Late Cements Research Articles

Facies-related diagenesis in the Main Claymore Oilfield sandstones

AbstractFour major sedimentary sequences of the Triassic and Upper Jurassic of the Main Claymore Oilfield of the North Sea each contain a characteristic suite of diagenetic minerals and fabrics. (1) Triassic Skagerrak Formation fluvial sandstones contain early authigenic pore-lining smectite, together with kaolinite and chlorite which form grain replacements and pore fills. Quartz and feldspar overgrowths are minor. Ferroan dolomite forms a late diagenetic patchy poikilotopic cement. Smectite is converted to illite-smectite in a 5 m thick zone beneath the sub-Jurassic unconformity. Smectite formed early in diagenesis prior to oil migration and destroyed permeability. Thus oil is not found in these sandstones although they occur in the oilzone. (2) The Piper Formation (late Oxfordian/early Kimmeridgian) paralic deposits mainly contain authigenic, pore-lining illite-smectite, vermicular kaolinite grain replacements and pore fills. Quartz overgrowths are generally well developed. (3) The Kimmeridge Clay Formation (early Kimmeridgian/early Volgian) comprises thin marine sandstone turbidites, contained within a thick siltstone/shale sequence. In the sandstones (the ‘Ten Foot Sandstone’) discrete double-ended quartz crystals (1–20 µm) developed prior to quartz, K- and Na-feldspar overgrowths. Only minor kaolinite and lllite-smectite are present. Late diagenetic dolomitic occurs as a patchy poikilotopic cement and as clusters of pore-filling rhombs. (4) The Claymore Sandstone Member (early to middle Volgian) thick marine sandstone turbidites are interbedded with thin siltstones/shales. Sandstones have well-developed quartz, K and Na-feldspar overgrowths, and kaolinite and illite-smectite occur as grain replacements and rarely as pore fills. Late-diagenetic dolomite and ferroan dolomite form poikilotopic cement and clusters of pore-filling rhombs. The major factors which control diagenetic features are depositional environment and associated porewater together with original mineralogy. Burial history and textural features of the sandstones also have important influences. Reservoir quality is controlled by a complex interplay of these features.

MVT Sulfide Mineralization, Carbonate Diagenesis, and Possible Relations to Hydrodynamic Evolution of Delaware Basin, Texas: ABSTRACT

Zinc-lead mineralization in the Sheffield channel is hosted in Early Permian shelf-marginal dolomites adjoining basinal facies, and inner shelf dolomites and evaporites. Sulfur isotopes, fluid inclusions, and burial-geothermal reconstructions suggest emplacement at approximately 96/sup 0/C during maximum burial in the late Mesozoic to early Tertiary. Sulfide mineralization can be placed into a paragenetic framework that is found in other areas of the Delaware basin. Early syndepositional and postdepositional dolomitization of shelf strata probably resulted from basinward reflex and, perhaps later, from Kohout convection. With progressive burial into the shallow mesogenetic realm, regional fluid-flow patterns reversed as a result of compactional dewatering of basinal shales. These fluids affected minor silicification, dolomitization, and dissolution of basinal and shelf-marginal strata. Maximum burial was coincident with (eastward) Laramide tilting of the Delaware basin, which created a gravity-driven flow system that discharged heated fluids to the Sheffield channel area. Sulfide emplacement at this time was accompanied by dissolution of the host dolomites, and late cementation by calcite and dolomite. Similar paragenetic trends recognized elsewhere may suggest regional controls on carbonate diagenesis, porosity history, and hydrocarbon migration related to the hydrodynamic evolution of the Delaware basin.

Light- and Heavy-Mineral Diagenesis in Cambrian Sandstones of Michigan's Upper Peninsula: ABSTRACT

Sandstones of the Cambrian Munising Formation have a complex history of postdepositional alteration, including primary porosity reduction and intrastratal framework grain dissolution leading to secondary porosity formation. Primary porosity loss by carbonatization is evident in calcite- and dolomite-cemented samples. Feldspar overgrowths on detrital potassium feldspars effectively reduce primary pore space in 5-10% of the samples. Similarly, multiple generations of quartz overgrowths, distinguished by opaque inclusion rings, reduce porosity further. Authigenic pore-filling clays occur in more lithic-fragment-rich sands. Cemented samples clearly show that secondary porosity is associated with diagenetic alterations of feldspar and garnet. Feldspar dissolution occurs in detrital feldspar cores and as slightly corroded grain boundaries. Intrastratal dissolution of garnet is also evident. Thin sections show oversized pores associated with the garnet, and grain mounts show delicate imbricate wedge marks on garnet surfaces. Calcite also occurs as a fracture filling in, and replacement mineral on, garnet. Other heavy minerals (zircon and tourmaline) appear unaffected by diagenesis. The textural relationships of garnet dissolution vary with respect to the formation of early calcite and late dolomitic cements. The association between garnet and other diagenetic events constrains the chemistry of the intrastratal solutions, which affected garnet and formed overgrowths on feldspar and quartz.

