Unraveling the early petroleum migration of the Potiguar Basin, Brazil: Constraints from fluid inclusions of the Ubarana and Lorena oilfields

https://doi.org/10.1016/j.marpetgeo.2021.105200Get rights and content

Highlights

  • A coupled study of fluid inclusions and one-dimensional basin modeling is presented.

  • Early oil filling of the onshore Lorena reservoir started between Barremian and Aptian.

  • Initial oil migration in the offshore Ubarana was linked to hydrothermal activity.

  • Ubarana fluid inclusion signatures are similar to Mississippi Valley Type deposits.

  • Possible scenarios for the onset of the atypical Ubarana petroleum system are discussed.

Abstract

A coupled study of fluid inclusions and one-dimensional basin modeling allowed the reconstruction of the early hydrocarbon filling history of the main petroleum systems of the Potiguar Basin, Brazil. Confocal laser scanning microscopy, microthermometry and fluorescence spectra measurements were used to model PVTx properties of petroleum inclusions in samples from the onshore Lorena and the offshore Ubarana oilfields. The fluid inclusion data and basin modeling suggest that the initial filling of the Lorena reservoir (Pendência Formation) occurred during the rift stage in two scenarios at 62.7–80.6 °C (11–16.8 MPa): (i) the late rift phase in the Barremian–Early Aptian (ca. 129–124 Ma) or during the beginning of the uplifting phase in the Aptian (ca. 118–117 Ma). In the Ubarana Oilfield, hydrothermal fluids migrated alongside petroleum during the reservoir initial filling. Fluid inclusion homogenization temperatures for both the Açu (mode interval of homogenization temperatures, Th(LV→L) mode interval = 124–128 °C) and Ponta do Mel (Th(LV→L) mode interval = 115–130 °C) formations are higher than the present-day reservoir temperature (110 °C). Isochoric modeling of saddle dolomite-hosted fluid inclusions from the Ponta do Mel Formation indicates trapping temperatures between 128.9 and 133.1 °C and pressures between 10.6 and 12.9 MPa. Temperatures and salinities (16 and 20 mass % equivalent in NaCl + CaCl2) of aqueous inclusions hosted in saddle dolomite are similar to those of the fluid inclusions trapped in hydrothermal minerals in Mississippi Valley-Type Pb–Zn deposits. No specific timing can be suggested for the hydrothermal event in Ubarana Oilfield. However, this atypical (hydrothermal) petroleum system could be linked to two possible scenarios: (i) the onset of petroleum migration of the Alagamar Formation during the Cenozoic Macau (ca. 48–8.8 Ma) magmatic event; or (ii) an earlier activity of the petroleum system, prior to the peak oil generation (50–10 Ma).

Introduction

A two-fold approach, combining fluid inclusion and basin modeling techniques, was applied to constrain the initial hydrocarbon migration history of the main petroleum systems of the Potiguar Basin, Brazil (Fig. 1A). The Potiguar Basin represents both a mature petroliferous province (Rodrigues and Suslick, 2009) and a key target for understanding the Brazilian Equatorial Margin (BEM), a current exploratory frontier (Mello et al., 2013). The BEM sedimentary basins (e.g., Barreirinhas, Ceará, Pará-Maranhão, Potiguar, and Foz do Amazonas) comprise rift basins originated during the Early Cretaceous continental break-up of the Western Gondwana (Matos, 1999). The BEM basins share many geological features, such as similar lacustrine source rocks, thermal history, and tectonic and sedimentary evolution (Mello et al., 1995; Dickson et al., 2016). In this scenario, the Potiguar Basin stands as the most studied sedimentary basins of the BEM (Dickson et al., 2016). Therefore, the understanding provided by the study of the onset of hydrocarbon migration in the Potiguar Basin may bring new insights into analogous petroleum systems.

Two petroleum systems were responsible for the majority of the hydrocarbon accumulations on both onshore and offshore sections of the Potiguar Basin (Rodrigues, 1983). These petroleum systems were sourced by the Neocomian Pendência and the Aptian Alagamar formations (Rodrigues, 1983). Although the previous studies focus on reconstructing the petroleum history of the Potiguar Basin, there is still a lack of knowledge regarding the early petroleum migration (Souto Filho et al., 2000; Behar and Penteado, 2004; Ducros et al., 2016). Aiming at studying the early petroleum migration history in these main petroleum systems of the Potiguar Basin, we selected two wells bearing oils sourced by the Pendência and Alagamar source-rocks, respectively: (1) well LOR-60 (onshore Lorena oilfield) and (2) well UB-40 (offshore Ubarana oilfield). We hope that the insights provided by this study may help reduce the uncertainty in the reconstruction of the analogous petroleum systems in the BEM area.

Section snippets

Geological setting

The origin and the evolution of the Potiguar Basin are directly related to the Western Gondwana break-up in the Early Cretaceous (Françolin and Szatmari, 1987; Matos, 1992). Originated upon the Precambrian basement rocks of the Borborema Province (Almeida et al., 1981; Arthaud et al., 2008), the Potiguar Basin evolution can be divided in three tectonic and stratigraphic stages (Fig. 1B): i) the rift stage, ii) the transitional stage, and iii) the drift stage (Françolin and Szatmari, 1987;

Sampling

A set of drill core samples (Fig. 2) was examined to identify samples suitable for the fluid inclusion study. Description of the sedimentary facies and petrographic characteristics was carried out to assess the main depositional and diagenetic features. A total of 90 doubly-polished thick sections (~120 μm thick) were produced to search for the most suitable samples for the microthermometric analysis which were analyzed in detail.

Fluid inclusion analysis

The fluid inclusions were analyzed using petrography,

Lorena Oilfield – Pendência Formation

The reservoir interval of the LOR-60 well consists of fine arkoses (Fig. 3A) interbedded with laminated mudrocks of the Pendência Formation (Fig. 2). The main diagenetic features of these sandstones are the early carbonate (calcite and dolomite) cementation, followed by the chloritization of mafic grains (i.e., biotite), quartz overgrowth, and albitization of detrital feldspars. Plagioclase replaced by albite (Fig. 3B) displays high microporosity and represents the main host mineral for

Assessment of uncertainties

Uncertainties in fluid inclusions studies may arrive from: (1) effects from post-entrapment modification; and (2) misrecognition of heterogeneous entrapment. Post-entrapment modifications, also known as fluid inclusion re-equilibration, are processes that often act on fluid inclusions by changing their shapes, volumes and, in some cases, their composition (Goldstein, 2001; Bodnar, 2003). The extent to which post-entrapment phenomena can change fluid inclusion assemblages varies according to the

Conclusions

  • 1.

    A contrasting initial migration timing and overall temperature condition between the Lorena and Ubarana oilfields of the Potiguar Basin are reported by the coupled study of fluid inclusions and one-dimensional basin modeling;

  • 2.

    Fluid inclusion data and basin modeling suggest that the initial filling of the LOR-60 reservoir (Lorena Oilfield) happened during the late rifting in two scenarios: the early entrapment happened either during the basin rift phase in the Barremian–Early Aptian (ca.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors thank Petrobras for the financial and technical support and for the authorization to publish this article. We also thank ANP (Brazilian National Agency of Petroleum, Gas, and Biofuels) for providing access to drill cores and the authorization to use the samples. We are especially grateful to the staff of the Petrobras sedimentology laboratory in Natal, State of Rio Grande do Norte, who helped us with sample handling and photography. Special thanks also to Dr. Elizabete Pedrão

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