Harry H. Posey

Fluid-rock interactions in the salt dome environment: An introduction and review

Abstract The salt dome environment in the Gulf of Mexico Coast, U.S.A, provides one of the most diverse and long-lived records of fluid-rock interactions in an active sedimentary basin dominated by saline formation waters. The geochemical record, most of which is isotopic, appears to span a temperature range between Earth surface temperatures and lower greenschist facies. Most mineral-forming processes in the salt dome cap rocks appear to involve mixtures of warm saline formation fluids from deep basin sources and cool dilute meteoric waters. Papers in this issue of Chemical Geology document some of the current research on fluid-rock interactions in the salt dome environment. These papers highlight the diversity of opinion about salt dome processes, offering clues for further research. The present paper reviews some of the key and more current literature on salt dome geochemistry, fluid convection around salt domes, and fluid-rock interactions within salt and within the salt dome environment.

Multiple fluid components of salt diapirs and salt dome cap rocks, Gulf Coast, U.S.A.

Abstract Salt diapirs contain a few percent of anhydrite that accumulated as residue to form anhydrite cap rocks during salt dissolutions. Reported 87Sr/86Sr ratios of these salt-hosted and cap rock anhydrites in the Gulf Coast, U.S.A., indicate their derivation from Middle Jurassic seawater. However, a much wider range of 87Sr/86Sr ratios, incorporating a highly radiogenic component in addition to the Middle Jurassic component, has been found in several Gulf Coast salt domes. This wide range of 87Sr/86Sr ratios of anhydrite within the salt stocks records Sr contributions from both marine water and formation water that has equilibrated with siliciclastics. During cap rock formation this anhydrite either recrystallized in the presence of, or was cemented by, a low-Sr fluid with a Late Cretaceous seawter-type Sr isotope ratio or simply lost Sr during recrystallization. Later, the cap rock was invaded by warm saline brines with high Sr isotope ratios from which barite and metal sulfides were precipitated. Subsequently, low-salinity water hydrated part of the anhydrite bringing to six the total number of fluids that interacted througout the history of salt dome and cap rock growth. The progenitor of these salt diapirs, the Louann Formation, is generally thought to have formed from marine water evaporated to halite and, rarely, higher evaporite facies. Salt domes in the East Texas, North Louisiana, and Mississippi Salt Basins have 87Sr/86Sr and δ34S values that corroborate a Mid-Jurassic age for the mother salt. However, salt domes in the Houston and Rio Grande Embayments of the Gulf Coast Basin have 87Sr/86Sr ration ranging to values higher than both Middle Jurassic seawater and all Rb-free marine Phanerozoic rocks. These anomalous 87Sr/86Sr ratios are probably derived from radiogenic Sr-bearing fluids that equilibrated with siliciclastic rocks and invaded the salt either prior to, or during, diapirism. Potential sources of the radiogenic 87Sr component include clay and/or feldspar (located either in older units beneath the Louann Formation or younger units flanking the salt diapirs) and K-salts within the Louann evaporites. Because partial Sr exchange in anhydrite had to take place in a fluid medium, admittance of radiogenic 87Sr-bearing fluids into the salt may have led to diapirism by lowering the shear strength of the crystalline salt. The slight number of anomalous 87Sr/86Sr values in the interior basins indicates that anomalous values are related to areally discrete structural or stratigraphic controls that affected only the Gulf Coast Basin.

A petrographic and geochemical model for the origin of calcite cap rock at Damon Mound salt dome, Texas, U.S.A.

