Abstract: Quantitative Petrologic Constraints on Basin Paleohydrologic Models

Raymond Siever, W. Naylor Stone

Quantitative petrologic analysis can measure the effects of diagenetic processes on the porosity of sandstones. By setting bounds on pressure solution and cementation, petrology also checks predictions based on computer models of basin tectonic and paleohydrologic evolution and so indirectly assesses model performance. Petrology can help modelers choose carefully among the many assumptions of even simplified models.

We have assessed the role of mechanical compaction, chemical compaction

(pressure and grain solution), and cementation of deeply buried quartz arenitic sandstones in three basins (Greater Green River, Anadarko, East Texas) and compared them with an earlier study of moderately deeply buried sandstones in the Denver basin. We analyzed for intergranular volume (IGV), intergranular pressure solution (IPS) and stylolitization, allowing prediction of porosity decrease and cementation as a function of depth of burial and thermal maturity. We find that mechanical compaction in early stages of burial reduces IGV to approximately 31%, while pressure solution further reduces IGV further to a range about 24%. Much of this IGV reduction is complete by relatively shallow burial (2 km). Reduction of porosity by later cementation in available pore space is more variable nd correlates well with thermal maturity (vitrinite reflectance). IPS and stylolitization cannot account for the amounts of quartz cement in these sandstones; hence most are silica importers.

The implication is that some mechanisms (convective? advective? chemical reaction?) are responsible for quartz cement increasing with thermal maturity. This seems incompatible with models that predict little in the way of deep fluid movement in the deep parts of basins (well below levels of meteoric water mixing). We suspect that there is more thermobaric fluid movement than predicted in many basins and that waters released from dehydration reactions (including smectite-illite and poorly crystallized to well crystallized illite and muscovite) are also significant over a greater range of depths than commonly assumed.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994