Article

Petroleum Geology & Experiment

, 37:12

First online:

Open Access This content is freely available online to anyone, anywhere at any time.

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

  • Qing YuAffiliated withState Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu University of Technology
  • , guosheng XuAffiliated withState Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu University of Technology
  • , Fanghao XuAffiliated withState Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu University of Technology
  • , Shenghui XuAffiliated withSINOPEC International Exploration and Production Corporation
  • , Lihui ZhengAffiliated withSINOPEC International Exploration and Production Corporation

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

Key words:

source rock; crude oil; geochemical feature; biodegradation effect; sedimentary environment; Llanos Basin; South America;

南美Llanos 盆地中部地区烃源岩与原油地球化学特征