Abstract Calcite cap rock at Damon Mound, Texas, consists of five calcite generations recognizable through standard petrography and cathodoluminescence. Stage-IA calcite formed by dissolution of anhydrite followed by calcite precipitation and was partly replaced by stage-IB calcite. Stage II formed above stage-IA and -IB substrates either filling open voids or cementing breccia fragments. Stage-IIIA and -IIIB calcites fill late fractures that crosscut all previously formed generations. Microprobe analysis indicates that each calcite generation is geochemically distinct. Sr/Mg ratios indicate that calcite precipitated from variable proportions of formation water, meteoric water, and/or seawater. Wide variation in calcite fluid-inclusion compositions (1.0–9.4 eq.wt.% NaCl) indicates mixing of high-salinity water and low-salinity meteoric water. The single-phase nature of most fluid inclusions and the homogenization temperatures indicate that calcite precipitated at temperatures of Calcite δ 13 C-values range from −31.1 to −14.4‰ (PDB), generally decrease with depth, and are most similar to crude-oil δ 13 C-values. Calcite δ 18 O-values range from −7.6 to −4.8‰ (PDB), generally increase with depth, and are compatible with derivation from local meteoric water or formation water at high temperature. Late-stage calcite can be either isotopically heavier or lighter than adjacent early-stage calcite with regard to both carbon and oxygen, and because isotopic values vary with depth, it appears that fluid mixing is the mechanism most responsible for isotopic variations. Although δ 13 C- and δ 18 O-values of stage-I calcite are similar to adjacent stage-II and -III calcite, 87 Sr 86 Sr ratios of adjacent calcite pairs are generally different, indicating that each formed from different fluids or a fluid whose 87 Sr 86 Sr composition varied through time due to mixing. Isotopic modeling indicates that calcite cap rock formed from either of two fluid mixtures: (1) formation water and meteoric water between 40° and 80°C, the source of light carbon being either thermogenic CH 4 or liquid hydrocarbons; or (2) formation water, meteoric water and seawater between 50° and 70°C with thermogenic CH 4 being the light carbon source. Sr was contributed by both anhydrite and fluid. Overall, it appears that calcite cap rock formed from top to base from mixtures of formation water, containing isotopically light carbon and heavy oxygen, and seawater and/or meteoric water, containing isotopically heavy carbon and light oxygen. During this progression the contributions of formation water increased at the expense of seawater and/or meteoric water. The correlation between δ 18 O and δ 13 C probably indicates that separate fluid pulses had different carbon abundances and/or slightly different δ 13 C-values.

Isotopic Characteristics of Brines from Three Oil and Gas Fields, Southern Louisiana: ABSTRACT

ABSTRACT Isotopic analyses of twenty brine samples from two salt diapir-related oil fields and one growth fault-related gas field in southern Louisiana lend support to the model proposed by Workman and Hanor (this volume) that brines from the zone of geopressure are mixing with hydropressured formation waters along the flanks of the Iberia salt dome and, within the limits of the sampling, suggest that this hydrodynamic process may be characteristic of the region. 18O, D and 87Sr/ 86Sr determinations suggest that formation fluids above 2000 m (6500 ft) depth have partly equilibrated with terrigenous clastic rocks. Fluids below 2000 m (6500 ft) appear to be mixed Oligocene/Miocene seawater and clay-mineral water or evolved hydrocarbon-bearing water. These fluid compositions vary with depth due to mixing and possibly to temperature variations. Some samples may contain constituents derived from salt dissolution.

Chapter 5 Halokinesis, Cap Rock Development, and Salt Dome Mineral Resources

Publisher Summary This chapter focuses on the salt domes, their cap rocks, the associated chemical environments, and the adjacent sedimentary rocks and structures form one of the most economically viable evaporite-related geologic settings. The economic resources and utilizations of the salt dome setting are remarkably diverse. The major economic products of the salt dome environment are oil and gas that occur on the margins of salt stocks, halite and potash salts, and cap rock-hosted native sulfur deposits. Some cap rocks are sources of base metals, limestone, gypsum, or anhydrite, and some are potential sources of celestite or barite. With the exception of halite, none of the salt dome resources would exist except for complex interactions between halokinesis, basinal fluids, and the considerable temperature differentials that are present in the salt dome environment. Whereas halokinesis creates structures that facilitate mineralization in diapirs, fluid interactions within the diapirs may drive diapirism, and fluid interactions in the proper temperature environment can lead to mineralization. In addition to their importance as mineral and hydrocarbon resources, salt diapirs are important for their controls on basin depositional architecture, formation water evolution, sediment diagenesis, and hydrodynamic environments.

A sulfur and strontium isotopic investigation of Lower Permian anhydrite, Palo Duro Basin, Texas, U.S.A.

