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abstract

Geochemical and petrographic studies of core and cuttings samples from the Upper Cretaceous Ericson Sandstone in the Forest Oil, Jonah Gulch well, 9-26n-107w, reveal the presence of two distinct enhanced porosity zones that can be differentiated by their position relative to a paleo-oil/water contact. The evolution of enhanced porosity in these two zones can best be explained by experimentally predicted organic-inorganic chemical interactions. Both intragranular carbonate dissolution within chert clasts in the upper diagenetic zone and mixed-layer clay dissolution in the lower zone are apparently the result of organic-inorganic interactions. These interactions are the natural consequence of progressive burial of a sedimentary prism containing sand and shale.

The sequences of inorganic diagenesis for the Ericson Sandstone reservoir and organic maturation in the adjacent shales were determined independently. Then, spatial, temporal, and textural relationships in conjunction with reaction temperatures were used to integrate the two sequences. The synthesis of the organic and inorganic systems shows that the two major porosity enhancement events can be directly tied to organic solvents produced by progressive maturation reactions.

Dissolution of mixed-layer clay in the lower zone was associated with a pulse of organic acids generated prior to the formation and migration of most liquid hydrocarbons. These organic solvents—primarily carboxylic acids and phenols—complexed aluminum in the mixed-layer clay. This complexing action caused the destabilization and dissolution of the clay resulting in porosity enhancement. In the upper diagenetic zone, porosity enhancement occurred after hydrocarbon migration. The carbonic acid that dissolved carbonate within the chert grains was produced by thermal destruction of reservoired organic fluids in the upper Ericson. Thus, both porosity enhancement events can be related to organic solvents generated during progressive maturation.

The interpreted organic-inorganic interactions and associated porosity enhancement events can be diagrammed using a clastic reaction pathway flow chart. The flow chart uses the four component system of CO2, organic acids, carbonates and aluminosilicates to illustrate the evolution of enhanced porosity through time. Use of such flow charts represents the initial step in predicting regions of maximum enhanced and preserved porosity in the subsurface.

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