Connate Fluids Research Articles

Palaeocirculation in the basin of southeastern France sub-alpine range: a synthesis from fluid inclusions studies

In the basin of southeastern France, (sub-Alpine range), circulation of palaeofluids relates to the geodynamic evolution of the western Tethyan margin. Alpine chain build-up from the Triassic to Present. Recent reconstructions allow distinction of two main provinces: 1) the western border of the basin, where the Jurassic extensional structures have not been affected by Alpine compression; and 2) the central and eastern parts of the basin, subjected to a partial tectonic inversion during Eocene and Miocene compressional events. In the Oxfordian shales, two main distinct areas are recognized on both sides of the Digne fault, grading from a diagenetic domain with temperatures lower than 150°C, corresponding with aliphatic oil in fluid inclusions, to a low grade metamorphism with temperatures up to 250°C and dry gas. Hydrothermal circulations are suspected, but not yet evidenced. During the Oligocene, rising Triassic diapirs were the setting of intense fluid circulation with mineral neoformations. Fluid inclusions indicate a heterogeneous trapping of NaCaKMgCI solution (up to 35 wt% NaCl equ.) saturated with CO 2 ± N 2 at temperatures up to 350°C, during successive episodes of fluid injection. Such gas may originate from deep sources (pervasive flow of CO 2 ± N 2), but local shallow reactions can not be rejected. P-T conditions of 250°C-1.5 kbars are deduced for the eastern diapirs, and 250°C−1.5 kbars to 360°C 2.2 kbars for western diapirs. Geodynamic reconstructions indicate a palaeoburial depth for the brine of about 10 km corresponding to a mean thermal gradient of 35°C/km. This is in agreement with the burial depth of Triassic evaporates, exceeding 8 km during Cretaceous, before the main diapir rise in the Oligocene. Mesozoic sediments were investigated by drilling near the Uzer fault (Balazuc). We recognize major diagenetic events involving mineral transformations at temperatures up to 150°C. This can be explained by greater burial during the Cretaceous with a post-Cretaceous erosion amounting to about 1.5 km. Late circulation of low temperature CaMgNaBa rich solutions in all the series is indicated by intense fracturing. According to its tectonic characteristics, this episode may relate to fault reactivation during the Eocene. In conclusion, the configuration of the basin and its western border allows trapping of connate fluids beneath the Triassic evaporate, and their mixing with deeper water. All these fluids circulated between the Hercynian crystalline basement and the Triassic evaporates, across Lias and middle Jurassic strata during the Middle Cretaceous, in infra-Triassic sedimentary layers. The estimated depth during this period is about 10 km. In the basin, a normal geothermal gradient account for oil and gas generation. Low temperature subsurface aqueous fluids did circulate along the basin border in the Palaeogene during an episode of fault reactivation.

Diagenesis of Barremian‐Aptian platform carbonates (the Urgonian Limestone Formation of SE France): near‐surface and shallow‐burial diagenesis

ABSTRACTThe diagenesis of carbonate platform sediments is controlled by the original facies and mineralogy, climate, sea‐level changes and burial history; these controls are clearly seen in the diagenesis of the Urgonian platform carbonates of SE France. Early diagenesis in the Urgonian platform included the precipitation of marine cements, dissolution of rudist shells and minor karstification. Diagenetic features produced during this phase were controlled by several falls in relative sea‐level during the Barremian to mid‐Aptian punctuating platform sedimentation, the original mineralogy of the sediment and the prevailing semi‐arid/arid climate in the region at this time. Following a relative sea‐level rise and further sedimentation, progressive burial of the platform led to minor compaction, followed by precipitation of coarse, equant, zoned to non‐luminescent, calcite cement. This cement was cut by later stylolites, suggesting a relatively shallow‐burial origin. Stable isotope (mean values ‐ 7.94%δ18O and 0.36%δ13C) and trace element (mean values of Fe 334 ppm, Mn 92 ppm and Sr 213 ppm) data suggest that these cements precipitated from meteoric fluids at temperatures slightly elevated relative to depositional temperatures.A variable thickness of replacive dolomite which occurs preferentially within the shelf‐margin facies of the lower part of the Urgonian post‐dates mechanical fracturing and chemical compaction, but pre‐dates the main phase of stylolitization. It is probable that the dolomitizing fluid was sourced by the early compaction‐driven release of connate fluids held within the underlying muddy units.The burial history of these rocks suggests that calcite cementation and dolomitization took place at relatively shallow burial depths (1–1.5 km). The overall diagenetic history of the Urgonian Limestone Formation is a reflection of the pre‐conditioning of the platform limestones by climate, sea level, tectonics and the shallow burial depths experienced by the platform during the later Mesozoic.

