牛庄洼陷第三系古沉积环境及其控油气作用

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

地质与地球化学分析结果表明 ,东营凹陷牛庄洼陷及其邻区的两套主力烃源岩即沙河街组三段中、下亚段与沙河街组四段上亚段具有不同的原始沉积环境。沙河街组四段上亚段烃源岩可溶物及其相关原油具有Pr/Ph小于 1、高含量伽马蜡烷、升藿烷“翘尾”及硫芴含量相对较高的还原相、咸化湖相成因特征 ;沙河街组三段中、下亚段烃源岩及相关原油具有Pr/Ph大于 1、藿烷和氧芴系列含量相对较高的淡水湖相成因特征。研究区甲基甾烷、甲藻甾烷分布具有重要的沉积环境与生物属种标定意义 ,沉积环境及相关生源指标显示沙河街组四段“咸水型”油气与南斜坡原地富藻类未熟 -低熟页岩没有直接联系。该区油气分布及其成因类型明显受控于烃源岩的性质及其展布。沙河街组三段“淡水型”原油主要分布于牛庄洼陷 ,属自生自储的“淡水型”透镜体油藏 ;沙河街组四段“咸水型”原油主要分布于偏离生油洼陷的南斜坡等周边地区 ,具有相对较远的油气运移距离。沉积环境指标反映广利洼陷是广利油田及八面河油田北部“咸水型”

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 Es 3 and Es 4 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 Es 3 and upper Es 4 members.Rock extracts of Es 4 member and the related oil are characterized by Pr/Ph<1, high gammacerane, sterane and dibenzothiophene contents, relatively low diasterane content,and high C 35/C 34 homohopane ratios, suggesting that they have been generated in a reduction and brackish to saline lacustrine paleoenvironment. However, rock extracts of middle to lower Es 3 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 Es 4 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 Es 4 source rock buried at a depth over 2700 m. Similar phenomenon has been observed in Es 3 mudstones deposited in fresh water,i.e.C 28, C 29, C 30 4α methyl-ααα-20 Rsterane of Es 3 mudstone distribute in the shape of “V” in the m/z 231 mass chromatogram, while those in Es 4 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 Es 3 and Es 4 are quite different. The Sr/Ba ratio,boron content,paleo-salinity and saltiness in Es 4 are relatively higher than those in Es 3.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 Es 3 source rocks.They are actually of lenticular oil reservoirs characterized by short-distance migration and indigenous oil generation and accumulation. The oils generated from Es 4 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.