THE RESERVOIR GEOCHEMISTRY APPLICATION IN THE DETERMINATION OF THE CRUDE OILQUALITY IN OFICINA AND MERECURE FORMATIONS OF THE WEST SOCORORO FIELD, MAJOR OFICINA AREA, EASTERN VENEZUELAN BASIN

 Authors: ELIANA ALVARADO*, LILIANA LÓPEZ AND SALVADOR LO MÓNACO

LABORATORIO DE GEOQUÍMICA DEL PETRÓLEO, CENTRO DE GEOQUÍMICA, INSTITUTO DE CIENCIAS DE LA TIERRA, FACULTAD DE CIENCIAS, UNIVERSIDAD CENTRAL DE VENEZUELA (*eliana_alvarado@hotmail.com)

Abstract

Most of the reservoirs in Socororo field showed low recovery factor as a result of these variations in the quality of their crude oils. Based on this context and as a contribution to the study of the reservoir geochemistry in Venezuela, the main objective was to characterize its core samples determining the quality of the accumulated hydrocarbon and variability of hydrocarbons through geochemistry parameters such as mineralogical composition, TOC/SOM concentrations, SARA composition, biomarkers and API gravities. Quartz and clays dominated mineral phases, a good correspondence between the concentrations of TOC and SOM have been observed. According to SARA composition, distinguished 5 trends in the variations of these compositions but not in a continuous way from base to top while Rock-Eval pyrolysis suggested that in reservoir mode can be a rapid analysis in determining the composition of the sorbed crude oils in the rock and its mobility. An origin of marine-type organic matter with terrigenous facies variations and an increase in maturity towards the base to the top and different levels of alterations were inferred, belonging to corroborate that the existence of different oil chargers.

Keywords: TOC, SOM, terrigenous facies, reservoir rock

 

Introduction

The main Area of Socororo has an area of 270 square Km, located on the southern flank of the Eastern Venezuela Basin and southwest of the Main Oficina Area, with a total of 93 drilled wells (Morales, 2002). The formations that make up the main stratigraphic column of the Area correspond to Mesa, Las Piedras, Freites, Oficina, Merecure, Tigre, Canoa (these two last ones are GrupoTemblador) and Carrizal, overlying the igneous-metamorphic basement of Venezuelan Guayana (Precambrian), being the main producer formations such as Oficina and Merecure (Useche and Villamediana, 2005).

The development of these oil fields started a beginning of the year 1940 and from that moment several studies have been carried out in order to solve the complexities from the static (structural and stratigraphic) models as well as dynamic (reservoir energy, quality and variability of fluids) due to the low productivity of its deposits seen in recent years.

This study will show to the reservoir geochemistry as an integrating tool in the recognition of significant differences in the composition of impregnated sands with crude oils, which can contribute to understanding to the history of reservoir filling in this part of Eastern Venezuela Basin.

Methods

The mineralogical composition was performed through X-ray diffraction (XRD) in the Bruker AXS Simens (D-8 Advanced) equipment, with a source of λ signal from kα of Cu. Total Organic Carbon (TOC) concentration was obtained by LECO equipment (C-144 model). Soluble organic matter (SOM) from crude oil impregnations extracted of seventeen core samples were determined by Soxhlet equipment and using 30 to 100 g using dichloromethane for 8 hours.

API gravity and hydrocarbons fractions from eight samples were obtained by Rock-Eval 6, reservoir method (Lafargue et al., 1998), using temperature program according to Trabelsi et al. (1994).

SARA composition: seventeen crude oil samples from West Socororo field were separated into maltene and asphaltene fractions by asphaltenes precipitation with n-heptane (40:1 ratio). The maltene fraction was separated into its saturated, aromatic and resin fractions by means of adsorption chromatography using packed columns (20 cm length 1.5 cm i.d.) with alumina as the stationary phase (20 g). The saturated hydrocarbons were eluted with n-hexane (30 mL), the aromatic hydrocarbons with toluene (20 mL), and a mixture (15 mL) of toluene: MeOH (70:30 v/v) was used to elute polar compounds (resins).

Saturate and aromatic biomarkers: Gas chromatography (GC) of the saturated hydrocarbon fractions was carried out on a 6890N Agilent Technologies network gas chromatograph using a flame ionization detector (FID) and DB-1 fused capillary columns (60m×0.25mm×0.25µm) to determined n-alkanes and acyclic isoprenoides (pristane and phytane). Analyses of biomarkers and aromatic compounds were performed by gas chromatography-mass spectrometry (GC-MS) by coupling the gas chromatograph to a 5975 Agilent Technologies mass spectrometer operated in single ion monitoring mode. The GC system was equipped with DB-1 or DB-5 fused silica capillary columns (60m×0.25mm×0.25µm) to analyze the saturated and aromatic fractions respectively. The monitored ions were m/z = 191 (terpanes), 177 (25-norhopanes and 17-nor-tricyclic terpanes), 217 y 218 (steranes), 259 (diasteranes), 253, 231 (mono and triaromatic steroids), 178 (phenanthrene), 192 (C₁-phenanthrenes), 184 (dibenzothiophene) and 198 (C₁-dibenzothiophenes).

