-seafloor heat flow variability. On the other hand, toward the area outside the extent of the MNG (<3 km overburden thickness), continuous BSRs are not linked either to a deeper BSR or sub-BSR enhanced reflections. The continuity of one of these BSRs is deflected upwards beneath a slump, suggesting an incomplete thermal re-equilibration of the GHSZ. Therefore, we conclude that the BSR responds to (1) the confinement and thickening of the free gas zone (FGZ) above the MNG depocenter due to the sealing effect of recent sedimentation close to the top of the GHSZ, (2) the seepage of gas-rich fluids from a thinned FGZ above the relatively thick MNG due to the allowing effect of faults cutting the GHSZ far from the top of the GHSZ, (3) the undisturbed state of the FGZ outside the extent of the MNG, and (4) the disequilibrium state of the base of the GHSZ due to the unloading of sediments in an unstable slope prone to failure.


ISSN: 1354-0793
EISSN: 2041-496X
Serial Title: Petroleum Geoscience
Serial Volume: Pre-Issue Publication
Title: Varying free gas zone and bottom-simulating reflection (BSR) response across a deep graben off Trujillo, central Peru
Affiliation: Pontifica Universidad Catolica de Valparaiso, Escula de Ciencias del Mar, VAlparaiso, Chile
Pages: article petgeo2023-132
Published: 20240819
Text Language: English
Publisher: Geological Society Publishing House for EAGE (European Association of Geoscientists & Engineers), London, United Kingdom
References: 83
Accession Number: 2024-061615
Categories: OceanographyApplied geophysics
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. sects., geol. sketch maps
S08°55'00" - S07°55'00", W80°45'00" - W80°00'00"
Country of Publication: United Kingdom
Secondary Affiliation: GeoRef, Copyright 2024, American Geosciences Institute. Reference includes data from GeoScienceWorld, Alexandria, VA, United States. Reference includes data from The Geological Society, London, London, United Kingdom
Update Code: 2024
Program Name: ODPOcean Drilling Program
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