Ezat Heydari

Permian–Triassic boundary interval in the Abadeh section of Iran with implications for mass extinction: Part 1 – Sedimentology

The uppermost Permian strata of the Abadeh section in Iran consist of 56 m of skeletal limestone (Abadehian), that grades upward into 18 m of grey, bioturbated lime mudstone (Djulfian), which in turn grades upward to 18 m of red, nodular wackestone (Dorashamian) containing an abundant pelagic fauna. The overlying lowermost Triassic includes two enigmatic layers at its base. Immediately above the boundary is a 1-m-thick layer of inorganically precipitated synsedimentary carbonate cement. The cement is composed of 10–20-cm-long crystals oriented either perpendicular to the bedding or as dome-shaped features resembling botryoids. About 0.5 m of wackestone overlies the cement layer. The second layer is a 1.5-m-thick grainstone overlying the cement horizon and consists of recrystallised spherical grains (ooids or peloids). The remainder of unit ‘A’ is composed of 100+ m of grey, bioturbated to nodular lime mudstone. The lithofacies succession of the uppermost Permian is interpreted to represent deposition under increasing water depth, related to rising of relative sea level, which led to drowning of the carbonate platform to below storm wave base. Sedimentologic characteristics, lithology, wide distribution, and slow sedimentation rates indicate that strata immediately below the Permian–Triassic (P–T) boundary (Dorashamian interval) were deposited in deep, oxygenated waters. The enigmatic lowermost Triassic synsedimentary carbonate cement and grainstone layers are interpreted to have been deposited in shallow waters, indicating a rapid and major drop in relative sea level at the end of Permian time in this area, followed by a relative sea-level rise during the earliest Triassic. Precipitation of synsedimentary cement immediately above the P–T boundary in Iran and elsewhere (South China) suggests a global change in ocean chemistry. One possible explanation is that dissolved calcium and bicarbonate concentrations in seawater increased sufficiently to promote spontaneous carbonate precipitation, a rare event in the Phanerozoic but common during Precambrian time. Alternatively, a massive heating event could have resulted in CO2 degassing and decreases in oceanic carbonate solubility, both leading to inorganic, synsedimentary carbonate precipitation. Inorganic precipitation of synsedimentary carbonate cement may indicate that biochemical production of carbonate was halted due to the P–T boundary events. Our study indicates that shallow and moderately deep waters during the latest Permian were well oxygenated in the open ocean setting of the central Tethys Sea in Iran. Therefore, P–T mass extinction scenarios that invoke upon widespread marine anoxia should be viewed with caution.

The Role of Burial Diagenesis in Hydrocarbon Destruction and H2S Accumulation, Upper Jurassic Smackover Formation, Black Creek Field, Mississippi

Organic-inorganic interactions during burial of the Smackover Formation at Black Creek Field, Mississippi, have resulted in nearly complete destruction of hydrocarbons. The formation has been buried to a depth of 6 km, has experienced temperatures of over 200 degrees C, and presently contains 78% H 2 S, 20% CO 2 , and 2% CH 4 . Three distinct stages of burial diagenesis correspond to three phases of organic matter maturation. Pre-oil window diagenesis was dominated by precipitation of prebitumen calcite cement. Diagenesis in the oil window was characterized by precipitation of saddle dolomite and anhydrite in water-filled layers and by formation of solid bitumen in the oil column. Diagenesis in the gas window was dominated by thermochemical sulfate reduction (TSR) resulting in hydrocarbon destruction, anhydrite dissolution, large amounts of H 2 S, CO 2 , and S generation, and postbitumen calcite cementation. During TSR, anhydrite reacted with H 2 S to produce S , which in turn reacted with CH 4 to generate more H 2 S in a self-reinforcing cycle. The lack of metal cations to stabilize H 2 S as metal sulfides, availability of sufficient sulfate to generate H 2 S, and a closed system to prevent H 2 S from escaping resulted in the continuation of the TSR cycle until nearly all hydrocarbons were consumed. In Mississippi, concentration of H 2 S is nearly zero in Smackover hydrocarbon reservoirs that have experienced temperatures of 120 degrees C for more than 50 m.y., suggesting that TSR is not a kinetic (time-dependent) process. High H 2 S concentrations initiate at temperatures above 140 degrees C and increase with temperature, indicating that TSR is a thermodynamic phenomenon. Reported high H 2 S concentrations at low temperatures (80-120 degrees C) from other locations may be explained by the following processes; (1) migration of H 2 S into these reservoirs, (2) high geothermal gradients or local thermal perturbations in the past, (3) a biochemical origin for the H 2 S, or (4) exposure of these reservoirs to temperatures greater than 150 degrees C and a rapid uplift. In Black Creek Field, burial cementation and pressure solution resulted in total destruction of porosity and permeability in limestone reservoirs, but not in dolomite reservoirs, which still possess up to 20% porosity and 100 md permeability. Secondary porosity was not created as a result of hydrocarbon migration. Abundant CO 2 derived during hydrocarbon destruction resulted in calcite cementation rather than carbonate dissolution. Late, secondary porosity development in carbonates may be related to acids generated by metal sulfide precipitation.