Abstract Lower Permian Wolfcamp and Wichita carbonates and anhydrites, Palo Duro Basin, Texas Panhandle, record a change from a normal marine to marine evaporite depositional environment. Isotopic compositions of S and Sr in anhydrite were determined to investigate the age of Wolfcamp and Wichita strata and the paragenesis of eight anhydrite forms. Bedded nodular mosaic (Wichita) and replacive nodular (Wolfcamp) anhydrites have S and Sr isotopic compositions that record precipitation from Early Permian (Wolfcampian) to Leonardian) seawater. Silicified nodular, coarsely crystalline nodular, fossil- filling, and vein-filling anhydrites have enriched S isotopic compositions relative to Permian seawater, whereas euhedral and anhedral replacive anhydrites have depleted S. The Sr isotopic composition of most anhydrite forms indicates an Early Permian marine origin, however, Sr in anhydrite veins is slightly radiogenic compared to Permian seawater. The Sr isotopic compoition of bedded nodular mosaic anhydrite indicates as Leonardian seawater source of material. The γ 34 S values are slightly enriched. relative to values predicted from the S age curve, suggesting an excursion of the S isotopic evolution trend of Early Permian seawater. Replacive nodular anhydrite 87 Sr/ 86 Sr ratios are slightly higher than those of bedded nodular mosaic anhydrite reflecting incorporation of Sr from older marine pore waters or replaced sediments or from dissolution of detrital siliciclastic minerals. Anhydrite paragenesis is interpreted from stratigraphic, mineralogic, petrographic, and isotopic data. Bedded nodular mosaic and replacive nodular anhydrite precipitated as marine evaporite strata prograded across underlying normal marine carbonate sediments. Siliified and coarsely crystalline nodules and anhydrite fossil-filling precipitated at essentially the same time as bedded and replacive nodular anhydrite but in environments where sulfate reduction was more extensive. Euhedral anhedral replacive anhydrite formed in shallower environments where S redox cycling occurred prior to CaSO 4 precipitation. Anhydrite vein-fills formed last and incorporated radiogenic Sr released during alteration of detrital siliciclastic grains.

Mineralogic and isotopic constraints on the origin of strontium-rich cap rock, Tatum dome, Mississippi, U.S.A.

Abstract The limestone portion of the salt dome cap rock at Tatum dome, Mississippi, is composed of an upper massive zone and a lower banded zone. The upper zone consists of equigranular fine-crystalline calcite with veinlets and disseminations of carbonaceous matter associated with minor amounts of detrital and authigenic quartz, sulfides and Sr minerals. The lower zone is composed of alternating light and dark calcite bands. The dark bands are composed of fine-grained and peloidal calcite, quartz, bitumen and disseminated sulfide minerals. The lighter bands consist of variable proportions of generally coarse-crystalline euhedral calcite, celestite and strontianite resulting in Sr contents of up to 30% locally. Solubility data for celestite, strontianite, calcite and anhydrite suggest that a decrease in temperature favors the replacement of Ca minerals by Sr minerals, which is consistent with the observed mineral textures and paragenesis. However, the source of cap rock Sr is difficult to determine. Anhydrite at Tatum dome contains 800 ppm Sr, but the abundance of Sr minerals in the limestone cap rock and the 87 Sr 86 Sr ratios of limestone cap rock minerals require a Sr source other than local anhydrite. Sr released by the replacement of anhydrite by calcite is capable of producing a molar ratio of celestite to calcite of only ∼ 0.001, yet locally this ratio is ∼ 3. A likely source of additional Sr is oilfield brines, such as those in central Mississippi that contain up to 3000 ppm Sr along with significant Pb and Zn. Episodic introduction of brines into the cap rock in combination with the action of sulfate-reducing bacteria probably caused the sequential production of the dark bands. The lighter bands probably reflect introduction of a later Sr-enriched fluid or evolution of the original fluid in combination with changes in the chemical parameters controlling mineral precipitation. Calcite was replaced by Sr minerals during this later Sr-rich event.

