Primary Migration of Hydrocarbons Through Microfracture Propagation in Petroleum Source Rocks

Petroleum is generated from finely grained source rocks rich in organic materials and accumulated and trapped in reservoir rocks with relatively higher permeability and porosity. Expulsion of petroleum through and out of source rocks is called primary migration. Primary migration, as a link between source rocks and carrier rocks, presents a vital challenge to the society of petroleum geosciences and exploration and attracts the research interests of many geologists and geochemists. Despite extensive research the effective mechanisms responsible for primary migration of hydrocarbons are still in intensive debate. Conversion of kerogen to oil and/or gas results in appreciable volume increase due to the density difference between the precursor and the products. Overpressure is developed as a natural consequence in well-sealed dense source rocks at great depths. When the overpressure reaches some critical value, bedding-parallel microcracks are initiated owing to laminated structure and strength anisotropy of source rocks. As transformation proceeds, microcracks are driven to grow subcritically by the overpressure. Such microcracks serve as migration conduits for hydrocarbon flow and may connect to other preexisting conductive fractures to form fracture networks or systems, which may facilitate further migration of hydrocarbons. Convincing evidence from observations in nature and laboratory experiments is found to support the idea that microcracks caused mainly by overpressure buildup from hydrocarbon generation functions as effective primary migration pathways. Based on those published findings, the present dissertation adopted an integrated approach consisting of petroleum geochemistry, petrophysics and fracture mechanics to assess the role of self-propagating microfractures as an effective mechanism for primary migration of hydrocarbons. Four models were developed: migration though subcritical propagation and coalescence of collinear oil-filled cracks, migration through subcritical propagation of an oil-filled penny-shaped crack in isotropic source rocks, subcritical growth of a penny-shaped crack filled by hydrocarbon mix in anisotropic source rocks, and a penny-shaped crack driven by overpressure during conversion of oil to gas. To predict the migration time and quantities of oil and natural gas, we use the reaction kinetics taking into account of pressure and temperature histories during continuous burial of sediments. To account for the compressibility of gas at high temperatures and pressures, we adopt an equation of state for methane, the predominant component of natural gas. To address the excess pressure buildup through volume expansion associated with kerogen degradation and initiation of microfractures, we employ linear fracture mechanics. To simulate the propagation of microcracks, hence the migration of hydrocarbons, we use a finite difference approach. The time period for pressure build-up, the overpressure evolution over time, and crack propagation distance and duration are determined using the coupled model where the interaction of hydrocarbon generation and expulsion is included. A detailed systematic parametric study is carried out to investigate the sensitivity of hydrocarbon migration behavior to variations in the input parameters including elastic and fracture properties of source rocks, richness and type of organic matter and burial history. Oil retained in the microfractures may be subjected to thermal cracking to form gas when the gas window is reached as the temperature and pressure continue to increase with the progressive burial. Numerical results are presented for the two cases: kerogen conversion to hydrocarbon mix and subsequently oil conversion to gas. The modeling results agree well with published geological observations which suggest that microfractures caused by overpressures mainly due to hydrocarbon generation serve as effective migration pathways for hydrocarbons within well-sealed source rocks under favorable burial conditions. The fully coupled multiphysics modeling allows us to gain some insight on the primary migration of hydrocarbons, which is essential for the exploration of source rocks