AAPG HEDBERG CONFERENCE 2-5 July, 2018. Sigüenza. Spain 1
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AAPG HEDBERG CONFERENCE 2-5 July, 2018. Sigüenza. Spain Kenya´s world class lacustrine source rock: A geochemical analysis and proven petroleum system Anderson, L1,*, Grimmer, J.1,*, Cajas, L.1, Rodrigo, G.1, Gómez, R.1, Cuñado, E.1, García, M.F.2 1 CEPSA E&P S.A. Torre Cepsa, Paseo de la Castellana 259A, 28046 Madrid, Spain. 2 CEPSA S.A.U. Centro de Investigación, Avenida Punto Com 1, 28805 Alcalá de Henares, Spain. * Corresponding author: [email protected], [email protected] Introduction Until Cepsa drilled Tarach-1 (2016) in Block 11A, Lotikipi Basin NW Kenya, the presence of a proven Tertiary source rock was restricted to the Tullow operated Lokichar Basin blocks some 200 kms southeast of Cepsa´s area of interest (figure 1). The prolific lacustrine petroleum system of the Lokichar Basin has now yielded around 1 BBO recoverable resources. Figure 1. Block 11A – Regional Setting East Africa Rift System Block 11A is located at the junction of the N-S Tertiary East Africa Rift System (EARS) and the NW- SE trending Cretaceous Rift System (CARS) (figure 1). The existence of both Tertiary and Cretaceous rift basins should not be ruled out, the lack of stratigraphic control in the immediate area should not exclude the possibility of more than one rifting event. The EARS is composed of two rift trends called the Eastern and the Western branches. The Eastern branch is located North and East of the Lake Victoria and corresponds to the volcanic system that forms the Kenya and the Ethiopian Rifts. In some places there are outcrops of Plio-Pleistocene deposits, dominantly fluvio-lacustrine sediments. However outcrops of older sedimentary sequences are rare, in the area of interest, Block 11A no sedimentary formations have been identified. 1 AAPG HEDBERG CONFERENCE 2-5 July, 2018. Sigüenza. Spain The main surface feature of the Block 11A is the Lotikipi Plain. This broad depression measures approximately 110 km from East to West. The Lotikipi Basin has been interpreted as either a thermal sag basin or a rift basin by Wescott et al. in 1999. Gravity data, acquired in the area by Cepsa in 2013, identified a sedimentary basin within the Block 11A area below the Lotikipi Plain. Using neighbouring basin evolution as an analogue the sedimentary fill was inferred to be a series of Miocene-age, stacked fluvial deltaic sandstones and lacustrine deposits, forming a sedimentary package of more than 3000m thickness. The likelihood of a deeper Cretaceous sequence should not be discounted. The Tarach-1 well was a frontier exploration wildcat and was located in the deepest part of the basin (based on initial gravity modelling), in a location best considered to test the presence of a working petroleum system and to provide stratigraphic control of the area. The well spudded on the 14th April 2016 and reached the prognosed 3010m TD on 11th June 2016 (59 days). The well penetrated an intercalation of thick layers of volcanic rocks (dominant volcanic/volcaniclastic sequence composed of basaltic, andesites and trachytes) interbedded with shales and siltstones. The sequence drilled by the well is Late Oligocene to Middle Miocene based on biostratigraphy. The well did not encounter reservoir facies, however it did encounter two excellent organic-rich shale intervals. Gas Shows The most important gas show was reported by gas-while-drilling in a shale interval from 1270 to 1325m MD with up to 0.16% total gas (TG) and the methane (C1) concentration exceeded 1000 ppm. Even ethane (C2) to pentane (C5) presence was registered. Wellsite geology also reported the presence of very dull hydrocarbons shows in this interval. A second shale interval contained less prominent gas shows, but again with the presence of C1 to C5. Isotope analyses of these gases indicated a thermogenic and oil associated gas type. Source Rock Rock Eval analyses revealed the presence of at least two organic-rich shale units (1270-1325 m MD and 1475-1505 m MD) with excellent source rock potential and kerogen quality (table 1). The depositional environment is lacustrine and indicative for a Type I source rock, which is strongly supported by biomarker analyses. TOC Depth (m MD) S2 (mg/g) HI (mgHC/g TOC) Tmax (ºC) (%) 1280 5.8 37 634 445 1480 3.6 30 846 444 Table 1. Rock Eval analyses indicate the presence of excellent source rocks. Organic petrography supported these analyses. The lacustrine depositional setting of the source rocks is demonstrated by abundant occurrence of Botryococcus (b) and Pediastrum (figure 2a). Botryococcus is a colonial algae typical of freshwater-to-brackish environments with intense yellow fluorescence in the samples. Even oil inclusions have been observed in the lower source layer (figure 2b). 