Abstract
The organic geochemical features of 30 sampled oils from the northern Bolivar Coastal Complex (Lake Maracaibo Basin, NW Venezuela) were examined by combining carbon isotope, classical biomarker, and extended diamondoid analyses to clarify source facies and to assess the thermal maturity and extent of biodegradation of the oils analyzed. In this work, oils are understood as a mixture of two episodes of petroleum generation from the La Luna Formation: a paleobiodegraded oil pulse during Paleogene times and a late pulse having a higher maturity in the post-Oligocene. For the oil samples analyzed, results revealed a mixing composed of different proportions of almost a terrestrially derived siliciclastic-sourced and a purely marine carbonate-sourced oil pulse. Moreover, two main groups of oils were identified by means of hierarchical cluster analysis. Finally, inter- and intrafield variations in the extent of biodegradation were also assessed using two classification schemes (Peters and Moldowan, and Manco scales).
Highlights
The Lake Maracaibo Basin (NW Venezuela) is among the world’s most productive petroleum producing regions
TJ-872, TJ-1323, and those from the Misoa B7 interval showed American Petroleum Institute (API) values from 16 to 26◦ and relatively similar SARA contents: SAT in the 34–41% range, ARO ranging from 26% to 30%, and POL ranging between 30% and 38%
The oils analyzed from two fields in the Bolivar Coastal Complex were generated from the same source facies (Upper Cretaceous marine La Luna source rocks deposited in low-oxygen settings), but they are the result of mixing two different petroleum charges, which have been subjected to distinct alteration processes: a biodegraded oil charge during Eocene-Oligocene times and a more mature refreshing during Miocene-Recent times
Summary
The Lake Maracaibo Basin (NW Venezuela) is among the world’s most productive petroleum producing regions. It covers approximately 50,000 km , with the Upper Cretaceous La Luna Formation its principal source rock, others have been identified in the region [1,2]. Carbon (TOC) ranging from 2.5 to 10.8% wt.). This unit was deposited under reducing conditions in a middle to outer neritic environment on the northwest part of South America [8,9]. In terms of organic facies, the La Luna Formation is more carbonatic towards the north, grading southeastward into more siliciclastic facies throughout the southern Lake
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