Abstract
The Middle-Upper Jurassic organic-rich carbonate mudrocks of the Tuwaiq Mountain Formation in the Jafurah Sub-Basin of the Central Arabian Basin have long been recognized as the source of hydrocarbons trapped primarily in Upper Jurassic carbonate reservoirs of Saudi Arabia (e.g., Giant Ghawar Field). With the recent development of commercial hydrocarbon production from unconventional reservoirs in North America, these organic-rich, carbonate mudrocks are now recognized as potential shale-gas and shale oil plays. Three conventional source rock intervals (the Tuwaiq Mountain (TQMN), Hanifa and Jubaila Formations) exist within the Jurassic petroleum system of the Jafurah Sub-Basin. The TQMN formation is being evaluated for shale-gas and shale-oil potential by Saudi Aramco because the TQMN Fm. holds excellent unconventional gas characteristics, such as high total organic carbon content (TOC) and low clay content, and proper maturity window for oil and gas generation. Five wells, representing the first unconventional gas exploration wells drilled in the Jafurah Sub-Basin have been selected for 1D and 2D basin modeling work. The work was performed to investigate the burial and thermal maturity evolution of the TQMN Formation in the Jafurah Sub-Basin. The maturity of the TQMN Formation in the study wells varies from 0.8 to 1.25%VRE (covering peak oil to wet gas generation stage), despite only slight differences in present-day burial depths. The present day temperature gradients have been calculated to be higher at well locations where the source rock is at higher maturity. Modeling results suggest that higher temperature gradient areas of the basin probably has higher basement heat flow which leads to satisfactory match between measured and predicted maturity and temperature at present day. This increased basement heat flow in areas of higher temperature gradient correspond to the interpreted change in crustal composition (most likely granitic basement) generating more radiogenic heat. The modeling results suggest that TQMN source rocks started generating oil at about 100 Ma and they reached the peak oil generation at about 70 Ma. Present-day they are in the wet-gas generation window in the northern and eastern parts of the Jafurah Sub-Basin, where the thermal gradients are > 28 degrees C/km or higher. The central and southwestern parts of the Sub-Basin are cooler, and here, the TQMN source rocks are within the peak oil generation window. The warmer parts of the Sub-Basin have higher shale-gas potential compared to the cooler parts of the Basin. We note that proper modeling and estimation of gas content in these source rocks, and probably in unconventional resource estimation work in general, some skilled modifications in basin modeling parameters are necessary. Variation of the oil saturation threshold is necessary until a gas saturation level equal to or higher than the gas adsorption capacity of the source rock is obtained, otherwise, modeling will lead to overestimation of the oil migration out of the source rock and thus the amount of the oil that can be retained and cracked to gas at higher temperatures will be miscalculated. Therefore, the eventual gas content estimations will may be severely underestimated. Following our basin modeling study, we note that improvement of the present day subsurface temperature data will be necessary in the future to estimate the maturity and shale-gas potential of the TQMN source rocks in the Jafurah Sub-Basin.