Diagenetic Study of Upper Member of the Smackover Formation (Upper Jurassic), Columbia County, Arkansas: ABSTRACT

ABSTRACT Upper Smackover member reservoir quality trends in Columbia County, Arkansas are the result of regional variations in the intensity of diagenesis. Major controls on diagenesis were marine-meteoric phreatic cementation and later stage brine dissolution and cementation associated with the maturation of hydrocarbons. Carbonates only altered by brine dissolution have higher porosity and permeability values than carbonates affected by marine-meteoric phreatic cementation. Cores from wildcat tests 1-Neill 23-8 and Pruet McMahen 25-12, Columbia County, Arkansas were analyzed. The core from 1-Neill 23-8 was affected by late stage dissolution and partial cementation by baroque dolomite, anhydrite, and poikilotopic calcite. The core from Pruet McMahen 25-12 ws altered by marine-meteoric phreatic cementation which effectively occluded pore space. Scanning electron microscope photomicrographs also reveal arogonitic needle-like structures in the core of Pruet McMahen 25-12 core. These structures are believed to have been preserved as a consequence of early cementation which occluded pore space and created a closed diagenetic system. Chemical analyses of both cores and the diagenetic overprints indicate brine development occurred nearer the location of 1-Neill 23-8 wildcat test and migrated updip toward the site of Pruet McMahen 25-12 wildcat test. Based on the results of this study, it is proposed that regional diagenetic control in the study area can be related to the mobility and location of the Louann Salt.

Prolific Overton Field Gas Reservoirs Within Large Transverse Oolite Shoals, Upper Jurassic Haynesville, Eastern Margin East Texas Basin: ABSTRACT

Late Triassic rifting along a northeast-southwest spreading center in east Texas resulted in basement highs along the eastern margin of the East Texas basin that became sites of extensive ooid shoal deposition during Late Jurassic time. Reservoirs within oolite facies at Overton field contain over 1 tcf of natural gas. These large shoals, each approximately 15 mi (24 km) long and 3 mi (4.8 km) wide, trend north-south as a group and northeast-southwest individually. They are oblique to the basin margin but parallel with Jurassic diffracted tidal currents within the East Texas embayment. Modern Bahamian ooid shoals of similar size, trend, and depositional setting occur at the terminus of the deep Tongue-Of-The-Ocean platform reentrant. Overton field reservoirs are in ooid g ainstone shoal facies and in transitional shoal margins of skeletal-oolitic-peloidal grainstones and packstones. Adjacent nonreservoir facies are peloidal-skeletal-siliciclastic wackestones and mudstones. Early diagenesis of grainstone reservoir facies included meteoric dissolution and grain stabilization, resulting in abundant chalky intraparticle porosity and equant and bladed calcite cements filling interparticle porosity. Subsequent burial diagenesis resulted in intense solution compaction and coarse equant calcite and saddle crystal dolomite that occluded remaining interparticle porosity. Whole-rock trace element analysis indicates greatest diagenetic flushing (less magnesium, strontium) in porous zones. Stable isotopes for grains and cements show strong overprint of later burial diagenesis, with greater depletion of ^dgr18O in reservoir facies. However, hydrocarbons were emplaced prior to late cementation, and unlike other Jurassic Gulf Coast reservoirs, deep burial iagenesis provided no late-stage formation of porosity. End_of_Article - Last_Page 257------------

Origin and Significance of Complex Authigenic Carbonates in Cretaceous Black Shales of the Western Alps

