Lacustrine Sandstones Research Articles

Geology and Hydrocarbon Reservoir Potential of the Pituil and Barreal Formations, Calingasta Valley, Western Argentina: ABSTRACT

The Calingasta basin is one of the north-south-trending intermontane basins informally known as the Bolsones. The stratigraphy consists of lower Paleozoic metamorphic basement overlain by sediments and volcanics of upper Paleozoic through Cenozoic age. Three distinct geological provinces are recognized within the Bolsones region: Sierras Pampeanas, Precordillera, and Cordillera Frontal. Outcrop samples from the Permian Pituil and Triassic Barreal formations from the Tamberias region of the Sierras Pampeanas province were analyzed to determine the composition, porosity type, and diagenetic modification. The Pituil formation is a shallow marine sequence overlying Carboniferous glaciomarine sediments. They grade eastward into nonmarine lacustrine, deltaic, and fluvial sandstones. The rocks are fine- to medium-grained litharenites with porosities of 6-10 %. Diagenetic modifications include quartz overgrowths, unstable grain dissolution, carbonate cements, pyrite, and kaolinite. Triassic deposits occur on the western flank of the Precordillera, overlying a basement of volcanics and metasedimentary rocks. The Triassic sediments can be several hundreds of meters thick; deposition occurred in fluvial to lacustrine environments. These clastic sediments are considered to be northern extensions of the hydrocarbon-productive sediments in the Cuyo basin. The Barreal formation ranges from clay-rich lithic wackes and shales to conglomeratic, volcaniclastic litharenites and sublitharenites. Framework grains consist of quartz,more » feldspars, rock fragments, and, rarely, glass shards. Cements include zeolites, carbonates, chalcedony, pyrite, and clays. Tuffs are found at certain intervals within the section; alteration to iron-rich smectite is common. Reservoir potential is highly variable. Porosities range from as low as 5% to greater than 25%.« less

Lacustrine Strata of Early Jurassic Age, Hartford and Deerfield Rift Basins, Massachusetts and Connecticut: Deposition, Maturation of Black Mudstones, Sandstone Diagenesis, and Hydrocarbon Occurrences: ABSTRACT

Lacustrine cycles occur in the 100-m Shuttle Meadow, 170-m East Berlin, and lower part of the 1,200-m Portland Formations in the Hartford basin and in the 600-m Turners Falls Sandstone in the Deerfield basin. Eleven exposed cycles range in thickness from 3 to 10 m. Black mudstones are mostly 1-2 m thick, deposited in the deeper parts of oligomictic perennial lakes. Gray sandstones accumulated on delta lobes, fan deltas, and from sheetfloods that spread across the valley floor to cover exposed lacustrine littoral muds. Both rift-basin fills are dominated by alluvial-fan, fluvial, and playa red beds. The lacustrine sandstones are arkoses and lithic arkoses (72 modal analyses). Omitting minor cements, diagenetic events are (1) compaction, (2) quartz and albite overgrowths with albitization of detrital plagioclase, (3) dolomite and ferroan dolomite cements, and (4) hydrocarbon migration with creation of secondary porosity. Consistently large intergranular volumes in the lacustrine sandstones, as high as 40%, suggest that cementation began prior to significant compaction. Porosity in the sandstones ranges from 0 to 11%, averaging 2%. Organic geochemical data for black mudstones in the East Berlin, Portland, and Turners Falls Formations, combined with published and USGS open-file data, show that organic richness and oil pronenessmore » increase southward in the Hartford basin whereas thermal maturity increases northward. Oil generation has occurred in the East Berlin Formation, as evidenced by relatively large amounts of extractable bitumen, high-molecular-weight hydrocarbons, and hydrocarbons present in the sandstones. The Deerfield basin is overmature for oil.« less

Petrology, depositional environments and structural development of the Mineta Formation, Teran Basin, Cochise County, Arizona

The Oligocene Mineta Formation of the Teran Basin in southeastern Arizona contains approximately 1000 m of interfingering alluvial-fan, fluvial, and lacustrine strata with minor intercalated volcanics. Sedimentation began prior to the middle Oligocene and ended with the eruption of the Oligocene-Miocene Galiuro Volcanics about 28 m.y. ago. Several related depositional facies are present in a cyclic pattern within the Mineta Formation. A complete cycle contains proximal, coarse-grained alluvial-fan conglomerates that fine upward into sandstone and pebble conglomerate of a distal-fan or braided-stream environment. Medium- to fine-grained sandstones are interpreted as sandflat deposits that lie stratigraphically above the distal-fan deposits and below lacustrine sandstone and algal limestone. The cycle culminates with deeper water lacustrine mudstone with intercalated thin-bedded turbidite sandstone. Discrete cycles of bedded gypsum and mudstone are found locally within the lacustrine deposits. These deposits represent a temporary playa-lake phase of the Mineta depositional system. The Mineta Formation filled a half-graben that developed in response to extensional stresses associated with the development of the Rincon/Catalina metamorphic core complex. Paleocurrents flowed southwestward from a source area of predominantly sedimentary and volcanic rocks. Sedimentation was contemporaneous with extensional deformation that produced low-angle normal faults and tilted the section toward the northeast. Maximum extension was directed about N55°E-S55°W, roughly parallel to the direction of paleocurrent flow, parallel to the mean slip direction for low-angle normal faults and perpendicular to the strike of the Mineta section.

