Hydrocarbon Prospects Under Lake Turkana Basin, NW- Kenya: Based on Geophysical and Geochemical Data Investigations

Hydrocarbon Prospects under Lake Turkana Basin, NW- Kenya: Based on Geophysical and Geochemical Data Investigations

Bernard Kipsang Rop

Accepted 8 August,2015

Department of Mining, Materials and Petroleum Engineering, JKUAT, P.O. Box 62000-00200, Nairobi, Kenya. E-mail: [email protected].

ABSTRACT

This study attempts to unravel subsurface structural features and further creates a better understanding of sedimentary rock sequences, believed to belong to Cretaceous-Tertiary age, which have been deposited under the vast Lake Turkana Basin (NW Kenya). The gravity anomaly maps and seismic profiles revealed the presence of several horst- and graben-like structural systems. It was also revealed that the basin could have attracted potential petroliferous sedimentary piles (? 2000 to 5200 m thick) which are deposited on basement rocks of Precambrian age. The stupendous outpouring of the lava flows has hidden under them a large terrain of older rocks comprising the Precambrian basement rocks and sediments filling the asymmetric rift basins. Thus direct observations on the possible Mesozoic and Tertiary sedimentary sequences are not possible, apart from few drilled Wells to the southwest and southeast of Lake Turkana basin (LT-1, LT-2 and C1, C2, C3, respectively). Some selective geochemical analysis of rock samples in Loperot-1 Well (denoted as LT-1 Well) for their total organic carbon (TOC) and other sedimentological parameters were useful in this research work. Understanding the subsurface structures and depositional environments conducive for hydrocarbon generation and trapping is essential as it forms the basis for exploration. The organic matter richness, facies type and degree of thermal alteration and/or maturation are factors useful in evaluating the potential source rocks. Rock-Eval pyrolysis was employed on selected rock samples with high TOC for quantification of data, and this would help in precise identification of phases of hydrocarbon generation.

Key words: Lake Turkana, Lokichar, Chalbi, sedimentary, hydrocarbon, petroliferous, basement, gravity, seismic and source rocks.

INTRODUCTION

Lake Turkana Basin lies between longitudes 35o45‟E and active and have also affected the Upper Oligocene-Lower 36 o48‟E and latitudes 2o25‟N and 4o38‟N. The basin was Miocene to Pliocene volcanics, forming the highlands on initiated as an asymmetric graben bounded by the N-S both the eastern and western side of the Basin (Figure 2). fault systems during the Tertiary time. At the centre of the basin lies Lake Turkana (formerly Lake Rudolf), one of GEOLOGICAL SETTING the largest lakes in the eastern arm of the East African Rift system (Figure 1). The Lake Turkana Basin forms a north-south broad depressional Horst-graben feature with The geology of Lake Turkana Basin consists of the an alkaline lake water. It is bounded both to the east and Precambrian basement rocks and the Tertiary volcanics to the west by the north-south trending fault systems of that have covered subsurface sedimentary deposits, the Kenya Rift (Tertiary). These fault systems are the which are now considered to be a potential petroliferous result of the rifting mechanism, which began in Late block in Northern Kenya (Rop, 2003). The basin is known Oligocene to Early Miocene time and later gradually to have evolved through extension tectonics that brought progressed eastwards with time to trend NNE-SSW out continental rifting as a part of the major (Barker et al., 1972; Rop, 2002, 2013; Wescott et al., Gondwanaland breakup in the Late Paleozoic time and 1993). Many of these major basinal faults have been continued in the Mesozoic and Tertiary (Figure 2). The Rop 42

Figure 1. General Geological map showing present study area.

Figure 2. Physiographical and Geological map of the study area showing major fault system.

region underwent uplift and subsidence, intermittently, where oil discovery has been made (Schull, 1988). The along major boundary faults even in the Miocene period. basin and its sub-basins continued as sites of deposition These movements were accompanied by the stupendous and extension also during the Late Tertiary and even in outpouring of the lava flows. The Mounts Kulal-Porr-Moiti Quaternary period. Sedimentary rock sequences and Loriu-Lothidok-Murua Rith volcanoes (Figure 2), belonging to (?) Cretaceous-Tertiary age may have been which erupted during the Quaternary, form the eastern deposited under the vast and little known Lake Turkana and western shoulders of the Lake Turkana basin, Basin (Rop, 2003, 2013). respectively. The Chalbi basin (Cretaceous), which forms a part of the Great North Anza Rift, is located some 40 km to the east of Lake Turkana. It is assumed that the PHYSIOGRAPHY OF LAKE TURKANA BASIN Anza Graben discontinuously extends northwestwards across (under) the Lake Turkana and Lotikipi basins The physiographical features such as the basement (Figure 3) into the Abu Gabra Rift of southern Sudan, ranges and inliers, Tertiary volcanic plateaus, Lake Res. J. Eng. Technol. 43

Figure 3. Map Showing Drilled Wells in Northwestern Kenya Rifts Basins.

