Complex Depositional Environments Research Articles

Pore Fractal Characteristics of Lacustrine Shale of Upper Cretaceous Nenjiang Formation from the Songliao Basin, NE China

The evaluation of pore characteristics in lacustrine shales is of significance for shale oil and gas exploration in terrestrial sedimentary basins. However, due to the complex depositional environments, there have been few reports on the quantitative evaluation of lacustrine pores. In this study, we carry out experiments, e.g., scanning electron microscopy (SEM), nitrogen adsorption, X-ray diffraction (XRD), and rock pyrolysis, on sixteen lacustrine shale core samples of the Late Cretaceous Nenjiang Formation (K2n1+2) in the Songliao Basin. At the same time, we bring in the fractal dimensions to quantitatively characterize the pores of shale samples in K2n1+2. The kerogen type of shales in the Nenjiang Formation is mainly type Ⅰ. The shales of K2n1+2 develop a large number of mineral intergranular pores and a small number of organic pores and mineral intragranular pores. The primary minerals of the samples are clay minerals and quartz. The total organic carbon (TOC) has a correlation with the pore physical parameters, whereas the content of minerals does not correlate significantly with pore development. The average pore fractal dimensions obtained from the analysis of nitrogen adsorption data are D1 = 2.5308 and D2 = 2.6557. There is a significant negative correlation between the fractal dimensions (D1 and D2) and the TOC content: the lower the fractal dimensions, the higher the TOC, and the more the free oil. In low maturity shales with higher TOC content, due to hydrocarbon generation, larger pores with a regular shape (small fractal parameters) can be produced, which can store more free oil. The shale reservoir evaluation of K2n1+2 in the Songliao Basin should be focused on shales with higher TOC and lower pore fractal parameters. Our work provides a reference for the shale oil evaluation of K2n1+2 in the Songliao Basin and complements studies on lacustrine pore characteristics.

Geochemistry and organic petrography of the Middle Permian Lucaogou alkaline lacustrine oil shale in the southern Junggar Basin, China: Implications for formation conditions and organic matter accumulation

Organic matter (OM) accumulation in terrestrial sediments shows not only a significant carbon sink in the Earth's carbon cycle but an important origin of fossil fuels, which is closely associated with the complex and diverse depositional environments and climate conditions. The Junggar Basin developed the most laterally extensive and thickest alkaline lacustrine sediments as well as the richest hydrocarbon source rock/oil shale interval of all the world during the deposition of the Lucaogou Formation and equivalent units. In this study, the sedimentary succession of 564 m with a mudstone and dolomite matrix of the Lucaogou oil shale from the Qi 1 well located in the southern Junggar Basin was investigated, which can be divided into two members based on lithology. This paper is a synthetic use of major element oxides, trace, and rare-earth elements as well as organic petrography data as proxies to evaluate the provenance, paleotectonic setting, paleoclimate and weathering conditions, paleoenvironment, and paleoproductivity as well as origin and accumulation of OM of the Lucaogou sediments.It is suggested that no recycling sediments in all the studied samples, based on parameters including the Th/Sc vs. Zr/Sc plot, the Chemical Index of Alteration (CIA) being linearly dependent on Weathering Index of Parker (WIP), and the high Index of Compositional Variability. The provenance mainly from intermediate-felsic volcanic rocks (e.g., granodiorite, andesite and dacite) is supported by the bivariate plots of Euanom vs. Th/Sc, La/Th vs. Hf, La/Sc vs. Co/Th, Cr/Th vs. Sc/Th, Y/Ni vs. Cr/V, and Nb/Y vs. Zr/TiO2 as well as triangular diagrams of mafic–felsic–weathering, M+–4Si–R2+ and Rb/V–Zr/Zn–Sc/Nb. The continental island arc is inferred from paleotectonic setting discrimination diagrams (i.e., the La–Th–Sc, Th–Co–Zr/10 and Th–Sc–Zr/10 ternary as well as Ti/Zr vs. La/Sc binary diagrams). The chemical weathering indices (i.e., the CIA, WIP, weathering index (W), Weathering Intensity Scale, Sodium Depletion Index, and Ga/Rb vs. K2O/Al2O3 binary diagram) indicate the paleoclimate conditions were cold/arid with weak chemical weathering in a warming and/or enhanced continental weathering episode. Besides, the Sr/Ba ratios and gallium concentrations indicate the paleosalinity was brackish to hypersaline and was higher in the lower member in relation to that in the upper member. However, the suboxic environment of the benthic water (inferred from the MoEF vs. UEF co-variations and Corg/P ratios as well as large sizes of framboidal pyrites) and the moderate paleoproductivity (evaluated via the Sibio, Babio, and Pbio values, as well as Ni/Al and Cu/Al ratios) caused by predominate algae and bacteria (inferred from the organic petrography) kept stable in the paleolake during this period. The OM accumulation was mainly controlled by the preservation conditions rather than the paleoproductivity and limited detrital inputs/low deposition rates, which was indicated by the plot of Co × Mn values vs. Cd/Mo ratios and the total organic carbon contents being negatively related to stable carbon isotope compositions of OM.

