Paul Pushkar

Formation waters from Mississippian-Pennsylvanian reservoirs, Illinois basin, USA: Chemical and isotopic constraints on evolution and migration

We have analyzed a suite of seventy-four formation-water samples from Mississippian and Pennsylvanian carbonate and siliciclastic strata in the Illinois basin for major, minor, and trace element concentrations and for strontium isotopic composition. A subset of these samples was also analyzed for boron isotopic composition. Data are used to interpret origin of salinity and chemical and Sr isotopic evolution of the brines and in comparison with a similar data set from an earlier study of basin formation waters from Silurian-Devonian reservoirs. Systematics of Cl-Br-Na show that present Mississippian-Pennsylvanian brine salinity can be explained by a combination of subaerial seawater evaporation short of halite saturation and subsurface dissolution of halite from an evaporite zone in the middle Mississippian St. Louis Limestone, along with extensive dilution by mixing with meteoric waters. Additional diagenetic modifications in the subsurface interpreted from cation/Br ratios include K depletion through interaction with clay minerals, Ca enrichment, and Mg depletion by dolomitization, and Sr enrichment through CaCO3 recrystallization and dolomitization. Ste. Genevieve Limestone (middle Mississippian) formation waters show 87Sr86Sr ratios in the range 0.70782–0.70900, whereas waters from the siliciclastic reservoirs are in the range 0.70900–0.71052. Inverse correlations between 87Sr86Sr and B, Li, and Mg concentrations suggest that the brines acquired radiogenic 87Sr through interaction with siliciclastic minerals. Completely unsystematic relations between 87Sr86Sr and 1/Sr are observed; Sr concentrations in Ste. Genevieve and Aux Vases (middle Mississippian) waters appear to be buffered by equilibrium with respect to SrSO4. Although there are many similarities in their origin and evolution, these formation waters are distinguished from Silurian-Devonian brines in the basin by elevated ClBr and NaBr ratios and by unsystematic Sr isotope relationships. Thus waters from these two major segments of the Illinois basin stratigraphic column form distinct geochemical regimes which are separated by the New Albany Shale Group (Devonian-Mississippian) regional aquitard. Geochemical evolution appears to have been influenced significantly by Paleozoic and Mesozoic hydrologic flow systems in the basin.

A strontium isotopic study of Smackover brines and associated solids, southern Arkansas

Abstract The isotopic composition of Sr has been measured in brine samples from the Upper Jurassic Smackover Formation in southern Arkansas; 87 Sr 86 Sr ratios range from 0.7071 to 0.7101. With one exception, the 32 Smackover brines contain Sr which is significantly more radiogenic than the Sr in Late Jurassic sea water, indicating sizable Sr contributions from detrital sources. Isotopic analyses of core samples from rock units associated with the brines and regional stratigraphic relationships suggest that the radiogenic Sr was released from detrital minerals in Bossier shale to interstitial fluids expelled from the underlying Louann Salt in the North Louisiana salt basin. These fluids migrated through the Bossier Formation updip to the South Arkansas shelf, where they entered the upper Smackover carbonate grainstone. The radiogenic fluids mixed with Sr-rich interstitial marine waters that had the isotopic composition of Late Jurassic sea water; mixing in variable proportions resulted in the random distribution pattern of variable 87 Sr 86 Sr ratios that is observed in Smackover brines within the 5000 km2 study area. Isotopic analyses of nonskeletal carbonate grains and coexisting coarse calcspar cement from the upper Smackover grainstone imply that the grains were diagenetically stabilized in the presence of interstitial marine waters, whereas the calcspar cement is a relatively late diagenetic phase precipitated after the arrival of radiogenic fluids.

A strontium isotopic study of formation waters from the Illinois basin, U.S.A.

Abstract The isotopic composition of Sr has been measured in 73 formation-water samples from Paleozoic strata in the Illinois basin; 87Sr/86Sr ratios range from 0.7079 to 0.7108. With the exception of four samples, the waters are more radiogenic than corresponding Paleozoic sea-water values. The relatively narrow range of slightly elevated 87Sr/86Sr rations is uniformly distributed in waters throughout the stratigraphic column and in Silurian waters across the basin. Isotopic analyses of core samples from reservoir rocks show an absence of water-rock Sr isotopic equilibration. Basin lithology and analyses of detrital rock units indicate that clay minerals in shales and in quartz sandstone matrices represent the only significant source of radiogenic Sr for the waters. Silurian and Devonian water show a two-component mixing relation which suggests that they comprise a single hydrogeological system that evolved when radiogenic water from New Albany shales entered Silurian-Devonian carbonate rocks and mixed with marine interstitial water. Regional migration of the waters and associated petroleum within the Silurian-Devonian strata, proposed in other studies, is consistent with the Sr isotopic data. Under favorable circumstances subsurface waters are capable of retaining a Sr isotopic recor of their evolution.

