Salinity of deep formation waters, Texas Gulf Coast - preliminary results

摘要

Vertical variation in salinity can be either systematic or unpredictable. The widely reported decrease in salinity near the top of geopressure led some workers to erroneously conclude that salinities in the geopressured zone are less than in the hydropressured zone. Indeed, salinities are lowest in the transition zone, but in many areas, salinities of geopressured reservoirs tend to gradually increase with depth. High salinities at depth have been explained by hyperfiltration and proximity to salt domes. Apparently both mechanisms are responsible for some high salinities in the Frio Formation; however, concentrated brines are not necessarily indicative of salt dissolution as shown by water analyses from south Texas. This area is far removed from any known salt accumulations and yet salinities of formation water from some fields are greater than 200,000 ppM. On a local scale, lateral variations in salinity are minor and wells within the same fault-bounded reservoir produce waters with nearly uniform salinities. For intermediate ranges (35,000-70,000 ppM), well to well variations are generally less than 20,000 ppM. Even reservoirs that are stratigraphically equivalent but separated by minor faults produce waters with similar salinities. The greatest differences in salinity are found in stratigraphically separated reservoirs.