Depositional and Provenance Controls on Diagenesis of Sandstones Associated with Green River Formation of Wyoming: ABSTRACT

Synchronous sandstones associated with the lacustrine Eocene Green River Formation of Wyoming from three distinct provenance areas and two general depositional environments were studied to determine provenance and depositional controls on diagenesis. Quartzitic fluvial sands in an area dominated by ground water flow were cemented early with calcite. Marginal lacustrine sands of the same detrital mineralogy became cemented with calcite and ankerite. Lacustrine sandstones with a micritic matrix contain no porosity, whereas fluvial sandstones with well developed secondary porosity registered porosities as high as 25% and permeabilities above 2,300 md. Arkosic sands deposited from rivers meandering across a fluvial floodplain developed mostly authigenic clay cement from the dissolution of feldspars. In contrast, arkosic beach sands intercalated with lacustrine carbonates were cemented with ankerite. Volcanogenic fluvial sands were cemented with zeolite and clay minerals, reducing the effective porosity to zero. On the other hand, environmentally controlled carbonate forms the principal cement of marginal lucustrine volcanogenic sandstones. Cementation by carbonate halted the degradation process of volcanic rock fragments to pore-filling clays and zeolites. Furthermore, some of these rocks exhibit excellent secondary porosity and permeability. In conclusion, provenance set the stage for diagenesis by providing the raw materials for subsequent sandstone alteration. However, depositional environment can cause sands of similar detrital mineralogy to follow different diagenetic courses leading to the development of significantly different porosity and permeability characteristics. Diagenetically produced porosity differences in a marginal lacustrine setting can yield a wide variety of reservoirs, seals, and traps. End_of_Article - Last_Page 540------------

Depositional Models, Intrabasin Tectonics, and Sea Level Changes in Petroleum Exploration: ABSTRACT

Recurrent movement on Precambrian-age basement-fault systems has influenced the origin, thickness, and distribution of Phanerozoic strata in the interior of the North American continent. In this structural setting, tectonic and sedimentation patterns are well known for major orogenies of the late Paleozoic, late Mesozoic-early Cenozoic (Laramide orogeny), and post-Oligocene. The influence of basement-fault movement on sedimentation and erosion during anorogenic times is generally not as well known. Subtle structural movement controlled topography in the depositional basins, which in turn influenced the distribution of high energy (reservoir) and low energy (nonreservoir) deposits. In addition, extensional fractures in strata overlying basement faults provided pathways for petroleum migration, either vertically or horizontally. Inversions of structural movement along fault zones are commonly observed, and they sometimes mask the recognition of exact time of sporadic fault movement. The most common petroleum reservoirs are deposited during rising or high sea level stands and are fluvial-deltaic, estuarine, shoreline, shelf, and lacustrine sandstones, and shallow marine and tidal-dominated carbonates. Depositional topography, which formed within the depositional basin during highstands, controls the distribution of unconformities during lowstands. Associated lowstand reservoirs are either base of slope or shelf sandstones, or secondary porosity in carbonates associated with paleokarst or dolomitization. Porosity in sandstones deposited during highstands may be infilled with diagenetic clays because of subaerial exposure during lowstands. Thus, recognition of the lowstand unconformity is of vital importance in exploration. All rock systems record sea level changes, but those most important to petroleum exploration in western cratonic basins are in the Ordovician, Devonian, Permian-Pennsylvanian, Jurassic, Cretaceous, and Tertiary. A major problem is to determine whether structural topography, or depositional topography and eustatic changes, exerted the primary control on sedimentation. Petroleum occurrences in well-documented sequences related to sea level changes are discussed for the Rocky Mountain and Mid-Continent regions. End_of_Article - Last_Page 1838------------