Turkana and lowlands with alluvial plains as well as to have developed tectonically during the Early Miocene- drainage systems are present in the study area (Figure Pliocene time, with major rifting of Pliocene age forming 2). The entire area is volcanically and seismically active, the north-south trending marginal faults on the even throughout the Quaternary period, shown by the northwestern and central parts of the lake (Ochieng et al., thick alluvial cover concealing the entire eroded surface 1988; Wilkinson, 1988). of the older sedimentary sequences as well as the It is bounded by the major N-S trending normal faults and basement rocks (Johnston et al., 1987;Barker et al., the Kenyan and Ethiopian domes abutting against the 1972; Rop, 2002, 2013). The landscape consisting of flat northern end of the Gregory Rift Valley of Kenya, the alluvial plains and high plateaus and ranges intervening entire depression between these two domes is rift- them indicates the control of block faulting. The drainage related; the rifting strongly influenced also the follows most recent strikes of faulting, north-south sedimentation history within this region. The Lake direction, but the tilts are asymmetric giving rise to rivers Turkana itself is said to be a part of the half-graben flowing in opposite directions. structure with intermittent structural highs, deepening progressively to the north because of tectonic subsidence (Johnson et al., 1987). Dunkelman (1986) suggested that Lake Turkana, Alluvial Plains and Drainage Systems the Lake Turkana is intersected obliquely also by some pre-rift structures of the North Anza Basin, which is a The Lake Turkana is the largest lake in the eastern arm NW-SE striking basin filled with Cretaceous sediments, of the East African Rift Valley. It is a tectonically predicted to be 2500 m to 4000 m thick. The flat lowlands controlled lake occurring centrally in the present study around the lake (for example, Koobi Fora to the area. About 240 km long and 50 km wide, it has a northwest) contain Quaternary sediments of Plio- maximum depth of 120 m (Johnson et al., 1987). The Pleistocene age (Brown and Feibel, 1986; Wilkinson, water level in the lake fluctuates with the variations in 1988; Ochieng et al., 1988). These areas are distinctly rainfall and the evaporation rates. The lake is fed by the transected by the east-west flowing small rivulets. These Omo River (draining the Ethiopian highlands) in the north rivers do not follow the main north-south drainage trend and the perennial Turkwell-Kerio river systems draining of this physiographic region. The low-lying alluvial areas the western part of the present study area (Figure 2). with Plio-Pleistocene sediments occur intermittently These rivers, which predominantly flow southwards and amongst the Tertiary volcanic plateaus and the basement northwards into the lake trace the north-south-trending ranges. The rivers flowing through these lowlands have major faults. The environment being very hot and arid, sudden major bends that are also tectonically controlled. the lake has neither a surface outlet nor a subsurface The regional rifting and faulting systems have imposed drainage. The water is alkaline (pH 9.2), moderately restraint upon the courses of the rivers. The lowlands are saline, with total dissolved solids of about 2500 parts per more dominantly seen along sections drawn at 3oN and million (Yuretich, 1986). The Lake Turkana is supposed 3o30‟N latitudes (Figure 2). Further south at 2o30‟N Rop 44

section line there are no mounts of very high altitudes like SUBSURFACE STRUCTURES AND TECTONICS Mount Kulal. UNDER LAKE TURKANA The terrain is low but highly undulatory. On the whole, the steep escarpments are seen along the northernmost The Lake Turkana Basin lies along the Kenya Rif Valley, section and on both sides of Mount Kulal along the which forms the eastern arm of the East African Rift section at 3oN latitude. The Turkwell and Kerio rivers, system (Figure 1). The rifting period in the Turkana basin draining the predominantly metamorphosed basement occurred between the Late Mesozoic to Early Tertiary source area, subsequently converge and form a delta on during which the sub-basins that formed extended in the the central southwestern shores of Lake Turkana. The north-south directions; the rifting being mostly in the east- Precambrian basement rocks are sparsely exposed as west direction. During this rifting, there was stupendous inliers and ranges, particularly at the Lapurr Ranges to outpouring of basaltic lavas which extensively covered the northwest, Kajong-Porr area to the southeast and at the entire northern Kenya (Figure 3). These lavas have the north Loriu Plateau to the southwestern shores of hidden under them a large terrain of older rocks Lake Turkana (Figure 2). These basement rocks are comprising the Precambrian basement rocks and probably extension of the Precambrian rocks of Samburu sediments filled in the rift basins (Rop, 2002; 2003). Thus gneissic groups to the south of the lake (Key et al., 1987). direct observations on the possible Mesozoic and Tertiary sedimentary sequences are not possible apart from few drilled wells (Figure 3) to the southwest and southeast of The Lapurr Range Sedimentary Sequences Lake Turkana basin (LT-1, LT-2 and C1, C2, C3, respectively). The Lake Turkana basin forms a part of the The basement rocks in the Lapurr Ranges (the „type‟ Early Oligocene-Miocene rifting. The Chalbi basin to the section for the surface geology in the present study) are east of Lake Turkana and the Lotikipi basin to the west of seen to be unconformably overlain by sediments initially the lake are not a part of this Tertiary rifting. They belong termed “Turkana Grits” by earlier workers. This term to the Late Paleozoic rifting (Key et al., 1989; Winn et al., denotes a generally coarse-grained siliciclastic sediments 1993) and are supposed to extend subsurface in the NW between the basement and the Tertiary volanics. direction towards Sudan, where oil discoveries have been Thesesedimentary sequences have yielded wood and reported (Schull, 1988). dinosaurian fossil remains (Savage and Williamson, The study of intracontinental rifting and its associated 1986; McJGuire and Serra, 1985) in the Lapurr Ranges, subsurface structural features caused by extensional to the immediate northwest of the Lake Turkana (Figure tectonic stresses is critical for effective hydrocarbon 2). These have been described as continental fluvial- exploration province (Rop, 2013; Harding, 1984). It is lacustrine and fluvial-deltaic deposits (up to 600 m thick) proposed therefore to examine in this paper the tectonic consisting of fine- to coarse-grained sandstones, pebble- structure of the Lake Turkana subsurface basin in the to cobble-conglomerates and siltstones or shales light of basement depth contours and gravity anomalies interbedded between the sandstones and conglomerates. from BEICIP, 1987 regional structural contour maps They have been designated as the Lapurr Range (Figures 4 and 5). The gravity anomaly maps and seismic Formation (Rop, 2003, 2013). profiles were most useful for the interpretations of The sediments to the east of Lake Turkana Basin subsurface structural features, thus unravelling the (around Kajong-Porr area) have been described as depositional province of the (? 2000 to 5200 m thick) admixtures of both siliciclastic and volcaniclastic strata sedimentary rock sequences, believed to belong to (?) termed as Sera Iltomia and Kajong Formations, Cretaceous-Tertiary age, which have been deposited in respectively (Savage and Williams, 1986; Rop, 1990, the vast Lake Turkana. The gravity values both to the 2013; Wescott et al., 1993). east and west of Lake Turkana basin are mainly positive, The siliciclastic Sera Iltomia sediments are believed to be to the order of 20 to 40 mgals (Figure 4). Over the Koobi of Upper Cretaceous (?) to Miocene in age, while the Fora and Moiti areas, to the east of the basin, the gravity Kajong sediments and volcanics are of Miocene age. The are positive (+40 and +20 mgals, respectively). Some voluminous basalts (Upper Oligocene-Miocene to negative gravity anomalies are seen in areas around the Pliocene) overlie the Lapurr Range and the Kajong-Porr North Island (-90 mgal) within the lake, Eliye Springs (-70 sediments and, in turn, are overlain by the post-volcanic mgal) to the southwest of the lake, and South Island (-70 sediments of Plio-Pleistocene to Holocene age, mgal) within the south part of the lake. particularly in the horst-graben like Koobi Fora sub-basin to the northwest of Lake Turkana Basin. The depositional environments within the margins of Lake Turkana basin Gravity Cross-Section along 4oN Latitude provided suitable life conditions for a prolific vertebrate fauna during the Tertiary and Quaternary times resulting The strike of the Bouguer gravity anomaly contours is in high potential for preservation in the hominids fossil mostly north-south. A cross-section along latitude 4oN records (Brown and Feibel, 1986; Harris et. al., 1988). (Figures 2, 4 and 6) through Lake Turkana basin reveals Res. J. Eng. Technol. 45