Fluid inclusion and isotopic systematics of an evolving magmatic-hydrothermal system

The Geysers, California, is the site of a long-lived hydrothermal system that initially developed 1.5–2 m.y. ago in response to the intrusion of a hypabyssal granitic pluton. Although wells drilled into The Geysers produce only dry steam, fluid inclusion, isotopic, and mineralogic data demonstrate that the present vapor-dominated regime evolved from an earlier and more extensive, liquid-dominated hydrothermal system. Circulation of these early fluids produced veins characterized by tourmaline ± biotite ± actinolite ± clinopyroxene within the pluton and adjacent biotite-rich hornfels, actinolite ± ferroaxinite ± epidote and epidote ± chlorite within the intermediate parts of the thermal system and calcite in the outer parts. Potassium feldspar and quartz are present in all assemblages. Pressure-corrected homogenization temperatures and apparent salinities of fluid inclusions trapped in vein minerals range from 440°C and 44 wt% NaCl equivalent within the hornfels (<600 m from the pluton) to 325°C and 5 wt% NaCl equivalent at distances of approximately 1500 m from the intrusion. We suggest that the shallow, moderate salinity fluids are connate waters modified by water-rock interactions while the high-salinity fluids are interpreted as magmatic brines. Halite-dissolution temperatures of inclusions in the hornfels and pluton indicate that the magmatic fluids were trapped at lithostatic pressures (300–900 bars). In contrast, homogenization temperatures of the connate fluids suggest trapping under hydrostatic pressures of less than several hundred bars. Whole-rock δ 18O values of samples from The Geysers display systematic variations with respect to depth, location within the field, and grade of alteration. At depths below +610 m relative to mean sea level, the δ 18O values are strongly zoned around a northwest-southeast trending low located near the center of the steam reservoir. As the pluton beneath The Geysers is approached, the δ 18O values decrease from approximately + 14 per mil near the surface to +4 to +7 per mil within the hornfels. The δ 18O values then increase to +8 to + 10 per mil at the intrusive contact, and thereafter, vary little within the pluton. Calculated rock-water fractionation factors suggest that temperature was the dominant control on the vertical decrease in the isotopic values of the reservoir rocks. In contrast, the increase in the δ 18O values within the hornfels suggest equilibration with an isotopically heavy fluid and/or decreasing water:rock ratios. The formation of vapor-dominated conditions is reflected in the abrupt appearance of low salinity (0.0–0.4 wt% NaCl equivalent) fluid inclusions with homogenization temperatures near 265°C. These inclusion fluids are thought to represent steam condensate that formed as the early liquid-dominated system boiled off. Bladed calcite, which is corroded in places, and late-stage clays provide mineralogic evidence of boiling and the formation of an acidic condensate in the upper part of the reservoir and in the overlying caprock. At greater depths, however, clear mineralogic evidence of boiling is lacking. The present vapor-dominated regime at The Geysers consists of two hydraulically connected steam reservoirs. Within the main steam reservoir, pressures are vaporstatic and temperatures are nearly constant at 240°C. Beneath this reservoir in the northwestern third of the field, temperatures as high as 342°C have been encountered. Wells drilled into this deeper reservoir discharge corrosive, Cl-bearing steam.

Can Regional Porosity Occurrence in Carbonate Slope and Basin Facies Systems be Predicted?: ABSTRACT

Subsurface carbonate depositional sequences (i.e., coeval platform-to-basin facies mosaics), which can most readily be identified by application of seismic sequence analysis, include porous units of complex geometry and occurrence. A highly sought-after occurrence of reservoir-grade porosity commonly, but not always, is associated with sequence boundaries. Therefore, porosity can be predicted to possible occur in already-lithified platform strata beneath significant unconformities of regional extent (e.g., cavernous porosity). By this method, however, the possible existence of porosity in carbonate slope and basin deposits may be overlooked because they do not lie beneath sequence-bounding unconformities insofar as they are not usually exposed subaerially during platform emergence. Yet, such facies compose locally significant hydrocarbon reservoirs in the Permian basin by virtue of high porosities and permeabilities and water-free production histories. Short of drilling or seismic wavelet analysis, can porosity in such facies be predicted a priori Reservoirs in carbonate slope and basin deposits in the Midland basin include both porous limestones and dolomites. Petrographic and geochemical studies indicate that syndepositional and shallow-burial diagenesis of such deposits mainly involved porosity reduction via calcite cementation, dolomitization, and compaction. Most of the porosity present in reservoirs in these facies was created by later dissolution in the deep-burialmore » environment as a consequence of rock interaction with connate fluids enriched in carbonic and sulfuric acid and/or organic acids generated during hydrocarbon maturation. These fluids migrated up and out from deeper parts of the basin through slope and basin deposits and into platform carbonate strata. Insofar as fluid migration pathways can be mapped or at least inferred by various means, it follows that porosity created (or destroyed) by these fluids also can be mapped predictively.« less

