南美Llanos盆地中部地区烃源岩与原油地球化学特征

Geochemical features of source rocks and crude oils in central Llanos Basin,South America

南美Llanos盆地中部地区为盆地内的重要产油区,对其烃源岩及原油的地球化学特征分析,能揭示出盆地内的排烃和运聚过程及原油的生物降解特征。通过岩石热解、原油组分分析等方法和手段,结合多种生标参数,对这一地区的烃源岩及原油的地球化学特征进行了系统和全面的分析,结果表明:Gacheta组烃源岩沉积在较少的陆源有机质、咸水和弱氧化—弱还原环境,为腐泥—腐殖混合型干酪根;Los Cuervos组烃源岩沉积在较多的陆源有机质、半咸水和氧化环境,为腐泥—腐殖混合型和腐殖型干酪根,2套烃源岩都处于低—中等的成熟程度,均具有较强的生烃潜力;盆地中部地区存在A、B、C三类原油,其中A类为白垩系烃源岩油;B类为古近系烃源岩油;C类系前两者的混合;在大部分的油样中含有丰富的重排甾烷,二苯并噻吩/菲含量低,而Pr/Ph值较高,这说明原油对应的烃源岩为海相页岩而非碳酸盐岩;原油组分受生物降解及二次充注过程的控制,从Santiago油田沿北东方向到La Gloria等油田生物降解作用呈减弱的趋势,而Cupiagua和Buenos Aires油田的样品受二次充注的轻质油的影响,其API远远超过La Gloria等油田。

The central Llanos Basin in South America is an important petroliferous province. The analysis of the geochemical features of source rocks and crude oils in the study area revealed the process of hydrocarbon expulsion,migration and accumulation as well as the biodegradation features of crude oils. According to rock evaluation and crude oil composition,and combined with a variety of biomarker parameters,the geochemical features of source rocks and crude oils were analyzed systematically and comprehensively. Several conclusions were made as follows. Firstly,source rocks in the Gacheta Formation were deposited in a saline-water and weakly oxidizing to weakly reducing environment with little terrestrial organic matter,and kerogen in the Gacheta Formation was of sapropel-humic type. Source rocks in the Los Cuervos Formation were deposited in an oxidizing brackish-water environment with abundant terrestrial organic matter,and kerogen in the Los Cuervos Formation was of sapropel-humic and humic types. Source rocks in the Gacheta and Los Cuervos formations were both low-maturity and medium-maturity,showing large hydrocarbongenerating potentials. Secondly,3 crude oil families were found in the central basin,named families A,B and C.Crude oils of family A originated from Cretaceous source rocks,while those of family B from Paleogene,and those of family C were a mixture of families A and B. Thirdly,for most of the crude oil samples,rearranged sterane was abundant,the dibenzothiophene / phenanthrene ratio was low,and the Pr / Ph value was high,indicating that the source rocks were marine-facies shales rather than carbonates. Finally,crude oil composition was controlled by biodegradation and recharge. From the Santiago Oilfield to the La Gloria Oilfield,biodegradation effect weakened toward the NE.Influenced by the recharge of light oils,crude oils from the Cupiagua and Buenos Aires Oilfields had much higher API gravity values than those from the La Gloria and other oilfields.