ALBERT

Description: Due to their favorable P-T conditions and organic-rich deposits, sub-seafloor sediments in the northern Gulf of Mexico are known to have a large potential for gas hydrate accumulations. The presence of gas hydrates within sediments of the Green Canyon block has been proven by various methods, incl. seismic imaging, geochemical analysis, and drilling conducted mainly as a part of Joint Industry Project (JIP) Phase II. Gas hydrates reported therein usually occur as tens up to hundreds of meters thick sections with moderate to high concentrations within a range of 50 – 70 vol. % of pore space, and hence, seem to offer a considerable natural deposit of methane gas. The main focus of this study was to explore the complex effects of a set of control- parameters responsible for hydrocarbon migration and storage within the Gas Hydrate Stability Zone (GHSZ) on the accumulation of gas hydrates. To investigate the processes of basin formation and its subsidence history, source rock maturation, hydrocarbon migration and expulsion, and to quantify the gas hydrate accumulation potential, 3-D numerical study has been conducted using PetroMod. The area of interest extends over ~14 km x 33 km and covers the edge of the Sigsbee Escarpment representing the main salt mobility front in the region. The simulation contains full depositional history of the Green Canyon block, incl. salt deposition and re-mobilization as well as its further implications for temperature field, fluids migration and sedimentary layers distribution. Methane generation has been resolved by in-situ POC degradation and deep thermogenic mobilization from two distinct hydrocarbon sources. As a result, we present a number of likely scenarios of gas hydrate formation and accumulation in the study area that have been calibrated against available data.

Description: Cruise M86/5 aimed at collecting data from potential fluid dewatering sites located in the deep-sea region of the western part of the Gulf of Cadiz and the adjacent deep sea plain. Previous work on mud volcanoes (cruises SO175 and MSM1/3) located on the accretionary wedge in the Gulf of Cadiz showed that mud volcano fluids are typically sourced at several km depth below the seafloor. In addition, the geochemical composition of fluids from the deepest mud volcano in this area which is located on a west-east trending transform lineament (SWIM1) indicated that these fluids are typical for having been altered by the reaction with oceanic crust. This implies that there is active flow connecting the oceanic basement and the seafloor. To date, such kind of hydrothermal circulation is only known for relatively young oceanic crust (〈60 Ma). Hence, the existence of a hydrological connection between old, sedimented oceanic crust and the seafloor is a phenomenon, which essentially has not been investigated in the past, and may represent a (missing) link between hot vents at mid-ocean ridges and cold seeps at continental margins. On the cruise, we followed an interdisciplinary approach characterized by extensive geochemical sampling in the water column and the sediment, heat flow measurements, and detailed mapping with AUV and ship-based hydroacoustic systems. We mainly investigated selected sites with conspicuous backscatter anomalies recorded on previous cruises along the SWIM1 lineament: (i) within the transition between the accretionary prism and the Horseshoe Abyssal Plain and (ii) on seafloor highs within the Horseshoe Abyssal Plain itself. The geotectonic environment in both sections is completely different from the situation on the accretionary wedge, where numerous mud volcanoes were detected on previous cruises. Within the transition zone three new mud volcanoes were discovered and extensively sampled during M86/5. Typal chemoautotrophic organisms were found here together with conspicuous methane anomalies in der water column. Preliminary pore water analyses show that the fluids indicate a deep origin, but show distinctive differences to those found on the mud volcanoes on the wedge. Future analyses will show, if this phenomenon is - as hypothesized - due to decreasing sediment thickness with increasing westward distance from the wedge, and hence decreasing sediment-water interaction overprinting the original fluid composition. On the seafloor highs along the SWIM1 lineament in the Horseshoe Abyssal Plain no seeps or mud volcanoes were discovered. However, the lineament seems to be active in terms of fluid flow as indicated by pore water and heat flow anomalies as compared to values measured off the lineament. Overall, the major goal of the cruise could be fulfilled: fluid seeps related to active faults were discovered in a new geotectonic environment. The overall significance in terms of fluid transport in old oceanic crust and/or the role of deeply-rooted fault systems needs to be further addressed in subsequent studies.

Description: The Alaska North Slope comprises an area of about 400,000 km2 including prominent gas and oil fields. Gas hydrates occur widely at the Alaska North Slope. A recent assessment by the USGS estimates 0.7-4.47 x 1012 m3 of technically recoverable gas hydrates based on well data and drilled hydrate accumulations. In spring 2012 a production field trial, testing CO2/N2 injection and depressurization, was conducted by USDOE/JOGMEC/ConocoPhillips at the Ignik Sikumi site. The 3D geological model of the Alaska North Slope developed by the USGS and Schlumberger is used to test the new gas hydrate module in the petroleum systems modeling software PetroMod®. Model results of the present extent of the gas hydrate stability zone (GHSZ) are in good agreement with results from well data. The model simulations reveal that the evolution of the GHSZ over time is primarily controlled by the climatic conditions regulating the extent of the permafrost during the last 1 Myr. Preliminary model runs predict the highest gas hydrate saturations near the major faults and at the bottom of the GHSZ, where thermogenic methane gas accumulates after migration through the most permeable stratigraphic layers (e.g. Sag River Sandstone Fm, Ivishak Fm). Gas hydrate saturations predicted for the Mount Elbert Stratigraphic Test Well and the Ignik Sikumi sites are basically controlled by the alternation of layers with different permeability and the fault properties (time of opening, permeability, etc). Further results including a total gas hydrate assessment for the Alaska North Slope will be presented during the conference.