Abstract: Reservoir Compartmentalization in the Livramento Oil Field, Potiguar Basin, Brazil

Magnier, C.; Trindade, L.A.F.; Cerqueira, J.; Souto Filho, J. - Petrobras/Cenpes

The main objective of reservoir geochemistry today is to better understand the presence of hydrocarbons in a reservoir, to evaluate the relative degree of compartmentalization, and to estimate the hydrocarbon charging history. This has direct impact on reservoir management.

This paper presents a study made with this objective and applied to the Livramento oil field in the Potiguar basin. The Potiguar basin lies in the northeastern part of Brazil and is the easternmost basin of the equatorial margin. Selected oils for the reservoir continuity study are known to be produced from different stratigraphic units in a complex geological setting where some faults are suspected to compartmentalize the field. The role of these faults and their impact on the hydrocarbon compositions will be discussed in terms of reservoir homogeneity in different production zones. In this respect, twenty oils have been analyzed combining different analytical techniques (GC of whole oils, GC-MS, IRMS and statistical analysis) to determine possible reservoir continuities across the field. The reservoir problem associated with the oils of Livramento has been firstly addressed by the method of comparison of whole oils fingerprints and with statistic cluster analysis for categorizing oil families. Then, biomarker data and the carbon isotope data of the whole oils were reviewed to attempt to supplement this field's particular hydrocarbon charging in terms of in-filling direction and migration paths.

From the gas chromatography of whole oils, a set of 50 well resolved peaks (excluding paraffins) were compared to make oil fingerprints (star diagrams). Peaks were all selected in the carbon range C₈-C₃₀. As some of the oils were affected by water washing and early bacterial activity (biodegradation), the ratios calculated in the C₁₅- hydrocarbon fractions were isolated as they involved compounds known to be more volatile and easily subjected to secondary effects. The fingerprinting method and the cluster analysis (excluding the sample from the B unit) highlighted four groups of hydrocarbons: 13 oils of zone A, D, C1 and C2, two oils in zone C1 (LV-22 and LV-26) and two others LV-1 1 and LV-20 of zone C1 and LV-5 and LV-1 8 of zone D and C1 respectively. As an example Figure 1 shows a cross section with three wells studied. This method identified zones where reservoirs are in connection (wells LV-2, LV-8) and where they aren't. In this particular case K is proposed that the fault is acting as a fluid barrier and bringing about compositional heterogeneities in LV-26 oil. The biomarker and isotope data suggested a freshwater lacustrine depositional environment for the hydrocarbons and generally low thermal maturity. Figure 2 shows the distribution of the C29 abb / C29 abb+aaa steranes in Livramento, with a tentative direction of infilling from the southeastern corner of the field.

Applied reservoir geochemistry such as this study is quite important for the development of the field and definition of production strategies.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil