Recognition Of Depositional Environments Research Articles

Modeling stochastic saline groundwater occurrence in coastal aquifers

The issue of freshwater salinization in coastal areas has grown in importance with the increase of the demand of groundwater supply and the more frequent droughts. However, the spatial patterns of salinity contamination are not easy to be understood, as well as their numerical modeling is subject to various kinds of uncertainty. This paper offers a robust, flexible, and reliable geostatistical methodology to provide a stochastic assessment of salinity distribution in alluvial coastal areas. The methodology is applied to a coastal aquifer in Campania (Italy), where 83 monitoring wells provided depth-averaged salinity data. A Monte Carlo (MC) framework was implemented to simulate depth-averaged groundwater salinity fields. Both MC stochastic fields and the mean across MC simulations enabled to the delineation of which areas are subject to high salinity. Then, a probabilistic approach was developed setting up salinity thresholds for agricultural use to delineate the areas with unsuitable groundwater for irrigation purposes. Furthermore, steady spatial patterns of saline wedge lengths were unveiled through uncertainty estimates of seawater ingression at the Volturno River mouth. The results were compared versus a calibrated numerical model with remarkable model fit (R2=0.96) and versus an analytical solution, obtaining similar wedge lengths. The results pointed out that the high groundwater salinities found inland (more than 2 km from the coastline) could be ascribed to trapped paleo-seawater rather than to actual seawater intrusion. In fact, the inland high salinities were in correspondence of thick peaty layers, which can store trapped saline waters because of their high porosity and low permeability. Furthermore, these results are consistent with the recognition of depositional environments and the position of ancient lagoon alluvial sediments, located in the same areas where are the highest (simulated) salinity fields. This robust probabilistic approach could be applied to similar alluvial coastal areas to understand spatial patterns of present salinization, to disentangle actual from paleo-seawater intrusion, and more in general to delineate zones with unsuitable salinity for irrigation purposes.

Sedimentological analysis and bed thickness statistics from a Carboniferous deep-water channel-levee complex: Myall Trough, SE Australia

This investigation presents an outcrop-based integrated study of internal division analysis and statistical treatment of turbidite bed thickness applied to a Carboniferous deep-water channel-levee complex in the Myall Trough, southeast Australia. Turbidite beds of the studied succession are characterized by a range of sedimentary structures grouped into two main associations, a thick-bedded and a thin-bedded one, that reflect channel-fill and overbank/levee deposits, respectively. Three vertically stacked channel-levee cycles have been identified. Results of statistical analysis of bed thickness, grain-size and internal division patterns applied on the studied channel-levee succession, indicate that turbidite bed thickness data seem to be well characterized by a bimodal lognormal distribution, which is possibly reflecting the difference between deposition from lower-density flows (in a levee/overbank setting) and very high-density flows (in a channel fill setting). Power law and exponential distributions were observed to hold only for the thick-bedded parts of the succession and cannot characterize the whole bed thickness range of the studied sediments. The succession also exhibits non-random clustering of bed thickness and grain-size measurements. The studied sediments are also characterized by the presence of statistically detected fining-upward sandstone packets. A novel quantitative approach (change-point analysis) is proposed for the detection of those packets. Markov permutation statistics also revealed the existence of order in the alternation of internal divisions in the succession expressed by an optimal internal division cycle reflecting two main types of gravity flow events deposited within both thick-bedded conglomeratic and thin-bedded sandstone associations. The analytical methods presented in this study can be used as additional tools for quantitative analysis and recognition of depositional environments in hydrocarbon-bearing research of ancient deep-water channel-levee settings.

Surface Gamma-Ray Profiling Technique Applied to Cretaceous Ferron Sandstone, East-Central Utah: ABSTRACT

The preparation of total count gamma-ray profiles using a hand-held scintillometer is a powerful supplement to standard measured vertical sections. Gamma readings taken at 1-ft intervals produce a composite profile which bears a striking resemblance to borehole well logs. Rock unit boundaries, marker beds, and subtle changes in rock composition can be detected with the profiling technique. Overall pattern shape and the distribution of gamma-ray measurements on histograms are useful in the recognition of depositional environments. Exposures of the Turonian Ferron Sandstone Member, located near the towns of Ferron and Emery in east-central Utah, were measured using the gamma-ray profiling technique. Various deltaic and barrier island subenvironments are shown to have characteristic profiles and histograms. Gamma-ray profiles of stream mouth bars, point bars, crevasse splays, barrier spits, tidal deltas, and washovers are presented. Vertical sequences are recognized, and correlations to adjacent measured sections have been established. The surface profiles also correlate favorably with nearby subsurface well logs, including those within the Ferron gas field. The integration of surface and subsurface data sets by the correlation of gamma-ray signatures results in a stronger, more sophisticated stratigraphic framework than either data set can yield independently.