Diagenetic mass transport in the southern San Joaquin basin, California, U.S.A.: Implications from the strontium isotopic composition of modern pore fluids

Three types of chemically and isotopically distinct pore fluids from the southern San Joaquin basin previously recognized by J.B. Fisher and J.R. Boles also have distinctive 87 Sr 86 Sr ratios and Sr concentrations. Meteoric fluids have stable isotopic compositions which lie on or near the meteoric water line and low chlorinities. Sr concentrations are between 0.01 and 2.6 mg l −1, and 87 Sr 86 Sr ratios range from 0.7061 to 0.7078. Diagenetically modified connate marine fluids have δD-and δ 18O-values more positive than −35‰ and 0‰, respectively, and have chlorinities generally comparable to seawater. Sr concentration are much higher than the meteoric group (16–198 mg l −1), although the 87 Sr 86 Sr ratios (0.7070–0.7081) are not distinctive. Mixed meteoric-modified connate fluids have δD, δ 18 O and chlorinity intermediate between the meteoric and modified connate groups. Sr concentrations are also intermediate, between 16 and 22 mg l −1, but 87 Sr 86 Sr ratios (0.7080–0.7087) are generally more radiogenic than either the meteoric or modified connate groups. All of the fluids have 87 Sr 86 Sr ratios comparable to or lower than Tertiary seawater. Alteration of detrital plagioclase is the probable origin of the low isotopic ratios. Mass-balance calculations based on the Sr data suggest that essentially no transport of Sr occurred during diagenesis of sandstones containing modified connate pore fluids, while large amounts of Sr have been transported out of meteoric reservoirs by fluid flow. The chemically anomalous mixed meteoric-modified connate fluids contain the most radiogenic strontium in the basin. These fluids are spatially associated with major faults, and may represent clay mineral dehydration waters which have been transported upward from greater depth. These results suggest that the three types of fluids identified by Fisher and Boles represent three distinct mass transport regimes: a largely stagnant deep-basin system containing modified connate pore fluids; an actively recharging meteoric system along the basin flanks; and a third system restricted to the southern basin which may be characterized by largescale cross-formational fluid flow, rather than dilution by meteoric waters.

The third age of wireline log analysis: application to reservoir diagenesis

Abstract Geophysical well logs were first developed to identify the porosity and fluid composition of potential petroleum reservoirs. Subsequently their use was extended to include diagnosing the depositional environment of reservoirs, as an aid to predicting their geometry. Both these techniques are now routine. A new age of the application of geophysical logs now dawns due to the enhanced understanding of the diagenetic processes that affect petroleum reservoirs, including cementation which destroys porosity and solution which enhances it. These ideas can be used to interpret both conventional porosity logs and the new types of log that, when carefully interpreted, allow vertical variations in mineralogy to be measured. Three main diagenetic effects can be recognized in geophysical well logs: facies-related diagenetic phenomena; cemented and leached zones related to migrating diagenetic fronts of acid meteoric or deep connate fluids; and diagenetic effects related to petroleum migration and entrapment.

Dissolved gases in the Milk River aquifer, Alberta, Canada

Geochemical relations between the noble gases, 40Ar/ 36Ar ratio, N 2 and CH 4 contents of groundwaters from the Milk River aquifer, Albert, Canada, have been investigated. The CH 4 contents for many of the groundwaters are several times their N 2 contents and cause degassing to occur as the hydrostatic pressure decreases during their ascent in the wells. Recharge temperatures, derived for those groundwaters that were not degassed, indicate that the youngest groundwaters were recharged under cooler climatic conditions than prevailed during earlier recharge. This conclusion is supported by the stable isotope compositions of the groundwater. The 4He concentration in the groundwater varies with depth according to the theoretical concentration/depth profile for accumulated radiogenic 4He in the aquifer and its adjacent shales. The corresponding flux of 4He to the atmosphere is <2% of the total crustal 4He production rate. Excess radiogenic 40Ar is associated with CH 4 and both gases originated within the adjacent shales. The CH 4 is biogenic and was produced in an aqueous environment which ranged from marine to freshwater as the connate fluids in these marine shales were replaced. The degassing process has been modelled to explain the residual dissolved gas contents of the groundwater and to show that the CH 4/N 2 ratio in situ was ∼12.