Future Petroleum-Productive Regions of USSR and Mongolia: ABSTRACT

The potential for major discoveries of oil and gas is high in the USSR, but remote in Mongolia. Development of the USSR's potential is plagued by six factors: (1) remoteness of prospective basins from commercial markets, (2) lack of adequate infrastructure within prospective basins, (3) inadequate drilling technology for economic development below depths of 3,200-3,800 m (10,500-12,500 ft), (4) poor-quality indigenous equipment, (5) absence of offshore capabilities, not only in the warm Caspian and Black Seas, but also in the ice-plagued Arctic and Pacific Oceans, and (6) a chronic manpower shortage. Imports of foreign technology are alleviating the problems gradually, but they are far from solved. European USSR: The most important objectives of the future are: (1) Jurassic through Devonian in the Greater Barents basin and Svalbard platform, offshore Barents Sea, (2) deep Carboniferous-Ordovician of Timian-Pechora basin, (3) subthrust plays of western Urals, (4) pre-Kungurian salt section of the Pricaspian basin, (5) Devonian of the Dnepr-Donets graben, (6) Jurassic through Paleogene of the Black Sea shelf, (7) pre-Tertiary formations of the North Caucasus trough, and (8) deep Tertiary objectives near Baku in the central and southern Caspian Sea. Asiatic USSR: The most important targets are: (1) deep Mesozoic and Paleozoic (Carboniferous, Devonian, Silurian) carbonates of the Nyurol'ka, Frolovo, and other depressions in and below the West Siberian basin, including the Kara Sea, (2) late Paleozoic-Mesozoic of the Aral' Sea, the Ustyurt depression, the Chu-Sarysu basin, and the Turgay and Syrdar'ya syneclises, most of them unexplored, (3) the pre-Upper Jurassic of the central Asian basins, (4) the deep Tertiary of the Cheleken district, (5) the subthrust and pre-Tithonian salt section of the South Tadzhik basin, (6) the Proterozoic-Silurian marine sequences of the Tunguska, Lena, and Sukhana basins, and of the Nepa-Botuoba arch, (7) the Carboniferous-Lower Jurassic of the Vilyuy, Lena-Anabar, Khatanga, and Yenisey basins, with the associated sub hrust plays of the Taymyr and Verkhoyansk ranges, and (8) the numerous late Paleozoic-Mesozoic offshore basins of the Arctic shelf plus the Tertiary basins of the Pacific, especially the Severny basin. Mongolia: Principal objectives are Jurassic and Cretaceous fluviatile and lacustrine sandstones in southeastern Mongolia, especially in the East Gobi basin and in the Hailar basin, which is shared with China. Similar basins in China have giant fields, such as Karamay in the Junggar basin and Daqing in the Songliao basin. Resources: Through 1983, the USSR had produced about 75 billion bbl of oil and condensate and 217 tcf of gas. Estimated proved plus probable liquids was 35-36 billion bbl and, of gas, about 800 tcf. Resource potential, above and beyond proved plus probable, is estimated at about 90 billion bbl and 1,000 tcf (these numbers will increase as offshore icepack technology is improved). In contrast, Mongolia's produced and proved oil is less than 2 million bbl, but the resource potential could be large. End_of_Article - Last_Page 1205------------

Oil Exploration in Nonmarine Rift Basins of Interior Sudan: ABSTRACT

In early 1975 Chevron Overseas Petroleum Inc. commenced a major petroleum exploration effort in previously unexplored interior Sudan. With the complete cooperation of the Sudanese Government, Chevron has acquired a vast amount of geologic and and geophysical data during the past 9 years. These data include extensive aeromagnetic and gravity surveys, 25,000 mi (40,200 km) of seismic data, and the results of 66 wells. This information has defined several large rift basins which are now recognized as a major part of the Central African rift system. The sedimentary basins of interior Sudan are characterized by thick Cretaceous and Tertiary nonmarine clastic sequences. Over 35,000 ft (10,600 m) of sediment have been deposited in the deepest trough, and extensive basinal areas are underlain by more than 20,000 ft (6,100 m) of sediment. The depositional sequence includes thick lacustrine shales and claystones, flood plain claystones, and lacustrine, fluvial, and alluvial sandstones and conglomerates. Those lacustrine claystones which were deposited in an anoxic environment provide oil-prone source rocks. Reservoir sandstones have been found in a wide variety of nonmarine sandstone facies. The extensional tectonism which formed these basins began in the Early Cretaceous. Movement along major fault trends continued intermittently into the Miocene. This deformation resulted in a complex structural history which led to the formation of several deep fault-bounded troughs, major interbasin high trends, and complex basin flanks. This tectonism has created a wide variety of structures, many of which have become effective hydrocarbon traps. End_of_Article - Last_Page 526------------