ABSTRACT Cretaceous black shales bearing complex authigenic carbonates are common in some Penninic nappes of the western Alps. In this paper, we discuss the petrography, mineralogy, chemistry, and origin of a typical occurrence of these diagenetic carbonates in the pre-Senonian Serenne Zone of the Embrunais-Ubaye area. At least three types of carbonates with the following average compositions were identified by microprobe analyses: Mn, Fe-rich dolomite: Ca1.02 Mg.42 Fe.24 Mn.32 (CO3)2; Ca-rich rhodochrosite: Ca.30 Mg1.2 Fe.07 Mn.51 CO3; and Mn, Mg-rich siderite: Ca.07 Mg.19 Fe.54 Mn.20 CO3. These carbonates occur as lami ae, nodules, and layers. Petrographic results combined with X-ray diffraction suggest that Mn, Fe-rich dolomite is a relatively early precipitate, occurring as fine-grained, unevenly distributed cement which incorporates zones of less-compacted shales. Its abundance, relative to Mn, Mg-rich siderite, increases towards the center of the diagenetic layers. Mn, Mg-rich siderite is a late cement, as suggested by large, evenly distributed rhombs occurring in lower concentrations but with increasing abundance toward the margins, relative to Mn, Fe-rich dolomite. The presence of Carich rhodochrosite could not be established with certainty, as its (104) XRD peak is located, in most samples, in the shoulder of the dolomite peak. Compositional trends suggest equilibrium precipitation between Ca-ri h rhodochrosite and Mn, Fe-rich dolomite. Isotopic compositions of carbon and oxygen vary between -6.2 and -11.0 (13CPDB) and between -4.4 and -9.6 (18OPDB), the oxygen being somewhat heavier for Mn, Mg-rich siderite (-6.1) than for Mn, Fe-rich dolomite (-8.8). The presence of iron in the authigenic carbonates and the carbon isotopic composition suggest that precipitation occured b low the lower limit of diffusion of sulfate from seawater, after the onset of methane production. Absence of sulfate and high metal and CO3-2 activities favored initial precipitation of Mn, Fe-rich dolomite, which was followed by formation of Mn, Mg-rich siderite when an unknown value of (Fe + Mn + Mg)/Ca activities was reached. The metals were probably derived from hemipelagic metal-rich sediments accumulated just above an oceanic crust. The accumulation and preservation of high organic matter contents, favored by episodes of an expanded oxygen-minimum layer during the mid-Cretaceous, may have promoted the mobilization of Fe and Mn and their incorporation into complex carbonates.

Depositional Controls on Porosity and Permeability Evolution in Upper Smackover Formation at Tubal Field, South Arkansas: ABSTRACT

Hydrocarbon production at Tubal field is from secondarily enhanced, preserved primary porosity in an interval of carbonate grainstones at the top of the Upper Jurassic Smackover Formation. These grainstones were deposited as an ooid shoal over a topographic high on the south Arkansas shelf. The degree of porosity and permeability preservation in these sediments ranges greatly and is controlled by both depositional and diagenetic processes. Porosity and permeability are reduced mostly by pressure solution and the precipitation of late sparry cements. The chemical stability of a grain type determines its resistance to pressure solution, whereas pore size is critical to retaining porosity and permeability after cementation. Therefore, the ability of particular facies types to preserve permeability is controlled chiefly by the types, sizes, and sorting of the constituent grains. Using these parameters, pay facies (facies in which porosity and permeability are commonly preserved) can be defined and identified. Pay facies at Tubal field include the poorly sorted coarse ooid facies, the ooid-composite grain facies, and the pellet-grainstone facies. By studying the distribution of analogous modern facies, the ability to predict the distributions of upper Smackover pay facies, and thus production, can be improved. End_of_Article - Last_Page 1212------------

Calcification of Encrusting Aragonitic Algae: Implications for Origin of Late Paleozoic Reefs and Cements: ABSTRACT

Encrusting aragonitic calcareous algae of the family Peyssonneliacae are the largest group of calcified red algae except for the well-known Mg-calcite coralline algae. They are distributed worldwide, are most heavily calcified in temperate and tropical waters, and grow as prone but arched sheets on soft mud substrates, on hard rock surfaces, as extensive bridgelike networks between corals in reefs, and as concentric layers in nodules. Calcification is entirely aragonite, species specific, and both intracellular and extracellular below the thallus as a hypobasal layer. The hypobasal layer develops outside the tissue as an encrustation of small aragonite botryoids attached to the lower surface between rhyzoids, and on living plants it may exceed the thickness of the thallus Calcification ranges from nonexistent in some cold-water forms to thallus calcification only, to both thallus and hypobasal calcification, to species in which the thallus is noncalcified but there is a hypobasal layer of aragonite botryoids. Although the confirmed fossil range of this family extends only to the Early Cretaceous, striking similarities between these aragonitic forms and some late Paleozoic phylloid algae suggest that they may be closely related. Their ability to grow on soft mud substrates and form structures composed of irregular arched sheets with extensive pore space, as well as their brittle nature so susceptible to fragmentation, are all characteristics of mound-forming fossil phylloid algae. The hypobasal layer of botryoidal aragonite, developed while the plant is still growing, could easily act as a nucleation site for further epitaxial submarine precipitation, thus explaining the common association of Paleozoic phylloid algae and extensive fossil reef cements that resemble botryoidal aragonite. The resence of hypobasal botryoidal aragonite on plants with a noncalcified thallus raises the possibility that some fossil reef cements may be related to now-vanished algae. End_of_Article - Last_Page 491------------