E T H I O P I A

Fig. 4: Bouguer gravity contour map over the Lake Turkana basin (Adopted from BEICIP- FigureMinistry of 4. Energy Bouguer Geological gravity Map, 1987) contour map over the Lake Turkana basin (Adopted from BEICIP-Ministry of Energy Geological Map, 1987). Map, 1987)

E T H I O P I A

Fig. 5: Basement contour map showing basement depths in Lake Turkana (Adopted from BEICIP-Ministry of EnergyFigure Geological 5. BasementMap, 1987) contour map showing basement depths in

Lake Turkana (Adopted from BEICIP-Ministry of Energy Geological Map, 1987).

a couple of sharp anomaly variations, which indicate 35o30‟E the gravity anomalies become steeply negative subsurface features such as the thickness of sediments, (ranging from -60 to -90 mgal). This is the area extending basement depth, and direction of shallowing and from the western margins of the N-S trending Murua deepening (Rop, 2002, 2003). To the east of longitude Rith–Lokitaung volcanic Hills (point X) towards east up to Rop 46

Figure 7. W-E Section of Bouguer Gravity and Basement Depth Values profile along Latitude 3o30‟N. Between Longitudes 31o15‟E and 37o30‟E through Lake Turkana north Chalbi Basin.

the immediate southwest of the North Island (point R). the lake (depth contours increasing from -3 km to -5 km). From point R eastwards the gravity anomalies become It implies that the sediment fill progressively deepens not only gently positive from -90 to -80 mgal up to point T. only eastwards under points R and S but also The anomalies show a jump and become positive (-75 northwards. The gravity picture of the Lake Turkana mgal to +75 mgal), representing horst structure covered basin is in sharp contrast with that towards the immediate by volcanic under point U on the section. From the eastern and western region. In the Lotikipi plains to the basement depth contour map (Figures 5 and 6), the west, the gravity anomalies showed hardly any variations basement is also seen to be deepening (from -1 km to -4 along this latitude, while to the east under the Koobi Fora km) towards the centre of the lake, west of North Island. region, highly positive anomalies are seen. Thus the This deepening is seen to have been accomplished in faults in the Turkana basin seem to be even mantle- two stages. From point X the basement rises from -2 km reaching, bringing about a down warp of the mantle. In to -1 km up to point P and then steeply deepens to -2 km comparison, the mantle has been up warped under the at point Q. The basement deepens further eastwards, Lotikipi plains and has been sharply thrown upwards first gently up to point R and then steeply to -4.2 km at under the Koobi Fora area. Between points X and P, point S. From point S the basement also steeply rises faulting has up thrown the basement but has affected the first from -4.2 km to -3 km at point T and then gently to mantle also. about -2 km at point U. From the above variations in the basement depth and the steep negativity of the gravity anomalies, one can conclude that (1) the area has Gravity Cross-Section along 3o30’N Latitude undergone vertical active tectonic and (2) the upper mantle in this region has steeply down warped. There are The cross-section along latitude 3o30‟N (Figures 2, 4 and a series of faults, some of them are deep and have 7) through the central part of the Lake Turkana Basin, at affected the basement also. The sense of movement the Central Island area (long. 36o05‟E) reveals, more or between point P and point S is that of a down throw. It less, similar subsurface features as those to the north can thus be concluded that the basin here represents a (cross-section 4oN latitude). The positive gravity graben structure bounded by faults underneath points X anomalies (+50 to +70 mgals) between longs. 35o40‟E and U. Other intrabasinal faults underneath points P, Q. and 35o45‟E over the Lothidok volcanic hills (points C and R and S can also be marked. The magnitude of the O) mark the western margin of the basin. From point O deepening, west of the North Island, is far less than eastwards the anomaly suddenly dips to -75 mgal at point towards the east. The region to the east of the shoulders E. Further eastwards the negativity varies gently towards of the basin beyond point U represents an up thrown the central part of the lake up to longitude 36oE (-85 horst structure, while the Turkana Basin itself seems to mgal) at point F, which is to the immediate west of the be half-graben. Central Island. It becomes gently positive from point F to The basement contours (Figure 5) also indicate a deeper point G (-85 to -75 mgal), where it suddenly (point H) half-graben structure towards the north western edge of becomes highly positive (from -70 to +70 mgal) in the Res. J. Eng. Technol. 47

Figure 8. W-E Section of Bouguer Gravity and Basement Depth Values profile along Latitude 3o00‟N. Between Longitudes 35o25.5‟E and 37o30‟E through Lokichar/Kerio, Lake Turkana and Chalbi Basins.