Stochastic analysis on the lateral buckling of a HPHT pipeline considering the spatial variability of seabed

Complex geological processes and depositional environments could bring great spatial variability of the engineering properties of seabed sediments, which may further cause uncertainty in the global buckling predictions of high-pressure high-temperature (HPHT) pipelines. Unlike previous investigations with the deterministic method, a stochastic finite element (SFE) model is established for predicting the lateral buckling of a pipeline considering the spatial variability of the seabed. A flow chart of the Monto Carlo simulation for the lateral buckling of pipeline is provided. The SFE results for the simplified homogeneous seabed are compared with the existing analytical solutions for model verifications. Monte Carlo simulation results indicate that the mean values of the critical temperature rise for triggering the lateral buckling of a pipeline on the spatially varied seabed are generally larger than the deterministic predictions. It is found that the critical temperature rise may reach its maximum value when the horizontal scale of fluctuation ranges from the value of the discrete-element size to that of the imperfection wavelength. As the horizontal scale of fluctuation increases, the buckling probability increases significantly, which could be up to about 50% while the horizontal scale of fluctuation approaching a quite large value or even infinity.

Mechanical Rock Properties Estimation for Carbonate Reservoir Using Laboratory Measurement: A Case Study from Jeribe, Khasib and Mishrif Formations in Fauqi Oil Field

Estimation of mechanical and physical rock properties is an essential issue in applications related to reservoir geomechanics. Carbonate rocks have complex depositional environments and digenetic processes which alter the rock mechanical properties to varying degrees even at a small distance. This study has been conducted on seventeen core plug samples that have been taken from different formations of carbonate reservoirs in the Fauqi oil field (Jeribe, Khasib, and Mishrif formations). While the rock mechanical and petrophysical properties have been measured in the laboratory including the unconfined compressive strength, Young's modulus, bulk density, porosity, compressional and shear -waves, well logs have been used to do a comparison between the lab results and well logs measurements. The results of this study revealed that petrophysical properties are consistent indexes to determine the rock mechanical properties with high performance capacity. Different empirical correlations have been developed in this study to determine the rock mechanical properties using the multiple regression analysis. These correlations are UCS-porosity, UCS-bulk density, UCS-Vs, UCs-Vp Es-Vs, Es-Vp, and Vs-Vp. (*). For example, the UCS-Vs correlation gives a good determination coefficient (R2= 0.77) for limestone and (R2=0.94) for dolomite. A comparison of the developed correlations with literature was also checked. This study presents a set of empirical correlations that can be used to determine and calibrate the rock mechanical properties when core samples are missing or incomplete.

Complex depositional environments on a siliciclastic-carbonate platform with shallow-water turbidites: The Natividade Group, central Brazil

