Sand Lithofacies Research Articles

Reservoir rock typing assessment in a coal-tight sand based heterogeneous geological formation through advanced AI methods

Geoscientists now identify coal layers using conventional well logs. Coal layer identification is the main technical difficulty in coalbed methane exploration and development. This research uses advanced quantile–quantile plot, self-organizing maps (SOM), k-means clustering, t-distributed stochastic neighbor embedding (t-SNE) and qualitative log curve assessment through three wells (X4, X5, X6) in complex geological formation to distinguish coal from tight sand and shale. Also, we identify the reservoir rock typing (RRT), gas-bearing and non-gas bearing potential zones. Results showed gamma-ray and resistivity logs are not reliable tools for coal identification. Further, coal layers highlighted high acoustic (AC) and neutron porosity (CNL), low density (DEN), low photoelectric, and low porosity values as compared to tight sand and shale. While, tight sand highlighted 5–10% porosity values. The SOM and clustering assessment provided the evidence of good-quality RRT for tight sand facies, whereas other clusters related to shale and coal showed poor-quality RRT. A t-SNE algorithm accurately distinguished coal and was used to make CNL and DEN plot that showed the presence of low-rank bituminous coal rank in study area. The presented strategy through conventional logs shall provide help to comprehend coal-tight sand lithofacies units for future mining.

Diagenetic alterations and deep high-quality reservoirs within deltaic distributary channel facies: a case study from the Permian Shihezi formation in the Hongde area, southwestern Ordos basin, China

Deep tight gas reservoirs are considered important hydrocarbon exploration targets. High-quality reservoir prediction is critical for successfully exploring and developing deeply buried tight sandstone gas. Previous research has found that the reservoir quality of deeply buried tight sandstones is controlled by diagenesis and sedimentary facies. However, the variation of diagenetic alterations in different facies is still poorly studied on deltaic tight gas sandstone. In this study, core analysis, wireline log data, and 3D seismic were studied for the characterization of diagenetic alterations and sedimentary facies. The tight sandstones were formed in braided river delta deposits. Gravel-bearing coarse-grained sandstone facies and cross-bedded sandstone facies developed in tight sandstones. The Gravel-bearing coarse-grained sandstone is formed in the mid-channel bar of deltaic distributary channels. The major diagenetic processes developed in the tight sandstone include compaction, cementation, and dissolution. Constructive diagenesis can generate secondary pores, mainly including dissolution and kaolinite metasomatism, which can effectively improve reservoir physical properties. Through the diagenesis alterations linked to different lithofacies in cored wells, there are obvious diagenesis variations in different lithofacies. Despite strong compaction, the reservoirs in coarse sandstone facies have developed internal dissolution. After compaction, cementation, and dissolution to increase porosity, the reservoir retains intergranular and secondary pores and forms relatively high-quality reservoirs. The fine sandstone facies with cross-bedding are strongly compacted, with internal ductile particles being compacted and deformed, and the particle orientation is clearly arranged. The reservoir is tight, and the development of dissolution in the reservoir is weak, resulting in poor reservoir quality. In addition, reservoirs located at the interface between sandstone and mudstone are often affected by diagenesis, resulting in the development of calcareous cementation, leading to poor reservoir quality. Therefore, high-quality reservoirs are mostly distributed in coarse sand lithofacies, mainly distributed in the mid-channel bar of distributary channel deposits. Sedimentary facies control the original physical properties of the reservoir with different content and texture; the quality of the original reservoir is subject to diagenetic alteration in different ways. The points in this study could offer insights better to predict deep tight reservoir quality in continental basins.

The Later Holocene Foraminifera of Liverpool Bay (Bae Lerpwl), British Isles: Morphospecies and Community Level Patterns

Abstract Two cores were studied from Liverpool Bay (∼40 m water depth). The lowest 80 cm of ISLF13 (290 cm long) comprised clay, gypsum and pyritised foraminifera (Surface Layer 2 Member, gypsiferous lithofacies GLF) from a tidal flat dominated by Cribroelphidium excavatum. Perturbation Detection Analysis (PDA) found two ISLF13 GLF abundance biozones (ABs) with assemblages in stasis. ISLF16 (80 cm long) and between 30–202 cm in ISLF13 were open marine, sandy with gastropods and bivalves (Surface Layer 1 Member, shelly sand lithofacies SSLF). Assemblages in the SSLF sections differed, despite their proximity. That in ISLF13 comprised dominant C. excavatum with lesser Ammonia batava, Asterigerinata mamilla, Cibicides refulgens and Bulimina elongata. PDA identified two ISLF13 SSLF ABs, both in growth stages. The ISLF16 assemblage was co-dominated by C. excavatum, A. mamilla and A. batava from a single AB in stasis. This difference might reflect sorting. Asterigerinata murraynhaynesi n. sp. is described.

Degradation of the plaquemines sub-delta and relative sea-level in eastern Mississippi deltaic coast during late holocene

The stratigraphic architecture of Plaquemines sub-delta lobe is identified from sedimentary cores and sub-bottom seismic data collected at Lower Breton Sound, Louisiana, USA. Four lithofacies were identified in cores from bottom to top: (1) organic-poor mud; (2) organic-poor silty sand; (3) massive mud; (4) organic-rich peat and root-rich soft mud. Overall, the lithofacies displays a typical progradational deltaic deposit at bottom with upward-coarsening feature, and then an upward-fining deposit overlying on top. Stratigraphic position of the organic-rich peat facies indicates that it is formed after the fluvial sediment source moves away from this region, and it is recognized as a termination of progradation of the Plaquemines sub-delta. Dated peat samples show that sub-delta progradation stops between 876 and 1384 yr AD, during which a 20–70 cm relative sea-level rise is detected. The geological framework reported in this study provides critical information for future modeling on river diversions. The stratigraphic architecture underlying Lower Breton Sound implicates that top three facies of root-rich soft mud, organic-rich peat and massive mud are likely to be eroded away before the development of new crevasse-splay or sub-delta, when sediment diversion is in operation. The deeper organic-poor silty sand lithofacies, however, can serve as a firm foundation to build new land.

Realization ranking of seismic geostatistical inversion based on a Bayesian lithofacies classification - A case study from an offshore field

Seismic inversion which is defined as the process of converting seismic boundary reflections into layer elastic properties plays a vital role in different methods of seismic reservoir characterization. It can be done through either deterministic or geostatistical approaches, and geostatistical inversion is the recommended approach for heterogeneous and complicated reservoir due to its capability in capturing the details of a reservoir with the associated uncertainties. However, a common issue with the geostatistical approach is generating a large number of realizations with the same probability which needs to be ranked based on some pre-defined criteria. This study applied a pre-stack geostatistical inversion on an offshore field in the Persian Gulf using a Bayesian framework to produce the joint posterior probability density function (PDF) of P-wave seismic impedance (IP) and S-wave seismic impedance (IS). This global PDF was then, decomposed into local ones at the trace locations using a sequential Gaussian simulation technique. Furthermore, these local PDFs were sampled and 250 realizations of IP and IS generated accordingly. These elastic properties were then converted into lithofacies, and were ranked based on the oil-producing volume lithofacies. Well log data showed a useful distinction of the oil sand lithofacies in the IP vs. VP/VS crossplot, and this domain was used to compute the PDFs for classifying lithofacies derived from the seismic inversion. Moreover, the oil lithofacies volumes were calculated, and the realizations were sorted to generate the probability of P10, P50 and P90 scenarios. Also, the probability of oil presence within the reservoir was computed using all of the classified realizations.

Automatic Determination of Sedimentary Units from Well Data

The issue of identifying stratigraphic units within a sedimentary succession is of prime importance for reservoir studies, because it allows splitting the reservoir into several units with specific parameters, thus reducing the vertical nonstationarity in simulations. A new method is proposed for semi-automatic determination of the sedimentary units from well logging that uses a customized geostatistical hierarchical clustering algorithm. A new linkage criteria derived from the Ward criteria (cluster minimum variance) is proposed to enforce the monotonic increase of dissimilarities. The discretized proportion of sand lithofacies calculated from the vertical proportion curve of the well is taken as input data. At each step of the procedure, the algorithm merges the most similar of two consecutive units of sand lithofacies, ensuring stratigraphic consistency. Finally, the number of units is deduced from the first most important step of the dissimilarity. The user can investigate a larger number of units by considering the clusters with lower levels of dissimilarities. The method is validated using two synthetic cases built for a fluvial meandering reservoir analog containing three and five units. The results from the synthetic cases show that the units are identified when the sand proportion contrast between units is larger than the internal variability within the units. For low sand contrasts between units or for a small number of wells, sedimentary unit limits may be found for lower clustering dissimilarities. Finally, the method is successfully applied to a field study, where the resulting cluster units are found to be comparable to the field interpretation, suggesting a limit between units defined by paleosols rather than close overlying lacustrine levels.

