Investigation of organic matter in black shales
Since shale rocks may be simultaneously source rock and reservoir rock for the hydrocarbons, it is important to determine petroleum characteristics of shale rocks, i.e. their capacity to generate, retain or expel the hydrocarbons or, in other words, to investigate the properties of petroleum systems.
Investigations of oil- and gas-bearing shale rocks fall into three groups:
- investigation of depositional environment where organic matter-rich sediments accumulate,
- investigation of organic matter origin with the determination of its thermal maturity, and
- the determination of the time and space of hydrocarbon generation and expulsion, as well as of the volume and type of generated hydrocarbons.
Investigation of oxygen conditions prevailing at the sea bottom during deposition is the key objective of studies on the origin of shale rocks. Other important topics are:
- investigation of organic matter accumulation zone and of the degree of organic matter transformation,
- the determination of the relative share of marine and terrigenous organic matter, and
- studies on the contribution of upwelling currents to the formation of shale rocks.
Tools for the determination of these environmental characteristics include sulphur isotopes and the studies of biomarkers, i.e. organic compounds, of which organic compounds found in living organisms are the precursors, that have preserved an unchanged or only slightly modified carbon skeleton of its biological precursor. Moreover, the analysis of the relationship between TOC and pyritic sulphur may also be helpful.
In addition to standard Rock Eval pyrolysis, optical microscope and biomarker examination methods, a study of the origin and thermal maturity of organic matter should include:
- an analysis of stable organic carbon and nitrogen isotopes,
- organic matter tests for chemical composition, and
- the content of extractable organic matter.
Combined techniques, such as for example Rock Eval combined with fluorescence, pyritic sulphur or extractable organic matter, are applied to establish space and time of hydrocarbon generation. Oil and source rocks can be correlated by the studies on stable carbon isotopes contained in saturated and aromatic hydrocarbons, while Lopatin Diagram enables an interpretation of the source rock burial history, as well as of the time and depth of hydrocarbon generation.
author: Przemysław Karcz