Fracture-permeability development in organically-rich sediments through methane generation
Monroe, John Napier
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The result of methane generation in low-permeability rock matrices is fracturepermeability development. Such expansion is the result of methane generation which, in turn, is the result of burial of organic matter under euxinic conditions. The fracturepermeability-development process has been demonstrated in the laboratory using a microwave oven to generate gas (water vapor) in well-indurated, low-permeability sedimentary rocks. The process has been quantified through modeling constrained by principles of chemistry and physics. The modeling process is applied to both shales and limestones and relates sediment expansion to sediment organic carbon content converted to methane. The model shows that the quantity of organic carbon required to be converted into methane for microfracture development is small compared to the amount commonly contained in hydrocarbon source rocks. A wide variety of fracture-producing mechanisms proposed to explain natural fracture development in hydrocarbon reservoirs is acknowledged. However, fracturepermeability-development that appears to occur selectively in low-permeability, organically-rich sequences has received much less attention. Additionally, unabsolved anomalies that persist when current explanations are applied call attention to the need for alternative explanations. The fracture permeability, including the distribution and orientation of those fractures, which some reservoirs exhibit seem to defy explanation until now. A better understanding of fracture-permeability development and related aspects of petroleum maturation will remain illusive until the methane-generation fracture-permeability process, which until now has not been adequately quantified, is fully appreciated. Sediment expansion through methane generation in low-permeability rock matrices explains fracture-permeability development in many naturally-fractured hydrocarbon reservoirs including cleat permeability in coalbed methane reservoirs. Evidence is presented for the importance of this fracture permeability process in producing the natural fractures in low-permeability matrix sediments ofthe Frio Formation (Oligocene); i.e., the Frio Formation's geopressured shales; and the low-permeability matrix sediments ofthe Spraberry Formation (Leonardian), the Clear Fork Formation (Leonardian), the Strawn Formation (Pennsylvanian), and the Austin Chalk Formation (Upper Cretaceous).Fracture-permeability development through methane generation is of utmost importance to the migration of liquid and gaseous hydrocarbons out of source beds into reservoir beds.