Moiti area forming the eastern margin of the basin (Figure Turkana basins. The North Kerio basin joins the Lake 4). From the basement depth contours map (Figures 5 Turkana basin at around longitude 36oE and extends and 7), the basement is also seen to be gently deepening southwards. The section reveals an N-S striking Bouguer (from -1.5 km to -4 km) from the western margin (point C) anomaly contours and similar subsurface features as of the basin to the eastern margin (point H). those of the latitude 3o30‟N cross-section. To the It is seen that the basement deepens nearer the eastern immediate east of longitude 36oE (Figure 4) the gravity margin of the basin from -1 km to -4 km (points C and H). anomaly dips at the Lokichar River from a low of -70 mgal From the above it can be concluded that the basin is to -80 mgal toward east, forming the western margin of bounded by faults that have affected the basement, and the Turkana basin (points C/B to point J in Figure 8). The have even down warped the upper mantle also. One can gravity anomaly then becomes gently positive eastwards also demarcate a series of faults striking north-south. (from -80 mgal to -75 mgal) up to point K then suddenly Among these, the faults under point C and point H form varies steeply to become positive (-75 to +75 mgal) under the western and eastern margin, respectively. Within points M and A on the section. From point A to point E, these two major marginal faults lie intra-basinal faults which is the eastern margin of the basin, the positive under points O, E, F and G. The western margin faults gravity anomalies gently decrease from +75 mgal to +45 (under points O, E and F) gently dip eastwards while the mgal. From the basement depth contours map (Figure 5) eastern margin faults (under points G, H and I) steeply and Figure 8, the basement is also seen to be gently dip westwards. Both marginal and intra-basinal faults deepening from -1.8 km at point H, at the eastern margin have subsequently contributed to the formation of half- of the Lokichar basin towards the east up to point K (-3.2 graben structure at the centre of the basin. From the km)) and gently rises to -2 km at point L. From point L it basement depth contours map (Figures 5 and 7) a half- takes another gently dip through point M (-2 km) to point graben structure can be interpreted, which is deeper to N (-2.2 km) where it suddenly becomes steep and the immediate east of the Central Island (-4 km). The positive up to point B (+0.5 km) on longitude 36o30‟E; in graben structure strikes in the N-S direction and appears the Kajong/Porr area, forming the south eastern shore of to deepen towards the north. Similar to the earlier section the Lake Turkana Basin (Figures 2 and 4). the magnitude of deepening, east of the Central Island, is The rifting and faulting in this area has been active in the far more than towards the west. The regions both to the Tertiary up to Quaternary time thus controlling also the western and eastern shoulders of the basin beyond river drainage system in this part of the basin. Comparing points O and H represent horst structures while the Lake the sections across latitudes 4oN and 3o30‟N, it can be Turkana Basin is a half-graben. concluded that the basemen depth shallows towards the south and deepens northwards (Figures 5, 6 and 7). It

Gravity Cross-Section along 3oN Latitude can be said that the sedimentary sequence in the Lake Turkana basin becomes thicker from south to north. The The cross-section along latitude 3oN (Figures 2, 4 and 8) region to the west of the shoulders of the lake (point H) cuts across the two subsurface basins: North Kerio and represents another graben structure (Lokichar/Kerio sub- side by a major fault and on the other side by a set of faults. The major faults define the boundaries with either a horst of basement rocks without volcanic cover or with a huge volcanic cover. The asymmetric rifts or half-grabens are intracontinental and the gravity profiles reveal that they characteristically occur over the crests of regional arches of the basement and the mantle, or, only on the continental crust with a trough-like mantle profile. Rop 48

Fig. 9: W-E Section of Bouguer Gravity anomaly values profile along Latitude 2o30’N. o BetweenFigure Longitudes 9. W-E Section 35 o of30’E Bouguer and 37 Gravityo00’E anomalythrough valuesLokichar profile-Kerio, along and Latitude Lake 2Turkana30‟N. Basins Between Longitudes 35o30‟E and 37o00‟E through Lokichar-Kerio, and Lake Turkana Basins.

