Evidences of early to late fluid migration from an upper Miocene turbiditic channel revealed by 3D seismic coupled to geochemical sampling within seafloor pockmarks, Lower Congo Basin

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Abstract

Using high quality 3D seismic data within the Lower Congo Basin (LCB), we have identified pockmarks that are aligned above the sinuous belt of a buried turbiditic palaeo-channel, 1000 m beneath the seafloor. Geochemical analyses on cores (GC traces), taken in the centre of four of these pockmarks along this channel, show no clear evidence for migrated oil. But, some features of the GC traces, including elevated baselines (UCM>34 μg/g) and a broad molecular weight range of n-alkanes with little odd–even preference, may be interpreted as indicating the presence of thermogenic hydrocarbons in the cores.

Seismic profiles show that these pockmarks developed above two main features representative of pore fluid escape during early compaction: (1) closely spaced normal faults affecting the upper 0–800 ms TWT of the sedimentary column. This highly faulted interval (HFI) appears as a hexagonal network in plane view, which is characteristic of a volumetrical contraction of sediments in response to pore fluid escape. (2) Buried palaeo-pockmarks and their underlying chimneys seem to be rooted at the channel–levee interface. The chimneys developed during early stages of burial and are now connected to the HFI.

This study shows that the buried turbiditic channel now concentrates thermogenic fluids that can migrate through early chimneys and polygonal faults to reach the seafloor within some pockmarks. Using a multidisciplinary approach within the Lower Congo Basin, combining 3D seismic data and geochemical analyses on cores, we trace the fluid history from early compaction expelling pore fluids to later migration of thermogenic hydrocarbons.

Introduction

Gases of thermal origin in near-surface sediments are believed to have been generated at greater depth and migrated to the surface (Leythaeuser et al., 1982, Leythaeuser et al., 2000). Detection of near-surface thermal hydrocarbons could obviously be of economic importance because it offers the possibility of direct geochemical hydrocarbon prospecting and exploration (Wenger and Isaksen, 2002; Abrams, 1992, Horvitz and Ma, 1988, Faber and Stahl, 1984, Horvitz, 1972, Horvitz, 1978). The origin of hydrocarbons in shallow sediments and the applicability of geochemical surface data in petroleum prospecting is still controversial due to difficulties in data interpretation and in the principal understanding of the effects of biodegradation and vertical migration of hydrocarbons from deep source rocks to the surface (Faber et al., 1998, Hunt, 1990, Fuex, 1977).

Due to the low matrix permeability of argillaceous mudstone, fluid flow through the sedimentary column is quite slow and diffusive but it is compensated for by long Ma timescale. However, active gas venting is clearly controlled by subsurface structures such as faults and faulted anticlines (Eichhubl et al., 2000). Evidence of focused fluid flow through the sedimentary column is seen (1) on the surface by pockmarks that are consistently located above faults of a polygonal fault interval (Gay et al., 2004) and (2) in the sedimentary column by seismic chimneys that are indicative of deeper reservoirs (Gay et al., 2003, Heggland, 1998). Seeps associated with diapiric features in offshore West Africa are often abundant and are often very large (visibly oil-stained sediments, very high petroleum concentrations), actively migrating, associated with high concentrations of gas with a thermogenic component and sometimes supporting oil slicks on the sea surface (Wenger and Isaksen, 2002). Even when seeps are authenticated, their presence does not prove economic hydrocarbon accumulations at depth, seal failure or gas displacement from reservoirs (Wenger and Isaksen, 2002). In the Lower Congo Basin (LCB), we have identified pockmarks related to a deep buried turbiditic channel (i.e. upper Miocene). Using a multidisciplinary approach, combining 3D seismic data and geochemical analyses on shallow sediments (Fig. 1), we trace the fluid history from early compaction expelling pore fluids to later migration of deep thermogenic hydrocarbons from the upper Miocene turbiditic channel. Although our examples are specific to the LCB, it is hoped that the dynamical model proposed may find applicability in other basins characterized by similar post-rift stratigraphy.

