Blocks 9 and 10A are in the Anza Graben of NE Kenya, a NW-SE oriented Mesozoic graben along trend from the prolific Mesozoic play of southern Sudan.

The Anza Basin is a NW-SE trending rift basin which forms part of a Late Jurassic-Cretaceous rift system which extends across central Africa. The basin is over 580 km long and 150 km wide with a potential prospective area in excess of 50,000 km². The basin is filled in places with more than 6,000 m of Mesozoic and Cenozoic sediments and locally by Plio-Pleistocene basalts. Bouger and residual gravity anomalies have highlighted several sub-basins separated by intra-basin highs.

A Karoo-aged, NE-SW trend rift occurred in the eastern part of Mozambique, Kenya, Ethiopia and Somalia, and renewed extension along this trend during Mid Triassic-Early Jurassic resulted in the separation of Madagascar from Africa and marine transgression into Eastern Kenya. The subsidence of the NW trending Anza rift began during the Late Jurassic at the time of the deposition of the marine limestone deposition in the central Anza Basin. Rift expansion during the Neocomian, during a continent-wide extension phase in the Anza Graben was contemporaneous with the formation of the along strike NW trending Muglad and Melut rift basins of Sudan. Further extension during the Late Cretaceous reactivated the subsidence in the Anza Basin and the Cretaceous saw the deposition of up to 6 km of the predominantly continental and fluvial lacustrine sediments in the deepest parts of the basin. Further rifting in the Paleocene-Eocene saw thick continental deposition in subsiding troughs.

During the Oligo -- Miocene, as a result of the tectonic movements related to the formation of the East African Rift System in Ethiopia and Northern Kenya, the Anza Basin was affected by significant compressional and/or transpressional movements. Some of the normal faults formed during Cretaceous-Paleogene rift phases were reactivated and large scale inversion occurred. New faults with different fault orientations were also formed which uplifted large basement blocks. Basin modelling and well data indicate that several thousand feet of sediments were locally eroded. Following basin inversion during the Micoene, thick lacustrine and continental fluvial sediments were deposited above the regional base Miocene unconformity.

The basin has undergone two periods of extensive flood basalt extrusion associated with the East African Rift System during the Latest Miocene-Early Pliocene and the Late Pliocene-Pleistocene. These basalts covered the whole area of Block 10A and the northern part of block 9 with thickness varying from 30-250 m. This volcanic activity is believed to have had only a limited effect on the petroleum system.

The Petroleum System of the Blocks has been tested by several wells and the presence of reservoir, seals and potential source rocks has been demonstrated. The Anza Graben is interpreted to be an extension of the prolific Sudan Basin, a Cretaceous rift basin system of north-Central Africa.

The low level of exploration activity in both blocks means that ages and thickness of the formations drilled to date are poorly constrained and no formal stratigraphy is defined. Source rocks, reservoirs and seals are known from exploration wells and there is only a partial penetration of most of the units in the basin. In addition, correlation between the wells is difficult because they are drilled in different sub-basins each containing units not present in other wells.



Block 10BB is located in NW Kenya in the region of Lake Turkana.

Sands within the Auwerwer and Lokhone formations have been identified as the main target intervals.

Shell's (previous operator) Loperot-1 well was drilled in the southern portion of the block in the Lodwar South sub-basin. The well found a total of 13 m net pay in thin, shallow landstone layers of the Auwerwer formation. It also penetrated two intervals of lacustrine shales which contained good to excellent source rocks. However, Shell's main objective, Lokhone reservoir sandstones immediately below the upper Lokhone source rock, was interpreted as water bearing. Age determination of the Lokhone reservoir is difficult due to the lack of good Tertiary palynological data. This section is overlain by volcanic rocks that have been dated as Mid-Miocene.

Shell's Eliye Springs-2 well, drilled in the northern portion of the Block, encountered Tertiary continental deposits, predominantly sandy in the upper 1700 m of the well, but with an increasing amount of claystones below. Porosities in the sandstones are 18-30%. Volcanic rock was penetrated at a depth of around 1797 mtvd. In addition, thin limestone stringers were encountered near TD. Age determination of these limestone stringers was difficult as only a few intervals yielded abundant fossil assemblages, the rest of the well being barren of any fossil assemblages. There was a reliable K/Ar date of 5.1±0.2 MY (Pliocene/Late Miocene) obtained from the volcanic layer at 1797 m. Depositional environments were fluvio-lacustrine, with two distinct developments. Thin source rock intervals were encountered between 750 to 2000 m. Samples of these intervals indicate immature, marginal to good quality source rocks.

Block 10BB is about 500 km northeast of the commercial discoveries in Blocks 1 and 3A in Uganda, and 600 km southeast of producing fields in the Melut and Muglad Basins in Sudan. Block 10BB is in a similar rift valley system; however it is separated from these producing basins by major fault zones.

1. Uganda

Heritage Oil, along with Tullow Oil, has two licenses in the Albert basin of the Western Rift Valley of Uganda -- known as Block 1 and Block 3A. They have focused on the northern part of Block 3A, on the eastern shores of Lake Albert, which straddles the border of Uganda and the Democratic Republic of Congo.

Their Kingfisher-1 well in Block 3A was drilled in 2006/2007 to a total depth of 3,195 m. Hydrocarbons were encountered in Tertiary-age reservoir sandstones and four intervals were tested, resulting in an overall cumulative maximum flow rate of 13,893 BOPD over a total net productive thickness of 54 m. The tested oil was light (~30° API) and sweet with low gas-oil ratio and some associated wax.

2. Sudan

The Melut Basin in the Sudan is one of the major sources of crude oil in Africa. The basin is located about 1,120 km south of Khartoum east of the Nile River. In 2007, the output from the basin was in excess of 500,000 BOPD and has the potential to increase to 800,000 BOPD in the bear future. The primary field in the Melut Basin is the Great Palogue Field which has estimated recoverable reserves of over 900 million barrels of good-quality light crude oil.

The Muglad Basin is the largest of the Central African rift Basins. It extends across an area of 120,000 km² and is up to 200 km wide and over 800 km long. Locally the basin contains up to 13 km of Cretaceous to Tertiary sediments. The two largest hydrocarbon accumulations, the Unity and Heglig fields, have combined recoverable reserves of 250-300 MBO.

Production of oil in the Muglad Basin started in June 1999. By the end of October 2004 the cumulative production from all fields was 461,000,000 BBLs. Eight fields are contributing to production, Heglug, Unity, Toma South, El Nar, El Toor, Bamboo, Munga, and Diffra. These fields have average flow rates of 325,000 BOPD.

For the prospects in the Loperot area of Block 10BB the expected production is medium (~29.4° API) gravity oil, which was recovered from RFT sampling in the Loperot-1 well. Analysis of heavy crude and tarry oil of 12.0° ~16.0° API gravity oil recovered from the shakers in Loperot-1 is considered not representative of the expected reservoir fluids. For the leads in the area near the Eliye Springs-1 well there is some uncertainty to the product type and quality because the well lacked any indications of commercial hydrocarbons.

In the Loperot-1 area prospects, the depth ranges of the Auwerwer and Lokhone sands are from 900 to 1700 m sstvd, with the primary target, Lokhone sands in the range of 1300 to 1700 m sstvd.