basins) while that to the east of the fault under point N On both the eastern and western sides of this basin, represents a horst (Kajong/Porr area). The Lake Turkana horst structures can be interpreted on which the surface Basin itself is a half-graben. outcrops of basaltic flows are located. This study of the intracontinental Lake Turkana rifting and its associated subsurface structural features caused by extensional Gravity Cross-Section along 2o30’N Latitude tectonic stresses is critical for effective probable petroleum prospective target areas (Figure 10) identified The cross-section along latitude 2o30‟N (Figure 9), further from the gravity profiles of the cross sections above. The to the south resembles those drawn on the northern Turkana basin revealed the presence of several horst- latitudes (Figure 2, 6 and 8). Both the Bouguer gravity and graben-like structural systems. It was also revealed anomalies and the basement depth dip eastwards from that the basin could have attracted potential petroliferous point A to point E. East of point E both the basement and sedimentary piles (? 2000 to 5200 m thick) which are gravity anomalies gently rise; the basement rises from - deposited on basement rocks of Precambrian age and 2.2 km to the surface at the southern margin of Mount later covered by Tertiary/Quaternary volcanics. Kulal (Figure 2) and the gravity anomaly rises from -80 mgal to -65 mgal. At the eastern margin of the basin the gravity anomaly rises to +65 mgal (point M). It can be Concluding Remarks on the Cross-sections concluded that the basin represents a simple graben bounded by faults at points A and M. Some intra-basinal From the above four cross-sections, it can be concluded faults can also be interpreted between points H and C. that the entire Lake Turkana subsurface basin is bounded The central part of this graben is the deepest region but by N-S striking faults and the thickness of the sediments the basement depth is the shallowest when compared to seems to be maximum to the north and less to the south, sections along the northern latitudes. It seems the Lake since the basin seems to be deepening northwards. The Turkana deepens from south towards the north. nature of the subsurface basin is like half-graben, which The faults interpreted have certainly affected the deepens and becomes more complex from south to sediments, basement and perhaps also the upper mantle. north. There seems to be variations in the depth of the Res. J. Eng. Technol. 49

Figure 10. Map showing probable petroleum prospective areas (shaded black) in the Lake Turkana rift- related asymmetric and half-graben sub-basins.

basement in this part of the basin from lats. 2o30‟N to complicated, characterised by irregular downthrows and 4o30‟N. It can be seen that between lat. 2o30‟N and 3oN up throws related first to the E-W faulting and later to the the basement shallows and is exposed to the surface. N-S faulting in the basin. This is a horst-like structure which seems to be controlled The gravity profiles of the Lake Turkana Basin present by E-W faults. Beyond latitude 3o30‟N, the basement terrain have indicated that within the basin are sub- deepens suddenly as has been interpreted earlier; it basins, which are asymmetric half-grabens bounded only becomes deepest between latitudes 4oN to 4o15‟N. The on one side by a major fault and on the other side by a subsurface structure between latitudes 2o50‟N to 3o45‟N, set of faults. The major faults define the boundaries with based on gravity profiles and basement depth, showed either a horst of basement rocks without volcanic cover or that the basin also deepens towards the east; deepest with a huge volcanic cover. The asymmetric rifts or half- towards the north between latitudes 3o30‟N to 4o30‟N. grabens are intra continental and the gravity profiles Thus the subsurface structure is therefore very reveal that they characteristically occur over the crests of Rop 50

Figure 11. Laboratory rock pyrolysis showing effect of heating an organic-rich sample at 1098 m of LT-1 Well (Tmax 435.3oC).

regional arches of the basement and the mantle, or, only carbonates. The remaining acid is drained using a on the continental crust with a trough-like mantle profile. vacuum pump. The sample is washed with distilled water and dried in the oven at a constant temperature of about 40oC. The difference in weight before and after HCl METHODOLOGY treatment or reaction is recorded. Once the system is started, it is left to run for one hour and calibrated for Selection of Samples for TOC Analysis TOC/CO2. The dried sample is then analyzed in a carbon determinator (IR–212) with an induction furnace and a Total organic carbon (TOC) is a measure of carbon control console or computer. Oxygen gas (for burning the present in a rock (not pecent) in the form of kerogen and sample) and nitrogen gas (for pneutomatics-lifting of bitumen. The Loperot-1 well (LT–1), drilled in Lokichar pedestal) are pressure-controlled at 35 pounds per basin, penetrated a total depth of 2960 m. The lithology square inch (psi). The combustion chamber pressure is established through drill core samples showed no also set between 11 and 12 psi. The CO2 produced is substantial presence of carbonaceous strata beyond passed through a carbon IR cell and its volume is 2000 m depths. Rock samples selected from 800 to 1800 recorded. The machine automatically records the m depths, showing some indications of the presence of corresponding TOC weight percent. Generally, TOC in organic matter, were analyzed using Rock-Eval shales, especially black shales, always exceed five times technique. The section between 800 and 1800 m depths that from carbonates or other beds of sediments (Selly, was selected because of the reported oil indications and 1985). The organic matter in carbonates has more abundance of rocks with high gamma ray values. The potential to generate hydrocarbons than those in shales. aim was to measure the total amount of organic carbon In the present case the LT–1 well does not show (TOC) and identify the possible source rocks. presence of carbonates. The lithology is mostly clastics dominated by sandstones with shaley horizons.

Methodology of TOC Analysis Rock-Eval Pyrolysis of LT–1 Samples The rock samples (about 1 gram) are first treated with hydrochloric acid - HCL (20% v/v) to remove inorganic The selected samples are first treated with organic Res. J. Eng. Technol. 51