A SPECTRAL METHOD OF LITHOFACIES DIFFERENTIATION WITHIN A HYDROCARBON RESERVIOR UNIT USING SEISMIC AND WELL DATA FROM TOMBOY FIELD NIGER DELTA

Seismic facies analysis is a key component of Seismic interpretation workflow, as much information on depositional process, environment, reservoir architecture and potential can be determined from the seismic data. This enhances the seismic visibility of subtle stratigraphic features, and prediction of new prospects for drilling due to lateral continuity of sand bodies (the hydrocarbon reservoir) deposited within the sedimentary column. This study presents the results of the application of seismic amplitude inversion and spectral decomposition to differentiate lithofacies within a thin-sand reservoir and show their lateral and vertical continuity using amplitude-derived attributes (rock properties) in the interpretation of 3D seismic and well log data from “TOMBOY” field Niger Delta. A thin-sand reservoir was delineated from lithologic log signatures within the interval 11,135.61-11,210.35ft which corresponds to the time window 2.752 - 2.768secs on the seismic data along an arbitrary line drawn to connect the six wells (01-06) in the field. Crossplot of acoustic impedance against density across the reservoir bed showed three compartments interpreted as gas filled sand, oil filled sand and shale. The interpretation was enhanced via spectral analysis by computing frequency maps of acoustic impedance, density and velocity using discrete Fourier transform (DFT) technique at top (2.752sec) of the reservoir in terms of magnitude, phase and frequency. Phase response for the three attributes (acoustic impedance, density and velocity) showed lateral continuity and discontinuity of sand lithofacies at the various well locations which were not evident on their original amplitude time slice, while frequency response showed bed thickness, with low frequency indicating presence of hydrocarbon or sand rich beddings, while high frequency indicates thin shale beddings. It was observed that the top (2.752sec) of the reservoir for wells 02, 05, 01 and 04 showed low frequency values for the three attributes (acoustic impedance, density and velocity) which indicates potential zones for hydrocarbon production and development in the field, while wells 03 and 06 within the same time slice (2.752sec) showed a higher value in acoustic impedance frequency, density frequency and velocity frequency and suggests the presence of thin shale facies at the top (2.752sec) of the thin-sand reservoir. By defining the lateral and vertical continuity of sand facies within the reservoir bed, hydrocarbon fairways were better defined and knowledge of this can be incorporated into drilling decisions for field development. The results gave enhanced structural disposition of the reservoir bed and more insights into the variation of sand lithofacies with depth. Keywords: Lithofacies, Amplitude inversion, Geologic transform, Seismic attributes, Cross plots and Spectral decomposition. DOI : 10.7176/JEES/9-3-14 Publication date :March 31 st 2019

Nature and possible origins of hyper-arid floodplain islands: exemplified by the Kuiseb river, Namibia

The present work considers morphological and sedimentary evidence from floodplain island sediments in the ephemeral Kuiseb river of central coastal Namibia. Morphologically the islands constitute point bars, curvilinear ridges and composite megaforms. The island sediments consistently comprise two lithofacies – a lower relatively thick often massive silt-very fine sand which is overlain by an upper, thinner, intermittent fine sand. The structure and texture of the lower beds suggest that early island deposition took place as a series of mass flows in association with low velocity stream flow. Originating from pre-existing upstream silt and very fine sand, the mass flows appear to have entered the main Kuiseb valley under unusual conditions of relatively heavy, coastal rainfall during the generally drier conditions of the Little Ice Age. Upon desiccation the sediments were soon partially indurated due to extreme evaporation effects. The upper fine sand lithofacies are regarded as being the product of more recent major flood events. The fine sands were deposited as a result of upper catchment rainfall leading to the formation of mainly overbank and levee deposits, in addition to eroding the islands into their current morphology. Hence the Kuiseb floodplain islands are characterised as hybrid deposits reflecting changing palaeo-climatic conditions over the last 600 years or so. This type of hybridisation involving infrequent earlier mudflow deposition, the induration and retention of the deposits and their later modification by renewed flooding, might be regarded as an important characteristic of moderate to low energy hyper-arid rivers worldwide.

