Immature oils as the products of early catagenetic transformation of bacterial-algal organic matter

Abstract

GC-MS were used to investigate oils from Eastern Sakhalin and Western Kamchatka spatially associated with immature biogenic siliceous rocks ( R o o = 0.4–0.6). Genetic unity of organic matter and oil was established after the biomarkers composition. The oils were distinguished as immature after the low rate of isomerization of steranes and hopanes. Biogenic siliceous rocks contain bacterial-phytogenic organic matter, rich in oxygen. From the sedimentation to the catagenetic stage the amount of oxygen in asphatlenes decreases (from 22 to 10%) as well as the content of heteroelements in kerogen (from 28 to 16%). According to X-ray structural analysis the percentage of condensed structures in asphaltenes is equal to 20% becoming 35–40% by the end of diagenesis. The transformation of the structure of asphaltenes is accompanied by the generation of hydrocarbons. It is supposed that sources of hydrocarbons at an early stage of generation are asphaltenes of bitumen and kerogen fragments containing heteroatomic elements. The migration of newly-formed hydrocarbons is on-going with water and CO 2 realization from organic matter. The transformation of the original silica forms which are on-going at the same level, accompanied by water realization is also favourable for hydrocarbons discharge.