The diagenetic history of the giant Lacq gas field, witness to the apto-albian rifting and the Pyrenean orogeny, revealed by fluid and basin modeling
Introduction
The circulation of diagenetic fluids in petroleum reservoirs can be strongly influenced by the geologic history of the basin (Beaudoin et al., 2014, Beaudoin et al., 2015; Salardon et al., 2017; Renard et al., 2019; Elias Bahnan et al., 2020). In the Aquitaine foreland basin, north of the Pyrenees Mountains (SW France), several hydrocarbon fields are located in Jurassic and Cretaceous reservoirs (Biteau et al., 2006). The giant Lacq field is part of the Lacq petroleum system with productive reservoirs extending over 320 km2, a gas column reaching 3100 m and reserves in the order of 8.9 trillion cubic feet (Biteau et al., 2006). The exploitation of this gas field was a pioneering achievement at the time of its discovery in 1951. The high pressure (675 bar) and temperature (135 °C) with a dangerous gas composition (15,2% H2S, 9,7% CO2; 69,2% C1, 5,3% C2+) (Le-vot et al., 1996) made it necessary to build an innovative desulfurization plant in 1957, still operational till present day.
The anticlinal structure of Deep Lacq is the result of the superposition of Jurassic rifting and Early Cretaceous rifting events, later inverted during the Late Cretaceous and the Paleogene (Sibuet et al., 2004; Jammes et al, 2009, 2010; Clerc and Lagabrielle, 2014; Mouthereau et al., 2014; Corre et al., 2016; Teixell et al., 2016; Vacherat et al., 2016). These events had significant control on the types, sources, chemistries and circulation mechanisms of the diagenetic fluids in the Aquitaine basin, as previously documented in the Rousse field (Renard et al., 2019), in the Upper Lacq oil reservoir (Elias Bahnan et al., 2020) and in the Chaînons Béarnais (Incerpi et al., 2020; Motte et al., 2021; Salardon et al., 2017). However, despite its importance as the most strategic reservoir in France during the 1970s (Biteau et al., 2006), no models were yet proposed to explain the diagenesis of Deep Lacq.
This study focuses on the Kimmeridigian-Barremian Deep Lacq gas field and aims to present a diagenetic model describing the different types, sources and circulation schemes of diagenetic fluids at different phases of the complex geological history that the Aquitaine basin has experienced. The diagenetic events described in this research are placed in a geodynamic perspective that allows a better understanding of fluid activity during a complete Wilson cycle. Starting from a tectonic quiescence state and proceeding to early rifting, hyperextension, post-rift subsidence and ending with orogenesis, fluid-rock interactions are examined at each step. The case study presented in this work can be used as an example to compare diagenetic models of petroleum reservoirs in basins that experienced a similar geodynamic history as the Aquitaine. Examples include the Dengying reservoirs of the SW Sichuan basin (Wu et al., 2016), the Jurassic carbonates of Mount Lebanon (Nader, 2003), the Mobile Bay Jurassic Norphlet Formation of the Gulf of Mexico (Mankiewicz et al., 2009), the Níspero deeply buried Lower Cretaceous carbonate reservoir in the Gulf of Mexico (Bourdet et al., 2010), the Devonian Southesk-Cairn carbonate complex in Alberta, Canada (Machel and Buschkuehle, 2008), etc … The significance of this work is the use of multi-scale modeling approaches to extrapolate μm-scale fluid inclusions to basin-scale geodynamic events of the Aquitaine basin.
Section snippets
Geology and stratigraphy
The Deep Lacq reservoir is part of the Lacq petroleum system in the Arzacq sub-basin, south of the Aquitaine basin (Fig. 1). The Upper Lacq reservoir (Elias Bahnan et al., 2020) is separated from Deep Lacq by the Sainte Suzanne Marls. Since the target of this work is Deep Lacq, only the Jurassic to Early Cretaceous formations will be discussed. The genesis of the Aquitaine basin and the complete stratigraphic column of Lacq have been extensively described and detailed in a previous work (Elias
Sampling
The studied core samples of Deep Lacq were obtained from wells LA-101 (Latitude: 43.420648°; longitude: −0.619980°) and LA-104 (Latitude: 43.420421°; longitude: −0.638811°) (Fig. 1, Fig. 3). Additional public data on these wells can be found at the French national geological survey website (BRGM: infoterre. brgm.fr). Following the same strategy as that described by Elias Bahnan et al. (2020) for Upper Lacq, these wells were selected after a preliminary evaluation of nearly 600 thin sections
Basin modeling
The geodynamic history of the Lacq petroleum system was modeled using Petromod 1D software. A synthetic well combining the entire stratigraphy of wells LA-101 and LA-104 was prepared from drilling reports and used as the main input source for the model. Boundary conditions were set by using the paleowater depths (PWD), sediment-water interface temperatures (SWIT) and basal heat flow (HF) as discussed by Elias Bahnan et al. (2020). Vitrinite data were collected from TOTAL archives and were used
Petrography of diagenetic phases
Precursor limestones consist of chalky mudstones, wackestones to bioclastic packstones (Fig. 4-A and B). Bioclastic material consists of microforaminifera, shells of mollusks, sponges, echinoderms and annelids, with sizes reaching up to 2 mm.
Fluid modeling
Fig. 9 gives strong petrographic evidence of primary aqueous and oil inclusions entrapment in the studied phases. The location of fluids between growth zones or in clusters limited to individual crystals indicate a primary origin of the inclusions. Also, microthermometry results provide additional supporting evidence that can complement petrographic observations. Fig. 10 shows different homogenization temperatures and salinities between the different cement phases. This difference indicates
Conclusions
This work highlights the power of applying a multi-modeling approach to reveal the diagenetic history in a complex geodynamic setting. After a detailed petrographic analysis of all the phases present, the multitude of equations of states used in AIT-PIT modeling provided valuable insight into the fluids P-T conditions. These in turn were then validated against the basin history to place age constraints on the timing of fluids circulation.
The diagenesis of the giant Deep Lacq reservoir has been
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
This work was funded by TOTAL EP-R&D and the Center de Recherches sur la Géologie des Matières Premières Minérales et Energétiques (CREGU), contract number FR00008500 - CREGU/T25. Sylvain Calassou and the colleagues from the Center Scientifique et Technique Jean Féger de TOTAL (CSTJF-TOTAL) are warmly thanked for providing access to their facilities and data archives to conduct this research. Olivier Fonta from GEOPETROL is thanked for providing access to the core samples. Pierre Cartigny from
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2023, Marine and Petroleum GeologyCitation Excerpt :Although these efforts, the failure of salt rock integrity has rarely been reported with direct evidence to explain the exact geological conditions (pressure, temperature, time) when ancient fluids did breakthrough. To this end, fluid inclusions PVTX modeling has been proven to provide useful information of ancient hydrocarbons such as their compositions, temperature, pressure and the charging time, in several basins worldwide (Kelly et al., 2000; Goldstein, 2001; Dutkiewicz et al., 2004; Parnell, 2010; Volk and George, 2019; Bahnan et al., 2020, 2021; Lima et al., 2020; Su et al., 2022). Collective of these studies explains how information could be obtained from the fluorescence colors, homogenization temperatures, salinities of fluid inclusions, and how modeling tools can be acquired to obtain their internal pressure, trapping time and temperature.
Quantitative diagenesis: Methods for studying the evolution of the physical properties of tight carbonate reservoir rocks
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