Origins of heavy oils in the Erlian basin, NE China

The Erlian Basin is located in the Central Asia-Mongolian fold belt between the Siberian and Sino-Korean Cratons. It is a Mesozoic continental rift ba sin composed of 52 individual fault-depressions. The main phase of rifting took place during the Early Cretaceous when a series of fluvial-lacustrine sediments were deposited. Each depression forms an independent sedimentary system and behaves as an independent petroleum system. Hydrocarbon source r ocks are found in the upper Arshan and lower Tengger Formations. These are mainly type II source rocks and are mainly located in oil generation 'windo w' at the present day. A series of oilfields and commercial oil hows have b een found in the basin, highlighting its good petroleum potential. Many of these oils are heavy. Six oil samples from the Anan and ten from the Jirgalangtu Depressions have been subjected to routine geochemical analytical techniques in order to ev aluate the origins. The methods used include gas chromatography of the satu rated and aromatic hydrocarbon fractions, gas-chromatography-mass-spectrome try of the saturated hydrocarbon fraction and stable carbon isotope analyse s. The trace metal elements of the biodegraded oils from the Jirgalangtu De pression were also analysed by atomic absorption spectroscopy. Two types of heavy oils: primary and biodegraded were identified on the bas is of these data. The former includes both immature and mature heavy oils. A 'filtering-and-spill' process was proposed to explain the origin of prima ry mature heavy oils (or tar-mat) in the Anan Depression. The biodegraded o ils from the Jirgalangtu Depression were ranked and classified in terms of the degree of biodegradation, using a series of geochemical parameters base d on the gas chromatographic concentrations and biomarker fingerprints of g as-chromatography-mass-spectrometry of the saturated hydrocarbon fraction. The relationship between oil saturation and porosity indicates that the hea vy oils in the Jirgalangtu Depression were biodegraded after they accumulat ed. (C) 1998 Elsevier Science Ltd. All rights reserved.