EemianWeichselian Pleniglacial fluvial deposits in S Poland (an example of the Vistula River valley in Kraków)

A fragment of the middle terrace in the VistulaRivervalley, nearby the railway station in Kraków, is formed by fluvial channel and overbank deposits of the PrądnikRiver, which bear a record of various environments affected by changing climatic conditions. The sedimentary succession includes two complexes that differ in lithofacies. The older complex comprises fining-upward deposits (channel sand and gravelly sand with medium- and large-scale trough cross-stratification) and, less frequently, sand with planar cross-stratification overlain by silt with intercalations of biogenic deposits of abandoned channels. Vegetation accompanying the deposition of biogenic layers was typical of boreal coniferous forests, dominated by Pinus sylvestris with a small admixture of Larix, Pinus cembra, Picea, Betula, and Populus . Periodically, the landscape passed into open areas overgrown by woody tundra. The complex developed as a result of activity of a meandering river under conditions of a moderately cool climate. The younger complex includes the sand lithofacies with horizontal stratification and low-angle cross-stratification, overlain by alternating sands and silts. The topmost part is represented by sands with large- and medium scale planar cross stratification. Lack of biogenic deposits and considerable amount of frosted quartz grains in alluvial sediments indicate aeolian processes of greater intensity, periglacial conditions and evolution towards a braided or transitional river. Pollen successions, absolute dating and studies of structural and textural features of the sediment suggest that the time of its deposition may be estimated at a range between the close of the Eemian Interglacial and the Weichselian Middle Pleniglacial (OIS 5e–OIS 3).

Early Holocene fluvial activity from the sedimentology and palaeohydrology of gravel terrace in the semi arid Mahi River Basin, India

Palaeocompetence analysis and palaeodischarge estimation techniques are applied to a late Pleistocene–early Holocene gravel terrace in the Mahi River Basin, western India. Terrace sedimentology, comprising gravels overlain by sand lithofacies suggests a gradual change in palaeohydrological conditions marking a switch from braided to meandering fluvial styles. The discharge values for the gravel bedforms based on the clast size and the cross bed set thickness are estimated between ∼150–180m3s−1 comparable with the present day observed values albeit with a much higher competence. Results indicate that fluvial aggradation occurred under low discharge conditions with intermittent high discharge events depositing longitudinal gravel bars. The incision of these gravel bars and the formation of terraces can be attributed to the higher discharge regime post 9.2ka. The study further indicates that whereas the aggradation of the gravel terrace during the early Holocene was controlled by the large sediment influx, the incision that followed was in response to the increase in the discharge and competence of the river flow.

The significance of deep-water cryptic bioturbation in slope-channel massive sand deposits of the lower Rio Dell Formation, Eel River basin, California

Deep-water massive (featureless) sands are commonly interpreted as indicators of flow conditions during or shortly after deposition and are almost never interpreted to reflect post-depositional homogenization due to bioturbation. Deep-water deposits of the lower Rio Dell Formation (Pliocene), Eel River basin (NW California), however, reveal previously unrecognized cryptically bioturbated fabrics in massive sandstone and mudstone. This study represents the first detailed documentation of cryptically bioturbated massive-appearing sandstone in deep-water channelized environments. Although fluid escape contributed to some of the massive-appearing textures in the Rio Dell Formation, we present criteria for recognizing cryptic bioturbation fabrics: (1) tubes with lithologically distinctive concentric linings or infillings, (2) meniscate backfill in burrows, (3) lithologically distinctive spreite, (4) concentric grain orientations, and (5) grains realigned adjacent to tubular structures. Using biostratigraphic data, detailed stratigraphic descriptions, core photos, and photomicrographs, we document ichnofabrics associated with two main lithofacies and their bounding surfaces: (1) diffuse muddy siltstone lithofacies (e.g. Planolites, Phycosiphon, Schaubcylindrichnus), (2) massive sandstone lithofacies (e.g. cryptic bioturbation , Planolites), (3) lithofacies crossers, (4) burrows that prefer the top-of-sand/base-of-mud interface (e.g. Planolites, Skolithos), and (5) a Glossifungites-demarcated discontinuity at the base of the massive sand lithofacies. We also establish two main lithofacies associations with characteristic sedimentologic traits. We use relative percentages of the lithofacies associations to interpret slope-channel depositional facies: distal channel margin, proximal channel margin, and channel axis. By comparing ichnologic characteristics to depositional facies, we conclude that proximity to the channel axis appear to control the degree and type of bioturbation observed in the massive-appearing deposits. Distal channel margin deposits preserve the smallest burrows, highly developed cryptic bioturbation fabrics, and the most complete sequence of background bioturbation. Proximal channel margin and channel axis deposits record larger burrows but incomplete cryptic bioturbated fabrics and an overall smaller proportion of preserved background bioturbation.

