Geology topics

Ana Garcia-Garcia

Publications and source records attributed to Ana Garcia-Garcia.

3 recordsLinked to original sources

USGS2007

Understanding shallow gas occurrences in the Gulf of Lions

New coring data have been acquired along the western Gulf of Lions showing anomalous concentrations of methane (up to 95,700 ppm) off the Rhône prodelta and the head of the southern canyons Lacaze-Duthiers and Cap de Creus. Sediment cores were acquired with box and kasten cores during 2004–2005 on several EuroSTRATAFORM cruises. Anomalous methane concentrations are discussed and integrated with organic carbon data. Sampled sites include locations where previous surveys identified acoustic anomalies in high-resolution seismic profiles, which may be related to the presence of gas. Interpretation of the collected data has enabled us to discuss the nature of shallow gas along the Gulf of Lions, and its association with recent sedimentary dynamics. The Rhône prodelta flood deposits deliver significant amounts of terrigenous organic matter that can be rapidly buried, effectively removing this organic matter from aerobic oxidation and biological uptake, and leading to the potential for methanogenesis with burial. Away from the flood-related sediments off the Rhône delta, the organic matter is being reworked and remineralized on its way along the western coast of the Gulf of Lions, with the result that the recent deposits in the canyon contain little reactive carbon. In the southernmost canyons, Lacaze-Duthiers and Cap de Creus, the gas analyses show relatively little shallow gas in the core samples. Samples with anomalous gas (up to 5,000 ppm methane) are limited to local areas where the samples also show higher amounts of organic matter. The anomalous samples at the head of the southern canyons may be related to methanogenesis of recent drape or of older sidewall canyon infills.

Gulf of Lions↗

USGS2006

Shallow gas off the Rhône prodelta, Gulf of Lions

Sediment cores acquired in 2004 off the Rhône prodelta show consistent anomalous methane concentrations of up to 87,440 ppm. Methane compositional and isotopic data support a biogenic origin, although there are a few sites that show strongly depleted δ 13 C values (− 53&permil; PDB) suggesting a mixed source for the gas (biogenic and thermogenic). Anomalous methane concentrations (samples with more than 90 ppm) are discussed and integrated with organic carbon data, sedimentary rates and ADCP profiles. Highest gas concentrations were found directly off the river mouth (20–40 m water depth) and where the IFREMER models point to the thickest accumulation (> 2 m) in response to the Rhône flood event. In areas unaffected by the high flux of organic matter and rapid/thick flood deposition, or in between flood events, the conditions for methanogenesis and gas accumulation have not been met; in these areas, the physical and biological reworking of the surficial sediment may effectively oxidize and mineralize organic matter and limit bacterial methanogenesis in the sub-surface. We propose that in the Rhône prodelta flood deposits deliver significant amounts of terrigenous organic matter that can be rapidly buried, effectively removing this organic matter from aerobic oxidation and biological uptake and leading to the potential for methanogenesis with burial.

Marine Geology↗

USGS2005

High-resolution surveys for geohazards and shallow gas: NW Adriatic (Italy) and Iskenderun Bay (Turkey)

The need for quantifying and understanding the distribution of shallow gas is both of academic interest and of relevance to offshore facilities. The combination of seafloor mapping, subbottom profiling, and multi-channel seismic data can provide information on regions of possible shallow gas, where the gas impacts the acoustic properties of the host material and the seafloor. In this paper, we present two case studies - one academic and one industry - that evaluate the distribution of shallow gas in two field areas in the Mediterranean. In the first case study, geophysical data from Iskenderun Bay, southeastern Turkey, indicate the presence and distribution of shallow gas. Pockmarks on the seafloor are associated with acoustic wipeout in the shallow subbottom data. Although deeper seismic data do not show bright spots or other indicators of possible gas, instantaneous frequency analysis clearly shows laterally restricted anomalies indicating gas-rich zones. The interpretation of possible shallow gas resulted in moving a proposed drilling location to a nearby area characterized by fewer (but still present) shallow gas signatures. In the second case study, cores acquired in the Po Delta, Adriatic Sea, provide quantitative ground-truthing of shallow gas - as suggested by geophysical data - and provide minimum estimates of the percentage of gas in the subsurface. Cores targeted on anomalous subbottom data yielded up to 41,000 ppm methane; cores with anomalous gas content are associated with thick recent flood deposits which may effectively isolate reactive terrigenous organic matter from biologic and physical re-working. ?? Springer 2005.

Marine Geophysical Research↗