Distinguishing Diagenetic Environments of Equant Calcite Cementation: Example from Lower Cretaceous Pearsall Formation in South Texas: ABSTRACT

Equant calcite is the most common cement in limestones. It occludes more pore space, and hence has more control over reservoir quality than any other carbonate cement. It is important, therefore, to be able to delineate the environments of precipitation of equant calcite cements. The Lower Cretaceous Pearsall formation in south Texas has a history of equant calcite cementation that began early in the shallow-subsurface meteoric environment and is continuing today in the deep-subsurface basinal environment. Trace-element analysis of Mg+2 and Fe+2 along with morphological characteristics of the Pearsall equant calcite cements delineate environments of precipitation. Cements precipitated in the early, shallow-subsurface meteoric environment are generally very fine to medium-crystalline equant (less commonly bladed) calcites. The very fine to fine-crystalline calcite cement generally rims grains and is gradational into the medium-crystalline equant calcite cement that fills intergranular and moldic porosity. Early cements commonly have irregular crystal boundaries as seen in thin section. Late, deep-subsurface precipitated cements are generally coarse to very coarse-crystalline equant calcites. Commonly they have straight crystal boundaries and form one or several large crystals in a pore space. There is generally a sharp change in crystal size with the fine to medium-crystalline equant calcites precipitated in the shallow-subsurface meteoric zone. Trace-element analysis (electron microprobe) shows a statistically valid difference of Mg+2 and Fe+2 content between the early and late equant calcite cements. The early calcite cements have a higher Mg+2 content (1.8 ± 0.3 mole % MgCO3) and a lower Fe+2 content (785 ± 184 ppm) than the late calcite cements (1.3 ± 0.3 mole % MgCO3 and 2.618 ± 1,952 ppm Fe+2). Also, semiquantitative probe analysis, due to low count rate, indicates that the early calcite cement is richer in Sr+2 (738 ± 286 ppm) than the late calcite cement 472 ± 189 ppm). Magnesium in the fine-crystalline rim cement shows a pronounced trace-element distribution pattern. In the incipient crystal growth next to the grain, a high Mg+2 peak usually occurs, followed by a decrease in Mg+2 in the late rim cement and coarser equant calcite cement. This initial high Mg+2 peak is attributed to early meteoric diagenesis as described by Benson in 1974 in meteoric cements from Barbados. The Fe+2 trace-element distribution pattern shows an opposite trend from that of the magnesium. The early cements show low Fe+2, whereas the late cements are Fe+2 rich, indicating a reducing environment of precipitation for the late cements. Early, shallow-subsurface equant calcite cements can be distinguished from late, deep-subsurface equant calcite cements by relative position in pores, crystal size, straightness of crystal boundaries, gradation between crystal sizes, and by trace-element content and distribution patterns. These parameters may be valid environmental indicators of equant calcite precipitation in other limestone formations. End_of_Article - Last_Page 506------------

