HISTORY AND MECHANISMS OF ACTIVE FLUID FLOW THROUGH LOW-PERMEABILITY SEDIMENTARY ROCKS

It is reasonable to hypothesize that mud rocks are closed to new fluids over geologic time, harboring only connate fluids. Diagenetic data from the Permian Basin, Midcontinent and Williston Basin, however, call that into question. The data show that systems were repeatedly open to fluid flow.

The mineral paragenesis in all three regions is complex, each showing 20-30 events. Textures in the Bakken-Scallion (Williston Basin) indicate early burial fluid escape from compaction. Dolomite has isotopic signatures and distribution indicating precipitation from outwardly migrating connate fluids at low temperature. Also during early burial, in the Permian Basin and Midcontinent, calcite, dolomite, and anhydrite have fluid inclusion homogenization temperatures and ice final melting temperatures indicating precipitation from low temperature and high salinity brines. Sr, O isotope, and U/Pb dates all point to regional brine reflux associated with deposition of Permian evaporites. This record is consistent with composition of modern basin brines. The brine reflux replaced most connate fluids, and brines evolved in composition through rock-water interaction. This diagenetic record is commonly followed by widespread fracturing, calcite, quartz, and dolomite. Fluid inclusions yield episodically high homogenization temperatures, moderately radiogenic Sr, and O isotope and fluid inclusion data indicating abnormal geothermal gradients and mixing. These data indicate multiple events of regionally advective hydrothermal fluid flow, linked to orogenic events. Later fracturing and calcite cementation, from fluids that show high homogenization temperatures and radiogenic Sr, suggest vertical fluid flow was possibly related to fault pumping or valving.

The diagenetic record from these regions shows that fluid flow was extensive including: (1) radial flow from compaction fluids; (2) density-driven downward reflux associated with evaporites; (3) regionally advective hydrothermal flow caused by tectonic uplift; and (4) vertical flow caused by fault pumping or valving. It is clear that the diagenetic system was repeatedly dynamic. The active fluid flow affected the diagenesis, porosity, thermal history, and hydrocarbon migration as well as providing a detailed record of tectonic and hydrologic drivers.