Abstract:

31 rock samples and 20 oil samples have been collected from Niuzhuang subsag and its southern slope for detail GC,andGC-MS analyses.The rocks have been sampled from Es₃ and Es₄ members, which are respecting the two main source rocks being deposited in freshwater lake and brackish to saline lake environments, respectively. The oils have been sampled from Bamianhe, Guangli and Shaziling oilfields in the southern slope of Niuzhuang subsag.Our oil-oil and oil-rock correlations show that there are two genetic types of oils in Niuzhuang subsag and its periphery.They are separately corresponding to the two major source rocks in middle to lower Es₃ and upper Es₄ members.Rock extracts of Es₄ member and the related oil are characterized by Pr/Ph<1, high gammacerane, sterane and dibenzothiophene contents, relatively low diasterane content,and high C₃₅/C₃₄ homohopane ratios, suggesting that they have been generated in a reduction and brackish to saline lacustrine paleoenvironment. However, rock extracts of middle to lower Es₃ and the related oil are characterized by Pr/Ph>1,and relatively high concentration of hopane and dibenzofuran contents,representing freshwater lacustrine genesis. It is found that methylsteranes including dinosteranes are of importance to indicating sedimentary environment and even specific species of organisms. The alge-rich Es₄ shales deposited in brackish water in the southern slope of Niuzhuang subsag are rich in C_28-30 4-methyl steranes and dinosteranes, and but very small amount of dinosterane in the Es₄ source rock buried at a depth over 2700 m. Similar phenomenon has been observed in Es₃ mudstones deposited in fresh water,i.e.C₂₈, C₂₉, C₃₀ 4α methyl-ααα-20 Rsterane of Es₃ mudstone distribute in the shape of “V” in the m/z 231 mass chromatogram, while those in Es₄ source rock have a linear distribution, suggesting that they have apparently different paleoenviornment and related geochemical environment. Analyses of formation water and trace element also show that the sedimentary environments in Es₃ and Es₄ are quite different. The Sr/Ba ratio,boron content,paleo-salinity and saltiness in Es₄ are relatively higher than those in Es₃.In a word,the development of the Tertiary lake in this area has been closely related to the tectonic movement and paleo-climates. Sedimentary environment and biomarker index show that the oil field in the deeper part of Niuzhuang subsag have mainly been sourced by the Es₃ source rocks.They are actually of lenticular oil reservoirs characterized by short-distance migration and indigenous oil generation and accumulation. The oils generated from Es₄ have mainly been accumulated in the tectonically high position,such as southern slope of Niuzhuang subsag and the adjacent area.They are characterized by relatively long-distance migration. The sedimentary environment indexes indicate that most of the oils discovered in the southern slope have been sourced from Niuzhuang subsag,only Guangli oilfield and the oil pools in northern Bamianhe oilfield have been sourced from Guangli subsag and adjacent area. It can,therefore,be concluded that papleoenvironment is one of the most important factors having controlled oil and gas distribution in the area.

Key words: Niuzhuang subsag, paleoenvironment, source rock, oil, biomarkers