Hydrocarbon Source Rock Potential Evaluation of Bokh Shale of Calub Area In the Ogaden Basin, Southeastern Ethiopia

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Addis Ababa University


The Ogaden Basin is situated in the Southeastern part of Ethiopia. It is the largest sedimentary basin with thick successions and has got an attention for petroleum exploration since the mid of twentieth century. Bokh shale is one of the source rocks in this basin and it is the most targeted formation as it is considered to be a source for Hilala and Calub gas deposit. The previous petroleum exploration and development activities were focused on the regional geology, basin evolution, as well as biostratigraphy of the basin but a little is done about the hydrocarbon potential. Therefore, this study iss proposed to determine the hydrocarbon potential of the source rock and to establish the depositional environment and to infer the provenance of Bokh shale. In this study a total of 12 core samples of Bokh shale from Calub 2, 3 and 5 were selected. The organic geochemical of six samples were analyzed using Rock-Eval pyrolysis analytical method. The results were used to determine organic richness and hydrocarbon generation potential, and kerogen type. The organic petrography of the two samples were studied, the results then applied to establish thermal maturity of the Bokh shale. The major, minor and trace elements of four samples were investigated using X-ray fluorescence (XRF) techniques. The results were used to infer the depositional environment and provenance of the source rock. From the analyzed samples the average total organic content (TOC) of the Bokh shale obtained is 0.521 wt% and ranges from 0.5 to 0.56 wt%. The pyrolysis result indicated S1 ranging from 0.02 to 0.23 mgHC/g rock with an average of 0.076 mgHC/g rock and that of S2 is from 0.05 to 0.27 mgHC/g rock with an average of 0.116 mgHC/g rock. The average generation potential of the source rock is 0.192 mgHC/g rock. The vitrinite reflectance of Calub 3 exhibits 1.79 Ro (random reflectance) whereas Calub 5 indicates 1.3 Ro. The overall organic richness and kerogen type of Bokh shale signify fair carbon quantity and poor gas generation potential. The studied sample shows kerogen type IV, i.e. it is dominated by inertinite macerals that generate little or no hydrocarbons upon further maturation. The examined vitrinite reflectance of samples suggest late maturate (Calub 5) to over-mature (Calub 3) organic matter in the Bokh shale. The V to Ni ratio of Calub 3 indicates it was formed of mixed marine and terrestrial organic matter while the Calub 5 was formed of marine organic matter. The examination of ratio of V to V+Ni suggests both samples were deposited under sub-oxic environmental condition. The Zr/Cr, La/Sc, and La/Co ratio indicates both samples were sourced from felsic preexisting rock.



Ogaden Basin, Bokh Shale, Calub, Quality and Quantity, Thermal Maturity