The Natividade Group is a Paleo-Mesoproterozoic metasedimentary sequence outcropping in the external zone of the Northern Brasília Belt, western margin of the São Francisco Craton, Brazil. The present study delimited eleven sedimentary rock types grouped into four rock assemblages: i) Sand-Silt-Carbonate; ii) Sand-Conglomerate; iii) Sand, iv) Silt-Clay. A geological map for the region was produced, and six stratigraphic columns were composed. Based on the descriptions of rock types and on lateral variation of rock assemblages, it was possible to establish four depositional environments for the Natividade Group, including mixed platform (siliciclastic-carbonate deposition), internal siliciclastic platform, open-marine siliciclastic platform, and a shallow water turbidite (mass flow slope deposition). The mixed platform was controlled by the basement paleogeography, which allowed the deposition of carbonates in warm, agitated, and clean, shallow water conditions in parallel to deposition in deeper water setting, where thin siliciclastic sediments were deposited. The siliciclastic internal platform consists mostly of quartzite, originating from sandy sediments, probably indicating deposition dominated by bedload currents under shallow water shoreface conditions. The open-marine siliciclastic platform is dominated by fine-grained sediments, indicating deepening water conditions under lower shoreface conditions below the fair-weather wave base. Shallow water turbidite occurs in the southern parts of the study area. The elevated paleorelief in the southeast of the Natividade basin is considered as the provenance area providing suitable slope environments for density flow initiation. The basin was shallower in the southern portion with a predominance of carbonate and gravitational flux deposits and deeper to the north, evidenced by the northward predominance of fine-grained sediments. Finally, the study shows that the Natividade Group was deposited in a basin controlled by thermo-flexural subsidence.

Using to Reconstruct Radiation Transport through Complex Archaeological Environments.

The DosiVox programme is used to reconstruct radiation transport in complex depositional environments. Using two archaeological case studies from ancient Egypt, the burial environments for a selection of ceramic vessels are reconstructed using the DosiVox programme, allowing the simulation of the emission and transport of radiation throughout these burial environments. From this simulation we can extract the external dose rate of the archaeological samples, a measurement necessary for determine a luminescence age. We describe in detail how DosiVox can be used to best advantage at sites with complex depositional histories and highlight that DosiVox is a valuable tool in luminescence dating. This work illustrates that DosiVox is, at present, unparalleled in reconstructing a more accurate and detailed external gamma dose rate which can significantly improve upon simplistic scaled geometric models.

Coping with uncertainties through an automated workflow for 3D reservoir modelling of carbonate reservoirs

Reliable 3D modelling of underground hydrocarbon reservoirs is a challenging task due to the complexity of the underground geological formations and to the availability of different types of data that are typically affected by uncertainties. In the case of geologically complex depositional environments, such as fractured hydrocarbon reservoirs, the uncertainties involved in the modelling process demand accurate analysis and quantification in order to provide a reliable confidence range of volumetric estimations. In the present work, we used a 3D model of a fractured carbonate reservoir and populated it with different lithological and petrophysical properties. The available dataset also included a discrete fracture network (DFN) property that was used to model the fracture distribution. Uncertainties affecting lithological facies, their geometry and absolute positions (related to the fault system), fracture distribution and petrophysical properties were accounted for. We included all different types of uncertainties in an automated approach using tools available in today’s modelling software packages and combining all the uncertain input parameters in a series of statistically representative geological realizations. In particular, we defined a specific workflow for the definition of the absolute permeability according to an equivalent, single porosity approach, taking into account the contribution of both the matrix and the fracture system. The results of the analyses were transferred into a 3D numerical fluid-dynamic simulator to evaluate the propagation of the uncertainties associated to the input data down to the final results, and to assess the dynamic response of the reservoir following a selected development plan. The “integrated approach” presented in this paper can be useful for all technicians involved in the construction and validation of 3D numerical models of hydrocarbon-bearing reservoirs and can potentially become part of the educational training for young geoscientists and engineers, since an integrated and well-constructed workflow is the backbone of any reservoir study.

Single-grain TT-OSL bleaching characteristics: Insights from modern analogues and OSL dating comparisons

Previous assessments of thermally transferred optically stimulated luminescence (TT-OSL) signal resetting in natural sedimentary settings have been based on relatively limited numbers of observations, and have been conducted primarily at the multi-grain scale of equivalent dose (De) analysis. In this study, we undertake a series of single-grain TT-OSL bleaching assessments on nineteen modern and geological dating samples from different sedimentary environments. Daylight bleaching experiments performed over several weeks confirm that single-grain TT-OSL signals are optically reset at relatively slow, and potentially variable, rates. Single-grain TT-OSL residual doses range between 0 and 24 Gy for thirteen modern samples, with > 50% of these samples yielding weighted mean De values of 0 Gy at 2σ. Single-grain OSL and TT-OSL dating comparisons performed on well-bleached and heterogeneously bleached late Pleistocene samples from Kangaroo Island, South Australia, yield consistent replicate age estimates. Our results reveal that (i) single-grain TT-OSL residuals can potentially be reduced down to insignificant levels when compared with the natural dose range of interest for most TT-OSL dating applications; (ii) the slow bleaching properties of TT-OSL signals may not necessarily limit their dating applicability to certain depositional environments; and (iii) non-trivial differences may be observed between single-grain and multi-grain TT-OSL bleaching residuals in some modern samples. Collectively, these findings suggest that single-grain TT-OSL dating may offer advantages over multi-grain TT-OSL dating in certain complex depositional environments.