Lacustrine Siliciclastic Rocks and Hydrocarbons: ABSTRACT

Siliciclastic rocks formed from sediments deposited in lake basins are source and reservoir units for large accumulations of oil and gas in North and South America, Africa, and Asia. Lacustrine strata of China consist primarily of siliciclastic units; those of Brazil, Angola, and Cabinda are principally of siliciclastic rocks with abundant carbonate beds; those of the United States consist of at least 50% carbonate rock. The sediments were deposited in, or peripheral to, ancient stratified lakes of a variety of ages, which for millions of years maintained a size comparable to that of modern inland seas. In siliciclastic fine-grained beds formed at the depositional center of these large lakes, values of organic carbon commonly average 3% or less. Those sequences of carbonate strata developed in open-lacustrine depositional settings commonly contain greater than 25% organic carbon. However, porosity values are generally very low in reservoir beds associated with the carbonate source rocks (generally cemented with carbonate minerals) relative to those associated with claystone source rocks. The world's largest lacustrine oil and gas fields are developed in depositional systems with very thick sequences of open-lacustrine clay mudstone that contain relatively low values (generally 3% or less) of organic carbon. In these fields, hydrocarbons migrated from a thick central core of claystone into peripheral and overlying lacustrine and alluvial sandstones at relatively shallow burial depths and prior to significant compaction and cementation of the beds. Siliciclastic rocks form the principal reservoir units in most of the world's ancient petroliferous lacustrine depositional systems. Lacustrine turbidite, bar, and deltaic sandstones are important reservoir units in Brazil, Africa, China, and the United States. Although nonmarine reservoir rocks are commonly described as being of a lacustrine origin, many were formed from sediment deposited at the edge of the lake or in settings well removed from the lake. A principal reservoir facies in the Uinta basin, Utah, is composed of siliciclastic beds that developed as the basal parts of coalesced fluvial channels at the fluctuating margin of Paleocene and Eocene Lake Uinta. Oil-bearing strata in some Chinese basins are channel-fill sandstones formed from sediment deposited in moderately sinu us streams on an alluvial plain several kilometers from the lacustrine shoreline. Depositional models of siliciclastic lacustrine rocks constructed to aid in the exploration and exploitation of indigenous hydrocarbon accumulations should be sensitive to type, richness, and thermochemical maturation of organic matter developed and preserved in the lake. Because siliciclastic open-lacustrine beds in many of the large petroliferous lake basins commonly contain relatively small amount of organic matter, thick sequences of such strata may be required to generate large accumulations of oil. Known accumulations are largest where migration has been to porous and permeable beds at relatively shallow burial depths. End_of_Article - Last_Page 462------------

Geochemical Evaluation of Proposed Ore Genesis Models for Colorado Plateau Tabular-Type Uranium-Vanadium Deposits: ABSTRACT

The elemental, isotopic, and mineralogic examination of core samples from the Tony M orebody, a typical Colorado Plateau tabular-type, uranium-vanadium deposit, located in the Henry structural basin of south-central Utah, allows a critical evaluation of proposed ore genesis models. Uranium mineralization is concentrated into two horizontally oriented tabular horizons hosted within the Salt Wash member of the Jurassic Morrison Formation. The ore zones are not strata bound and rise stratigraphically toward the center of the ancestral Henry basin. Preservation of fossil plant debris and the lack of oxidative destruction of iron disulfide minerals in the interval between the uranium-enriched units argue this zone is not the oxidized tongue formed by a roll-type mineralizing p ocess. The selenium-molybdenum trace element pattern is also inconsistent with the roll-front model. The necessary interbedding of gray lacustrine mudstones and nearshore lacustrine sandstones, as required by the lacustrine-humate model, is present within the Tony M deposit. However, absence of quantitatively significant amounts of transported humic substances either associated or remote from mineralization suggests that mudstone-derived organic acids were not involved in uranium localization. Vanadium oxide is the major vanadium-bearing phase within the lower uranium lense whereas vanadium is partitioned within chlorite in the supra-adjacent barren zone. Such a sharp vertical break in vanadium mineralogy implies vanadium deposition within two chemically different environments. The lower ore zone is characterized by isotopically light (^dgr34S ^cong -26^pmil - 46^pmil) FeS2. Bacterial sulfate reduction is shown to be the most likely fractionating agent. The uniformity of the sulfur isotopic composition of the iron disulfide minerals requires a non-depletable sulfate reservoir. Dissolution of gypsum (^dgr34S ^cong + 14^pmil) occurring below the Tony M orebody is demonstrated to be a plausible sulfur source thereby establishing the presence of a sulfate brine. From this data we conclude that the solution-interface model, which postulates uraniferous meteoric fresh water flowing over a denser brine, best explains the genesis of this deposit. End_of_Article - Last_Page 436------------

Stratabound copper-lead-zinc mineralisation in the Permo-Triassic of central East Greenland