Recognition of Depositional Environments Using Programmed Pyrolysis: Sparta Formation, South-Central Louisiana: ABSTRACT

Programmed pyrolysis of mudstone provides a rapid and effective technique to recognize environments of deposition in the subsurface. Ninety-three mudstone samples from four downdip Sparta Formation wells in south-central Louisiana were analyzed using a Rock-Eval pyrolysis analyzer with total organic carbon (TOC) module. TOC and the hydrogen index were found to be the most useful parameters for distinguishing depositional environments. Detailed conventional core analysis demonstrates that the Sparta Formation in the study area represents a prograding barrier island complex consisting of reservoir quality shoreface, storm-washover, and tidal-inlet sandstone. The sandstone overlies open marine mudstone and is capped by lagoonal mudstone. Mudstone of both facies is characteristically dark brown to black, laminated, and organic rich. However, pyrolysis results reveal geochemical contrasts that can be used to distinguish between the marine mudstone facies and the lagoonal mudstone facies. Lagoonal mudstone has TOC values averaging 1.37% and hydrogen indices averaging 160. In contrast, offshore marine mudstone has much higher TOC values, which average 2.61%, and a dramatically higher average hydrogen index of 341. Average T/sub max/ of mudstone from both facies is 431/degrees/C, and S/sub 1//S/sub 2/ ratios are well below unity, showing that the Sparta Formation is thermally immature in the studymore » area. Thermal maturity differences cannot be invoked to explain the observed geochemical differences between environments. The clear contrast of organic geochemical differences between environments. The clear contrast of organic geochemical parameters is interpreted to reflect dissimilarity in kerogen types and in modes of preservation between mudstone facies. Lagoonal mudstone contains mostly terrestrial kerogen, whereas more oil-prone marine kerogen predominates in marine mudstone.« less

Recognition of Depositional Environments Through Thin-Section Analysis of Detrital Sediments: ABSTRACT

Petrographic analysis can be a powerful tool in the recognition and delineation of depositional environments of detrital sediments. A study of varied fluvial-deltaic, and interdeltaic environments in the Eocene of the Gulf Coast and the Cretaceous of Montana-Wyoming shows that individual environments may be segregated by applying univariate and multivariate statistical techniques to petrographic data (composition and texture). These results are supported by data collated from Holocene environments in the Mississippi alluvial valley and delta, and Galveston barrier island. Holocene environments that can be segregated with a minimum of 80% reliability are point bar, natural levee, lake, bay, lower shoreface, middle shoreface, upper shoreface-beach, dune, and lagoon. Consequently, full diameter cores may not be necessary for precise environmental interpretation everywhere in the subsurface. Small samples, such as side-wall cores, taken throughout an interval, may yield significant and reliable environmental information. An example is provided by the Lower Cretaceous Muddy Sandstone of Montana in which the subenvironments of the petroliferous barrier-bar complex at Bell Creek field can be recognized from thin-s ction analysis and verified by a study of full-diameter cores. The presence of the Bell Creek barrier bar can be predicted through thin-section analyses of sediments in dry holes which surround the bar. If thin-section data are integrated with paleogeographic knowledge of the Lower Cretaceous, the relative position and strike of the barrier system may be ascertained. End_of_Article - Last_Page 843------------

Ecologic Criteria for Recognition of Depositional Environments in Carbonate Rocks: ABSTRACT

Carbonate skeletons of many Recent and fossil species show morphologic characters which can be related to specific factors in their environments. Similarly, the mineralogy and chemistry of the carbonate from the skeletons are known to reflect a variety of ecologic factors. Few attempts have been made to utilize the ecologic information from the physical and chemical properties of skeletal carbonates in the analyses of depositional environments of carbonate rocks. Data are presented to illustrate their usefulness in recognizing certain ecologic factors in the depositional environment of carbonate rocks. In this presentation, particular emphasis is placed on comparative functional morphology of carbonate skeletons. Ecologic factors to be considered are habitat, derivation of constituent grains, rates of sedimentation, turbidity, micro-hydrography, consistency of the sediments, temperature, and depth of the accumulating sediments. End_of_Article - Last_Page 473------------