solvents (Chloroform, Dichloromethane, Menthol range of these samples. Or whatever hydrocarbon that Acetone, etc) to remove bitumen. Bitumen content TOC was generated has been expelled and/or migrated. For also represents the oil content that can be dissolved from expulsion to occur the source rocks must get saturated a rock. Subsequently, they are subjected to pyrolysis first, a condition which is reached only on maturity of (Rock-Eval) after drying. The tried sample (~100 mg) is kerogen and temperature. put inside a flame ionization detector (FID) and is heated in a stream of helium at relatively low temperature (up to 300oC) for the first five minutes, in order to remove free or RESULTS AND DISCUSSION absorbed hydrocarbons (bitumen) that were present in the rock sample before pyrolysis (Figure 11). The Some evaluation of the Eliye Springs-1 and Loperot-1 analysis records two peaks, representing the volumes of Wells (denoted as LT-1 and LT-2 Wells, respectively) two components of the organic matter (the volumes being drilled in the southwestern part of Lake Turkana (Figure proportional to the areas below the peaks). The expelled 3) in terms of lithology, comprised mainly sand-shale hydrocarbons, which usually volatilize below 300oC are sequence with some volcanic intercalations which have represented by S1, which provides a measure of already been interpreted. The section of LT-2 Well from 2900 m generated hydrocarbons bitumen (North, 1985; Barker, up to the total depth (2964 m) apparently contains mainly 1999; Tissot and Welte, 19844). There are not many shales of lacustrine origin and volcanics; possibly related samples, which give higher S1 values. Most of the to extensional episodes. The sands comprised poorly to samples showed S2 values only up to 20 kg/g and low S1 well sorted, sub-angular to rounded, very fine to coarse values (<0.3). The low S1 values show the paucity of quartz grains with some feldspars (8 to 20%). The wire hydrocarbons (bitumen) generated below 300oC. S2 logs from this well (LT-2) which were of reasonable values >10 kg/g are supposed to be more potential for quality were helpful in this study. Porosities were derived hydrocarbons (kerogen) generation. from the sonic log but the density logs were not easily On further heating the sample at the rate of 25oC per available for this study. Whereas two potential reservoir minute up to 600oC, the main pyrolytic thermal intervals were identified in the Loperot-1 (LT-2) Well breakdown of kerogen occurs, and is represented by which comprised the oil sandstones of Lower/Middle peak S2 (Figure 11). The measure of the S2 area, Miocene and the main reservoir sandstones of produced at higher temperatures (550–600oC), decides Oligocene/Lower Miocene age, from which high TOC the actual potential for generation of hydrocarbons by the were geochemically analyzed in the section above. sample (Rop, 2003). The area S2 is a measure of the The deltaic and fluvial-lacustrine sedimentation in these remaining hydrocarbon generating ability of the organic intracratonic rift basins was controlled by intrabasinal and matter. Oxygen bearing volatile compounds (CO2 and marginal faults, some of them reaching also the H2O) are usually passed to a separate (thermal basement (Figure 12). The source rocks of well LT-1 conductivity) detector, which produces S3 response. identified above belonged to Oligocene to Lower Miocene However, this procedure was not done in the case of LT– age. These source rocks range from depths 800 to 1797 1 Well sample. The two areas of S1 and S2 are used to m. However, the interval section which proved to have determine the maturation level of kerogen in the source effective source rocks with sufficient organic matter (OM) rock. They are expressed in milligrams per gram of that could generate potential oil and gas, is considered to original rock (mg/g), or kilograms per tonne (kg/ton). be at depths 1650 to 1800 m (considering the normal Rocks with S1 + S2 values < 2 kg/ton are not considered temperatures gradients). The stratigraphic column is potential source rocks for hydrocarbon generation. shown in Figure 12. The porosity at shallow depths (800 Between 2 and 5 kg/ton a significant amount of petroleum to 1760 m) is high (12 to 40%) but decreases with depth may be generated but it would be too small to result in (5 to 10%) at 1760 to 1800 m. No wells have so far been expulsion (Rop, 2013). Source rocks with S1 + S2 values drilled in the North Lokichar and South Kerio basins. between 5 and 10 kg/ton have the potential to expel Future drilling in the deeper parts of both the Lokichar some portion of the generated oil. Only source rocks with and Kerio-Turkana basins could indicate potential areas values >10 kg/ton are considered rich for sufficient oil for hydrocarbons (presuming that the source rocks could expulsion (Allen and Allen, 1990). Thus, these rocks had have reached the natural temperatures gradient in these the potential of oil generation as well as expulsion. parts). The actual oil and gas generation can take place only The thickness of different lithologies (Figure 12) when the threshold temperature (60oC) is reached. It also according to age and time is seen distinctly in well LT-1. depends on the maturity level of kerogen, given by the The oldest strata of Paleocene or younger age in LT-1 temperature maximum (Tmax). The Tmax at which the has a thickness of 360 m (2600 to 2960 m), indicating a S2 generation peak occurs is also recorded in degree thicker sedimentary sequence with the exception only Celsius and is an indicator of source maturity (a function seen in the section of Lower Miocene age, which anyway of the degree of maturation). Perhaps the temperature has neither the source rocks nor the oil shows. The did not reach the threshold of 60oC considering the depth sections in which oil and gas indications in LT-1 occur Rop 52

Figure 12. LT-1 Well cross-section showing probable prognostic site to the east.