Genesis of arsenic in groundwater of North Bengal Plain using PCA: A case study of English Bazar Block, Malda District, West Bengal, India

AbstractThe study area is located on the western part of the alluvium‐filled gap between the Rajmahal hills on the west and the Garo hills on the east. Groundwater occurs under unconfined condition in a thick zone of saturation within the Quaternary alluvial sediments. Three hydrochemical facies with distinct characteristics have been identified which are dominated in general by alkaline earths and weak acids. The major‐ion chemistry of the area is controlled by weathering of silicate minerals, rainfall recharge, ion‐exchange processes and anthropogenic activities such as irrigation return flow and the application of inorganic fertilizers and pesticides. A stoichiometric approach suggests that mineral dissolution and anthropogenic activities contribute 79% and 21% of the total cations dissolved in groundwater. Principal component analysis (PCA) of 42 groundwater samples using 13 chemical parameters indicates that the combined processes of recharge of groundwater from rainfall, sediment water interaction, groundwater flow, infiltration of irrigation return water (which is arsenic rich due to the use of arsenic‐bearing pesticides, wood preservatives, etc. and the pumping of arsenic‐rich groundwater for agriculture purpose), oxidation of natural or anthropogenic organic matter and the reductive dissolution of ferric iron and manganese oxides play a key role in the evolution of groundwater in the study area. Factor 2 scores, associated with the infiltration of irrigation return water and spatial distribution of arsenic concentration reveal that the groundwater of the municipal area will not be affected by arsenic in the future in spite of heavy groundwater abstraction. Another PCA with geologic, geomorphic, anthropogenic, geochemical and landuse factors indicates that arsenic concentration in groundwater increases with increasing area of mango orchards, sand lithofacies and nitrate and decreases with increasing distance of paleochannel from the monitored well and depth of bore wells. High loading on nitrate may be attributed to the use of fertilizer, pesticides, etc. in mango orchards and agricultural land. High loadings on log pCO2, mango orchards (with negative sign) and phosphate (with positive sign) indicate that mango orchards provide the organic waste material which is decomposed to form organic carbon. The organic carbon undergoes oxidative carbon degeneration by different oxidants and increases the concentration of CO2 in the aquifer. The reducing condition thus developed in the aquifer helps to dissolve the arsenic adsorbed on iron hydroxide or oxy‐hydroxide coated margins of sand, iron rich heavy mineral grain margins, clay minerals and Fe–Mn concretions present in the aquifer matrix. Copyright © 2007 John Wiley & Sons, Ltd.

Terrestrial glacial sedimentation on the eastern margin of the Irish Sea basin: Thurstaston, Wirral

This paper provides the first comprehensive description and interpretation of Pleistocene glacigenic deposits exposed in a cliff section at Thurstaston on the Wirral Peninsula, NW England. The section occupies a strategic position where the Irish Sea ice sheet impinged on the English Midlands and North Wales coast. The section comprises six lithofacies: diamicton, gravel, sand, laminites, mud and cobble pavements. The diamicton lithofacies can be divided into an upper, clast-poor sandy diamicton and multiple units of a lower, clast-rich sandy diamicton. Both diamictons exhibit cobble pavements. The two diamicton lithofacies are distinguished on the basis of textural composition, clast lithology, clast shape, clast surface features and clast macrofabrics. Between the diamicton lithofacies are interbeds of sands and gravels of variable thickness. Minor mud and laminites also occur in close association with the sands and gravels. Both the diamictons are interpreted as basal, deformation tills with the interbeds of the sand, gravel and mud lithofacies as indicators of subglacial meltwater flow and ponding. The cobble pavements are interpreted as the result of clasts sinking within, or to the base of, the deforming layer. The sedimentary succession at Thurstaston is best explained by the advance and subsequent recession of a single terrestrially based ice sheet during the Late Devensian. There is no evidence at Thurstaston to suggest a glaciomarine origin for the Late Devensian deglaciation sediments on this margin of the Irish Sea basin. The evidence at Thurstaston points to constructional deformation with the net accretion of till and interbeds caused by the upward migration of the deforming layer base.