Stratabound and stratiform copper-lead-zinc-mineralised horizons confined to specific sedimentary fades in the Permo-Triassic Jameson Land Basin of East Greenland were revealed during recent exploration and sedimentological studies. The occurrences are divided into fault-bounded-stratabound and stratabound-stratiform mineralisation. The first group comprises lead-zinc-copper mineralisation in Upper Permian limestone; the remaining mineralisation falls in the second group which is subdivided into mineralisation hosted in mudstones, in sandstones with mudflasers and in sandstones and conglomerates. A lithogeochemical programme helped to define the mineralised horizons in the Triassic. During the interpretation of the geochemical data an empiric statistical function was introduced which is an estimate of how anomalous the 95 per cent fractile is for individual elements compared with the frequency distribution around the median. The Upper Permian sediments host copper in basal shoreline conglomerates, zinc-lead-copper in lagoonal mudstones and lead-zinc-copper in carbonate buildup and shelf fades. The Lower Triassic contains copper-lead mineralisation in alluvial fan sediments, the Middle Triassic hosts lead-zinc-copper in sandy shoreline limestones and lagoonal mudstones and copper-lead-zinc in gypsiferous lacustrine sandstones and mudstones while the Upper Triassic contains copper in both dolomitic lacustrine sandstones and mudstones and in overlying carbonate-rich fluvial channel sandstones.

Stratigraphy of Northeastern Cady Mountains and Its Implications for Cenozoic Volcanic Evolution of Mojave Desert: ABSTRACT

The Cenozoic rocks of the northeastern Cady Mountains can be divided into two separate sequences--an unnamed volcanic succession in the eastern Cady Mountains, and the volcanic and sedimentary strata of the Hector Formation in the northern Cady Mountains. In the eastern Cady Mountains, approximately 3,280 ft (1,100 m) of volcanic flows, flow breccias, domes, and laharic breccias overlie a basement complex of Mesozoic quartz diorite and quartz monzonite. These volcanic rocks have a continuous compositional variation from basaltic andesite to rhyodacite, although dacites and rhyodacites predominate. The age of this sequence is probably Oligocene and/or Miocene. In the northern Cady Mountains, approximately 2,395 ft (730 m) of strata assigned to the Hector Formation (Oligocene? to Miocene) lie unconformably on a basement complex of Mesozoic quartz monzonite and metamorphic rocks. The upper part of the Hector Formation also unconformably overlaps the volcanic rocks of the eastern Cady Mountains. The Hector Formation consists of alluvial sandstone and conglomerate plus lacustrine and paludal sandstone, siltstone, mudstone, and limestone. Laharic breccias, ash-flow and air-fall tuffs, and andesitic to basaltic lava flows are interbedded with the sedimentary rocks. Geochemical data indicate that the volcanic rocks from both areas are part of a calc-alkalic suite. The model proposed by various authors for the Cenozoic tectonic evolution of the western United States suggests that there should have been a transition from andesitic-type volcanism to basaltic-type volcanism, concurrent with the termination of subduction and the initiation of regional faulting. No such volcanic transition, however, can be identified within the northeastern Cady Mountains, nor has one been clearly identified in other late Oligocene to Miocene volcanic sequences within the Mojave Desert region. If this type of transition did take place within the Mojave Desert, it must have occurred after 18.6 ± 0.2 m.y.B.P. This is the date obtained from the youngest dated volcani rock that is unquestionably part of the calc-alkalic suite observed in the northeastern Cady Mountains. End_of_Article - Last_Page 1695------------

Triassic Paleocaliche in Red Beds of Dolores Formation, Southwestern Colorado: ABSTRACT

Early diagenesis of Upper Triassic lacustrine mudrocks and sandstones in southwestern Colorado includes pedogenic accumulation of calcium carbonate. Petrocalcic horizons, 0.4 to 1.5 m thick, are dominated by unlaminated micritic caliche concretions which range from pellets 1 mm in diameter to nodules 30 mm in diameter. Evidence for in-situ origin of these limestone concretions includes: irregular shape, floating texture, inverse grading, and truncation of back-filled burrows. Mature paleocaliche profiles are capped by calcium carbonate plugged horizons which contain rootlets 1 to 2 mm in diameter. These plugged horizons also contain paleofracture systems filled with reddish brown mudrock. Locally paleofracture morphology suggests plant-root control in its development. In very fine sandstones, micritic caliche nodules coalesce in upward coarsening, sinuous vertical stacks that suggest a rhizo-concretion origin. A maximum of nine petroclastic profiles were observed over a vertical interval of 56 m in one outcrop north of Durango, Colorado. The most mature profile observed exhibited the Stage III development of Gile and others. Caliche nodules and pellets occur throughout the Dolores Formation in southwestern Colorado either in pedogenic profiles or as transported clasts in fluvial, lacustrine deltaic distributary, or fan delta deposits. Literature descriptions suggest that caliche nodules are also present in the correlative Chinle Formation of New Mexico, Arizona, Utah, and Colorado. Based on the areal distribution of modern caliche, a semiarid climate is suggested for the Late Triassic in the Four Corners region of the western United States. End_of_Article - Last_Page 678------------

Petrology of Tertiary Sandstones of Southern Piceance Creek Basin, Colorado--Implication for Provenance and Depositional Processes: ABSTRACT

Comparative petrographic analyses of channel-form and tabular sandstone bodies in the upper Wasatch Formation (Paleocene-Eocene) and lower Green River Formation (Eocene) in the southern Piceance Creek basin show that compositional and textural variability is primarily a reflection of provenance and the environment of deposition. Sandstones from the upper Wasatch Formation and lower Green River Formation are generally similar in texture and composition and have varying concentrations of (1) angular to well-rounded monocrystalline quartz grains, some with abraded overgrowths; (2) fresh and slightly altered potassic and sodic feldspars; and (3) volcanic lithic fragments, mostly andesite. Wasatch sandstones contain slightly more lithic fragments than those of the lower Green River, which are more quartzose. This difference is attributed to the fluvial mode of deposition of the Wasatch in contrast to the marginal-lacustrine nature of the Green River sandstones. Lacustrine sandstone also commonly contains accessory analcime and pyrite. The sampled intervals of the Green River Formation permit an evaluation of the source terrane and its evolution during development of Lake Uinta. Paleocurrent data suggest that the sources for most of the sediment were on the south, southwest, and southeast. Petrographic similarities among the samples imply a relatively constant source terrane during deposition of the Green River Formation that was composed of Mesozoic and Paleozoic sedimentary rocks and late Mesozoic and early Cenozoic silicic volcanic and plutonic rocks. End_of_Article - Last_Page 836------------

Sedimentary and Tectonic Controls of Uranium Mineralization in Morrison Formation (Upper Jurassic) of South-Central Utah: ABSTRACT

Sedimentologic studies in the Henry basin of south-central Utah indicate that uranium deposits in sandstones of the Morrison Formation are related to the depositional environment in which the host sandstone bed and nearby mudstones were deposited and to the tectonic setting at the time of deposition. Thus, an understanding of environments of deposition and contemporaneous tectonism may be helpful exploration guides for uranium deposits in sandstones. Discontinuous tabular uranium orebodies are present in fluvial and marginal lacustrine sandstone beds that lie just above, below, or a short lateral distance from offshore lacustrine gray mudstone beds. The mudstones are present only in the distal part of the lowermost alluvial-plain sequence of the Morrison where fluvial energy regimes and rates of sedimentation were low, allowing lakes to form while the Henry basin was slowly subsiding. The lakes were restricted to the east side of the basin where the least amount of clastic sedimentation by streams occurred. Palynomorphs, carbonized plant debris, and scarce pyrite in the mudstones indicate that the lakes were sufficiently persistent and deep during most of their lifetimes for reducing conditions to persist in the offshore muds. The also indicate that the mudstones were originally gray and are not bleached red mudstones. The gray mudstones are not present in other alluvial-plain sequences of the Morrison because little or no basin subsidence occurred after deposition of the lowermost sequence. Plant debris and palynomorphs in the gray lacustrine mudstones suggest deposition in humus-producing lakes in which humic and fulvic acids (degradation products of plant tissues) were generated in the lake sediments. The close spatial association of the ore-bearing sandstones and the gray mudstones suggests that pore fluids containing these organic acids were expelled by compaction or seepage from the mudstones into nearby sandstones where they were fixed as tabular humate deposits. Subsequently, uranium in groundwater passing through the sandstone was concentrated by the humate to form the ore deposits. Gray mudstones which cannot be used as exploration guides are bleached red mudstones and primary gray mudstones containing the alga Botryococcus. Sandstone beds adjacent to these gray mudstones are barren of uranium because (1) the gray color in bleached red mudstones is an alteration feature and does not indicate primary reducing conditions, and (2) Botryococcus thrives in humus-free lakes whereas humus-producing lakes are considered necessary to form ore deposits. Thus, the requisites for mineralization appear to have been active crustal downwarping to cause ponding of fluvial sediments, and formation of humus-producing lakes on the alluvial plain, in the most sediment-starved part of the area undergoing basinal downwarping. End_of_Article - Last_Page 837------------

Red-bed diagenesis in the East Berlin Formation, Newark Group, Connecticut Valley