Porosity Types in Limestones: ABSTRACT

A suite of selected oolitic limestones ranging in age from Devonian to Pleistocene was studied petrographically with light microscopy and SEM observation of resin pore casts, and petrophysically with measurements of porosity, gas and liquid permeability, and with mercury injection capillary pressure curves. A new genetic classification of porosity types and related processes in oolitic limestones is presented which is based on the chronological order of their occurrence in the natural history of the rock, following the terminology and concepts of Choquette and Pray. The 11 subdivisions of the proposed classification are: Primary Porosity: pre-deposition, deposition; Secondary Porosity: eogenetic dissolution, eogenetic drusy cementation, eogenetic compaction, mesogenetic cementation, mesogenetic dissolution, mesogenetic compaction, late mesogenetic cementation, telogenetic recrystallization and telogenetic dissolution. Furthermore, it is possible to subdivide the investigated samples into five evolutionary stages showing the gradual reduction of primary porosity mainly through cementation, and into six stages showing the modification of secondary porosity mainly through dissolution. Concurrently, 11 laboratory experiments were performed in which samples of a tightly cemented Mississippian oolitic calcarenite were submitted to simulated burial conditions, and undersaturated carbonic acid solution was forced to pass through them. The result was selective dissolution of the ooid cortical layers, with the sparite cement preserved undissolved. It is concluded that oomoldic porosity can result from textural variation between components and does not necessarily imply that the ooids had an unstable mineralogy. The understanding of the complex time and space relationships between the different types of porosity in oolitic limestones is critical for reconstructing their depositional-diagenetic history and for evaluating their economic importance as potential hydrocarbon reservoirs. End_of_Article - Last_Page 540------------

Brines, Clay Minerals, and Equilibria: Predicting Diagenetic History and Reservoir Quality in Oligocene Frio Formation of Texas: ABSTRACT

Sandstone reservoirs of good quality, displaying abundant deep secondary porosity, exist on the upper Texas coast, whereas secondary porosity and permeability in sandstones of the lower coast are occluded by authigenic ferroan calcite and chlorite. This difference in regional reservoir quality is controlled by bulk mineralogy, temperature, pressure, and pore-fluid chemistry. Concentrations and activities of major species show depth dependent trends that correspond to pore pressure gradients and associated thermal gradients. Salinities decrease near the base of hydropressure, but increase at intermediate pressure gradients between 0.465 and 0.7 psi/ft (10.5 and 15.8 kPa/m). At higher pressure gradients salinities decrease with depth. The Ca/Na ratio is lowest at top of geo ressure. Predictions from solution-mineral equilibria using approximately 130 analyses of Frio brines add new insight on relative mineral stabilities and in-situ pH, and are consistent with the diagenetic sequence developed from petrographic data. Kaolinite is stable in geopressured waters relative to Ca-montmorillonite and plagioclase; it is abundant on the upper coast as a late stage cement. Lower temperature and in-situ pH (high PCO2) explain the general absence of chlorite on the upper coast; its formation on the lower coast is promoted by higher temperature, a mineralogy rich in volcanic and carbonate detritus, and inferred higher pH. The key to predicting reservoir quality at depth is the deep hydropressured waters. Activity indices are indicators of reservoir quality. Waters of the lower coast plot more deeply into the stability field of chlorite than do those of the upper coast. End_of_Article - Last_Page 492------------

Deposition and Diagenesis of Glauconite Sandstone, Berrymore-Lobstick-Bigoray Area, South-Central Alberta: ABSTRACT

The depositional environments of the Glauconite sandstone in the Berrymore-Lobstick-Bigoray area are distributary channels, delta platform, distributary mouth bars, and interdistributary bays. The diagenetic mineralogy is consistent with the formation water chemistry . A simplified model for evolution of Glauconite sandstone water compositions includes (1) original derivation in a deltaic setting giving composition of early pore waters as brackish to normal marine, (2) alteration due to inorganic and organic chemical diagenesis, and (3) dilution through time due to meteoric water recharge. The formation waters now evolved have such a composition as to be (1) oversaturated with respect to hematite, kaolinite, and illite (late-stage cements); and (2) near equilibrium to undersaturated with respect to quartz, calcite, siderite, and dolomite. The early diagenetic mineralogy is a function of early pore waters and thus the subenvironment. For example, ankerite cement forms early at the base of distributary channels. The intermediate to late diagenetic cementation is a function of early diagenetic mineralogy. For example, hematite is formed by the oxidation of siderite due to meteoric water recharge. Oxidation of pyrite is quantitatively unimportant. Occurrence of late stage hematite is associated with structural highs which are most affected by meteoric water recharge. Delta platform deposits contain gas, and distributary channel and distributary mouth bar deposits contain water. Delta platform deposits are isolated from distributary channel and distributary mouth bar deposits by a vertical permeability barrier of ankerite cement and a lateral permeability barrier of siderite and kaolinite cement, respectively. End_of_Article - Last_Page 606------------

Zoned calcites in Jurassic ammonite chambers: trace elements, isotopes and neomorphic origin