 Results and Discussion

As a result of the evaluation of these parameters to determine the variability of these crude oil extracts, firsly the mineral phases in the samples corresponded to quartz and clays such as kaolinite and illite, carbonates such as calcite and siderite; Also in some intervals, microcline, dolomite, albite, sanidine, ankerite and goethite, mineralogy representative of the column of the Oficina Formation. There was a correspondence between the concentrations of TOC and SOM, being a measure of the crude oil recovery, if SOM is almost all the TOC, it is inferred that 100% was recovered and if SOM is lower than that of TOC, possibly heavy compounds remained and absorbed to the minerals and in the smaller porous spaces of the space filled by the first hydrocarbons charges (Table 1). From SARA composition, the saturates showed low concentrations towards the top and bottom of the column, just as the aromatics showed their lowest concentration towards the base, and the resins and asphaltenes showed the highest concentration values also towards the final part of the stratigraphic column, distinguishing 5 trends in the variations of these compositions but not in a continuous way from base to top. The values of API gravity estimated through Rock-Eval 6 in reservoir mode were related to amounts of light hydrocarbons and NSO + A compounds, indicating a good correspondence between these components and the estimated API gravity.

The hydrocarbons composition of the SOM is a reflection of the SARA composition, where it was determined that the SOM is enriched in resins and asphaltenes (equivalent to NSO + A pyrolysis) and suggested that Rock-Eval pyrolysis in reservoir mode can be a rapid analysis in determining the composition of the sorbed crude oil in the rock and its mobility, with a view to the application of recovery methods. The analysis of biomarkers in the saturated and aromatic fractions and their application in the samples of crude oil extracts of the ES-452 core allowed to infer an origin of marine-type organic matter with terrigenousfacies variations due to the presence of 18α(H)-oleanane, the lithology of the source rock from marine shales to limestones to marls and sedimentation conditions from anoxic to suboxic. There was a correspondence between the thermal maturity ratios calculated from the aromatic and saturated hydrocarbons markers. The relationships, C₃₂22S/(22S + 22R) and C₂₉(20S/20S+20R) showed thermal equilibrium towards the base and top of the ES-452 core, C₃₀moretane/(C₃₀moretane + hopane), MDR ratio,methylphenanthrene ratio and calculated vitrinite reflectance (Rc) versus depth suggested an increase in maturity towardsatthe base and topas well (Figure 1).To the alteration or post-generation processes, crude extracts samples of the ES-452 core showed a distribution of n-alkanes and pristane and phytane with some loss of n-alkanes of low molecular mass which would indicate an incipient biodegradation but the observation of 25-norhopanes in all of these and slight alteration of regular steranes (C₂₇ to C₂₉), indicate a severe biodegradation according to the Peters and Moldowan scale (1993). The presence of compounds with different susceptibility to biodegradation suggested the existence of more than one crude oil charges.

Conclusions

According to geochemistry parameters as SARA composition, the extracted ES-452 crude oil samples from Socororo field showed different SARA composition trends towards base and top of stratigraphic column, as well as the main minerals founded as quartz, clays and carbonates. Based on saturates and aromatics biomarkers, these crude oil samples showed variations of organic matter facies (marine and terrestrial) and a maturity from peak of generation window to more mature towards base and top of the reservoirs. Regarding to alteration or post-generation processes, inferred that all crude oil samples showed some loss of n-alkanes of low molecular mass with 25-norhopanes presence, suggesting oil mixing.

Figure 1. Mass fragmentograms for terpanes and steranes (m/z = 191/ 217) and aromathics-C₁-phenanthrenes (m/z = 192) of crude oil 4462 sample. Oleanane and C₃₀ steranes relation indicated organic matter variations (marine to terrigenous) and maturity variations from vitrine calculated (Rc) versus phenanthrene index

Units	Depth (ft)	TOC (%)	SOM (%)	SOM/TOC (%)
H-4,5,6	2914	1,03	0,04	3,52
H-4,5,6	2914	1,48	0,06	3,97
I-3	3008	1,98	0,06	3,06
I-4	3046	2,36	0,17	7,29
J-1	3104	2,03	0,02	1,02
J-1	3122	1,40	0,08	5,50
J-1	3159	1,92	0,26	13,26
J-3	3226	3,16	0,91	28,97
J-3	3247	2,02	0,12	5,92
M-1	3514	4,47	0,13	2,89
N-2	3724	3,06	0,04	1,26
R-0	3950	0,69	0,07	10,90
T-L	4418	1,75	0,06	3,77
U-1U	4427	5,31	0,61	11,51
U-1M	4462	2,20	1,84	83,93
U-2U	4504	2,29	0,05	2,03

 

References

Lafargue, E., Marquis, F., Pillot, D., 1998. Rock-eval 6 applications in hydrocarbon exploration, production, and soil contamination studies. Revue de l’institutFrançais du Pétrole 53, 421-437

Morales, E., 2002.Stratigraphic interpretation of the West Socororo Area, operated by PetroUCV, Anzoátegui State, Venezuela. Final Project. Central of VenezuelaUniversity. Faculty of Engineering. Geology, Geophysics and Mines School, 1-107

Useche, D., y Villamediana, G., 2005. Stratigraphic-Sedimentological Study in the Oficina and Merecure Formations of the East Socororo field, South flank, Eastern Basin of Venezuela, Anzoátegui State, VenezuelaFinal Project. Central of Venezuela University. Faculty of Engineering. Geology, Geophysics and Mines School, 1-2.

Trabelsi K., Espitalie J. y Huc A.Y., 1994. Characterisation of extra heavy oils and tar deposits by modified pyrolysis methods. In: European Symposium on Heavy Oil Technologies in a Wider Europe, Proceedings, June 7&8, 1994, 30-40