2 AAPG HEDBERG CONFERENCE 2-5 July, 2018. Sigüenza. Spain Figure 2a. Botryococcus and Pediastrum identified in the source rocks of Tarach-1. Figure 2b. Oil inclusions are present in samples of the lower source rock layer. The maturity of the two presented source layers ranges from immature for the upper to oil-mature for the lower unit. This is supported by vitrinite reflectance (VR), homohopane (H) biomarkers and the analysed gas isotopes. Tmax data indicate a higher maturity, which seems to be less likely. 3D petroleum systems modelling calibrated with VR, H, gas isotopes and well temperatures shows that the oil kitchen very likely sits in a deeper setting further to the southwest of the well location. Conclusions The Tarach-1 well has provided the first and only stratigraphic control in Block 11A; and has confirmed the existence of a sedimentary fill in the basin (organic rich shales). This sedimentary section is interbedded with a dominant volcanic/volcaniclastic sequence composed of basaltic, andesites and trachytes. Most of the sequence drilled by the well is Late Oligocene to Middle Miocene, in age and suggests that lakes were developed at several levels and were subjected to a number of strong volcanic episodes. Reservoir facies have not been drilled. However, two excellent organic-rich shale intervals at 1270-1325 m MD and 1475-1505 m MD have been discovered. The upper unit had the most important gas shows. The lower unit contained oil inclusions. These intervals are world-class lacustrine source rocks (Type I) and are believed to be equivalent in age to the Lokhone Shale found in the Lokichar Basin to the southeast. These units are immature to oil mature at the well location. The presence of these source rocks and the resulting analyses indicate a working petroleum system in the Lotikipi Basin with certain exploration potential. References Wescott, W.A., S.T. Wigger, D.M Stone, and C.K. Morley, 1999, Geology & Geophysics of the Lotikipi Plain, in C.K. Morley ed., Geoscience of Rifts Systems-Evolution of East Africa: AAPG Studies in Geology No. 44, p. 55-65. 3 AAPG HEDBERG CONFERENCE 2-5 July, 2018. Sigüenza. Spain Coniacian-Santonian organic-rich Loma Chumico Formation of N-Costa Rica: An expression of Oceanic Anoxic Event 3? Andjic, G., Baumgartner, P. O.*, Baumgartner-Mora, C. Institute of Earth Sciences, University of Lausanne, Géopolis, CH1015 Lausanne, Switzerland. * Corresponding author: [email protected] Introduction The Loma Chumico Formation (LCF) of N-Costa Rica (Fig.1), long known for its hydrocarbon potential (Erlich et al. 1996), has been considered by many as of Albian age, based on an single Ammonite determination (Azéma et al. 1979). Hence, this formation has been considered as a prime source rock formed during OAE 1 (Astorga 1987). Later, first radiolarian dating (Erlich et al. 1996) suggested a younger, poorly defined Cenomanian to Campanian age for this formation, more compatible with the age of the underlying basement. Our systematic radiolarian research in both organic-rich shales (Loma Chumico Facies) and volcanogenic green chert and siliceous mudstones (Berrugate Facies, Andjic et al. 2018) revealed identical, well-preserved radiolarian assemblages (fig.2) of Coniacian- Santonian age for the lower part of the formation. This age is in agreement with Ar/Ar ages of igneous rocks measured in the underlying basement re-defined by us as the Manzanillo Terrane (Andjic et al. 2018). Geologic and plate tectonic setting The Nicoya Peninsula in N- Costa Rica exposes part of a complex puzzle of terranes that are set between the Mesquito Composite Oceanic Terrane (Baumgartner et al. 2008) to the N and the main body of the Caribbean Large Igneous Province (CLIP) to the S (Fig. 1b). Until now, the basements of the whole Nicoya Peninsula were comprised with the Nicoya Complex (Denyer & Baumgartner 2006, Denyer et al. 2014). In our recent work (Bandini et al. 2008) we proposed a subdivision into different terranes which has been consolidated in Andjic et al. (2018). The Nicoya Complex s. str. (sensu Bandini et al. 2008) comprises only the NW-part of the Nicoya Peninsula (Fig 1 c). It is characterized by multiple plateau basalt and intrusive events that can be grouped into three phases of plateau volcanism (Ar/Ar ages: 139–132 Ma, 111 Ma and 95–83 Ma). The youngest phase only (95–83 Ma) is related to the CLIP main pulse (Sinton et al. 1997; Hoernle et al. 2004). 100 m to km sized blocks of ribbon-bedded radiolarite are incorporated into the igneous rocks, and chilled margins indicate generally an older age of radiolarites with respect to encasing igneous rocks. Radiolarian biochronology indicates ages ranging from Bajocian/Bathonian (±168 Ma) to Coniacian- Santonian (±85 Ma, Baumgartner 1984, Denyer & Baumgartner 2006, Baumgartner et al. 2008). No pre-Campanian arc-derived detrital material has been observed in this unit. Unconformable, middle Campanian overlap shallow water and off-shore carbonates encroach directly on the Nicoya Complex s.