The isotopic composition of strontium in brines from petroleum fields of southeastern Ohio

Abstract 87 Sr 86 Sr ratios were determined for one oil and fourteen brines collected from three petroleum reservoirs in southeastern Ohio: the North Logan and Triadelphia fields, both of which are contained within the Clinton sandstone; and the Macksburg field which is contained within the Berea and Macksburg sandstones. Only the Clinton sandstone yields brines that could reasonably be assumed to be uncontaminated with other fluids. The data suggest that in the North Logan field this brine has a characteristic value of 0.7103 ± 0.0005 which persists for distances of at least several kilometers. This persistence, however, does not extend to the Triadelphia field 125 km away where the brine from the Clinton sandstone has a somewhat lower ratio of 0.7092 The 87 Sr 86 Sr ratio in one sample of oil from the Clinton sandstone in the North Logan field is identical to that in the associated brine. The data from the brines collected from the Macksburg and Berea sandstones in the Macksburg field indicate that these brines are severely contaminated with other fluids. The ratios in all the brines are generally similar to those in the local ground and stream waters, so that the ratios in all the fluids may be governed by the same factors. In southeastern Ohio, the presence of carbonate minerals in the enclosing rocks seems to be the dominant factor in controlling the 87 Sr 86 Sr ratios of the fluids.

87Sr/86Sr ratios in some volcanic rocks and some semifused inclusions of the San Francisco volcanic field

Two cinder cones and associated basaltic lava flows near Williams, Arizona, contain numerous inclusions of ultramafic and of felsic material. Many of the felsic inclusions are semifused and contain high 87 Sr/ 86 Sr ratios, yet the enclosing basalts do not show any enrichment in 87 Sr when compared with other local basalts. Brown glass within a quartz monzonite inclusion has a much higher 87 Sr/ 86 Sr ratio than the inclusion as a whole. This is believed to reflect the domination of early-forming melts by biotite. If so, separation and assimilation of this glass by a magma offers a mechanism to increase the 87 Sr/ 86 Sr ratio of that melt without significant bulk assimilation of granitic material.

Geochemistry of Strontium in the Scioto River Drainage Basin, Ohio

Ground water that emanates from carbonate bedrock in the Scioto River drainage basin is characterized by 87 Sr/ 86 Sr ratios in the range of 0.708 to 0.709; usually high Sr/Ca ratios in this water identify celestite lenses within the carbonate bedrock as the dominant source of strontium. Ground water from clastic bedrock, principally shale, has 87 Sr/ 86 Sr ratios that vary from about 0.710 to about 0.713 and shows low Sr/Ca ratios. Thus, there are two basic ground-water types that emanate from bedrock within the basin. They can be identified by these two parameters. Most ground water that has been in contact only with glacial till, which covers the northern two-thirds of the basin, has carbonate-type 87 Sr/ 86 Sr ratios as well as high Sr/Ca ratios. Celestite is apparently present in the till throughout much of the Scioto basin. Ground water that contains celestite-derived strontium, whether from the carbonate bedrock or the till, has so great a strontium content as to control completely the 87 Sr/ 86 Sr ratios of surface water northwest of the glacial boundary. This fact limits the usefulness of the 87 Sr/ 86 Sr parameter as a tracer in water studies within the basin.

Application of Strontium Isotopes to Origin of Smackover Brines and Diagenetic Phases, Southern Arkansas: ABSTRACT

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 End_Page 553------------------------------ (~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------------

Origin of the Quaternary Basalts from the Black Rock Desert Region, Utah: Strontium Isotopic Evidence

The basalts of the Black Rock Desert region in Utah have Sr 87 /Sr 86 ratios that vary from 0.7044 to 0.7059. Although this variation may be explained by derivation from inhomogenous mantle, it is also broadly compatible with a fractionation-assimilation model proposed for these basalts. Basalts of the Ice Spring field show an increase in Sr 87 /Sr 86 with decreasing Rb and Rb/Sr as a function of eruptive age. Such a trend may be explained by mixing of magmas having different differentiation-assimilation histories.