The hydrocarbon potential of the offshore Talara Basin, Peru

The offshore Talara Basin is the western extension of the hydrocarbon producing onshore fields since the mid-1800s area of Peru and is also located above the subduction zone of the active continental margin of South America. The offshore portion was evaluated using high quality 3D seismic where mapping horizons are all unconformities within the Eocene as well as the unconformities at the top Paleocene and top Cretaceous. Possible source rocks are the Cretaceous black marine shales of the Campanian Redondo Formation, the limestones of the Albian Muerto Formation, and the marine shales of the Paleogene. The primary target offshore is expected to be deep-water turbidites of Paleocene/Eocene age with a depositional source from the northeast from highlands created by the compressional uplift of the Andes. The main seals offshore are expected to be shales of the upper Eocene Lagunitos Formation and shales in the Chacra Formation, which are also seals in the onshore Litoral field. Thermal maturation modeling shows that two hydrocarbon kitchens exist in the offshore portion of the Talara basin, one in the north and one in the south. The probable Cretaceous source rocks reached the onset of maturity (VR = 0.63%) at a depth of 3,250 to 3,285 m (10,663 – 10,778 ft) between 30 and 39 Ma (Late Eocene to Oligocene). Importantly, the Cretaceous source rocks stay within the oil window once they enter it in the late Eocene. Satellite studies show a large offshore present-day oil seep in the southern part of the basin and 3D seismic shows direct hydrocarbon indicators (DHIs) imaged as flat spots and bottom simulating reflectors (BSR). Basin modeling suggests hydrocarbon migration pathways would have been updip (to the east) into the onshore traps and would therefore have first filled the offshore traps along the migration pathway. We conclude that the Talara Basin offshore offers excellent exploration opportunities in a proven productive area where multiple prospects have been mapped.

Distribution of Methane Plumes on Cascadia Margin and Implications for the Landward Limit of Methane Hydrate Stability

Nearly 3,500 methane bubble streams, clustered into more than 1,300 methane emission sites, have been identified along the US Cascadia margin, derived both from archived published data and 2011, 2016–2018 dedicated multibeam surveys using co-registered seafloor and water column data. In this study, new multibeam sonar surveys systematically mapped nearly 40% of the US Cascadia margin, extending from the Strait of Juan de Fuca in the north to the Mendocino fracture zone in the south, and bounded East–West by the coast and the base of the accretionary prism. The frequency-depth histogram of the bubble emission sites has a dominant peak at the 500 m isobar, which extends laterally along much of the Cascadia margin off Oregon and Washington. Comparisons with published seismic data on the distribution of bottom simulating reflectors (BSR), which is the acoustic impedance boundary between methane hydrate (solid phase) and free gas phase below, correlates the bottom simulating reflectors upward termination of the feather edge of methane hydrate stability (FEMHS) zone and the newly identified bubble emission sites off Oregon and Washington. The Cascadia margin off northern California, where the BSR ends seaward of the FEMHS, has fewer sites centered on the 500 m isobaths, although data are more limited there. We propose that the peak in bubble emission sites observed near the 500 m isobath results from migration of free gas from beneath the solid phase of the BSR upslope to the FEMHS termination zone, and suggest that this boundary will be useful to monitor for a change in methane release rate potentially related to a warming ocean.

On the lack of widespread bottom simulating reflectors in the Mediterranean Basin

<p>In the Mediterranean Basin, gas hydrate bottom simulating reflectors (BSR) are absent, with very few and spatially limited exceptions occurring in Eastern Mediterranean mud volcanoes and in the Nile deep sea fan. This is in spite of widespread occurrence of hydrocarbon gases in the subsurface, mainly biogenic methane, from a wide range of stratigraphic intervals.<br>In this study we model the methane hydrate stability field using all available information on DSDP and ODP boreholes in the Western Mediterranean and in the Levant Basin, including the downhole changes of pore water salinity. The models take into account the consequent pore water density changes and use known estimates of geothermal gradient. None of the drilled sites were located on seismic profiles in which a BSR is present.<br>The modelled base of the stability field of methane hydrates is located variably within, below, or even above the drilled sedimentary section (the latter case implies that it is located in the water column). We discuss the results in terms of geodynamic environments, areal distribution of Messinian evaporites, upward ion diffusion from Messinian evaporites, organic carbon content, and the peculiar thermal structure of the Mediterranean water column. <br>We conclude that the cumulative effects of geological and geochemical environments make the Mediterranean Basin a region that is unfavorable to the existence of BSRs in the seismic record, and most likely to the existence of natural gas hydrates below the seabed.<br><br></p>