Chapter 10 Diagenesis and its Relation to Mineralization and Hydrocarbon Reservoir Development: Gulf Coast and North sea Basins

Publisher Summary This chapter explains an integrated hydrothermal model (developed for the Gulf Coast Basin) that has been successfully applied to the North Sea Basin. This comprehensive model relates initiation of salt diapirism, cap-rock formation and cap-rock mineralization to sequential generation of diagenetic minerals, hydrothermal fluids, hydrocarbons, and overpressures that formed as a consequence of source rock and mother-salt burial. Comprehensive modeling may reveal novel techniques for locating salt-dome-related metal deposits, hydrocarbon fields, fractured reservoirs, or deep over-pressured geothermal aquifers, which have significance for future geopressured-geothermal energy production. The North Sea Basin consists of a faulted basement overlain, respectively, by a deformed sediment shell that has been distorted by reactivated basement faulting, and a weakly to unreformed sedimentary carapace, which developed above during thermal subsidence. In the North Sea Basin, the smectite–illite transformation and hydrocarbon generation are shown to be related in temperature, and salt diapirism and overpressuring are related in time. Furthermore, several clay mineral transformations in the North Sea Basin release ions necessary for late diagenetic cementation and dissolution processes.

Successive pore fluid generations in a Lower Permian brine aquifer, Palo Duro Basin, Texas Panhandle, U.S.A.

Abstract The successive presence of four compositionally distinct pore fluids in Lower Permian carbonate strata of the Palo Duro Basin, Texas Panhandle, is interpreted from (1) lithological, minerlogical, and petrographic evidence for depositional conditions; (2) isotopic compositions of C, O, and Sr in limestone and dolomite; (3) limited data on fluid inclusions in sphalerite; and (4) chemical and isotopic analyses of formation water. Wolfcamp carbonate mudstone containing normal marine fauna was initially deposited on an open, shallow shelf. The C and O isotopic compositions indicate precipitation from Permian seawater with minor terrestrial and meteoric influence; Sr isotopic compositions document a Leonardian seawatersource of material. Conditions gradually became more restricted and seawater was concentrated by evaporation, resulting in deposition of penecontemporaneous dolomite and anhydrite in overlying Wichita strata. Reflux of Wichita brine dolomitized underlying Wolfcamp carbonate mudstone and generated the upper Wolfcamp aquifer. Isotopic compositions of Sr in Wolfcamp dolomite suggest that the reflux brine was evaporatively concentrated Leonardian seawater. Expulsion of warm, saline formation water from deeper in the basin through the upper Wolfcamp aquifer is recorded in fluid inclusions in sphalerite. This change in pore fluid composition occured before and during regional Tertiary uplift and tilting of the Texas Panhandle. More recently meteoric water has recharged at least the western one-third of the upper Wolfcamp aquifer. This modern flow regime was established within the past 10–15 Ma.

Provincial Variations in Cap Rock Source Materials of Gulf Coast Salt Domes: ABSTRACT

Isotopes of strontium, carbon, and oxygen are used to model hydrocarbon, brine, and meteoric fluid interactions during cap rock evolution. Provincial isotopic variations occur between older salt domes of the east Texas (ETx) and northern Louisiana (NLa) basins and the younger domes of the Texas-Louisiana (Tx-La) coastal basin. ETx and NLa cap rocks exhibit normal, mid-Jurassic seawater values (87Sr//86Sr = 0.7068 to 0.7076), very wide ^dgr13C ranges (-5 to -49 per mil PDB) and ^dgr18O values (-6 to -11 per mil PDB) that are slightly lighter than Tx-La (-4 to -10 per mil). Tx-La domes yield remarkably high 87Sr/86Sr ratios (0.7073 to 0.7100), and their ^dgr13C values (-8 to -41 per mil) have means which are 5 to 15 per mil heavier than ETx and NLa domes. Detailed studies of the Hockley dome (Tx-La basin) reveal chemical diversity not recognized in domes farther inland. Anhydrite from the salt stock (mid-Jurassic Louann evaporites) mixed with two separate strontium sources during calcite formation. Calcites near the domes center formed from an intermediate Sr ratio fluid (87Sr/86Sr ^cong 0.7090), which, based on heavier than average ^dgr13C values, was enriched in CO2 relative to CH4; peripheral calcites evolved from a high Sr ratio fluid (87Sr/86Sr ^cong 0.7105) with a lower CO2/CH4 ratio. High 87Sr/86Sr ratios in other Tx-La anhydrite cap rocks compared with normal mid-Jurassic type values in ETx and NLa cap rocks suggest End_Page 297------------------------------ that the Tx-La basin was periodically isolated from normal seawater during Louann deposition; radiogenic fluids, derived either from local red beds or from meteoric waters, equilibrated with seawater prior to anhydrite precipitation. End_of_Article - Last_Page 298------------