Fluid evolution during uplift of the Annapurna Himal, central Nepal

Fluid inclusions from post-metamorphic quartz show that there has been considerable CO 2-rich fluid flux during uplift of greenschist-amphibolite facies rocks along the Main Central Thrust (MCT). Maximum fluid CO 2 content increased with metamorphic grade of the host rock, ranging from ∼ 20–30 mole% in the greenschist facies to both the north and south of the MCT, to 70 mole% in the amphibolite facies. The highest CO 2 content is in late-stage veins which cross-cut uplift-related schistosity in the sillimanite zone. This contrasts with the syn-metamorphic fluid which was probably dominantly aqueous. Fluid immiscibility occurred in quartz veins near the brittle-ductile transition, at 280–310°C and 700±300 bar fluid pressure. An upper limit of 1600 bar lithostatic pressure was determined from a CO 2-rich isochore. The high thermal gradient implied by P- T data is due to rapid uplift along the MCT. MCT movement ceased in the Miocene, and the rocks have experienced slower uplift along a lower thermal gradient. During this slower uplift CaNa brines, some with freezing point depression below −20°C, have traversed the rock. At first the brines mixed with CO 2, then brines dominated until diluted by incursion of lowsalinity, presumably meteoric water near the surface. Brines and CO 2 may be reaching the surface today, in diluted from as numerous warm and ambient springs. Brines were probably derived from connate fluids in the Tibetan carbonate/clastic sedimentary succession to the north of the MCT. CO 2 was derived from metamorphosed equivalents of these sediments, and perhaps from similar sediments beneath the MCT.

Procesos de dolomitización-dedolomitización de la formación Láncara (Cámbrico inferior-medio) en la región de pliegues y mantos de la zona Cantábrica, NW de España

The Lancara Formation is a carbonate sequence within the Cantabrian zone with two members, the lower of primary dolomite nature, and the upper mainly composed of limestone beds. This formation has two basic features worth mentioning: 1) Represents the sliding and fracturing surface of most nappes and thrust-sheets of the Cantabrian zone; 2) Hosts several ore deposits, mainly Pb, Zn, Ba and Hg. A representative section of this Formation has been defined at Caranga de Arriba, Asturias (Spain). The samples obtained from this section have been characterized using several analytic techniques (transmission microscopy, X-ray diffraction, electron microprobe and chemical analysis). A certain dedolomitization afecting the singenetic dolomites has been observed in the lower member, whereas the upper member shows a complete dolomitization. On the other hand, in the surroundings of the site described, and hosted by the refered Formation, the stratabound Pb-Zn ore deposito named Grupo Cantabria, is found. These facts seem to indicate that the Lancara Formation has undergone a great circulation of connate fluids, from the bottom to the top of the Formation, resulting in an intense cation leaching (such as magnesium, very abundant in the lower member) towards the upper member. This is probably due to the fact that the Lancara Formation represents the sliding surface of most nappes of the Cantabrian zone.

Regional Variations in Formation Water Salinity, Hollin and Napo Formations (Cretaceous), Oriente Basin, Ecuador

Salinity concentrations of formation waters in Cretaceous reservoirs of the Oriente basin of Ecuador range between 500 and 130,000 ppm NaCl equivalent. These values conform to salinity gradients detected within and vertically between stratigraphically discrete sandstones. Well logs and drill-stem test data document a consistent pattern of increasing salinity from east to west within individual sandstones and vertically from deeper to shallower sandstones. A model describes the hydrodynamic flow of meteoric water entering the basin through elevated and exposed section to the west, with expulsion occurring along the basin's eastern shallow rim. The outcropping Hollin sandstone at the base of the Cretaceous section provided the only passage through which meteoric water could have initially entered the buried section. This freshwater-bearing sandstone is the source for fresh water within the younger Cretaceous sandstones of the Napo Formation. Connate fluids associated with these younger sandstones were increasingly displaced vertically and to the east where the entire Cretaceous section thins and the sandstones merge into a single column. This zone of merger is the principal vertical pathway for fluid communication between older and young r sandstones and explains the lateral and vertical gradients observed from salinity maps and electric-log profiles.