Time‐lapse crosswell seismic tomogram interpretation: Implications for heavy oil reservoir characterization, thermal recovery process monitoring, and tomographic imaging technology

Time‐lapse crosswell seismic data acquired with a cemented receiver cable have been processed into P‐ and S‐wave tomograms which image heavy oil sand lithofacies and changes as a result of steam injection. Twenty‐seven crosswell surveys were acquired between two wells over a 3.5 month period before, during, and after a 34‐day, 30 MBBL [Formula: see text] steam injection cycle. Interpretation was based on correlations with reservoir data and models, observation well data, and engineering documentation of the production history and steam cycle. Baseline S‐ and P‐wave tomograms image reservoir sand flow units and areas affected by past cyclic steam injection. S‐wave tomograms define lithology and porosity contrasts between the excellent reservoir quality, “high flow” turbidite channel facies and the interbedded “low to moderate flow” bioturbated levee facies. The reservoir dip of approximately 20° is defined by the velocity contrast between lithofacies. P‐wave baseline tomograms image lithology, porosity, structure, and several low velocity zones caused by past steam injection. Previous steam‐heat injection caused the formation of gas which reduced velocities as much as several thousand ft/s (600 m/s), an amount which obscures the velocity contrast between lithofacies and smaller velocity reductions as a result of temperature alone. Time‐lapse and difference P‐wave tomograms document several areas with small decreases in velocity during steam injection and larger decreases after cyclic steam injection. Velocity reductions range from 300 to 900 ft/s (90 to 270 m/s) adjacent to and above injectors located 20 to 50 feet (6 to 15 m) from the tomogram cross‐section. Poisson’s ratio tomograms show a significant decrease (.10) in the same area, and include low values indicative of gas saturation. Continuous injectors located 50 to 350 feet (15 to 100 m) from the survey area also caused a progressive decrease in velocity of the “high flow” channel sands during the time‐lapse survey. Interdisciplinary interpretation indicates that tomograms not only complement other borehole‐derived reservoir characterization and temperature monitoring data but can be used to quantitatively characterize interwell reservoir properties and monitor changes as a result of the thermal recovery process. Monitoring results over 3.5 months confirms that stratification has controlled the flow of steam, in contrast to gravity override. This suggests that tomographic images of reservoir flow‐units and gas‐bearing high temperature zones should be useful for positioning wells and optimizing injection intervals, steam volumes, and producing well completions.

Clay-Mineral Suites in Cyclic Miocene Sediments: A Model for Continental-Margin Deposition in a Mixed Siliciclastic-Phosphatic-Dolomitic-Biogenic System

ABSTRACT Clay-size mineral assemblages from lithologically diverse Miocene shelf and upper-slope sediments of the North Carolina continental margin show strong environmental zonation. Three distinct clay-mineral suites are recognized in the Miocene sediments of the Pungo River Formation. (1) A detrital kaolinite suite was deposited proximal to siliciclastic point sources in muddy quartz sand lithofacies. This assemblage reflects intense chemical weathering on the adjacent Coastal Plain and Piedmont Provinces. (2) A detrital illite and smectite suite deposited in phosphatic lithofacies is distinguishable from the clay mineralogy of muddy (distal) terrigenous and biogenic-dominated lithofacies only because illite is more highly crystalline. (3) Lithofacies characterized by authigenic dolomite co tain a clay-mineral suite with palygorskite and sepiolite. These Mg-clay minerals formed authigenically in the upper decimeters of sediment, penecontemporaneously with dolomitization, and in association with diatom dissolution, which provided the silica source for neoformation. Clay-mineral assemblages have not been significantly altered by burial or late-stage diagenesis and therefore appear to be original signal depositional indicators of changing environmental conditions through major Miocene fluctuations in sea level. The kaolinite suite is present in a fourth-order cycle immediately preceding the sea-level lowstand. Palygorskite and sepiolite are restricted to organic-rich dolomitic sediments, deposited under highly dysaerobic bottom conditions associated with the highstand and formation of the condensed section.

Influence of late holocene pyroclastic eruptions on the sedimentary geochemistry of Lake Rotoiti, North Island, New Zealand

Lake Rotoiti in Taupo Volcanic Zone was formed by damming of the drainage system through the floor of Okataina Caldera. Basin sediments are predominantly silt or sand, with mineralogy consistent with derivation from local silicic rocks and airfall tephras. Sandy lithofacies around the shoreline are wave worked deposits. Sand and gravel lithofacies in deeper water and on steep slopes are largely relict or airfall tephras, or both. Profundal sediments are diatomaceous silts. Organic-rich diatomaceous silts also accumulate in near-shore wave-damped zones under weed beds.