ABSTRACTZoned calcites were found in the phragmacone chambers of three Sonniniid ammonites from marine Middle Jurassic sandstones (Isle of Skye, U.K.). Each ammonite has a unique sequence of up to nine zones of calcite which fill or partially fill the chambers. Zones are defined by changes in the density of minute opaque inclusions and variation in trace‐element composition. Proximal (early) calcites have undulose extinction and some exhibit the specific fabrics of fascicular‐optic and radiaxial fibrous calcites. Microdolomite inclusions are found in one specimen. Early calcites, interpreted as replacements after a single isopachous fringe of acicular carbonate (probably high magnesium calcite), are succeeded by blocky ferroan calcite cement. In one specimen there are two distinct generations of calcite, in the others there is a continuous mosaic incorporating both early calcites and late cement.Isotopic composition of the early calcite zones demonstrates the initial importance of organic derived carbon (δ13C =— 26‰, δ18O ‰ O). Further cementation and mineralogical stabilization took place at increased temperatures and probably after modification of the pore water isotopic composition (calcites with δ13C =— O‰, δ18O∼— 10‰). The distinctive fabrics and zonal patterns probably developed during the replacement of the precursor cement and are not primary growth features. Reversals in isotopic and trace element trends are believed to be related to the rate of neomorphic crystal growth and hence to the degree of exchange with external pore waters. Further increase in temperature, probably during Tertiary igneous activity, gave rise to the extremely light δ18O values of the late cements in the ammonite which had previously had least contact with external waters (cements with δ13C ∼ O, δ18O ∼— 20‰).

Petrology of Pennsylvanian Tensleep Sandstone, Lost Soldier Field, Wyoming: ABSTRACT

The Pennsylvanian Tensleep Sandstone in the Lost Soldier field of Sweetwater County, Wyoming, is composed of texturally homogeneous, fine-grained sandstones and interbedded dolomite beds. Geologic setting and sedimentary features indicate that the Tensleep was deposited in supratidal-eolian environments. Detailed, petrographic, cathodoluminescence, and SEM analyses of core samples show nine diagenetic events in the Tensleep: (1) early calcium sulfate cementation; (2) precipitation of feldspar overgrowths; (3) poikilotopic nodular calcite cementation; (4) nodular anhydrite cementation; (5) late anhydrite cementation; (6) precipitation of silica overgrowths and silicification of carbonates; (7) alteration of feldspar; (8) dolomite cementation and dolomitization; and (9) hyd ocarbon migration and accumulation. Four diagenetic facies with specific patterns of alterations have been identified: (1) early carbonate-precipitation facies; (2) early anhydrite-cementation facies; (3) primary carbonate-grain facies; and (4) nodular-cementation facies. Diagenetic alterations in each diagenetic facies were primarily controlled by lithologic characteristics within the original depositional environments. The carbonate-rich diagenetic facies are closely associated with the supratidal sabkha depositional facies. The anhydrite-rich facies is related to the interdune sabkha depositional facies, and the nodular-cementation facies is present within rocks deposited in the eolian-dune facies. Reservoir qualities (porosity and permeability) of the Tensleep Sandstone in the Lost Soldier field vary considerably and, with the mineral composition of the Tensleep, were modified significantly by diagenetic alterations. They can be estimated from knowledge about the distributions and associations of depositional and diagenetic facies. End_of_Article - Last_Page 565------------

Isotopic and trace element evidence for submarine lithification of hardgrounds in the Jurassic of eastern England

ABSTRACTGeochemical and petrographic data suggest early submarine cementation of hardgrounds from the Lincolnshire Limestone Formation, Middle Jurassic, England. The three hardgrounds, from Cowthick, Castle Bytham and Leadenham quarries, developed in tidal‐inlet, on‐barrier and lagoonal sub‐environments of a carbonate barrier‐island complex. At Cowthick early composite (acicular‐bladed) radial‐fibrous cements, which pre‐date aragonite dissolution, completely fill intergranular pore‐space at the hardground surface; away from it isopachous fringing cements decrease in thickness. Microprobe analyses demonstrate zoning within the fringes with magnesium concentrations (> 2 wt % MgCO3) higher than those in allochems or later, ferroan cement (≅0.5 wt % MgCO3, 1.7 wt % FeCO3). At Castle Bytham early granular isopachous cements, which post‐date aragonite dissolution, occur within 5 cm of the surface. At Leadenham early lithification is superficial and represented by ferruginous crusts and micritic internal sediment. Late blocky cement fills residual pore‐space in all three examples. Carbon and oxygen isotopic composition of whole‐rock samples taken at intervals away from each hardground surface demonstrate the increasing proportion of late 18O depleted cements (δ18O – 8 to – 10). Early cements must have a marine isotopic composition; different δ18O values from each hardground reflect the intensity of early lithification and exclusion of late cements at the hardened surface. There is no isotopic evidence for subaerial cement precipitation during possible emergence at Castle Bytham. Oyster samples (with δ18O, – 2.9 and δ13C, 2.4) give estimated palaeotemperatures of 22–25°C. Early cements from Cowthick are enriched in 18O and 13C (δ18O = 0 δ13C ≅ 3‰) compared to the oyster values. In conjunction with trace element data this is interpreted as evidence for high‐magnesium calcite precursor cements which underwent replacement in a system with a low water: rock ratio. The intensity of early lithification is related to depositional environment: maximum circulation of sea‐water producing the most lithified hardground (Cowthick). This is directly analogous to the formation of Recent hardgrounds.