Diagenetic origin of carbon and oxygen isotope compositions of Permian-Triassic boundary strata

Bulk carbonate δ^(13)C and δ^(18)O compositions of profiles across Permian–Triassic (P–T) boundary sections in China, Italy, Austria, and Iran show wide varieties of trends. The δ^(13)C depletions occur in all sections and range from 2 to 8‰ PDB in magnitude. These excursions take place over intervals ranging from less than 0.1 to more than 40 m. The δ^(18)O values may increase or decrease toward the P–T boundary, but decrease sharply by 2–9‰ PDB at or above the boundary. Cross-plots of δ^(13)C and δ^(18)O values from all sections show positive covariance. Wide differences in magnitudes, trends, and position of the excursions relative to the boundary, as well as the covariance patterns suggest that P–T boundary δ^(18)O and δ^(13)C values are partially or entirely diagenetic in origin, formed in association with exposure surfaces. This interpretation implies that P–T boundary sections studied till date were subaerially exposed before, during, and after the mass extinction, resulting in the removal of strata containing key information about the extinction mechanism. This inference is consistent with the paleontological studies that have shown the presence of gaps at the boundary, and further supported by the sharp lithologic changes observed at virtually all P–T boundary sections. Subaerial exposures are documented by detailed sedimentologic and isotopic studies from central Tethyan sections in Abadeh and Shah Reza in Iran. Proposed P–T boundary extinction models are based on isotopic values that are diagenetic in origin and stratigraphic sections that are incomplete, leading to extinction mechanisms with little physical supporting evidence.

Porosity Loss, Fluid Flow, and Mass Transfer in Limestone Reservoirs: Application to the Upper Jurassic Smackover Formation, Mississippi

Ooid grainstones of the Upper Jurassic Smack over Formation are buried to a depth of over 6 km and exposed to temperatures in excess of 200°C at Black Creek field, Mississippi. Combined effects of mechanical compaction, intergranular pressure solution, and cementation have reduced intergranular porosity of these ooid grainstones to 0%, indicating that porosity reduction has gone to completion. Modal analysis of 24 samples lacking preburial cements indicates that from the original 40% porosity, 13 porosity units (range: 4 to 21) were lost by mechanical compaction, 15 porosity units (range: 8 to 23) were reduced by intergranular pressure solution, and 12 porosity units (range: <1 to 26) were destroyed by cementation. Intergranular pressure solution caused an average of 28% (range: 15 to 51%) vertical shortening in Smackover ooid grainstones. Under ideal conditions, the 28% vertical shortening will generate enough calcium carbonate to precipitate 10% calcite cement. This is close to the measured volume of cements in Smackover grainstones (12%), suggesting that intergranular pressure solution provided most of the calcite cement present. No external sources of calcium carbonate are required. Fine-grained samples that experienced high degrees of intergranular pressure solution and contain only small amounts of cement occur at the top of the reservoir, whereas coarse-grained samples with abundant cement and low degrees of pressure solution occur in the middle and basal parts of the reservoir, suggesting that the fine-grained intervals acted as sources and the coarse-grained intervals as sinks for calcium carbonate. Mass transfer of pressure solution-generated calcium carbonate from the top of the unit to precipitation sites in the middle and basal parts of the reservoir could have occurred by a non-Rayleigh-type convection cell. Due to calcites reverse solubility with respect to temperature, the cooling, upward-moving limb of the convection cell would become progressively more undersaturated, and hence able to transport more dissolved calcium carbonate released by intergranular pressure solution in the upper portion of the reservoir. Pore fluids descending in the downward-moving limb of the cell would become progressively more supersaturated, and calcium carbonate would tend to precipitate as cement in the middle and basal parts of the reservoir as fluids become progressively hotter.