Metamorphosed Plio-Pleistocene evaporites and the origins of hypersaline brines in the Salton Sea geothermal system, California: Fluid inclusion evidence

The Salton Sea geothermal system (SSGS) occurs in Plio-Pleistocene deltaic-lacustrine-evaporite sediments deposited in the Salton Trough, an active continental rift zone. Temperatures up to 365°C and hypersaline brines with up to 26 wt.% TDS are encountered at 1–3 km depth in the sediments, which are undergoing active greenschist facies hydrothermal metamorphism. Previous models for the origins of the Na-Ca-K-Cl brines have assumed that the high salinities were derived mainly from the downward percolation of cold, dense brines formed by low-temperature dissolution of shallow non-marine evaporites. New drillcores from the central part of the geothermal field contain metamorphosed, bedded evaporites at 1 km depth consisting largely of hornfelsic anhydrite interbedded with anhydrite-cemented solution-collapse shale breccias. Fluid inclusions trapped within the bedded and breccia-cementing anhydrite homogenize at 300°C (identical to the measured downhole temperature) and contain saline Na-Ca-K-Cl brines. Some of the inclusions contain up to 50 vol.% halite, sylvite and carbonate crystals at room temperature, and some halite crystals persist to above 300°C upon laboratory heating. The data are consistent with the trapping of halite-saturated Na-Ca-K-Cl fluids during hydrothermal metamorphism of the evaporites and accompanying solution collapse of interbedded shales. We conclude that many of the salt crystals in inclusions are the residuum of bedded evaporitic salt that was dissolved during metamorphism by heated connate fluids. Therefore, the high salinities of the Salton Sea geothermal brines are derived in part from the in situ hydrothermal metamorphism and dissolution of halides and CaSO 4 from relatively deeply-buried lacustrine evaporites. This fact places important constraints on modeling fluid-flow in the SSGS, as brines need not have migrated over great distances. The brines have been further modified to their present complex Na-Ca-K-Fe-Mn-Cl compositions by on-going sediment metamorphism and water-rock interaction.

Active ore formation at a brine interface in metamorphosed deltaic lacustrine sediments; the Salton Sea geothermal system, California

The Salton Sea geothermal system is an area of active hydrothermal metamorphism of Pliocene to Pleistocene deltaic, lacustrine, and evaporitic sediments deposited in a modern continental rift zone. Temperatures up to 365 degrees C, hypersaline metalliferous brines with up to 26 wt percent total dissolved solids and active base metal ore formation accompany low-pressure greenschist and amphibolite facies metamorphism at depths less than 3.2 km. Drill cores recovered by the Salton Sea Scientific Drilling Project shed new light on the nature and origin of ore formation in the eastern portion of the Salton Sea geothermal system.Base metal ore mineralization occurs in vertical fractures that comprise the major form of fluid permeability at 1 to 3 km in the modern geothermal system. Type 1 veins are dominated by carbonate, contain pyrrhotite and other sulfides, are completely sealed, and occur in wall rock showing minimal alteration. Fluid inclusion data indicate that they were deposited under a thermal gradient much higher than that now found in the eastern part of the system. Type 2 veins are dominated by silicates and hematite, are permeable, and occur in wall rocks showing pervasive chloritization and epidotization. Fluid inclusion data indicate that they are forming now from the geothermal brines under the modern thermal gradient.Fluid inclusions record an apparent steep, progressive salinity increase in the vein systems in the upper 2 km of the system. However, fluid production data from Salton Sea geothermal wells indicate that reduced, metalliferous hypersaline brine averaging 23 wt percent total dissolved solids is overlain by more oxidized, metal-poor fluid averaging 5 wt percent total dissolved solids. A sharp interface exists between the fluids at 1 to 2 km, paralleling the 250 degrees C isothermal surface.A model is developed for type 2 vein ore formation involving mixing at the interface between the two fluids. Magmatic intrusion induces fracturing, heating, and the expulsion of reduced connate fluids to cause early type 1 vein mineralization. Following the rising thermal front, deep hypersaline brine rises diapirically and mixes with overlying fluids in vertical fractures transecting the ascending interface, forming type 2 veins. The hypersaline brine rises to a level of density-temperature stabilization, causing pervasive greenschist metamorphism of the host sediments. The diapir then begins to cool and descend. Shallow, oxidized brine is drawn in as the hypersaline brine descends, promoting late-stage hematite in type 2 veins. The eastern Salton Sea geothermal system presently appears to be in this latter mode of cooling and retrograde brine descent.Ore formation in the system is caused by the coincidence of transaxial entry of a major river into an active rift zone, deposition of metal-bearing deltaic sediments to form a closed-basin sedimentary environment, episodic lacustrine evaporite formation, and injection of heat and elements by rift-related magmatic intrusions at depth. The presence of an upwelling brine diapir in the system may be analogous to conditions immediately prior to the exhalation of metalliferous fluids that formed stratiform Fe-Zn-Pb-Cu sulfide and oxide deposits in paleorift zone environments.

Holocene Cementation Along the Central Texas Coast

ABSTRACT Sandy biosparite and shelly biomicrite gravel thinly covers a significant portion of the inner shelf from the Rio Grande to Sabine Pass, covering an area of at least 3,000 km2, and similar gravel is also a conspicuous component of beach sediment along rapidly eroding Holocene deltaic headlands of the central Texas coast. Although gravel clasts occur in a normal marine setting, most include shells of brackish to freshwater fluvial-deltaic and estuarine molluscs. Contained cements record lithification in two distinctly different diagenetic settings. Biosparites contain intergranular and intragranular cement that precipitated from meteoric fluids during subaerial exposure, whereas biomicrite cements precipitated in the presence of marine and marine-derived fluids, possibly mix d with connate fluids from depth. Most meteoric cement consists of equant crystals of low-Mg calcite spar precipitated during the partial dissolution and calcitization of molluscan aragonite. Marine cements are predominantly equant to elongate crystals of highMg calcite, but they also include acicular aragonite and radial barite. Unlike most normal marine calcite cements, Texas shelf highMg calcite is enriched in iron and manganese, is luminescent, and is isotopically depleted in carbon. Marine cementation, including baffle precipitation, took place at shallow depths below the sediment/water interface. The presence of brackish to freshwater molluscs in sediment lithified by both meteoric and marine fluids records a complex sequence of Quaternary cementational and erosional events. Meteoric lithification of Pleistocene shelly, deltaic, and estuarine coquinas took place during the Wisconsinan lowstand, whereas marine cementation of other, as yet unlithified, shelly layers began with the subsequent Holocene transgression of the Gulf shoreline to its present position.

Structural aspects of fluid-rock interactions in detachment zones

Metamorphic core complexes and associated detachment faults of the North American Cordillera represent gently dipping, normal-displacement shear zones (detachment zones) along which hot, deeper levels of the crust were transported upward and outward from underneath a brittlely distended upper plate. Structures that formed during the ductile-to-brittle evolution of detachment zones can be used to reconstruct the relative magnitudes of fluid pressure and deviatoric stress at different levels within the shear zones. Mylonitization, which occurred along deeper segments of detachment zones below the brittle-ductile transition, was locally accompanied by tensile failure, indicative of low deviatoric stress and high fluid pressure. This condition persisted into the earliest phases of brittle deformation, after which shear fractures formed due to both a reduction in fluid pressure and an increase in deviatoric stress. Brittle deformation of upper-plate rocks occurred largely under conditions of low fluid pressure. Structural and geochemical data suggest that normal displacement on detachment zones results in establishment of two fluid systems: (1) an upper-plate system driven by convection and dominated by meteoric and connate fluids at near-hydrostatic pressures and (2) a system within deeper levels of the shear zone, where fluids are largely derived from igneous sources and fluid migration is aided by dilatancy pumping. The late phases of normal displacement on detachment zones structurally juxtaposed rocks affected by the two fluid systems and locally caused the shear-zone rocks to be overprinted by mineralization related to the upper-plate fluid system.

Ancient fluid flow within foreland terrains

Summary Dewatering of the front Ranges of the Canadian Cordillera in the Campanian and Maastrichtian was achieved by expulsion of connate fluids from thrust sheets during active translation. Each major thrust sheet behaved as a separate hydrodynamic unit. Flow was consistently from hinterland to foreland sections of basal-sheet aquifers. The rate of fluid expulsion exceeded that of rock translation during hydrofracturing. Two flow systems, separated in time, utilized basal-sheet aquifers. Fluid flow at the margin of foreland basins in the Southern Pyrenees in the Oligocene was focused within both the crystalline infrastructure and the sedimentary suprastructure. Ascending metamorphic fluids passed from shear zone conduits in the basement into and through the basin margin, creating ghost vertical and lateral fluid fronts within the sedimentary pile. Connate fluids escaped via cleavage network channelways. Ponding of metamorphic and connate volumes of the fluid budget occurred at high levels close to the leading edge of natural strain within the basin margin, where excess fluid pressures were achieved.

