Cuvette Central Blocks

In relation to the stretching dynamics, faults develop at the level of the craton: faults with tilted blocks and transfer with, as a result, the presence of several horsts and grabens. The Cuvette Centrale underwent extension in the Upper Proterozoic and was reactivated in the Mesozoic.

A new fault event occurred in the Upper Jurassic. It is during this movement that the period of deposits of bituminous shale of the Kisangani stage is located.

Another fault event took place in the Cretaceous in relation to the system of grabens leading to the rupture of the craton and the opening of the South Atlantic. It was during this period that the bituminous layers of Kipala were deposited.

The Congo craton has recorded the effects of at least 3 orogenic episodes, centred on the mobile belts that surround it. These are the Kibarian, Pan-African and Hercynian cycles.

The superimposition of these cycles results in two families of pre-existing faults in the substratum:

• A family of orientation faults perpendicular to the direction of stretching, replaying as a normal fault, controlling a tectonics of the tilted blocks during the Pan-African episode, and leading to the genesis of collapse grabens with an asymmetrical profile.

• Another family of orientation faults parallel to the same direction (stretching) during the Hercynian episode, defining transfer zones which can be either in translation or in transgression.

In the first case, it is a set of strike-slip faults that controls the genesis of “pull-apart” type grabens oriented North-East;

In the second case, it is a set of faults that led to the genesis of a transfer anticlinorium oriented northwest. The sedimentation reflects the structural evolution with the consequent presence of several horsts and grabens.

Exploration History

The Cuvette Centrale was explored from 1952 to 1986 using geophysics (gravimetry, magnetometry) and by the interpretation of satellite images (Landsat), data from surface geology, seismic (refraction and reflection) , geochemistry, bio-stratigraphy and drilling (exploratory and stratigraphic).

Prospectivity of the Basin

The recent interpretations contained respectively in the reports "Cuvette Centrale Basin geological evolution and petroleum system analysis" of Eni (2011) and "Petroleum system Review of Busira and Lokoro Sub-basins, Congo Central Basin" 2006 and HRT/2007 "High Resolution Technology” (2006 and 2007) gravimetric and magnetometric data from the centrale basin led to the identification of the depocenters represented on the map below.

Geochemical and Geomicrobial Study

The results of the analysis carried out of microbial gases, LTE fluorescence of the soil by SONAHYDROC with the technical support of GMTI confirm the presence of an active petroleum system in the Lokoro sub-basin, and in particular in the areas of Tolo and by Nkolé.

Biogenic methane is present in high concentrations throughout the study area, particularly along the Lukeni River. The composition of Tolo and Nkole thermogenic hydrocarbons varies from dry gas to associated gas and/or oil or condensate.

The results of the microbial gas and sediment analysis confirm the presence of one or more active thermogenic petroleum systems in the Lokoro subbasin. The strongest and largest hydrocarbon microspreading anomalies occur in the Tolo region, mainly north and east of the Lukeni River. The study identified over 6 discrete seepage anomalies extending a distance of 5 km southeast of Tolo.

5 to 6 potentially significant anomalies have been identified northwest and northeast of Ilanga Nkole

Although most samples taken along the Lukeni River contain only biogenic methane, three potentially significant areas of thermogenic hydrocarbon micro-spill have been identified. The smallest is located at the beginning of the sampling along the same river, it is a small anomaly defined by samples L-01 to L-05, the second and largest is about 25-30 kilometers to the northwest of Tolo, along the Lukeni River.

The presence of high microbial values in these two areas provides information on the presence of a thermogenic hydrocarbon micro seep.

The hydrocarbon composition of the migrating gases associated with the Tolo and Ilanga Nkole microscopic anomalies ranges from thermogenic dry gas to gases associated with light oil and/or condensate based on LTE and GeoPac fluorescence data.

Chromatographic analysis of soil extracts from the Tolo and Nkole seeps yielded low concentrations of nC13 to nC30 normal alkanes (i.e. paraffins), thus independently documenting the presence of oil in these samples. Isoprenoid to n-alkane ratios suggest that this oil was generated from source rock rich in algal organic matter and deposited under reducing conditions. There is insufficient data to determine whether the source was deposited in a marine or lake environment.

Petroleum System

Source Rocks

The geochemical analyzes of the source rocks of the Cuvette Centrale were supplemented by studies carried out by the companies JNOC, Robertson Reseach, Amoco and Unocal.

The section explored for possible source rocks includes:

• The Infracambrian in the Gilson and Mbandaka surveys, outcropping in the SW and NE part of the DRC;

• The Permo-Carboniferous in the Dekese borehole (Lukuga formation);

• Jurassic/Cretaceous (Stanley town/Loia) in all boreholes.

After analysis, the source rocks identified are:

• Jurassic/Cretaceous (Stanleyville/Loia) sediments contain type I organic matter. Analysis by JNOC in 1986 gave a TOC of over 8%. Analysis by Robertson Research International Ltd (RRI) gave a TOC of 20%.

• Permo-Carboniferous sediments (Lukuga formation) contain type III organic matter. These sediments contain gas.

The results of TOC analysis indicate: 1.6% at 1,480 m in the Dekese borehole (Amoco, 1984), 1% to 3.08% on samples (Robertson Research International Ltd) and 1.92% on outcrops.

Reservoirs

Little information is available on the properties of the reservoir rocks in the Cuvette Centrale basin. JNOC (1986) analysed various stratigraphic units which outcrop in the vicinity of Kisangani. Good porosity has been measured in Jurassic, Cretaceous and Permo-Carboniferous sandstone.

An average porosity of 30.5% for the Lukuga formation but with low permeability while for the Infracambrian sandstone there is an average porosity of 7%. Unocal also analysed the samples and cores from the Dekese drilling, it turns out that the Lukuga formation has porosity values between 10.5 and 15.9% in its upper part.

In short, the reservoir potential is likely to be best in the upper subgroup deltaic sandstone. The Banalia arkoses were analysed in the boreholes of Samba and Dekese, they were described as very fine grained rocks with authigenic clay and calcite cement.

The effective porosity is largely intergranular ranging from 9 to 13% (permeability from 0.18 to 9.5md). The secondary permeability was created by leaching of the feldspars.

For the Galamboge sandstone, the porosity-permeability measurements were carried out on samples brought back during field campaigns and the results are as follows: the porosity varies on average between 19% and 11.8% and the permeability varies up to 14md. Galamboge sandstone exhibits characteristics of a good reservoir.

Studies conducted in Blocks 7, 8, and 9 of the Cuvette Centrale reported major inversion tectonics suggesting a reactivated accommodation zone that likely enhanced the quality of the clastic reservoir and accentuated fracturing.

Traps and Leads

Structures in tilted blocks, faults and anticlinorium and variations in lithologic facies constitute excellent means of possible trapping.

The reprocessing of the geophysical data made it possible to highlight areas of high prospectivity in several sub-basins of the basin. The work carried out by ECL and HRT made it possible to highlight several Leads on a few blocks of the basin of the Cuvette Centrale basin.

Leads defined by ECL - the work carried out by ECL has highlighted the following 7 leads: