Upper Part Of Cycle Research Articles

Characteristics and origin of the tight grainstone reservoirs of the Yijianfang Formation, middle Ordovician in the Shunnan area, Tarim Basin

Tight carbonate reservoirs are widely distributed in the world and have great economic exploration potential. The Ordovician Yijianfang Formation grainstones in the Shunnan area, Tarim Basin are considered to be hydrocarbon reservoirs. However, there are few visible pores from thin sections and cores, resulting in poor understanding of the pore systems in these tight limestone reservoirs. Core samples collected from 8 wells were prepared to characterize pore networks by combining the optical microscope, scanning electron microscope, helium gas porosimeter, nuclear magnetic resonance and micro-CT analysis. Carbon and oxygen isotopes and fluid inclusion thermometry were adopted to determine the timing of pore-related dissolution. For Yijianfang grainstones, porosity mostly ranges from 0.2 to 3.9%; permeability mostly ranges from 0.01 to 1 mD. Petrographic observations revealed that both intragranular macropores and micropores within grains constitute effective reservoir space. Besides, micropores could account for 50–90% of the total porosity. The intragranular pores only appear in the upper part of the cycle, and the micropores developed in upper and middle of the cycle. There are no pores in the lower part of the cycle. During brief subaerial exposure shortly after deposition, meteoric dissolution created intragranular pores and vugs in grainstones in upper part of the sedimentary cycle. Meanwhile, influenced by the freshwater-involved mineralogical stabilization, micropores transformed from between micrites to between calcite microcrystals within grains in grainstones in the upper and middle part of the cycle. In the burial diagenetic environment, vugs and interparticle pores were destroyed by compaction and burial cements. For packstones in the lower part of the cycle, mineralogical stabilization occurred without freshwater during progressive burial, leading to tight fabric with no or minor micropores. In summary, whether tight limestones can be effective reservoirs depends on whether freshwater is involved in dissolution and mineralogical stabilization. In general, tight grainstone reservoirs of shallowing-upward cycle could provide abundant micropores with few macropores due to the freshwater-involved dissolution and stabilization during subaerial exposure.

Climatic signature of cyclic fluvial architecture from the Quaternary of the central Po Plain, Italy

Detailed investigation of middle-upper Quaternary deposits from central Po Plain was carried out on the basis of integrated sedimentological and pollen analyses of a 114 m-long core, and stratigraphic correlations of well data. Stratigraphic architecture of alluvial strata beneath modern Po River reveals distinctive cyclic changes in lithofacies and channel stacking patterns. Each cycle includes basal, silt–clay overbank deposits with thin and lenticular fluvial-channel sands, with upward transition to increasingly amalgamated and more laterally extensive fluvial-channel sand bodies. Lower cycle boundaries, corresponding to the top of laterally extensive fluvial complexes, are locally associated with organic-rich, paludal clays. Pollen records show distinctive cyclic changes that parallel facies architecture. Overbank deposits in the lower parts of cycles are invariably associated with forest expansions. These indicate that major phases of channel abandonment and widespread floodplain aggradation took place at the onset of warm-temperate (interglacial) climatic conditions. The middle portions of cycles record transition to pollen associations diagnostic of glacial periods. Lack of pollen data from sandy facies prevents climatic characterization of laterally extensive fluvial bodies in the upper parts of cycles. On the basis of the overall stratigraphic framework, internal facies architecture and distinctive pollen attributes, the cycles identified in the subsurface of the study area are interpreted to fall in the Milankovitch (100 ka) band, and inferred to be correlative with the transgressive–regressive sequences recently recognized in the coeval successions of the Po coastal plain.

Evolution and Distribution of Porosity Associated with Subaerial Exposure in Upper Paleozoic Platform Limestones, West Texas