IDENTIFICATION OF GAS HYDRATES AND BOTTOM SIMULATING REFLECTORS IN FAR OFFSET SEISMIC IMAGES

The presence of marine gas hydrates is routinely inferred based on the identification of bottom simulating reflectors (BSRs) in common depth-point (CDP) seismic images. Additional seismic studies such as amplitude variation with offset (AVO) analysis can be applied for corroboration. Though confirmation is needed by drilling and sampling, seismic analysis has proven to be a cost-effective approach to identify the presence of marine gas hydrates. Single channel far offset seismic images are investigated for what appears to be a more reliable and cost-effective indicator for the presence of bottom simulating reflectors than traditional CDP processing or AVO analysis. A non-traditional approach to processing seismic data is taken to be more relevant to imaging the gas/gas hydrate contact. Instead of applying the traditional CDP seismic processing workflows from the oil industry, we more carefully review the significant amount of information existing in the data to explore how the character of the data changes as offset angle increases. Three cases from different environments are selected for detailed analysis. These include 1) stratigraphy running parallel with the ocean bottom; 2) a potential bottom simulating reflector, running parallel to the ocean bottom, and cross-cutting dipping reflections, and 3) a suspected thermal intrusion without a recognizable bottom simulating reflector. This investigation considers recently collected multi-channel seismic data from the deep waters of the central Aleutian Basin beneath the Bering Sea, the pre-processing of the data sets, and the methodology for processing and display to generate single channel seismic images. Descriptions are provided for the single channel near and far offset seismic images for the example cases. Results indicate that BSRs related to marine gas hydrates, and originating due to the presence of free gas, are more easily and uniquely identifiable from single channel displays of far offset seismic images than from traditional CDP displays.

ANALYSIS OF SEISMIC ATTRIBUTES TO RECOGNIZE BOTTOM SIMULATING REFLECTORS IN THE FOZ DO AMAZONAS BASIN, NORTHERN BRAZIL

ABSTRACT. Foz do Amazonas basin is located at the northern portion of the Brazilian Equatorial Margin, along the coastal zone of Amapá and Pará states. This basin has been subjected to several studies, and the presence of gas hydrates has been demonstrated locally through sampling, and over broader areas using seismic reflection data. Seismic reflection is one method to identify the occurrence of gas hydrates, as they give rise to well-marked reflectors that simulate the seafloor, known as Bottom Simulating Reflectors (BSR). This study aims to investigate BSRs associated with the presence of methane hydrates in the Foz do Amazonas Basin through the application of seismic attributes. It was compared seismic amplitudes from the seafloor and the BSR to validate the inferred seismic feature. Then, Envelope and Second Derivative were chosen for highlighting the BSR in seismic section. The results showed an inversion of polarities in the signal between the seafloor (positive polarity) and the BSR (negative polarity). The integrated use of these approaches allowed validating the level of the BSR in line 0239-0035 and inferring the presence of gas hydrates, revealing to be a useful tool for interpreting the distribution of the gas hydrates in the Foz do Amazonas Basin.Keywords: Gas hydrates, envelope, second derivative of envelope, Brazilian Equatorial Margin.RESUMO. A Bacia da Foz do Amazonas é localizada na porção norte da Margem Equatorial Brasileira, ao longo da zona de costa dos estados do Amapá e do Pará. A presença de hidratos de gás é comprovada localmente através de amostragem, e em áreas mais distantes através de dados de sísmica de reflexão. A sísmica de reflexão é eficaz para identificar hidratos de gás, pois refletores que simulam o fundo do mar, Bottom Simulating Reflectors (BSR), são utilizados para inferir a presença dos hidratos de metano. Este estudo pretende identificar feições sísmicas associadas aos hidratos de metano na Bacia da Foz do Amazonas através da aplicação de atributos sísmicos. Foram comparadas as amplitudes sísmicas do fundo do mar e do BSR para validar a feição sísmica inferida. Então, os atributos Envelope e Segunda Derivada do Envelope foram escolhidos por destacarem o BSR. Os resultados mostraram uma inversão das polaridades no sinal entre o fundo do mar (positivo) e o BSR (negativo). O uso integrado dessas abordagens valida a localização do BSR na linha 0239-0035 e infere a ocorrência de hidratos de gás, revelando ser uma ferramenta útil para interpretação da distribuição de hidratos de gás na Bacia da Foz do Amazonas.Palavras-chave: Hidratos de metano, envelope, segunda derivada do envelope, Margem Equatorial Brasileira.