How do marlstone oil shales differ from clay-rich oil shales?

Oil shales are classified based on their mineral constituents, which vary significantly depending on their sedimentary origin. The primary difference between marlstone and clay-rich oil shales lies in their dominant mineral composition:1

• Clay-rich oil shales: These are considered true shale deposits where clay minerals are the predominant inorganic component. They typically form in environments where muds and silts are deposited by gentle currents.12
• Marlstone oil shales: These deposits are characterized by a high content of carbonate minerals, specifically dolomite or calcite. While they also contain silicate minerals such as clay, quartz, and feldspar, the presence of carbonates distinguishes them from clay-dominant shales.1

These mineralogical differences often reflect the depositional environment; for example, many marlstone deposits are associated with large lake basins, whereas clay-rich oil shales are frequently found in shallow marine settings.3