Middle Pennsylvanian-Lower Permian limestones in the subsurface of west Texas were studied to determine how subaerial exposure and freshwater diagenesis (karstification) affected porosity distribution in meter-scale cycles. Approximately 87 depositional cycles are present in the gross reservoir interval (depths of 2600-3000 m), and each cycle is interpreted to represent a glacio-eustatic sea level fluctuation. Using recent radiometric age dating, average cycle duration is estimated at 160,000 yr per cycle. Reservoir-grade porosity (>4%) occurs in 5-25% of the gross reservoir section. Porosity is stratified, occurring in 1-6-m-thick intervals in the upper part of cycles that were subaerially exposed; however, many cycles that were subaerially exposed now lack porosity. Diagenesis and porosity development have distinct patterns related to duration of subaerial exposure. Four stages of porosity development are identified. (1) Very brief or no subaerial exposure (estimated at less than 5000 yr) caused little or no diagenetic change. (2) Brief to moderate subaerial exposure (estimated at 5000-50,000 yr) resulted in most primary pores being filled with calcite cement, and dissolution creating fine matrix pores (molds and intercrystalline pores). (3) Moderately long subaerial exposure (estimated at 50,000-130,000 yr) resulted in cements filling primary pores and some fine secondary pores, and dissolution creating small conduit pores (vugs, fractures, fissures). (4) Prolonged subaerial exposure (estimated at greater than 130,000 yr) resulted in most primary and secondary matrix pores being filled with calcite cement, but dissolution enlarged conduit pores (vugs, fractures, fissures). Present subsurface porosity in this field preferentially occurs in thick grainstones, phylloid algal boundstones, and a few wackestone/packstones in cycles subjected to brief subaerial exposure (stages 2 and 3). Matrix porosity (molds, intercrystalline pores) is dominant because most conduit pores formed during prolonged subaerial exposure were filled with either shale during subsequent transgressions or burial cements derived from pressure solution associated with the shales. The distribution of porosity in the Southwest Andrews area indicates that duration of subaerial exposure and supply of clastics are major factors determining ultimate porosity in limestones subjected to subaerial exposure and karstification.

Integrated chemo- and biostratigraphic calibration of early animal evolution: Neoproterozoic–early Cambrian of southwest Mongolia

AbstractFive overlapping sections from the thick Neoproterozoic to early Cambrian sediments of western Mongolia were analysed to yield a remarkable carbon-isotope, strontium-isotope and small shellyfossil (SSF) record. Chemostratigraphy suggests that barren limestones of sequences 3 and 4, which lie above the two Maikhan Uul diamictites, are post-Sturtian but pre-Varangerian in age. Limestones and dolomites of sequence 5, withBoxonia grumulosa, have geochemical signatures consistent with a post-Varangerian (Ediacarian) age. A major negative δ13C anomaly (feature ‘W’) in sequence 6 lies a shortdistance above anAnabarites trisulcatusZone SSF asemblage with hexactinellid sponges, of probable late Ediacarian age. Anomaly ‘W’ provides an anchor point for cross-correlation charts of carbon isotopes and small shelly fossils. Trace fossil assemblages with a distinctly Cambrian character first appear in sequence 8(PurellaZone), at the level of carbon isotopic feature ‘B’, provisionally correlated with the upper part of cycle Z in Siberia. A paradox is found from sequence 10 to 12 in Mongolia: Tommotian-type SSFs continue to appear, accompanied by Nemakit-Daldynian/Tommotian-type87Sr/86Sr ratios but by increasingly heavyδ13C values that cannot be matched in the Tommotian of eastern Siberia. The steady rate of generic diversification in Mongolia also contrasts markedly with the Tommotian ‘diversity explosion’ in eastern Siberia, which occurs just above a major karstic emergence surface. One explanation is that sequences 10 to 12 in Mongolia preserve a pre-Tommotian portion of the fossil record that was missing or removed in easternSiberia. The Mongolian sections certainly deserve an important place in tracing the true course and timing of the ‘Cambrian radiation’.

Facies and cyclicity of the Late Permian Bainmedart Coal Measures in the Northern Prince Charles Mountains, MacRobertson Land, Antarctica

ABSTRACTThe Late Permian Bainmedart Coal Measures form part of the Permo‐Triassic Amery Group, which crops out in the Beaver Lake area of the Northern Prince Charles Mountains, MacRobertson Land, Antarctica. The exposed strata are believed to have formed in graben or half‐graben sub‐basins on the western edge of the Lambert Graben, a major failed rift system. Sedimentological analysis has revealed that these rocks formed in alluvial environments in which swiftly flowing rivers of low sinuosity (represented by Facies A1 and A2) flowed northward down the axis of the basin, and were associated with waterlogged floodbasin and peat‐forming wetlands (Facies B1‐B4). A third Facies Association (comprising Facies C1‐C3), interpreted as the deposits of lake floor and delta environments, is exclusively developed within a distinctive, fine‐grained interval here named the Dragon's Teeth Member. The proportion of Association B facies within the succession increases markedly above the level of the Dragon's Teeth Member (at about 300 m above the base of the formation).Flat, low‐angle and undulatory bedding structures preserved within channel deposits are suggestive of sediment deposition in flow conditions which were often critical or supercritical. Presence of massive and chaotic intervals of sandstone further implies some deposition from high‐concentration aqueous flows. Alluvial channel bodies show evidence of incision into underlying substrates, both during initiation and at later stages in channel belt construction. The lack of interfingering between channel deposits and coals suggests that thick peats formed only in areas and at times of minimal clastic sediment supply.Analysis of well‐developed cyclicity within the coal measures suggests that the dominant control on sequence architecture was climatic, related to precessional Milankovitch fluctuations of c. 19‐kyr periodicity. Cycles began abruptly with the deposition of coarse‐grained material in high‐energy alluvial channels, which contracted with time in response to changes in water supply (rainfall). Upper parts of cycles are dominated by finer‐grained sediments and then coal, indicative of progressively reduced coarse sediment input. Tectonic processes overprinted this pattern at least once during the period of sediment accumulation, to form the Dragon's Teeth Member.

Cycle Stratigraphy and Porosity in Pennsylvanian and Lower Permian Shelf Limestones, Eastern Central Basin Platform, Texas

Pennsylvanian and Lower Permian shelfal limestones were studied in core and wireline logs on the eastern side of the Central Basin platform in west Texas. Sixty-three (63) cycles were delineated in the study interval, which includes 200250 m of Canyon (Missourian), Cisco (Virgilian), and Wolfcamp strata. Four general lithofacies are present: fossiliferous wackestones and packstones, grainstones, phylloid algal boundstones, and shales. These lithologies typically occur in 118-m-thick cycles bounded by subaerial exposure surfaces. Grainstones in the upper part of some cycles indicate a shallowing of environments prior to subaerial exposure. Many cycles have subaerial exposure surfaces developed on subtidal fossiliferous wackestones or packstones suggesting rapid falls in se level. Long-term transgressive intervals (transgressive systems tracts or TST) are dominated by thick (>4 m) cycles, whereas long-term regressive intervals (highstand systems tracts or HST) are dominated by thinner cycles. Stable carbon isotope data suggest that thick cycles in TSTs were subjected to short periods of subaerial exposure, whereas thin cycles in the HSTs were subjected to much longer subaerial exposure. Porosity is quite variable beneath subaerial exposure surfaces. At any given well, reservoir-grade porosity (>4%) is present below only 1045% of the identified subaerial exposure surfaces. Where pres-ent, reservoir-grade porosity occurs in the upper part of cycles, 0.35 m below subaerial exposure surfaces. Porosity in thick cycles of the TST is relatively widespread and not facies-selective. Porosity is less abundant and facies-selective in the HST, with porosity occurring only in relatively thick (1.54 m) grainstones, which are concentrated near the shelf margin. Thin (

Environments and Dynamics of Clastic Sediment Dispersal Across Cambrian of Grand Canyon: ABSTRACT

End_Page 1004------------------------------The transgressive Grand Canyon Cambrian contains basal fluvial sands (Tapeats Sandstone) and overlying shallow marine sands and shales (Bright Angel Shale) in which textures and structures permit a detailed reconstruction of clastic sediment dispersal across a broad cratonic margin. Dominantly trough cross-bedded basal Tapeats Sandstone, containing buried regolith in bed-rock depressions, a low-variance, unimodal paleocurrent trend down the regional paleoslope, and well to moderately sorted medium to coarse-grained sands, records pre-vegetation bed-load fluvial sedimentation. Bed-load sands (> 200 mµ) mature from arkosic to orthoquartzitic within 5 to 10 m of the base. Finer, suspension-transported sands remain subarkosic through the entire 250 to 500 m of Cambrian section implying rapid dispersal. The Bright Angel Shale is composed of shallowing upward sedimentary cycles 1 to 6 m thick. Lower parts of the cycles are thinly interbedded fine sand and shale layers whose sedimentary structures and textures record level-bottom storm resuspension, dispersal, and deposition (sand layers) alternating with quiescent periods of finer silt and clay dispersal and accumulation. Bed-load transported sands are absent. Sand bed thickness up to a maximum (5 to 20 cm) represents variation in storm intensities with maximum thicknesses controlled by water depth, which limits volume in suspension. Upper parts of cycles are cross-laminated fine sands with glauconite and U-shaped or vertical burrows. These represent shoaling phases with increased bottom agitation, decreased sedimentation rate, and winnowing bypass of silts and clays. Organism structures, tadpole ripples, and rare mud cracks record shallowing. Cycle caps are 1 to 20-cm thick layers of medium to very coarse quartz sand, hematitic ooids, phosphatic, and dolomite cemented clasts that record phases of emergence and complete bypass of all suspension load. Recently recognized dunes 3 to 5 m in height provided bed load dispersal at the close of sedimentation cycles. Structures and textures suggest dunes are eolian. End_of_Article - Last_Page 1005------------