Meteoric versus burial control on porosity evolution of the Smackover Formation

Whole-rock 13C, 18O, and Sr compositions of the uppermost ooid grainstones of the Smackover Formation at Black Creek field in Mississippi reveal characteristics of a meteoric system in this area. The trend in 13C signatures indicates that meteoric water acquired its low carbon values at a soil zone and was buffered completely with the host rock before entering the meteoric lens. Identical 18O data suggest that allochems stabilized at the same temperatures in waters of similar 18O compositions during early meteoric diagenesis; that is, calcitization continued to completion before the onset of burial. Sr concentrations reveal differential transformation mechanisms. Low Sr values in the phreatic zones are attributed to slow stabilization by exchange with the bulk solution. High Sr compositions in the vadose zone resulted from rapid, incremental transformation.Meteoric cementation was sparse; therefore, grainstones entered the burial realm with all of their porosity intact. Modification of porosity was a burial process and was directly related to the intensity of chemical compaction, which itself was controlled by three factors: (1) the degree of early cementation, (2) grain type, and (3) grain size. Early-cemented intervals experienced low to moderate compaction regardless of grain size or grain type. In the absence of early cementation, grain type and grain size were the dominant factors of porosity control. Coarse-grained intervals that were composed of micritic grains experienced less compaction than fine-grained, oolitic intervals.Average volumetric percentages of cements as determined by point counting of 63 samples are as follows: (1) marine and meteoric cements = 1.5%, (2) prebitumen calcite = 6.5%, (3) saddle dolomite = 1%, (4) postbitumen calcite = 4.3%, (5) pyrobitumen = 1%. Total cement (sum of 1–5), total burial cement (sum of 2–5), and total burial carbonate cement volumes (sum of 2–4) average 14, 13, and 12%, respectively. Out of an original porosity of 40%, 13 porosity units were lost by cementation, and 27 porosity units were reduced by compaction (mechanical and chemical).Present-day intergranular porosity of these grainstones is zero, whether they belonged to the vadose zone or the phreatic interval. Burial cementation and compaction destroyed all intergranular porosity, regardless of early diagenetic overprint or textural variables. The reservoir became compartmentalized during burial. Some intervals were active producers of calcium carbonate; others were passive recipients.

Massive Recrystallization of Low-Mg Calcite at High Temperatures in Hydrocarbon Source Rocks: Implications for Organic Acids as Factors in Diagenesis

We document massive recrystallization of low-Mg calcite lime mudstone source rocks at moderately high temperatures in the Smackover Formation of Mississippi. This process was driven by organic acids produced during kerogen maturation and holds the implications for studies of primary migration of hydrocarbons, secondary porosity generation in reservoirs, and global environmental change. This interpretation is based on contrasting the following petrographic and bulk-rock geochemical characteristics of organic-poor vs. organic-rich lime mudstones:

The Mars Science Laboratory (MSL) Mast cameras and Descent imager: Investigation and instrument descriptions