Section snippets

Data base, sample selection and analyses

This study was primarily based on 3D seismic datasets from the Lower Congo Basin (LCB) acquired by the TOTAL oil company (Fig. 1). The selected 3D-dataset covers an area of 4150 km2 with a line spacing of 12.5 m, a CDP distance of 12.5 m and a vertical resolution of 4 ms. The data were loaded on a workstation and interpreted using the SISMAGE software developed by TOTAL. The bathymetric and reflectivity maps were acquired with a Simrad EM12 dual multibeam. Complementary data collected more recently

Geological settings

The Lower Congo Basin is one of the numerous sub-basins that developed during the opening of the West African passive margin in the early Cretaceous (130 Ma) (Marton et al., 2000; Jansen et al., 1984a, Jansen et al., 1984b). Following the deposition of thick evaporites during mid-Aptian time (Karner et al., 1997), the margin developed in three phases between the late Cretaceous and the Present (Seranne et al., 1992) (Fig. 2):

  • (1)

    From late Cretaceous to early Oligocene. This period was characterized

Organisation of seafloor pockmarks

In the southern part of the study area, the seafloor is characterized by small circular pockmarks, ranging from 100 to 300 m in diameter, and from a few meters to a maximum of 20 m in depth (Fig. 3). These pockmarks appear as small patches on the reflectivity map. These could be due to the presence of carbonate concretions and chemosynthetic communities within the depression (Gay et al., 2006). Although pockmarks seem unorganised, they are aligned along a band, 5–10 km wide, oriented E–W (Fig. 3).

Cartography of the upper Miocene turbiditic channel

Due to the discontinuous character of turbiditic channels and infills, automatic picking of sand bodies is difficult within these intervals. Based on the amplitude of reflectors and their continuity, the SISMAGE software developed by TOTAL allows the calculation of the ‘Chaotism’ amplitude from a 3D seismic block, using a running window semblance-based algorithm. ‘Chaotism’ sliced in time or draped on a surface helps define faults and stratigraphy by looking for discontinuities or boundaries

Role of polygonal faults

The representative seismic section EF (Fig. 6) shows the seismic expression of a polygonal fault interval in the first 0–800 ms TWT below seafloor. It is characterized by numerous closely spaced normal faults, which have small offsets (5–30 m) and an average spacing of 100–500 m (Gay et al., 2004). A dip map of a horizon located in the northern part of the study area discloses a dense fault network with polygonal pattern in plane view (Fig. 6). The polygons size ranges from 1 to 3 km and they share

Early formation of conduits

The seismic profile GH (Fig. 7) across a pockmark (core Cb location) shows two seismic chimneys branching on both sides of an upper Miocene channel (see Fig. 3 for location). Their bases are located at the channel–levee interface, where they seem to take root because of the lack of any deeper seismic anomaly. At about 250 m above the turbiditic channel, the seismic chimneys end abruptly at the same time: they are covered by circular depressions corresponding to palaeo-pockmarks, now sealed by

Geochemical analyses on cores within isolated pockmarks

In addition to possible biodegradation in the reservoir, hydrocarbons are known to be oxidized by bacteria in the first 10 m below seafloor, in the aerobic zone and in the sulphate-reducing interval (Wenger and Isaksen, 2002, Devol and Ahmed, 1981, Bernard et al., 1978, Claypool and Kaplan, 1974). It is thus important to understand the processes controlling the distribution of migrated and in situ hydrocarbons in surficial sediments to detect anomalous migrated hydrocarbons. A number of

Discussion

In the area of isolated pockmarks, there is a close relationship between pockmarks, underlying chimneys, upward deflection of the BSR (if present) and an upper Miocene turbiditic channel. In the Lower Congo Basin, turbidite sands are associated with three main stages of deposition (Anderson et al., 2000): Burdigalian (17.5–15.5 Ma), Tortonian (8.2–6.3 Ma) and Messinian (6.3–5.5 Ma). The stacking patterns of the upper Miocene channels comprise fine to medium grained massive sands overlain by a

Conclusion

This detailed study of pockmarks within the Lower Congo Basin shows that the fluid migration history is complex due to the inter-relation of tectonic features and sedimentary bodies. We have shown that seafloor pockmarks, which seem isolated are most often related to an underlying upper Miocene turbiditic channel. Geochemical analyses conducted on four cores taken within pockmarks have led to the conclusion that fluids that are expelled on the present day seafloor are a mix of biogenic and

Acknowledgements

The authors would like to express their gratitude to TOTAL for their financial support, data and for supporting the publication of this work. We gratefully acknowledge the IFREMER team for the time and care it has taken in reviewing this paper.

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      Aligned type I pockmarks commonly follow the maximal flexural syncline axis of the minibasins (No. 1 in Fig. 1a; Andresen et al., 2011) or occur as sinuous trends marking the ancient pathways of the Congo canyon (No. 2; Gay et al., 2003). A biogenic origin is also suspected for the giant (∼2000m diameter) Regab pockmark, located in the deep abyssal plain at a water depth of 3150 m (No. 3; Gay et al., 2006a). The origin of the gas expelled at the salt front remains undetermined (No. 4; Wenau et al., 2014).

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