ABSTRACT The East Berlin Formation is a 200-m fluvial and lacustrine sequence of red and gray sandstone and mudstone that accumulated in a tropical rift valley during Early Jurassic time. The hematite pigment that colors the red beds is authigenic, produced by four post-depositional processes. (1) Brown and yellow-brown limonite that stained the surfaces of the detrital particles of sand and mud converted to hematite by aging. (2) In the sandstone, pervasive intrastratal solution of Fe-silicate grains, especially pyroxene, amphibole, epidote, chlorite, and biotite provided iron for precipitation of hematite, or a red ferric oxide precursor that then aged to hematite. These two processes were volumetrically the most important in generating hematite pigment. (3) In all the red beds, abundant mag etite grains were oxidized to hematite; ilmenite grains were oxidized to hematite-rutile. (4) In the sandstones, replacement of Fe-silicate grains by dolomite cement yielded additional iron for hematite cement. Conversion of limonite stains on clay particles to hematite by aging was the major source of hematite in flood-plain grayish red mudstone, a darker red (lower value and less yellow hue) than the interbedded stream channel sandstone of pale red and pale yellowish brown colors. Intrastratal solution of Fe-silicate grains in the fluvial and lacustrine sandstones produced an average of about 3 percent by volume of hematite cement. Pyroxene, amphibole, epidote, chlorite, and biotite grains were consistently protected from post-depositional solution in the impermeable dolomite concretions and grayish red mudstone of the flood-plain deposits and also in lacustrine gray mudstone. The limonite surface stains on detrital particles in the lakes were removed in the reducing, organic-rich bottom w ter so that the impermeable lacustrine gray mudstone and black shale are not reddened by aging of limonite to hematite nor by intrastratal solution.

SIMPSON DESERT SUB-BASIN—A PROMISING PERMIAN TARGET

Sediments of three major basins occur in the Simpson Desert region of central Australia:Cambro -Ordovician dolomites and sandstones, and Siluro- Devonian conglomerates, sandstones and shales, related to the Amadeus Basin:Permian conglomerates, sandstones, shales and coals of the Simpson Desert Sub-basin, the extensive eastern lobe of the Pedirka Basin:Mesozoic sandstones and shales of the Eromanga Basin.Principal petroleum exploration interest is presently directed toward the Permian sediments, which have many features in common with the petroleum producing Permian section of the neighbouring Cooper Basin.Lower Permian sediments known from drilling in the Simpson Desert Sub-basin comprise glaciofluvial conglomerates and sandstones overlain by fluvial and lacustrine sandstones, silt-stones, shales and coals. The maximum thickness encountered in wells is 1,479 ft (448 m) in Mokari 1.Recent seismic exploration 50 to 100 mi (80-160 km) west of Poeppel Corner in the deeper part of the Simpson Desert Sub-basin indicates that an additional sediment package up to 1,500 ft (350 m) thick occurs at depths of 6,500 to 7,500 ft (2,000-2,300 m) between Lower Permian and Lower Jurassic sections. This sediment package, nowhere penetrated by drilling, may be Middle to Upper Permian and/or Triassic in age. It is of great significance to petroleum exploration in the sub-basin and substantially upgrades the hydrocarbon prospects of the region.Permian sediments in the Simpson Desert Sub-basin thin by onlap, wedge out and stripping over the crests of anticlinal growth structures. Crestal sediments probably comprise mainly porous sandstones, grading off-structure into thicker sequences containing carbonaceous shales and coals. Such carbonaceous potential source rocks are probably best developed in the deepest part of the sub-basin, where Triassic cap rock may also be present. Two particularly promising drilling targets—the Colson Anticline and the East Colson Anticline—have been revealed by recent geophysical surveys in this portion of the sub-basin. Wells drilled on these structures may intersect Permo-Triassic sediments up to 2,200 + ft (670 in) thick which are comparable in age and type with producing sections in the Cooper Basin.

Differentiation of Lacustrine and Fluvial Sandstone by Analysis of Paleocurrent Patterns: ABSTRACT

Oil-impregnated intervals, up to 75 ft thick within a stratigraphic interval of about 250 ft in the Garden Gulch and Parachute Creek Members of the Green River Formation (Eocene), are exposed in beds that dip gently northward in the P. R. Spring area of the southeast Uinta basin, Utah. Reserve estimates indicate that there may be about 3.7 billion bbl of oil in place. It is necessary to distinguish lacustrine from fluvial sandstone in these intertonguing beds because most of the oil is in lacustrine sandstone. Paleocurrent study indicates that paleocurrent patterns can be used for these distinctions. A total of 308 paleocurrent measurements was made at 13 localities in the P. R. Spring area; 123 from sandstone beds of fluvial origin and 185 from lacustrine sandstone bodies. Of 9 fluvial paleocurrent patterns, 7 indicate that streams flowed northward into Lake Uinta in the P. R. Spring area. The considerable scatter in the paleocurrent measurements suggests that the streams had low gradients and were meandering. Many of the fluvial sandstone bodies are oriented approximately north-south and contain northward-inclined foreset laminae. Of 10 lacustrine paleocurrent patterns, 9 have significant intervals in the south half of the compass. These directions are interpreted to be dominantly the result of onshore lake currents. The shorelines of Lake Uinta probably trended northeast through much of the P. R. Spring area, but on the northeast, shorelines were oriented northwest-southeast. The paleocurrent patterns of fluvial and lacustrine sandstone are both unimodal. The environments can be differentiated, however, on the basis of paleocurrent orientations; the fluvial currents flowed northward; the lacustrine currents were southerly. End_of_Article - Last_Page 540------------