Abstract The Mars Science Laboratory Mast camera and Descent Imager investigations were designed, built, and operated by Malin Space Science Systems of San Diego, CA. They share common electronics and focal plane designs but have different optics. There are two Mastcams of dissimilar focal length. The Mastcam‐34 has an f/8, 34 mm focal length lens, and the M‐100 an f/10, 100 mm focal length lens. The M‐34 field of view is about 20° × 15° with an instantaneous field of view (IFOV) of 218 μrad; the M‐100 field of view (FOV) is 6.8° × 5.1° with an IFOV of 74 μrad. The M‐34 can focus from 0.5 m to infinity, and the M‐100 from ~1.6 m to infinity. All three cameras can acquire color images through a Bayer color filter array, and the Mastcams can also acquire images through seven science filters. Images are ≤1600 pixels wide by 1200 pixels tall. The Mastcams, mounted on the ~2 m tall Remote Sensing Mast, have a 360° azimuth and ~180° elevation field of regard. Mars Descent Imager is fixed‐mounted to the bottom left front side of the rover at ~66 cm above the surface. Its fixed focus lens is in focus from ~2 m to infinity, but out of focus at 66 cm. The f/3 lens has a FOV of ~70° by 52° across and along the direction of motion, with an IFOV of 0.76 mrad. All cameras can acquire video at 4 frames/second for full frames or 720p HD at 6 fps. Images can be processed using lossy Joint Photographic Experts Group and predictive lossless compression.

Paleoceanographic and Paleoclimatic Controls on Ooid Mineralogy of the Smackover Formation, Mississippi Salt Basin: Implications for Late Jurassic Seawater Composition

ABSTRACT The Late Jurassic Smackover Formation in the Mississippi salt basin consists of two 150 m thick shoaling-upward cycles, each capped by ooid grainstones. During deposition of the lower cycle, originally calcite ooids (preserved radial fabric, 18O = -3.8 PDB, 13C = 4.5 PDB, Sr2+ = 315 ppm) formed on the seaward side of the basin and former aragonite ooids (calcitized textures, 18O = -3.0 PDB, 13C = 5.5 PDB, Sr2+ = 1790 ppm) were precipitated on the landward side. In the upper cycle, originally calcite ooids (preserved radial fabric) were precipitated on both the seaward (18O = -2.5 PDB, 13C = 4.2 PDB, Sr2+ = 300 ppm) and the landward (18O = -2.5 PDB, 13C = 3.2 PDB, Sr2+ = 250 ppm) sides of the basin. Because kinetic variables (Mg2+/Ca2+, rate of CO32- supply, PO43-, SO42-) are incapable of totally preventing aragonite formation, we suggest that Smackover calcite ooids were precipitated from seawater with low carbonate saturation state (possibly undersaturated relative to aragonite). The shift from seaward calcite to landward aragonite ooids in the lower cycle was controlled by a shoreward increase in seawater salinity. The net effect of the salinity gradient was a landward increase in the carbonate saturation state in response to decreasing dissolved CO2 and increasing CO32-, Ca2+, and temperature. In seawater supersaturated with respect to both arago ite and calcite, kinetic variables favored dominance of aragonite over calcite. The landward increase in seawater salinity reflects extensive evaporation in an arid climate, resulting in antiestuarine circulation. The monomineralogic (calcite) nature of ooids of the upper cycle suggests that the salinity gradient across the basin was not sufficient to alter the seawater saturation state. This is attributed to a less arid climate and/or a less restricted circulation.

Geochemical Studies of Crude Oil Generation, Migration, and Destruction in the Mississippi Salt Basin

ABSTRACT The main source for crude oil in the Mississippi Salt Basin is the laminated lime mudstone facies of the Lower Smackover. Crude oil generation and migration commenced at a level of thermal maturity equivalent to about 0.55% vitrinite reflectance. Short-range lateral migration of crude oil was focused to Upper Smackover and Norphlet reservoirs, but vertical migration also charged some overlying Cotton Valley, Rodessa, Lower Tuscaloosa, and Eutaw reservoirs. Following migration from the Lower Smackover, thermal maturity history of reservoir rocks controls the preservation of crude oil, gas-condensate, and methane. Slow thermal cracking of crude oil occurred in deep Upper Smackover reservoirs, resulting in formation of gas-condensate and precipitation of solid bitumen. The maximum thermal maturity for preservation of condensate is equivalent to about 1.3% vitrinite reflectance. Only methane, pyrobitumen, and nonhydrocarbon gases including hydrogen sulfide persist at higher levels of thermal maturity. Early destruction of methane in deep Upper Smackover reservoirs near the Wiggins Arch is driven by thermochemical sulfate reduction. Lesser availability of sulfate in Norphlet reservoirs in Lower Mobile Bay could account for methane preservation at higher levels of thermal maturity. One basic geochemical strategy for further exploration of the Mississippi Salt Basin is to focus exploration effort on traps with reservoirs in the thermal maturity window for hydrocarbon preservation. Another strategy is to avoid drilling traps with overmature reservoir rocks.

Zonation and Geochemical Patterns of Burial Calcite Cements: Upper Smackover Formation, Clarke County, Mississippi

ABSTRACT Cathodoluminescence (CL), trace element concentrations, and isotope compositions of luminescently zoned poikilitic and mosaic calcite cements of the upper Smackover Formation are studied in the Harmony, Pachuta Creek, East Nancy, Goodwater, and Garland Creek fields in Clarke County, Mississippi. These calcites precipitated during burial in a depth range of 100 m to 3 km, a temperature range of 30-100°C, and a time span of 60 my. The calcium carbonate for calcite cementation was primarily derived locally by pressure dissolution of the Smackover Formation. Luminescent patterns are consistent in all samples within each field. However, calcites from individual fields differ in number of zones and CL intensity. There is no compelling evidence that equivalent zones in separate fields precipitated simultaneously. It is therefore unwise to correlate individual zones or groups of zones as time horizons from one field to another. Magnesium content of all calcites ranges from about 500-1000 ppm in the oldest zones and increases to about 3000-4000 ppm toward the younger zones. The low Mg2+ content of the older zones is the result of precipitation from waters with low Mg2+/Ca2+ ratio, typical of subsurface waters. The increase in Mg2+ toward the younger zones may have resulted from precipitation over time at progressively higher temperatures and the increase in DMg2. from a pore water whose Mg2+/Ca2+ ratio was declining. Strontium varies from 100-300 ppm and does not exhibit systematic variation across zones but increases with increasing Mn2+ content. Low Sr2+ content of the calcites is the result of very slow precipitation at high temperatures from a water with low Sr2+/Ca2+ ratio. Co-variation of Sr2+ with Mn2+ may reflect preferential Sr2+ substitution for Ca2+ in order to reduce lattice distortion caused by the substitution of Mn2+ for Ca2+. Non-luminescing to dull zones contain less than 30 ppm Mn2+, while bright zones have 300-1500 ppm Mn2+. Iron is nearly absent in all zones regardless of CL intensity. Low Mn2+ content of older zones is attributed to calcite precipitation in a reduced environment where Mn2+ was not available. The increase in Mn2+ content in younger zones is the result of importation of Mn2+ through local conduits from the underlying Norphlet Formation, coupled with an increase in DMn2 due to a progressive increase in temperature during burial. Absence of Fe2+ in all zones may have been the result of precipitation of iron-sulfides when Fe2+ - and Mn2+-bearing waters came into contact ith H2S generated by organic maturation of the lower Smackover Formation. 18O decreases and 87Sr/86Sr becomes more radiogenic toward the younger zones, while 13C remains constant. The lower 18O and more radiogenic 87Sr/86Sr composition of the younger zones suggest calcite precipitation at higher temperature and influx of radiogenic strontium from the underlying Norphlet Formation. The absence of any CL quenchers, lack of any CL activators other than Mn2+, and geologic and geochemical considerations suggest that CL intensities were primarily controlled by the Mn2+/Ca2+ ratio of pore waters and temperature.