Summary

The enhancement of sedimentary rock heterogeneity along lithology boundaries through diffusive transport of matter is demonstrated using the basin simulator CIRF.B. The process of layered mineralization, including Liesegang banding, is simulated. CIRF.B is a 3-dimensional numerical basin simulator capable of modeling chemical and mechanical compactions, fluid flow and kinetically controlled water-rock interactions.

The concept under scrutiny here is the mineralization along the lithologic boundaries due to the disequilibriated state of minerals and water persisting across lithologies, and/or wthin the same lithology due to changing temperature and/or imposed fluid. Such conditions are conducive to local water-rock interaction: Resulting changes in pore water chemistry then drives the diffusive migration of solutes across the lithologies, further enhancing the local processes.

Simulations of shale-sandstone assemblages indicate that significant porosity modification immediately adjacent to the lithology boundaries can be produced by such means. It is also evidenced that vastly varying paragenetic relations can evolve depending on the initial sediment compositions.

The effect of subsidence, differential compaction of shale compared to sandstone, and temperature are shown to have complex effects. Temperature increases the chemical reactivity, while shale-derived water contributes greater amount of solutes to sandstone. Together, they can affect the pattern and the widths of mineralization bands.

The immediate benefit of such study is in assessing the paleo-flow characteristics between the sandstones (reservoirs) and the enclosing shales, and in assessing the effectiveness of resulting diagenetic seals for retarding or diverting the flow characteristics of petroleum out of the source rock.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah