Detecting surface oil slick related to gas hydrate/petroleum on the ocean bed of South China Sea by ENVI/ASAR radar data Yunpeng Wang ⇑ , Duofu Chen, Zhiguang Song State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, Wushan 510640, China article info Article history: Available online 31 August 2012 Keywords: Oil slick ENVI/ASAR Remote sensing Gas hydrate Oil gas South China Sea abstract For exploring the distribution of oil slick related to gas hydrate or petroleum in the northern part of South China Sea and evaluating the potential of ENVI/ASAR radar data for detecting oil slick, this paper tries to detect the oil slick on ocean surface of some potential areas in South China Sea and map oil slick from anomaly area in radar image. Seven surface oil slicks in Qiongdongnan Basin and four potential targets of oil slicks in Taixi–Dongsha Basin were detected and extracted by using ENIV/ASAR data. The anomalies of the radar image representing oil slick are validated by in-site sampling and laboratory analyzing. The Gas Chromatograph (GC) results of the extracted oil from the water samples gathered from the sites of detected oil slick demonstrate a typical composition of oil rather than that of ocean microbial origins, and Total Organic Carbon (TOC) concentrations in two designed vertical profiles keep steady in different depths, showing that the organic contents in water column are steady from the deep area to ocean sur- face. These results suggest the detected oil slick is a thin floating oil rather than a thick spill. Influencing factors to the results of oil slick detection including wind speed, current, small islands and capes as well as deep-sea petroleum reservoirs are discussed. The results in this study exhibit that extracting the anomaly signals related to oil slick related to oil/gas seeps using image processing techniques from ENVI/ASAR data is helpful to predict anomaly areas as potential targets for further exploration in this area. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction It is widely accepted that hydrocarbon can migrated through shallow marine sediment on the ocean bed of continental margins and form oil layer on ocean surface throughout the world (Kvenv- olden, 1993). Over geological time, the hydrocarbon seep process returns large amounts of carbon from thermogenic sources to the biosphere (Macgregor, 1993). The formation and dissociation of gas hydrate deposits are important components of hydrocarbon seeps because very large quantities of methane can accumulate geologically in recent sediments (MacDonald et al., 2002). Typi- cally, gas and oil escape into the water from discrete vents within larger seep sites, which are released as small bubbles (Leifer and MacDonald, 2003). When oil is present, it usually coats the walls of the bubbles or rises as gassy droplets (Leifer and MacDonald, 2003). When oil reaches the surface, it spreads into thin, very elon- gated layers that coalesce and are then recognized as ‘‘slicks’’ be- cause they tend to suppress surface wavelets. Remote sensing technologies have been used to quantify the total magnitude of seepage and its distribution of the northern slope (Kornacki et al., 1994; MacDonald et al., 1996; Mitchell et al., 2000), and pro- vided evidence for seepage rate, reaching 0.4–1.1 Â 10 8 L/y in the Gulf of Mexico (Mitchell et al., 1999). To present, a number of methods have been used to study ocean petroleum and gas hydrate, including laboratory simulation, geo- physical detection, numerical simulation and modeling, ocean dril- ling (ODP), deep sea survey, as well as remote sensing. Remote sensing technologies used have covered very broad wavelength from UV (ultraviolet), visible, IR (infrared), thermal IR to radar bands. Satellite remote sensing imagery can readily detect layers of floating oil that form over active seeps (Espedal and Wahl, 1999; Hu et al., 2009), which provides a means to survey the num- bers of hydrocarbon seeps across oil-producing regions (Kornacki et al., 1994; MacDonald et al., 1996). Radar remote sensing is an indirect way for detecting ocean petroleum and gas hydrate. The basis of ocean petroleum and gas hydrate detecting by radar re- mote sensing is the existence of gas/oil seepage from the petro- leum reservoirs or gas hydrate in deep to ocean surface. Previous studies demonstrate that there exist many styles of gas venting at natural seeps. Results in the Gulf of Mexico show that bubbles of gas, which are often contaminated with oily coatings, escape from a shallow deposit of gas hydrate at 570 m water depth; single 1367-9120/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jseaes.2012.08.016 ⇑ Corresponding author. Address: P.O. Box 1131, Guangzhou, Wushan 510640, China. Tel./fax: +86 20 85290197. E-mail address: wangyp@gig.ac.cn (Y. Wang). Journal of Asian Earth Sciences 65 (2013) 21–26 Contents lists available at SciVerse ScienceDirect Journal of Asian Earth Sciences journal homepage: www.elsevier.com/locate/jseaes