Abstract
Abstract 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 hydrocarbon-generating 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
Highlights
The central Llanos Basin in South America is an important petroliferous province
C26三环萜烷比值,低 C35 / C34 升藿烷比值,高甾烷 / 藿烷比值,低—中伽马蜡烷 / C30 藿烷比值,这样的 生物标志物组合指示较多的陆源有机质,半咸水和 氧化环境。 原油中 C27 -C28 -C29 甾醇同系物的相对丰度可 以反映烃源岩有机质的甾烷分布。 Huang 和 Mein⁃ shein 根据现代海相和陆相沉积物的研究结果,提 出使用 C27 -C28 -C29甾醇同系物做的三角图来区分 不同烃 源岩来源的原油[6] 。 甾烷的来源比较复杂,一般认为 C27 甾烷来自水生生物,C28 甾烷为海 洋藻类繁盛的标志,C29 甾烷指示高等植 物 的 输 入。 尽管只使用 C27 -C28 -C29甾烷三角图不能厘定烃源 岩的年代,但仍能够通过它把早白垩世和古近纪同 更老时代的样品区分开[7] 。 根据 C27 -C28 -C29 甾烷 三元图( 图 5) 显示,Gacheta 组样品 C27 优于 C29 甾 烷,这表明该烃源岩以海相有机质为主;而 Los Cu⁃ ervos 组样品 C29 优于 C27 甾烷, 这表明烃源岩以陆 相有机质为主。
(4) Llanos 盆地中部地区原油受生物降解作用 及二次充注过程的控制。 从 Santiago 油田向北东 方向到 La Gloria 等油田,25 -降藿烷 / C30 藿烷比值 和沥青质含量下降,反映出沿该方向生物降解作用 呈减弱的趋势,而 Cupiagua 和 Buenos Aires 油田的 样品受二次充注的影响明显,后期大量充注的轻质 油,使得 Cupiagua 和 Buenos Aires 样品的 API 远远 超过 La Gloria 等油田。
Summary
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. Los Cuervos 组有机碳含量为 0.95% ~ 2.1%,平均为 1.7%。 2.2 有机质类型、成熟度及生烃潜力 这 2 套烃源岩具有各自不同的生物标志物组 合,反映出它们具有不同的有机质来源和沉积环境。 Gacheta 组烃源岩总体表现为中—低 Pr / Ph,低—中 C24 四环萜烷 / C26 三环萜烷比值,低—中 C35 / C34 升 藿烷比值,低甾烷 / 藿烷比值,高伽马蜡烷 / C30 藿烷 比值,这样的生物标志物组合指示较少的陆源有机 质,咸水和弱 氧 化—弱还原环境。 Los Cuervos 组 烃源岩总体表现为高Pr / Ph,中—高C24 四环萜烷 / 图 3 Llanos 盆地 Gacheta 组和 Los Cuervos 组 烃源岩热解峰温度—氢指数相关图
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
