Chapter 30
Sulfur Isotope Data Analysis of Crude Oils from the Bolivar Coastal Fields (Venezuela) 1
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B. Manowitz , H. R. Krouse , C. Barker , and Ε. T. Premuzic 1
Department of Applied Science, Brookhaven National Laboratory, Upton, NY 11973 Department of Physics, University of Calgary, Calgary, Alberta T2N 1N4, Canada Geosciences Department, University of Tulsa, Tulsa, OK 74104
Downloaded by UNIV LAVAL on February 25, 2013 | http://pubs.acs.org Publication Date: June 29, 1990 | doi: 10.1021/bk-1990-0429.ch030
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Oils in the Bolivar Coastal Fields of Venezuela have been divided into five major oil classes believed to reflect largely variations caused by biodegra dation in the reservoirs. Classes are based on variations in composition of hydrocarbons, NSO components (includ ing asphaltenes) and API gravity, as well as other considerations including the geological framework and reservoir depths and temperatures. Based on sulfur isotope data reported here, the o i l s f a l l into two groups, a non- or little-biodegraded group with δ S values averaging +7.56 ‰and a heavily biodegraded group in which δ34S values average +5.1 ‰. Thermal alteration effects also are probable with reservoir temperatures ranging from 20°C to 209°C. Various pos sibilities for explaining the isotopic data are considered. The r e l a t i v e l y narrow range in δ34S values suggests reasonably uniform source-rock charac ter and rather minor isotopic changes from all alteration processes. 34
The B o l i v a r Coastal F i e l d s (BCF) of eastern Lake Maracaibo, Venezuela, c o n t a i n f i v e c l a s s e s of o i l as r e f l e c t e d by t h e i r API g r a v i t i e s , C + saturates-andaromatics contents as w e l l as t h e i r t o t a l n i t r o g e n , s u l f u r , and oxygen (NSO) compositions. Biodégradation appears t o have had a major r o l e i n c o n t r o l l i n g the 15
0097-6156/90/0429-0592$06.25/0 © 1990 American Chemical Society
In Geochemistry of Sulfur in Fossil Fuels; Orr, W., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
Downloaded by UNIV LAVAL on February 25, 2013 | http://pubs.acs.org Publication Date: June 29, 1990 | doi: 10.1021/bk-1990-0429.ch030
30.
Sulfur Isotope Data Analysis ofCrude OUs
Μ Α Ν Ο W I T Z E T AL.
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compositions of the v a r i o u s c l a s s e s of o i l . The geology and geochemistry of crude o i l s and source rocks i n the BCF have been d e s c r i b e d (1-3). In the Cretaceous, the area was p a r t of the p l a t f o r m of a l a r g e geosyncline, and by the Eocene i t was near a coast where a s e r i e s of l a r g e sandy d e l t a s was depos i t e d , with t e r r e s t r i a l sediments on the south and t h i c k marine shales t o the north. At t h i s time, c o n d i t i o n s f o r o i l generation i n the shales and m i g r a t i o n t o the sands may have been e s t a b l i s h e d , and the subsequent Oligocène f a u l t i n g , u p l i f t , and e r o s i o n c o u l d have allowed meteoric water t o penetrate i n t o r e s e r v o i r s . During the Miocene and P l i o c e n e , the b a s i n was t i l t e d f i r s t west and then south, and f i l l e d with c o n t i n e n t a l sediments from the r i s i n g Andes. T i l t i n g i s s t i l l cont i n u i n g and o i l i s moving up along the Oligocène unconformity, forming s u r f a c e seeps. Most of the o i l f i e l d s are l o c a t e d i n sands above t h i s unconformity or i n f a u l t b l o c k s immediately below i t . T h i r t y crude o i l s from the BCF were c o l l e c t e d (1) along two p a r a l l e l and g e n e r a l l y southwest-northeast t r e n d s . The a r e a l extent of the BCF showing l o c a t i o n s of w e l l s sampled i s shown i n F i g u r e 1. These o i l s were c h a r a c t e r i z e d by t h e i r API g r a v i t y , percent s a t u r a t e s , aromatics, NSO and asphaltene compounds, gas chromatograms f o r whole o i l s , C -C f r a c t i o n s , and aromatics. Concurrently, 24 a s s o c i a t e d waters were a l s o sampled and analyzed f o r Ca , Mg**, Na , HC0 , C0 , SO ", pH, and t o t a l d i s s o l v e d s o l i d s (TDS) (1). In the present work, twenty-seven of these o i l s were s e p a r a t e l y analyzed f o r s u l f u r content and s u l f u r i s o t o p e r a t i o (
