About This Item
Share This Item
The abundance of the isotope 87Sr is variable in nature, as it is the radiogenic product of 87Rb decay. The relative amount of this Sr isotope that is dissolved in a brine, as expressed by the 87Sr/86Sr ratio, might be used as a tracer of the origin and subsequent history of the brine, including its diagenetic effects in petroleum reservoirs. Strontium isotopic analyses of 40 brines from oil fields in southern Arkansas have been conducted to investigate the sources of the dissolved Sr, the pathways of brine migration, and the relationship between the brines and diagenetic phases in the Jurassic upper Smackover Formation. The 87Sr/86Sr ratios of 33 brines from the upper Smackover lime-grainstone range from 0 7071 to 0.7101; seven brines from formations stratigraphically above the Smackover range from 0.7090 to 0.7112. Thus the Sr in these brines is variably more radiogenic than Jurassic sea water
(~0.7070). The brines are from 15 oil fields within an area of approximately 2,000 mi2 (5,200 km2); the isotopic variability shows no geographic pattern in the study area. The variation in 87Sr/86Sr ratios of brines occurs to a lesser degree within individual oil fields. For example, over a distance of about 10 mi (16 km) at Walker Creek, the isotopic ratios of 11 Smackover brines range from 0.7080 in the east to 0.7086 in the west.
The observation that Smackover brines are variably more radiogenic than Jurassic seawater is important because it indicates that a significant proportion of the Sr dissolved in these brines has been acquired from a source material that has not formed wholly by precipitation from Jurassic seawater. That is, some radiogenic Sr must have been added to the brines from a detrital source material. The nature and distribution pattern of the 87Sr/86Sr ratios indicate the acquisition of variable amounts of radiogenic Sr on a local basis. If the Smackover brines originated in the Louann Salt, with 87Sr/86Sr equivalent to that of Jurassic seawater, their present isotopic compositions may be the result of varying degrees of subsequent interaction with d trital sediments or they may have been produced by mixing in variable proportions with solutions containing more radiogenic Sr. Potential sources of the radiogenic Sr are the Norphlet Formation and the lower Smackover argillaceous lime-mudstone, both of which lie stratigraphically between the Louann salt and the upper Smackover, as well as the Bossier shale which interfingers with the upper Smackover in the North Louisiana salt basin. Anhydrites from the Werner and Buckner formations and from northern Louisiana salt domes, which constitute additional potential sources of brine Sr, yield 87Sr/86Sr ratios equivalent to those of Jurassic seawater.
Diagenetic phases of the upper Smackover, such as post compaction calcspar cement and baroque dolomite, have 87Sr/86Sr ratios more radiogenic than Jurassic seawater, suggesting their subsurface precipitation in Sr isotopic equilibrium with Smackover brines. However, ooids and oncolites from the upper Smackover lime-grainstone yield 87Sr/86Sr ratios indicating isotopic equilibrium with Jurassic seawater.
End_of_Article - Last_Page 554------------