Sedimentology, Ichnology, and Paleoenvironments of the Upper Cretaceous Wenonah and Mt. Laurel Formations, New Jersey

ABSTRACT The Wenonah and Mt. Laurel Formations of the New Jersey Coastal Plain comprise a shallow subtidal, coarsening-upward, regressive sequence that accumulated along a wave-dominated coast in Late Cretaceous (Maestrichtian) time. The highly bioturbated, muddy, very fine to fine quartz sands of the Wenonah Formation were deposited mainly in the offshore zone of an inner shelf, below storm wave base. Distinctive trace fossils include Rosselia socialis and a planar form of Zoophycos. The Mt. Laurel Formation consists of two lithofacies. In the northern New Jersey Coastal Plain, the formation is characterized by burrowed, clean quartz sands that are thinly interbedded with muddy sand and mud. Characteristic trace fossils include Ophiomorpha nodosa, Rosselia socialis, two forms of Skolithos, and a delicate branching burrow. Deposition occurred mainly in the offshore-shoreface transition zone between fair weather and storm wave base. Storm deposits include burrowed sands displaying trough cross-stratification, hummocky cross-stratification, lenticular and wavy ripple-bedding, and parallel lamination. Fair weather deposits consist of thoroughly bioturbated, muddy sand and thin-bedded mud. At updip exposures, shoreface deposits include trough cross-strati ied, burrowed, fine to medium sands. Paleocurrents were bipolar, with a predominant northwest (onshore) mode. A massive sand lithofacies characterizes the Mt. Laurel Formation in the central and southern New Jersey Coastal Plain and is characterized by highly bioturbated, medium to coarse sand. Rare cross-stratification indicates a polymodal paleocurrent pattern with predominantly northwest transport. Distinctive trace fossils include Ophiomorpha nodosa and Skolithos linearis. This lithofacies accumulated mainly as shoals or bars in the transition and offshore zones of the inner shelf. A shoreface ebb tidal delta facies was locally developed. The Marshalltown, Wenonah, and Mt. Laurel Formations comprise a depositional sequence (in the sense of Haq et al. 1987) that accumulated during a third order, eustatic sea level cycle (UZA-4.4). The Wenonah-Mt. Laurel regression occurred during a sea level high-stand as sediments began to overflow estuaries created during the Marshalltown transgression. Subsequent falling sea level intensified the regression, and ultimately led to erosional truncation of the Mt. Laurel sediments. Then eustatic rise in sea level during the Late Maestrichtian (UZA-4.5) led to ravinement. During this ravinement event, a fossiliferous and pebbly, glauconitic quartz sand lithofacies was deposited as a transgressive lag in the transition and offshore zones of the inner shelf. These transgressive sheet sand ediments comprise the lower part of the Navesink Formation. The depositional model developed herein for the Wenonah-Mt. Laurel regressive interval integrates both sedimentologic and ichnologic data. This model is compared with the well-known Gallup and Blackhawk Cretaceous regressive sequences of the North American Western Interior. It should be applicable in the evaluation of other regressive sequences in Cretaceous strata of the U.S. Atlantic Coastal Plain and Western Interior, as well as epeiric sea cycles from other similar settings around the world.

Thermal Maturity and Kerogen Type of Miocene Sands, Offshore Vermilion Parish, Louisiana: ABSTRACT

Representative lithofacies of the Miocene (Burdigalian) Robulus L zone from the Texaco 6 well, Vermilion Block 31, offshore Louisiana, were sampled and subjected to mean vitrinite reflectance measurements and programmed pyrolysis. Vitrinite reflectance and pyrolysis T/sub max/ results are consistent; they indicate organic matter in the Robulus L sands is thermally immature to marginally mature with respect to oil generation. Based on low hydrogen indices (HI) and high oxygen indices (OI) measured during pyrolysis, kerogen in the Robulus L interval is type III with no meaningful oil source potential. Maceral analysis supports this assertion. Crude oil in these reservoirs has migrated from deeper and more mature source rocks. Total organic carbon in the interval ranges from 0.26% in the sand lithofacies to 0.99% in the interbedded sand and shale lithofacies. Because of the predominance of recycled organic matter in the coarse size fractions, finer size fractions ( 10 ..mu..m) and whole-rock samples. The presence of type III kerogen at a pregenerative stage of maturity suggests this reservoir interval is a possible source of carboxylic acids. The carbonate dissolution observed in the robulus L sands may have beenmore » caused by this organic acid production. Hence, organic matter deposited in association with reservoir facies may be important in porosity modification and should be considered in studies concerned with geochemical mass balances.« less