Late Burial Diagenesis of Niagaran (Middle Silurian) Pinnacle Reefs in Michigan Basin

Core samples from Middle Silurian pinnacle reefs in northern Michigan exhibit a regionally consistent assemblage of late diagenetic phases including geopetal diagenetic sediment, disseminated pyrite, pyrobitumen, rhombic dolomite cement, and equant calcite spar. High (> 80°C) fluid inclusion homogenization temperatures and inclusions of Silurian-sourced oil in these diagenetic phases indicate that they formed between the Mississippian and Jurassic, when pinnacle reefs resided in the deep subsurface. Fluid inclusion salinites and stable carbon/oxygen isotopic ratios suggest that late diagenetic carbonates did not precipitate from connate fluids but from basinal brines that were conducted to pinnacle reefs by two regional carbonate aquifers. These data confirm that ate burial diagenesis can affect carbonate rocks residing in high-salinity, low-permeability fluid environments.

Wilcox sandstone diagenesis, Texas Gulf Coast: a regional isotopic comparison with the Frio Formation

Summary The burial diagenesis of Eocene Wilcox sandstones differs significantly from the burial diagenesis of other Gulf Coast terrigenous clastic formations. Differences between the onshore Wilcox and Oligocene-Miocene Frio Formations are larger than regional variations within either formation. Younger, offshore units have undergone less diagenetic alteration than either of the older, onshore units, whereas Mesozoic sandstones are generally more extensively altered. Cementation by quartz and calcite was the first diagenetic modification of volumetric significance to affect either the Wilcox or Frio Formations. The average δ 18 O of Wilcox quartz cement is approximately +25‰ SMOW, in contrast to + 31‰ for the Frio. In both formations, quartz cement is more abundant in the overpressured zone. Calcite has an average δ 18 O of −10.8‰ PDB in the Wilcox Formation (compared to −7.2‰ for the Frio). 87 Sr/ 86 Sr values suggest that calcite was derived from coeval nannofossils, carbonate rock fragments of Mesozoic age, or by local input of fluids from underlying Mesozoic carbonates. Most calcite cement (and therefore quartz which pre-dates calcite cement) was apparently emplaced prior to extensive alteration of detrital silicates which would have released strontium with a high 87 Sr/ 86 Sr ratio. The most common carbonate cement in the Wilcox Formation is ankerite which replaces calcite with increasing depth. The δ 13 C of ankerite is essentially identical to that of calcite at the same depth, and is also similar to Frio and younger carbonate cements. In contrast, δ 18 O values of Wilcox ankerite (average −9.8‰) indicate emplacement at higher temperature or from more depleted water than was true of Wilcox calcite. 87 Sr/ 86 Sr values for Wilcox ankerite are very radiogenic (&gt;0.7100), indicating that ankerite emplacement occurred during or after active silicate diagenesis. Both ankerite and dolomite are very uncommon phases in Frio and younger sandstones despite massive smectite stabilization, suggesting that the conversion of smectite to illite was not the source of iron and magnesium for the late cements. The volume of secondary porosity is similar in both Wilcox and Frio sandstones. Albitization and K-feldspar removal from both formations are essentially complete below 3000 m and essentially no unaltered detrital feldspar occurs below that depth. Differences between the Gulf Coast formations are attributed to different geothermal gradients, differences in the basinal sediments over which the units prograded, and to the changing nature of connate fluids in the units themselves. Diagenesis of the Wilcox Formation, like other Gulf Coast terrigenous wedges, is understandable only in terms of interaction with underlying units during the large-scale evolution, in both time and in space, of the Gulf Coast diagenetic system.

Hydrocarbon Migration and Diagenesis in Miocene Marine-Shelf Deposits: ABSTRACT

Miocene marine-shelf deposits typically found in the Gulf Coast basin are composed of thin (0.1-10.0 cm), horizontally bedded or cross-stratified quartzarenites to subarkoses, laminated silty clays up to 50 cm thick, and bioturbated admixtures of these 2 end members. Some of the coarser grained sand units may contain appreciable quantities (up to 50%) of shell fragments. These lithotypes exert a significant control on the diagenetic mineral products and amount of secondary porosity observed in specimens that have been subjected to temperatures in excess of 120°C. Low-magnesian calcite, maximum microcline, high albite, and a kaolin mineral (possibly dickite) are the major diagenetic products in the sandstones. A regular mixed-layered illite/smectite (rectorite) is dom nant in the clay-rich materials. Secondary porosity is most common in those rocks that originally contained numerous shell fragments. Quartz overgrowths are ubiquitous. The diagenetic differences are striking when the closeness of the sand and clay association is considered. The thin clay seams may have obtained small quantities of potassium from some of the associated sands. The sands illustrate considerable reaction with connate fluids, during the albitization process. Kaolinitic minerals are most abundant in the sand with the highest original porosity. The original composition and diagenetic products define the optimum conditions for hydrocarbon migration. End_of_Article - Last_Page 254------------