belong to the Upper Oligocene-Lower Miocene and to the better reservoir rocks. Thus, the areas in the basin west lower part of the Lower-Middle Miocene. The LT-1 well of LT-1 could be projected as the prognostic areas for showed good seals provided by lacustrine shales at future exploration targets (Figures 12 and 10). It can be depths where the oil shows have been found (Figure 12). visualized to some extent also from the seismic profiles. The black shales have high gamma ray log values (120 Northwards also, the Lokichar basin deepens with API units) and low porosity (< 5%). They intermittently intermediate horst-like structures separating the North were encountered in LT-1 well at 850 to 900 m, 980 to Lokichar (Lodwar) from the South Lokichar. 1057 m and 1360 to 1420 m depths (Rop, 2003, 2013). The other potential area for future exploration would be to However, the numerous interpreted and intrabasinal N-S the east of Lodwar fault (Figure 10) in the North Lokichar trending faults in Lokichar basin could have caused the basin. These two areas could also be decided after giving escape of whatever hydrocarbons generated. Thus, even due consideration to the east-west lineaments and/or if the good sandstones were of good reservoirs and faults controlling the drainage (Turkwell River). It must be quality, as intermittently seen between depths 800 to recalled that the well LT-2 (in the North Kerio basin) to 1760 m, no oil accumulation took place perhaps due to the east of the basement ridge went dry with no location lack of fault closures. The maturation of the organic of either the potential source rocks or the reservoir rocks matter in these source rocks (fluvio-lacustrine) was at depth. There is however a chance also to get some consequent to high heat flow associated with mantle rocks with potential hydrocarbons at a deeper level in the upwelling and rifting. area to the east of LT-2. The gravity and seismic data The proximity of volcanic dykes locally also enhanced the (gamma and sonic ray logs) further give clue to the maturation temperature. The well LT-1 was close to the porosity of the rocks. The studies by the drilling company eastern margin of South Lokichar basin, perhaps on a (AMOCO) in the Chalbi Basin did reveal the presence of tilted fault block (Figure 12). Further east of LT-1 location good reservoirs and source rocks mainly in the Upper is a small horst-like structure represented by the Cretaceous. The sections with source rocks identified in basement ridge. To the west of LT-1 numerous north- C1 well are represented by 1500 to 1800 m and 2300 to south intrabasinal faults have been interpreted, which 2390 m depths. These rocks have a high gamma ray extend to the basement. The South Lokichar basin is values (up to 75 API units) and p-wave velocity values of deeper in this part along the marginal major N-S trending 3.3 to 3.9 km/s. fault (Lokichar). The sections of source rocks in LT-1 The porosity of the upper source rock interval is good extending to the deeper regions towards west, could (27%) while the lower one is relatively low (10 to 15%). have achieved the temperature realms of 60 to 150oC. The depth at which they occur would make the organic Their maturity and cooking at that depth would make matter in these sediments (with TOC >5%) undergo these OM-rich sediments more potential for hydrocarbon changes to produce hydrocarbons since the temperature generation. The extension of the section with gradient would be a contribution by the igneous intrusive hydrocarbon-shows in the well LT-1 would also provide activity of a later date. The section with reported oil and Res. J. Eng. Technol. 53

Figure 13. Cross-section of C1 Well showing basement depth and Bouguer gravity curves.

gas shows in well C1 would similarly constituted of good Cretaceous stratigraphic part. The higher gamma ray reservoir rocks with good to fair porosity (30%). Since the peaks indicate the presence of radioactive elements with source and reservoir rocks are not much separated organic matter which enhance the possibility of reaching vertically, there is a possibility that the oil and gas have temperatures conducive for hydrocarbon generation. The not migrated much in this well section. Both the source areas to the east of C2 have experienced faster rocks as well as the reservoir rocks occur only within the subsidence as they are near the Kargi fault. Upper Cretaceous stratigraphic section. The Lower In the western section there is less possibility of potential Cretaceous sequence, the entire younger section of the source rocks but migration of oil from east to west along Upper Cretaceous as well as the Tertiary sections, have tilted faults cannot be ruled out. It will depend upon the no potential rocks. However, in the areas west and east channels, on the porosity, and on the intercalations of of this well C1 (Figure 13), there could be a migrated oil shales with sandstones. Unlike the C1 and C2, the C3 show, taking into consideration the various intrabasinal well was drilled near the Chalbi fault towards the western faults. part of the basin (Figures 7 and 3). It can be seen that it The potential rocks towards the north could be judged by was again drilled in the deeper part of the Chalbi basin the section of C2 drilled immediately to the north of C1. but much to the north. Between locations C1, C2 to the This well has also revealed depths of the order of 3500 south and C3 to the north, a few faults have been m. The source rocks have been represented in the interpreted striking WNW – ESE to the north of Mount interval depth 2230 m to 3400 m. They are rich in organic Kulal. The deepening of the basin towards the west in matter with up to 2% weight TOC content. This section this section is a departure from what is shown in C1 and has been characterized by high gamma peaks ranging C2 wells. Therefore it seems logical to visualize that this from 75 to 90 API units and p-wave velocities of 3.8 to crisis-cutting faults should have also contributed to the 4.1 km/s. Some good to fair porosity reservoir rocks are subsidence of the basin. The source rocks in the depth also present within this source rock section at range 2300 m to 3100 m is comparable with the depths depths2730 to 3370 m. The reservoirs have same reached in C1 and C2. The source rocks show high gamma ray and p-wave velocity characteristics as source gamma ray (60 API units) and p-wave values of 3.6 to 4.5 rocks. The source rock section, if extended further into km/s. The reservoir rocks at C3 well are at shallow the eastern deeper areas of C2, would probably have depths (1660 to 1700 m and 1860 to 1960 m), and are better oil and gas potential (Figure 14). The entire section characterized by low gamma ray (36 to 40 API units) and containing source rocks is mostly confined to the Upper p-wave values 2.9 to 3.2 km/s. Rop 54

Figure 14. Cross-section of C2 Well showing basement depth and Bouguer gravity curves.

Gas shows encountered in C3 are within a section which Extension Basin. The geothermal gradients of the is younger (Upper Cretaceous). However, both the Cretaceous and those of the Tertiary basins should have source rocks as well as the reservoir rocks in which gas been different, consequent to the non-uniform mantle up shows are found are within the Upper and Lower warping as revealed by the gravity anomaly profiles. Cretaceous stratigraphic section. This is a departure from Intracratonic basins of these types are poor prospects for the wells C1 and C2 where the source rocks as well as the hydrocarbon exploration, but they may contain the reservoir rocks are within the Upper Cretaceous adequate potential reservoir rocks, which can trap section only. Immediately to the east of well C3, there is a whatever hydrocarbons that were generated by the possibility that the Lower Cretaceous section containing chiefly continental organic matter buried with the these potential rocks reaches deeper parts of the basin, sediments. The examined salient structural features which will have higher temperatures and possible containing source-reservoir-seal rocks in the oil/gas- radioactive elements. bearing strata based on gravity, seismic and gamma ray profiles, TOC and other sedimentological parameters helped in characterizing possible future prognostic CONCLUSION targets and their implications for hydrocarbons prospecting and exploration in the Lokichar-Turkana- The examined subsurface stratigraphy was intended also Chalbi rift basinal systems. to assess the prognostic oil and gas potential of this basin and extrapolate the information to other parts of the northwestern unexplored areas of the Lotikipi basin to the ACKNOWLEDGEMENTS northwest of Lake Turkana. Their infilled sediments are predominantly non-marine but it is possible that there This paper is published with the permission of the was some marine influence during the initial sediments Managing Director, National Oil Corporation of Kenya. filling of the Chalbi Basin (Cretaceous), in the north Anza The fieldwork, data analyses and interpretation formed Res. J. Eng. Technol. 55

