Formation of hydrocarbons in the seawater - mineral substance - living matter system is modeled. Some systems of various living matter compositions and stoichiometry and also at several sets of T,P parameters are simulated. During diagenesis kerogens and hydrocarbons are formed. The equilibrium constant of the reaction between oily kerogen and oxidized one, methane and CO2 is evaluated: Φ1C292H288O12(kerogen, H/C 0.99, O/C 0.041) → Φ2C128H68O7(kerogen, H/C 0.53, O/C 0.055)+xСH4+yCO2+zH2O, where Φ1 and Φ2 moles of kerogens; x, y, z – stoichiometry coefficients. In thermodynamically closed system the aqueous natural hydrocarbons are stable up to 300оС (at pressure above water saturated vapor) due to reductive environment formation (lg f H2 ≈ 10-4 бар). Equilibrium mineral assemblages of the system include quartz, kaolinite, muscovite, ankerite, siderite, pyrite, (Ca,Mg)CO3, (Ca,Sr)CO3 and paragonite. High salinity Cl-HCO3-Na aqueous solution also contains CO2 of tens g/l and ammonium nitrogen.