The Geology and Hydrocarbon Potential of Lugh-mandera Basin (Somalian Sector)

Abstract

This study attempts to evaluate the petroleum potential of the Somalian sector of the broad Lugh-Mandera basin based on the available geological and geophysical (Seismic) data sets. The basin has not been studied in detail, and is classified as a frontier exploration area with only 3 wells drilled in an area of approximately 70,000 . It is infilled with thick Jurassic-Cretaceous formations whose source, reservoir, and sealing configurations and distribution are still poorly understood. The main aim of this research was to undertake an integrated structural and stratigraphic study of all geological data, in particular, interpretation of fourteen (14) 2D seismic profiles with a total length of 605 km and tied to the Hol-1 well and also evaluates the source rock potential of several levels of interest through Time-Temperature Index (TTI) Modeling. In addition, the investigations also undertook a regional correlation with formations from neighboring Ogaden basin in Ethiopia. Some of the tasks of the objectives that guided this study included: a) determination the of rock units succession and the types of proven petroleum systems within the basin compared to the neighboring Ogaden basin found in Ethiopia, through interpretation of the seismic sections and well log data; b) understanding the aspects of structural style and it’s impact on hydrocarbon entrapment, and delineate the subsurface target horizon in the study area by using (depth maps, thicknesses maps, and the interpreted fourteen (14) seismic profiles); c) to evaluate the hydrocarbon potential of the area by creating and analysing burial curves of the source rock intervals of the basin by calculating the TTI values. The stratigraphy of Hol-1 well encountered mainly the Jurassic section and lowermost of Cretaceous formations. The lithologies are commonly represented by interbedded layers of grey limestone, lime mudstone, shales, sandstones, pale marls, with minor dolomite and anhydrite occurrences. vi The result of the seismic interpretation shows that the deformed Garbaharey fold belt formed as a result of wrench tectonics and comprises anticlines with a wavelength of about 10 km that are associated with a transpressional right strike slip movement. The presence of positive flower structures that are viable hydrocarbon traps can be observed from the interpreted seismic profiles. A TTI analysis was done for the Lower Jurassic source rock interval encountered by Hol-1 well and yielded a value of 1,319. This value corresponds to Vitrinite Reflectance (Ro) of more than 2. Which indicates that these source rocks are over mature and are located within the dry gas window, which initiated in the Sinemurian (approximately 196 Ma) times? Maturation and expulsion began in the Aalenian (172 Ma). The Middle Jurassic source rock interval gives a TTI value of 20 which suggests maturation within the peak oil window. The Upper Jurassic source rock is positioned within the early oil window, as indicated by a TTI value of 5.1, maturation and outflow began Oxfordian (159 M.a) and is still ongoing. Faulted anticlines formed after the initial hydrocarbon expulsion and migration might have occurred and entrapment relies on only re-distribution of hydrocarbons. Potential hydrocarbon traps developed in the Somalian sector of the Lugh-Mandera basin are presented by structural (wrench faults, folds), and hydrodynamic types (Baidoa formation). The current study proposes other drilling locations in the south-eastern part of the study area that will test the pontential of the supposed hydrodynamic trap.