Freshwater Cementation of Holocene and Jurassic Grainstones: ABSTRACT

Freshwater cementation of Holocene sands in the Bahamas provides a modern analog to enhance our understanding of some cements in Jurassic grainstones of southern Arkansas. The Joulters Cays, three late Holocene islands on Great Bahama Bank, formed when ooid sands were subaerially exposed and lithified by freshwater cements. Cement fabric above the standing water table (vadose zone) and below (phreatic zone) is strikingly different. Vadose cements, characterized by End_Page 719------------------------------ patchily distributed spar most common at grain-contacts, change abruptly across the water table to phreatic cements, displaying a uniform rim of rhombohedrons surrounding each grain. Vadose cements preserve primary porosity and increase variation in permeability more than phreatic cements. The updip Smackover grainstone reservoirs in southern Arkansas are characterized by (1) early cements that predate hydrocarbon emplacement and that resemble the Joulter Cays freshwater cements, (2) preserved primary intergranular porosity, and (3) leached moldic porosity. Vadose imprint is characterized by poorly developed cement rims around grains, a grain-contact meniscus fabric producing rounded pores, and a patchy distribution of block spar with crystals that increase in size away from the grains. The meniscus fabric is only partly preserved where grain interpenetration has occurred during burial. Phreatic cements occur as moderately to well-developed non-isopachous rims around most or all of the grain margins. They line pores forming jagged boundaries, and are patchy to extensive y developed showing an increase in crystal size away from the grain. The cement rims are commonly broken and separated from the grains during compaction. Compaction features and late cements are not distributed uniformly in the grainstones, owing perhaps to heterogeneous porosity and permeability patterns established by early cements. End_of_Article - Last_Page 720------------

Mineralogic Composition and Diagenesis of Tertiary Sandstones along Texas Gulf Coast: ABSTRACT

The upper Gulf Coast Wilcox Formation is composed dominantly of quartz with feldspar, metamorphic and volcanic rock fragments, and mud clasts. The lower Gulf Coast Wilcox Formation contains more quartz and volcanic rock fragments. Lower Gulf Coast Yegua and Jackson Formations are composed of quartz, volcanic, and lesser carbonate rock fragments. Northward they become more quartz-rich and contain metamorphic rock fragments and mud clasts. Lower Gulf Coast Frio and Vicksburg Formations are similar to the Yegua and Jackson. Northward, they become more quartz-rich, but carbonate rock fragments are absent. Despite variations in composition, Tertiary sandstones exhibit a similar diagenetic sequence idealized as follows. Surface to shallow subsurface diagenesis (0 to 4,000 ft; 1,200 m) begins with pedogenic clay coats and with leaching and calcite replacement of feldspar. Minor kaolinite, feldspar overgrowths, and iron-rich carbonate material are also precipitated. Porosity is commonly reduced by compaction from the original 40% to less than 30%. Moderate subsurface diagenesis (4,000 to 11,000 ft; 1,200 to 3,300 m) involves leaching of early carbonate cements and subsequent cementation by quartz overgrowths and later carbonate cement. These stages commonly reduce porosity to 10% or less, but this trend may be reversed by late leaching of feldspar, rock fragments, and carbonate cements. Restoration of porosity to more than 30% can occur, but this may be reduced by late cementation by kaolinite and iron-rich dolomite and ankerite. Deep subsurface diagenesis (11,000 ft; 3,300 m) is a continuation of late iron-rich carbonate cements. End_of_Article - Last_Page 440------------

