Numerical modeling of microfracturing and primary hydrocarbon expulsion in the Jurassic Lower Tuwaiq Mountain Shale: A conceptual framework

This paper presents a geomechanical model of microcrack growth coupled with thermal maturation data for the Tuwaiq Mountain Formation, Jafurah Basin, Saudi Arabia. We set out to capture the basin evolution in terms of resource capacity and pore pressure. The overpressure that evolves in the organic matter-rich units during kerogen-oil conversion controls the creation and growth of horizontal and vertical microcracks between 65 and 60 Ma, according to our model. The crack coalescence time varies between 2 and 5 Ma. Vertical microcracks formed between 55 and 50 Ma, at a markedly higher overpressure than that for the horizontal microcracks. The rate of microcrack growth, organic matter content, and thermal maturity control the timing of microcrack coalescence that creates a rock-spanning cluster and enables primary migration of some of these hydrocarbons out of the source rock. With vertical permeabilities of 300 to 500 nd and expulsion efficiency of 70%, primary migration stopped between 30 and 25 Ma. Plastic bitumen and brittle solid pyrobitumen, produced during secondary cracking of oil to gas, remain in isolated clusters that sever flow pathways and limit loss of pore pressure. The flow of gas microbubbles and oil droplets through the plastic bitumen preserves overpressure for millions of years, with a pressure decrease of 5 to 10 MPa. Balancing the hydrocarbon mass, and assuming that 10 Ma is the shortest time required to disconnect flow, we estimate the final pore pressure (Pp) in the Tuwaiq Mountain Limestone Formation to be between 6500 and 9000 psi (45 and 62 MPa), Pp/Sv = 0.47 - 0.7, where Sv is the vertical stress. The methane gas storage capacity of the Tuwaiq Mountain Formation is 500-700 SCF/t source rock (14.2-20 m3/t source rock).