Mechanical and Chemical Compaction in Fine-Grained Shallow-Water Limestones

ABSTRACT Artificial compaction of in-situ cores of modern sediments from differing sedimentary environments resulted in: 1) reduction of sediment thickness by 50 percent and more; 2) reduction of initial porosities of 65 to 75 percent to 35 to 45 percent; 3) creation of megascopic textures almost identical to many ancient lime mud- and wackestones; 4) creation of organic, wispy stylolite-like layers that in nature probably would initiate dissolution and stylolite development given burial and time; 5) chemical compaction, evidenced by thin sections showing quartz grains piercing mollusc shells without causing fractures and SEM evidence of solutional interfitting of 1- to 4-µm-size aragonitic carbonate grains; 6) obliteration of pellets and birdseye or fenestral voids in those sediments here early cementation was lacking (pellets were preserved in slightly cemented Bahama Bank muds compacted to half the original thickness); 7) obliteration of identifiable marine grasses and vertical root tube voids; 8) mashing of sediment-filled circular burrows to produce ellipsoidal structures. Significant mechanical compaction resulted from pressures simulating less than 1,000 ft (305 m) of burial. Increasing loads to an equivalent of more than 10,000 ft (3,400 m) did not significantly increase compaction or reduce sediment core length. Chemical compaction (pressure dissolution) was detected only in sediment cores compacted to pressures greater than 10,000 ft (3,400 m) of burial. These short-term experiments suggest that chemical compaction would begin at much shallower depths given geologic time. Compaction experiments that caused chemical compaction lend support to the well-established hypothesis that cement required to produce a low porosity/low permeability fine-grained limestone is derived internally. Dissolution, ion diffusion, and reprecipitation are considered the m st likely processes for creating significant thicknesses of dense limestones in the geologic record. Continuation of chemical compaction after significant porosity reduction necessitates expulsion of connate fluids, possibly including hydrocarbons.

Geochemical and stable isotopic studies of the Crown and Mararoa reefs, Norseman, Western Australia

The quartz reefs of Archean age at Norseman, Western Australia are restricted to the flatter-dipping sections of reverse faults and the gold mineralization is mainly stratabound. Structural and petrographic data indicate that the reefs were emplaced prior to the peak of metamorphism, but gold distribution and wallrock mineralogy were modified by metamorphism and subsequent deformation events. Bucky' quartz crosscuts laminated quartz and was deposited during an episode of hydraulic fracturing. Gold and sulfides are concentrated on fractures in the laminated quartz reefs. Wallrock assemblages of the Crown and Mararoa reefs require two temporally distinct phases of alteration synchronous with the emplacement of laminated and bucky quartz reef. The first involved the deposition of hydrothermal actinolite and biotite, whereas chloritic overprinting of these assemblages characterizes the second phase of alterarion. Both potassic and chloritic assemblages were modified by the crystallization of metamorphic actinolite and tremolite. In general, single stage alteration is characterized by the addition of volatiles, K 2 0 and CaO, and the loss of Na 2 O. Changes in whole-rock geochemistry during chloritic overprinting of the potassic assemblages include losses of CaO, Al 2 0 3 , Na 2 0 and K 2 0, and a gain of MgO. The δ 18 O values of the Norseman wallrooks range from 5.8 to 9.1 ‰ with a mean value of 7.2 ±0.9 ‰ (16 sampes ). Reef quartzes also exhibit a narrow range of δ 18 O values (11.7 ± 0.4°‰, 26 samples). The wallrock assemblages are cornpatible with a mineralization temperature ≥ 400°C. The calculated δ 18 O value of the water in equilibrium with the quartzes at these temperatures is 7‰. Waters involved in the subsequent metamorphism have identical δ 18 O values which suggest they have a common source, connate fluids and hydrous minerals in the greenstone pile. The observed timing relations and isotope systcmatics suggest that the ore fluids were connate brincs which were mobilized during tectonic dewatering of the volcano-sedimentary pile and channelled up early faults.