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The AAPG/Datapages Combined Publications Database

AAPG Special Volumes

Abstract


Pub. Id: A071 (1965)

First Page: 342

Last Page: 366

Book Title: M 4: Fluids in Subsurface Environments

Article/Chapter: Saline Waters of Sedimentary Rocks

Subject Group: Oil--Methodology and Concepts

Spec. Pub. Type: Memoir

Pub. Year: 1965

Author(s): Donald E. White (2)

Abstract:

Most saline waters of marine sedimentary rocks were probably similar initially to present-day ocean water. Many early diagenetic changes in sediments and waters are related to organic content and bacterial activity; ion exchanges and perhaps some other early changes are inorganic. Diagenetic and later changes in sedimentary rocks cannot be understood without considering the associated fluids, which are mobile and leave little direct and easily interpretable evidence of their changing compositions with time.

Compaction of sediments and escape of interstitial water start at the time of deposition and probably continue for millions of years. The evidence is now convincing that fine-grained sediments behave as semipermeable membranes, permitting selective escape of water and concentrating dissolved components in remaining pore fluids. The initial driving force is Previous HitlithostaticNext Hit Previous HitpressureTop; after maximum compaction has been attained, salt-filtering may continue under certain circumstances of topography, structure, and lithology, with meteoric water providing the driving energy.

As salinity of the retained pore fluids increases, proportions of some constituents also change, with calcium in particular tending to increase relative to sodium in normal marine sediments. The usual explanation involves liquid-solid cation exchange reactions. This paper suggests that mobilities of individual dissolved constituents differ greatly and that CO2, B, and perhaps also NH4+ and H2S normally are relatively high in the escaping water. When cation-exchanging clay minerals are tightly compressed, their fixed negative charges evidently repel anions including Cl-, HCO3- and CO3-. The compositions of thermal and mineral waters of sedimentary rocks, however, strongly indicate that CO SUB>2 does escape through clays, perhaps as uncharged H2CO3 molecules. There is a little experimental evidence to indicate that an excess of cations over anions in the fluid passing through the membrane is balanced by a net return flow of H+ions. In carbonate-free sediments the system presumably soon comes to a steady state, but if carbonates are in the sediments, they may dissolve in the H+-enriched environment behind the membrane, thereby maintaining a supply of HCO3- and H2CO3 for further escape. The chemical evidence suggests that Ca++ (mostly from calcite?) is less mobile than Cl-, probably because of the double charge of Ca++, and is enriched in the retaine brine.

Original ocean water in contact with normal sediments evolves to connate (as redefined) or fossil water vastly different from its initial composition, with rates of migration and chemical evolution depending upon its physical and chemical environment. Membrane-filtered and membrane-concentrated types of connate water are recognized, and all gradations can be expected between extremes of these two types.

Chemical and isotopic criteria also clarify the origin of other saline waters in addition to those of normal marine sediments. These other types include connate waters of marine and nonmarine evaporites, several kinds of waters of relatively low salinity that have dissolved evaporites, sulfate and bicarbonate waters of oil fields, and waters driven out of sedimentary rocks during progressive metamorphism. Relatively complete analyses of waters from oil fields and other sources are included to illustrate some principles, but much additional field and experimental study is needed.

Waters escaping from depth into a lower temperature environment will normally exchange dissolved K for Na from the solid phases. In some circumstances the K/Na ratio of a water serves as a crude geothermometer for water temperature.

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