Source and reservoir rocks of the Block 18 oilfields, Sab atayn Basin, Yemen: Source rock evaluation, maturation, and reservoir characterization

A total of 183 core and cutting samples from seven exploratory wells were selected to be analyzed by Rock-Eval pyrolysis which have been drilled through the Lam and Meem Members of the Madbi Formation and which contain the major source rocks of Yemen´s sedimentary basins. Contents of total organic carbon were measured and Rock-Eval pyrolysis was performed to evaluate the hydrocarbon potential of Block 18 oilfields in central Yemen. Most of the studied samples have fair to excellent petroleum generation potential regarding the results of PP, PI, HI and TOC. They have also sufficient TOC values with an average value of 1.48 wt% and a maximum value of 12.34 wt% with a good petroleum potential averaging a value of 4.54 kg HC/ton of rock and a maximum value of 44.78 kg HC/ton of rock. HI values of the Madbi Formation in its full thickness range from 16 mg to 1114 mg HC/g TOC with an average value of 273 mg HC/g TOC. Kerogen types II and III, and a small amount of I, can be observed. These kerogens are thermally mature and mostly within the hydrocarbon generation zone. In a numerical simulation approach the thermal and burial histories of the Alif-1 well, which are representative for this area were modelled. Using vitrinite reflectance data, as means of calibration, the modelling results were subjected to a sensitivity analysis concerning paleo-heat flow, radiogenic heat production, basement thickness and eroded overburden. The calculated temperature during deepest burial of the source rock layers of the Madbi Formation reached 158 °C for the Lam Member and 182 °C for the Meem Member. Compliance with the measured R 0 values, used as calibration parameter, could be achieved by assuming an erosion of 1100 m of the Upper Tawilah Group. The heat flow is maximized at the onset of basin rifting in Late Jurassic to 90 mW/m 2 having a base through time of 60 mW/m 2. Thus, the Lam Member has reached the main oil to wet gas window at its deepest burial whereas the Meem Member has undergone the wet gas window in its full thickness. The isopach map of the Alif Member, as main reservoir in the studied area, indicates thickening of the sandstone unit towards the eastern and western parts of Block 18 around the Dostour Al-Wahdah gasfield in the east and at the Alif oilfields in the west.