Abstract: Permeability Control in Cretaceous Gas-Bearing Sandstones
Movement of subsurface waters in structural basins and domes is a result of both mechanical and chemical forces. In the short term, and for larger permeabilities, water moves laterally in response to mechanical pressure differences. Over longer time periods (thousands of years), water moves vertically as a result of chemical differences in fluids.
In marine basins, the result of chemical contrasts is the movement of strong brines upward by compaction and fresh waters downward by rehydration. Rewatering or dilation examples are the Bowdoin dome, Montana, and Wattenberg field, Colorado. The Upper Cretaceous sandstones currently are experiencing erosion and imbibition of fresh water. As a result of gas-saturated waters in reservoir rocks, free carbon dioxide spontaneously attracts fresh water from the meteoric-water zone. The resulting H2CO3 attacks carbonate cement to yield bicarbonated waters and increased permeability. Owing to loss of gas volume the pressure drops, causing more water to move into the newly created paths and to dilute the previous sodium-chloride brines. The entering water is shown to be f esh by its low sulfate content, for deeper waters have large gypsum components. Consequently, a search for high HCO3- and slightly reduced pressure gradients is a search for better permeability. The overall process is exothermic and yields unusually high temperatures as in the Piceance basin and Wattenberg field, Colorado (where the process evidently is under way).
In the compacting basin, brines forced upward are enriched by the leaching of marine shales and possibly bedded salt. The easiest volume reduction is caused by water emerging from fractures as the basin settles. This water is dominantly a Na+Cl- type with significant CA+ +SO4- - and other compounds, which may be used to identify the source whenever waters are mapped in three dimensions.
The precipitation of CaCO3 cement or the opposite, the dissolution to form bicarbonate, is a valuable basin-dynamics indicator, and it may be evaluated directly from well logs. A water-traffic map may be constructed for the Mowry Shale, for example, by contouring the vertical-resistivity gradient in this middle Cretaceous sedimentary rock.
Considerable exploration information is available from well logs using vertical profiles which are related to water chemistry. The interpretation of water chemistry can provide clues for the location of oil and gas deposits.
AAPG Search and Discovery Article #90964©1978 AAPG Rocky Mountain Section Meeting, Salt Lake City, Utah