Oil & Gas Geology ›› 2003, Vol. 24 ›› Issue (3): 269-273.doi: 10.11743/ogg20030315

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PALEOSEDIMENTARY ENVIRONMENT IN TERTIARY AND ITS EFFECT ON PETROLEUM DISTRIBUTION IN NIUZHUANG SUBSAG, DONGYING SAG

Li Sumei1,2, Li Xue3, Zhang Qinghong4, Ju Lirong5, Ma Xiaochang 3   

  1. 1. Research Center of Basin & Reservoir, University of Petroleum, Beijing;
    2. Key Laboratory of Hydrocarbon Accumulation Mechanism, Ministry of Education, University of Petroleum;
    3. Qinghe Oil-recovery Devision, Shengli Petroleum Exploration Bureau, Dongying, Shandong;
    4. Petroleum Exploration and Production Research Institute, SINOPEC, Beijing;
    5. Binnan Geological Institute, Shengli Petroleum Exploration Bureau, Dongying, Shandong
  • Received:2003-05-26 Online:2003-09-25 Published:2012-01-16

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 C28-30 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

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