Sedimentologic and Tectonic Factors Controlling Growth and Dimensions of Submarine Fans: ABSTRACT

The morphology, development, composition, and structure of submarine fans reflect five controlling factors. (1) Grain-size distribution--sandy debris results in radial fans, whereas mud-dominated debris promotes elongate fans. (2) Eustacy--during high sea-level stands, fan growth is curtailed as sediment, especially coarse sediment, is trapped in paralic settings, whereas during low stands, fan growth is accelerated owing to direct access of sediment to canyon heads and the flushing of debris from shelf areas. (3) Basin architecture--confined or unconfined. (4) Penecontemporaneous tectonics from seismic triggering to migrating source terranes. (5) Postdepositional tectonics that bias or alter lithofacies preservation--bias in sediment subduction results in the preferentia loss of distal facies. Some examples that demonstrate these five controlling factors include the following. (1a) The Navy and Arguello fans of the California continental borderland are sandy, radial fans, whereas (1b) the Bengal and Indus fans are muddy elongate fans. (2) The Eocene Farello fan, southern California borderland, is composed of thick intervals of sand and conglomerate lithofacies, corresponding to low sea level stands, and thin fine-grained intervals, corresponding to high sea level stands. (3) Relatively unconfined fans are most typical of passive or trailing continental margins, e.g., the Bengal, Amazon, and Mississippi fans, although even here diapir fields, seamounts, and fracture zones offer local impediments. In contrast, fans in confined basins are characteristic of convergent or transf rm continental margins, e.g., borderland, forearc, and backarc basins, although passive margins may also have confined basins. The Laurentian fan, for example, is blocked by the Kelvin Seamounts. (4) Large infrequent earthquakes (one to several per millenium) may trigger large depositional events, e.g., the 1929 Grand Banks shock. Shingling of depositional units occurs in pull-apart basins of strike-slip fault systems; such sequences may have stratigraphic thicknesses (not vertical accumulations) of 10 km/m.y. (33,000 ft/m.y.). Examples include the Neogene Ridge basin in California and the Cretaceous Izumi basin in Japan. Where a canyon head lies seaward of a transform fault, a large fan develops with multiple sources, e.g., the Monterey and Delgada fan systems. Where a canyon lies landw rd of a transform boundary, multiple small fans of one source terrane are strung out along a continental margin, e.g., the Baranoff fans of southeastern Alaska. (5) Inferred subduction complexes, e.g., the Torlesse of New Zealand, are composed principally of thousands of meters of tectonically thickened (thrust faulted) thick-bedded sandstone lithofacies. This composition may be due to decoupling of the more distal, fine-grained facies from the proximal facies and its transportation into the mantle along with the underlying oceanic crust. End_of_Article - Last_Page 488------------

Recent and Pleistocene beach/dune sequences, western Australia

Wave-dominated sandy shores occur along much of the coast of Western Australia. Despite local variations there is a characteristic distribution of lithofacies (corresponding to different geomorphic zones). Five lithofacies are recognised: (1) trough-bedded sand/gravel; (2) laminated sand; (3) laminated/bubble sand; (4) laminated/disrupted sand; and (5) aeolian cross-stratified sand. The trough-bedded sand/gravel lithofacies is being deposited in the shallow shoreface below LWL. The laminated sand and laminated/bubble sand lithofacies are sands with gravel layers being deposited on the foreshore swash zone; extensive bubble (or vesicular) sand is common towards HWL especially in berms. The laminated/disrupted sand lithofacies is being deposited on the backshore between HWL and storm water levels and consists of horizontally layered to homogeneous sands with storm debris, especially wood, weed and floatable skeletons (e.g. Sepla and Spirula). The aeolian cross-stratified sand lithofacies is forming in beach ridge/dune areas and consists of fine sands with large-scale, generally landward-dipping forests; soils and rootlets are common. Recognition of these lithofacies within a sedimentary sequence enables reconstruction of gross shoreline conditions in terms of wave and eolian environments, tidal and storm heights, and palaeogeography. Each of these lithofacies with their characteristic features is recognised in Pleistocene sequences in Perth Basin. The Pleistocene sequences fit a model of coastal progradation with the trough-bedded sand/gravel lithofacies at the base and the aeolian sand lithofacies at the top. The value of such a stratigraphic sequence, however, extends beyond the Pleistocene.