Exploration History of South Texas Lower Cretaceous Carbonate Platform

The search for hydrocarbons in reservoirs of the Lower Cretaceous of south-central Texas has been continuous for more than 60 years. Accumulations have been found in significant quantities in only four areas: (1) the very shallow fault traps high on the San Marcos arch in Caldwell and Guadalupe Counties, (2) a fault trend stretching across central Atascosa County, (3) a fault trend extending from southeastern Atascosa County to southern Gonzales County, and (4) a narrow, elongate band extending across the entire area known as the Stuart City reef trend. Additional minor discoveries are widely scattered in the Maverick basin and San Marcos arch. Reservoirs which contain the hydrocarbons were deposited in many environments, all related to a broad carbonate shelf covered by an extremely shallow sea. The sea deepened dramatically at the shelf margin parallel with a reef trend. Dolomites contain the best accumulations in the fault trends, and porosity and permeability are reasonably good. Generally, limestones in the reef trend were not extremely porous initially, and late cementation has diminished even that porosity making the reservoir of lesser quality. Oil is the dominant hydrocarbon in the shallow fields, is less dominant in the other fault trends, and is nonexistent in the reef reservoirs. Proved ultimate hydrocarbons for the platform are about 363 million bbl of oil and 1.7 Tcf of gas. Reserves for the reef trend's dry gas reservoirs are difficult to estimate because of highly variable reservoir conditions, but they should fall between 1 and 1.5 Tcf. Exploration has been most intense for the uppermost Lower Cretaceous rocks. The Sligo limestone still holds the promise of success, but lies at considerable depths over much of the area. Edwards and Glen Rose rocks are more densely explored, but there are ample opportunities for new plays even in these formations. Geologists who examine cores and cuttings, determine depositional patterns, understand modern carbonate sedimentology, and study patterns of diagenesis will have an advantage in developing new concepts for exploration.

Alpha Radioactivity of the Constituent Particles in Some Pelletal Phosphorites from Safaga and Other Localities in Egypt

ABSTRACT The uranium content of diverse pelletal phosphorite beds of Maestrichtian age in Egypt, as indicated by alpha radioactivity determined by autoradiographic nuclear emulsion techniques, is usually unequally distributed among the constituents of the phosphate rock. The most radioactive parts are those dark, almost opaque bituminous, blackish, brownish or reddish pellets, probably after original fecal pellets with a relatively high content of organic matter. Bone fragments, chiefly after fish and reptilian teeth, are less radioactive, but the amorphous (mostly isotropic under crossed nicols) clear or zonally to patchily colored collophanite pellets have intermediate radioactivities. Non-phosphatic limestone or chert lithoclasts as well as epigenetic intergranular carbonate cements or late diagenetic theft cements in the pelletal phosphorites are the least radioactive. External surfaces of a minority of large phosphatic pellets, and particularly of large reptilian bones are more radioactive than their interiors. Average specific radioactivity values are given for the different constituents of some pelletal phosphate beds near Port Safaga.

Diagenesis and Porosity Distribution in Deltaic Sandstone, Strawn Series (Pennsylvanian), North-Central Texas: ABSTRACT

ABSTRACT Extensive diagenetic changes have occurred in deltaic sandstones of the "Gray" interval, Strawn Series (Pennsylvanian) from Taylor County, Texas. The present burial depth of the "Gray" interval is 4500 feet (1370 m), and maximum burial was about 7500 feet (2300 m). Sandstone beds were deposited in delta slope, bar crest, and distributary channel environments in a high-constructive elongate delta. Petrographically, the rocks are fine to very fine, submature to mature, shale-bearing sublitharenite. Except for clay clasts, the framework grains, more than 95% quartz, are very mature mineralogically. Good primary porosity occurred in sediments deposited in high energy environments such as bar crest and channel. These sediments had large mean grain size (x = 2.8 or .15 mm), contained an average of only 1.49% detrital clay matrix, and were located farther away vertically from shale beds. It is estimated that depositional porosity of about 35% was reduced to 22% by compaction. Cementation began with authigenic chlorite rims, followed by quartz overgrowths. Calcite cement filled the remaining pore space and replaced unstable grains such as feldspar. Later, dissolution of calcite produced secondary porosity. Further compaction was prevented by quartz cement, so the secondary porosity was preserved. Kaolinite, barite, and ferroan dolomite precipitated as late cements that reduced secondary porosity. Samples from high energy environments contain the most cement, but they also retain the highest porosity. Original textures and depositional environments in a formation may be used to predict where porosity will survive diagenesis.