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Diagenetic Evolution of Organic Matter Cements in Unconventional Shale Reservoirs


Organic matter cements in the form of bitumen and pyrobitumen are commonly observed in scanning electron microscopic images in many U.S. unconventional shale reservoirs that range in age from Cretaceous to Ordovician. Organic matter cements are distinguished from kerogen based on petrographic identification of cement as a void-filling material within matrix pores, microfossil internal voids, and microfractures. The character of organic matter cements and the impact on reservoir quality changes with increasing thermal maturity as illustrated by the organic-rich interval of the lower Eagle Ford Formation in south Texas. In thermally immature (<0.50%Ro) outcrops of the Boquillas (Eagle Ford) Formation, meniscus-type organic matter cements partially fill interparticle pores within coccolith-rich lamina. The origin of this organic matter cement is interpreted as pre-oil generation bitumen created at the initial stage during the conversion of kerogen to oil. In the subsurface, migrated residual oil (migra-bitumen) fills matrix pores and foraminifera chambers forming solid organic matter plugs that may serve to form updip lateral seals along the updip edge of the oil window. This soluble bitumen may be partially removed by hot solvent (toluene) during Dean Stark extraction on crushed rock samples (GRI method), that could result in overly optimistic porosity measurements. Down dip at higher thermal maturity (>1.0%Ro), organic matter cements in the form of pyrobitumen develop a well-connected secondary porosity network, often mistakenly described as “kerogen” porosity. The organic matter pores are interpreted to form as a result of gas generation during the thermal cracking of oil retained within primary matrix pores preserved prior to oil generation and migration. Mineral cements observed within foraminifera chambers (e.g. calcite, quartz, kaolinite) predate the surrounding organic matter cement. This relationship suggests that mineral cementation may be terminated during primary oil migration as oil replaces water expelled from primary pores within the source rock.