part of the Ph.D. research-related thesis work submitted Ochieng‟ JO, Wilkinson AF, Kagasi J, Kimomo S (1988). Geology of to University of Pune, under the guidance of Prof. A.M. Loiyangalani area. Republic of Kenya, Mines and Geological Patwardhan, whom I deeply thank for the valuable Department: Report No. 109, p.53. Rop BK (2002). Subsurface geology of Kenyan Rift Basins adjacent to comments and discussions during my research period in Lake Turkana based on gravity anomalies. In the extended Abstracts India. of International Seminar on Sedimentation and Tectonics in Space and Time, S.D.M. College of Eng. & Tech. Dharward, 16th-18th April, 2002, pp. 83-85. Rop BK (2003). Subsurface stratigraphical studies of Cretaceous- REFERENCES Tertiary basins of northwest Kenya, Ph.D. Thesis, University of Pune, p.174. Allen AA, Allen JR (1990). Basin Analysis: Principles and Applications. Rop BK (2013). Oil and Gas Prospectivity: Northwestern Kenya Blackwell Scientific Publications, Oxford London-Edinburgh, p.451 (Revised Eds.), Verlag/Publisher: LAMBERT Academic Publishing Barker BH, Morh PA, Williams LAJ(1972).Geology of the Eastern Rift ISBN 978-3-659-49008-8. p.152. systems of Africa. Geol. Soc. Am. Special Paper 136, p.67. Rop BK.(1990) .Stratigraphic and sedimentological study of Mesozoic- Barker, C (1999). Petroleum Geochemistry in exploration and Tertiary strata in Loiyangalani area, Lake Turkana district, NW development: Part 1-Principles and processes. The Leading Kenya, M.Sc. Thesis, University of Windsor, p.128. Edge,18:678-684. Savage RJG, Williamson PG.(1986). Early rift sedimentation in the BEICIP (1984,1987) Petroleum potential of Kenya 1984 follow-up, Turkana basin, northern Kenya. In: Frostick, L.E. (Eds.), Ministry of Energy and Regional Development, P.100 Sedimentation in African Rifts, Geol. Soc. Special Publication No. 25, Brown FH, Feibel CS (1986).Revision of lithostratigraphic nomenclature pp. 267-283. in the Koobi For a region, Kenya. JG Soc. London,143:297-310. Schull TT (1988). Rift Basins of interior Sudan: Petroleum Exploration Dunkelman JT (1986). The structural and stratigraphic evolution of Lake and Discovery. The AAPG Bull. 72:1128-1142. Turkana, Kenya, as deduced from a multichannel seismic survey. Tissot PB, Welte DH (1984).Petroleum Formation and Occurrence, M.Sc. Thesis, Duke University, p.64. Second Revised and Enlarged Edition, Springer-Verlag Berlin Harding TP (1984).Graben hydrocarbon occurrences and structural Heidelberg New York Tokyo, p.699. styles. The AAPG Bull. 68:333-362. Wescott WA, Morley CK, Karanja FM (1993).Geology of the Turkana Harris JM; Brown FH, Leakey MG; Walker AC, Leakey RE(1988). Grits in Loriu Range and Mt. Porr areas, southern Lake Turkana, Pliocene and Pleistocene Hominid-Bearing Sites from West of Lake Northwestern Kenya. JAE Sci.16(4):425-435. Turkana, Kenya. Sci.239:27-33. Wilkinson AF (1988). Geology of Allia Bay area. Republic of Kenya, Johnston TC, Halfan JD, Rosendahl BR, Lister GS(1987). Climatic and Mines and Geological Department: Report No. 110, p.54. tectonic effects on sedimentation on a rift Valley lake: evidence from Winn RD, Steinmetz JC, Kerekgyarto WL (1993).Stratigraphy and Rift high-resolution seismic profiles, Lake Turkana, Kenya. Geol. Soc. History of Mesozoic-Cenozoic Anza Rift, Kenya. The AAPG Bull. Am. Bull.98:439-447. 77:1989-2005. Key RM, Charsley TJ, Hackman BD, Wilkinson AF, Rundell CC (1989). Yuretch RF(1986). Controls on the composition of modern sediments, Superimposed Upper Proterozoic collision-controlled orogenesis in Lake Turkana, Kenya. In: Frostick, L.E. (Eds.), Sedimentation in the Mozambique orogenic belt of Kenya. Prec. Res. 44:197-225. African Rifts, Geol. Soc. Special Publication No. 25, pp.141-158. Key RM, Rop BP, Rundle CC (1987) .The development of the Late Cenozoic alkali basaltic Marsabit Shield Volcano, northern Kenya. JAE Sci. 6:475-491. Mcjguire M, Day R, Serra S (1985). Dinosaur Fossils of the Lapurr Range, Turkana District, Kenya: Amoco Co., Africa and Middle East Region, Report No. 85002. North FK (1985). Petroleum Geology. Boston Unwin Hyman Publishers, p.631.

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APPENDIX: EXPLANATION FOR GEOLOGICAL COLOURED MAPS IN FIGURES 4 AND 5 (Adopted from BEICIP-Ministry of Energy Geological Map, 1987).