琼东南盆地深水东区渐新统烃源岩发育模式

Developmental Model of Oligocene Source Rock in the Eastern Deep-water Area of Qiongdongnan Basin

琼东南盆地深水东区勘探程度低,钻井样品受油基泥浆污染严重,目前关于该区浅海相和海陆过渡相烃源岩的有机地球化学研究较为局限。通过对深水东区渐新统受油基泥浆污染的泥岩岩屑进行有效地洗油实验及有机地球化学分析测试,结合东部浅水区渐新统烃源岩样品的有机岩石学、有机地球化学分析测试等地质资料,系统剖析了深水东区渐新统不同沉积相烃源岩的地球化学特征、发育控制因素和发育模式。研究结果认为,琼东南盆地深水东区渐新统烃源岩的发育模式可分为海陆过渡相三角洲前缘模式和浅海相模式。海陆过渡相三角洲前缘烃源岩为中等—高有机质丰度,有机质类型为Ⅲ型,具有Pr/Ph普遍大于3、(nC21+nC22/nC28+nC29)普遍小于1.2、高OL/C30H、高T/C30H、低C23TT/C30H、低C27ST/C29ST和相对较低的Ga/C30H、C35/C3422S的特征,烃源岩的TOC和硫含量相关性极好,沉积水体为氧化性淡水—半咸水环境。三角洲前缘烃源岩的TOC和母源输入参数相关关系较好,而与Ga/C30H、C35/C3422S参数相关关系较差,说明三角洲前缘烃源岩的有机质丰度主要受陆源有机质输入的控制。浅海相烃源岩有机质丰度中等,有机质类型为Ⅱ2~Ⅲ型,具有Pr/Ph普遍小于3、(nC21+nC22/nC28+nC29)普遍大于1.5、低OL/C30H、低T/C30H、高C23TT/C30H、高C27ST/C29ST和相对较高的Ga/C30H、C35/C3422S的特征,且烃源岩的TOC和硫含量相关性较差,沉积水体为弱氧化性半咸水—咸水沉积环境。浅海相烃源岩的TOC和母源输入参数、Ga/C30H、C35/C3422S等相关关系均较好,说明浅海相烃源岩有机质丰度受水生有机质输入和弱氧化性半咸水—咸水沉积环境控制。滨海相烃源岩的有机质丰度为差—中等,有机质类型以Ⅲ型为主,且TOC受母质来源或沉积水体的氧化还原性影响较小,其烃源岩发育模式是一种破坏性模式。

The eastern deep-water area in Qiongdongnan Basin (QDNB) is in the early stages of exploration stage, and some drilling samples collected from this area are severely contaminated by the oil-based mud. Little research regarding the organic geochemistry of the Oligocene source rock deposited in the marine or transitional environments in this area has been performed so far. In this paper,effective oil-cleaning experiments and organic geochemical analysis were carried out for Oligocene mudstone cuttings contaminated by oil-based mud in the eastern deep-water area,coupled with organic geochemical and petrological data of mudstone cuttings in the shallow-water area,and the geochemical characteristics,paleoenvironment,controlling factors,and developmental model of Oligocene source rocks from different sedimentary facies in the eastern QDNB were analyzed. Two distinct models of delta front source rock and neritic source rock were developed according to their organic matter sources and geochemical characteristics. Transitional delta front source rock has fair to good quality and gas-prone type organic matter,and its biomarkers are characterized by pristane/phytane bigger than 3,( nC21 + nC22 / nC28 + nC29 ) smaller than 1.2,high ratios of oleanane/C 30 hopane,T-bicadinane/C30 hopane,low ratios of C23 tricyclic terpane/C30 hopane,C27 sterane/C29 sterane,gammacerane/C30 hopane,and C35 homohopane/C 34 homohopane. There exists a strong correlation between total organic carbon (TOC) and total sulfur,which means water in the delta front subface was of oxidation fresh or brackish condition. Organic matter abundance in the delta front source rock is heavily controlled by terrigenous organic matter input. Neritic source rock has fair quality and an organic matter type prone to generating both oil and gas,and the biomarkers are characterized by pristane/phytane smaller than 3,(nC21 + nC22 / nC28 + nC29 ) bigger than 1.5,low ratios of oleanane/C30 hopane,T-bicadinane/C 30 hopane,high ratios of C23 tricyclic terpane/C30 hopane,C27 sterane/C29 sterane,gammacerane/ C30 hopane,and C 35 homohopane/C 34 homohopane. However,there exists no correlation between TOC and total sulfur,which means water in the neritic face was of the weak oxidation brackish or saline condition. Organic matter abundance in the neritic source rock is controlled by aquatic organic matter input and influenced by the water environment. Littoral source rock is generally of poor to medium quality and is seldom influenced by organic matter sources and water condition,and the source rock depositional model is considered to be a destructive model.