Oil-Impregnated Lacustrine and Fluvial Sandstone in Green River Formation (Eocene), Southeastern Uinta Basin, Utah: ABSTRACT

Oil-impregnated sandstone, in units up to 75 ft thick, are present in the lower Green River Formation in the P.R. Spring area, southeastern Uinta basin. Reserve estimates indicate that there may be about 3.7 billion bbl in place. A total of 308 paleocurrent measurements were made at 13 localities in the upper Wasatch Formation (Paleocene-Eocene) and the lower Green River Formation. Of these measurements, 123 were from fluvial sandstone and 185 were from lacustrine sandstone. Streams flowed northward into Lake Uinta in the P.R. Spring area. The considerable scatter in the fluvial paleocurrents suggests that the streams had low gradients and were meandering. Many of the fluvial sandstone bodies are oriented approximately north-south. Lacustrine paleocurrents are generally southeasterly, representing dominantly onshore lake currents. Thus, the shorelines trended northeast through much of the area, although in local embayments shorelines were oriented northwest. The paleocurrent patterns of fluvial and lacustrine sandstone are both unimodal with equal distribution of directions. The 2 environments can be differentiated, however, on the basis of paleocurrent orientations. Fluvial currents flowed northward; lacustrine currents were southerly. Both fluvial and lacustrine sandstones are mostly arkose. Potassium feldspar is more abundant than plagioclase. Quartz-feldspar ratios are greater for lacustrine sandstone than for fluvial sandstone. Lacustrine sandstone contains more quartz and authigenic carbonate and less feldspar, coarse mica, and matrix than fluvial sandstone. Some lacustrine sandstone is also characterized by intraclasts, oolite, fossil fragments, and analcime in contrast to fluvial sandstone. Most of the oil-impregnation is in lacustrine sandstone. Detailed sedimentological study has led to the discovery of new intervals. Such studies will also be useful when these and similar reservoirs are developed. End_of_Article - Last_Page 357------------

Nearshore Lacustrine Sandstone, Green River Formation (Eocene),Uinta Basin, Utah

Nearshore Lacustrine Sandstone, Green River Formation (Eocene),Uinta Basin, Utah

Reservoirs of Lacustrine Origin in Rocky Mountains--Exploration Criteria: ABSTRACT

Large reserves of oil and gas have been found in rocks of lacustrine origin in Rocky Mountain basins, and these strata remain largely unexplored. Except for the productive limestone of the Sheep Pass Formation (eastern Nevada), lacustrine carbonate reservoir rocks found to date have poor porosity and permeability. Sandstone is the best reservoir rock of the lacustrine suite. Most of the productive sandstone beds originated in deltaic complexes. Sandstone bodies paralleling old lake shores are much poorer objectives than their marine counterparts because of lower transport energy and relatively unstable water level in lakes. Large deltaic complexes are prime objectives, and directional criteria can be observed within the lacustrine facies as well as in the equivalent fluvial facies. The oil- and gas-producing sandstone units of the western Green River basin (commonly interpreted to have originated as beaches and bars along the west shore of a Paleocene-Eocene lake) are almost certainly of fluvial origin. These sandstone beds have no low-energy lateral facies equivalents which are characteristic of lacustrine deposits. Typically the main body of lacustrine deposits consists of very finely and uniformly laminated, well-indurated shale and very argillaceous microcrystalline carbonate rocks. Brown bituminous shale and oil shale are common. Proximal to the deltaic facies, shale becomes more variable. Induration decreases, sorting decreases, lamination and fissility decrease, siltstone is common, and slump structures and load casts are present in some places. The distal margins of sandstone bodies are well-cemented, well-sorted, very fine-grained (grading to siltstone), and thinly and horizontally bedded with abundant small ripple marks. They contain abundant shale partings on well-defined bedding planes. The main body of a typical lacustrine sandstone contains well-cemented oolitic laminae and thin beds alternating with thick to massive beds of porous, friable sandstone. Sorting is fair to good. Proximal to the fluvial facies, sandstone beds are poorly sorted with abundant clay matrix in many beds. Soft clasts (e.g., shale fragments) are common. End_of_Article - Last_Page 2033------------