ALBERT

Description: © The Author(s), 2017. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Ecosphere 8 (2017): 10.1002/ecs2.2017, doi:10.1002/ecs2.2017.

Description: Historically low temperatures have severely limited skeleton-breaking predation on the Antarctic shelf, facilitating the evolution of a benthic fauna poorly defended against durophagy. Now, rapid warming of the Southern Ocean is restructuring Antarctic marine ecosystems as conditions become favorable for range expansions. Populations of the lithodid crab Paralomis birsteini currently inhabit some areas of the continental slope off Antarctica. They could potentially expand along the slope and upward to the outer continental shelf, where temperatures are no longer prohibitively low. We identified two sites inhabited by different densities of lithodids in the slope environment along the western Antarctic Peninsula. Analysis of the gut contents of P. birsteini trapped on the slope revealed them to be opportunistic invertivores. The abundances of three commonly eaten, eurybathic taxa—ophiuroids, echinoids, and gastropods—were negatively associated with P. birsteini off Marguerite Bay, where lithodid densities averaged 4280 ind/km2 at depths of 1100–1499 m (range 3440–5010 ind/km2), but not off Anvers Island, where lithodid densities were lower, averaging 2060 ind/km2 at these depths (range 660–3270 ind/km2). Higher abundances of lithodids appear to exert a negative effect on invertebrate distribution on the slope. Lateral or vertical range expansions of P. birsteini at sufficient densities could substantially reduce populations of their benthic prey off Antarctica, potentially exacerbating the direct impacts of rising temperatures on the distribution and diversity of the contemporary shelf benthos.

Description: Division of Polar Programs Grant Numbers: ANT-0838466, ANT-0838844, ANT-1141877, ANT-1141896; Vetenskapsrådet Grant Number: 824-2008-6429; H2020 Marie Skłodowska-Curie Actions Grant Number: 704895; U.S. National Science Foundation; European Commission; University of Alabama at Birmingham

Description: We have developed a stochastic multifault method for analysis of the impact of stratigraphic uncertainty on cross-fault leakage at sand-sand juxtapositions. This method assumes that all sand-sand juxtapositions leak across the fault. Stratigraphic uncertainty is modeled by stochastic variation of stratigraphic stacking. Structural uncertainty is addressed through variation of the input. Our objectives were to quantitatively predict the impact of uncertainties in stratigraphic and structural input and to simulate the complex system of structural spills and juxtaposition leak points that control hydrocarbon contact levels in traps with stacked reservoir systems and many faults. Three examples demonstrate how this stochastic multifault method has helped us evaluate uncertainty and understand complex leak fill-and-spill controls. The Ling Gu prospect demonstrates that widespread cross-fault leakage on two crestal faults with throw changes that exceed seal thickness causes only a single hydrocarbon column to accumulate in multiple-stacked reservoirs. This column is controlled by a juxtaposition leak point on a third, deeper fault. We have learned from examples like Ling Gu that the relative size of throw change and seal thickness is a fundamental control on the probability of cross-fault juxtapositions. An example at prospect A demonstrates the sensitivity of hydrocarbon entrapment to small faults in a sand-prone interval with thin seals. The prospect A analysis shows that if seals are thin, faults or channel incisions below seismic resolution can leak hydrocarbons out of stacked reservoirs that are interpreted as unfaulted on seismic data. This introduced significant predrill uncertainty and risk. Guntong field demonstrates that a thin sand in a juxtaposed seal interval can introduce large uncertainty in the prediction of hydrocarbon columns. These examples and many other analyses using the method demonstrate how small changes in stratigraphic and structural input to a fault-seal analysis can introduce significant uncertainty in the predicted range of hydrocarbon volumes. Such uncertainties need to be directly and systematically accounted for in a fault-seal analysis. Bill James earned his B.S. degree in geology from Earlham College and a Ph.D. from Northwestern University in 1968. He moved on to careers at the Corps of Engineers and the U.S. Geological Survey before starting at Exxon Production Research (now ExxonMobil Upstream Research) in 1979. He worked there, specializing in statistical applications in geology, assessment, and seal analysis, until his recent retirement.Lee Fairchild has a B.A. degree in geology from the University of California, Berkeley, and an M.S. degree and a Ph.D. from the University of Washington. He joined Exxon Production Research (now ExxonMobil Upstream Research) in 1985, working on structural geology and fault-seal analysis. In 1999, he moved to Starpath Exploration as a geophysicist, prospecting in south Texas. In 2001, he began an independent consulting business. Gretchen Nakayama earned her B.S. and M.S. degrees from the State University of New York, Rochester, and her Ph.D. in geology from the University of California, Davis, in 1990. She started her career at Exxon Production Research (now ExxonMobil Upstream Research) immediately, specializing in fault-seal analysis. We are saddened by our recent loss of Gretchen to cancer. Susan Hippler has a B.A. degree in geology from Augustana College and a Ph.D. from the University of Leeds (1989). She then joined Exxon Production Research (now ExxonMobil Upstream Research) as an expert in fault-zone characterization and fault-zone migration. She transferred to ExxonMobil Exploration Co. in 1996, specializing in applications of integrated trap analysis to exploration, development, and production problems. Peter Vrolijk earned his B.S. and M.S. degrees from the Massachusetts Institute of Technology and his Ph.D. in geology from the University of California, Santa Cruz, in 1982. In 1989, he joined Exxon Production Research (now ExxonMobil Upstream Research), doing research on a wide range of topics, including most recently fault-seal analysis and fault transmissibility.