A cell-centered multipoint flux approximation method with a diamond stencil coupled with a higher order finite volume method for the simulation of oil–water displacements in heterogeneous and anisotropic petroleum reservoirs

The simulation of fluid flow and transport in heterogeneous and anisotropic oil and gas reservoirs poses a great challenge from the numerical point of view, due to the modeling of complex depositional environments, including inclined laminated layers, channels, fractures and faults and the modeling of deviated wells, making it difficult to build and handle the Reservoir Characterization Process (RCM), particularly by using structured meshes (cartesian or corner point), which is the current pattern in petroleum reservoir simulators. Under certain hypotheses, the mathematical model that describes the fluid flow in petroleum reservoirs includes an elliptic equation with heterogeneous, possibly discontinuous, coefficients for the pressure field and a non-linear hyperbolic equation for the saturation field. In the present paper, these equations are solved via an Implicit Pressure Explicit Saturation (IMPES) procedure. To solve these equations, we use a full cell-centered finite volume formulation. The pressure equation is discretized by a non-orthodox Multipoint Flux Approximation Method with a Diamond type stencil (MPFA-D) and used for the first time for the solution of two-phase flow problems in heterogeneous porous media. It is very robust and capable of reproducing piecewise linear solutions exactly by means of a linear preserving interpolation with explicit weights that avoids the solution of locally defined systems of equations. For the solution of the saturation equation, we use a Monotone Upstream Centered Scheme for Conservation Laws (MUSCL) method based on a gradient reconstruction obtained by a least square technique in which monotonicity is reinforced by an appropriate slope limiter. The method can be used with general polygonal meshes, even though we restrict ourselves to conforming triangular and quadrilateral grids. In order to validate and show the robustness of our formulation, we solve some problems including heterogeneous and anisotropic reservoirs and displacements with high mobility ratios. Our results compare well with others found in literature.

Empirical insights into multi-grain averaging effects from ‘pseudo’ single-grain OSL measurements

In this study we assess the signatures of multi-grain averaging effects for a series of sedimentary samples taken from the archaeological site of Hotel California, Atapuerca, Spain. We focus on the special case of equivalent dose (D e) measurements made on single-grain discs that contain more than one quartz grain in each of the individual grain-hole positions with the aims of (i) providing insight into the nature and extent of averaging effects in very small multi-grain aliquots of sedimentary quartz, and (ii) assessing the suitability of 'pseudo' single-grain D e measurements for this particular dating application. Pseudo single-grain OSL measurements made on standard discs loaded with 90-100 μm grains (equivalent to ∼30 grains per hole) yield significantly different D e distribution characteristics and finite mixture model (FMM) burial dose estimates compared with single-grain OSL measurements. Grains with aberrant luminescence behaviours, which are routinely rejected during single-grain analysis, exert strong averaging effects on the pseudo single-grain and multi-grain aliquot D e distributions. Grain-hole averaging effects arising from pseudo single-grain measurements also give rise to 'phantom' dose components and are apt to provide bias assessments of quartz signal characteristics and grain type classifications. Though this is a site-specific study, it serves as a cautionary note for interpretations of other pseudo single-grain OSL and D e datasets - particularly those obtained from measurements of discs containing several tens of grains per hole and those derived from complex depositional environments. The use of custom single-grain discs drilled with smaller sized grain holes is recommended as a means of limiting grain-hole averaging effects when dealing with very fine (<180 μm) sediments. © 2012 Elsevier Ltd. All rights reserved.

3D hydrocarbon migration by percolation technique in an alternate sand–shale environment described by a seismic facies classified volume

Modelling hydrocarbon migration in complex depositional environments is a difficult task. This paper illustrates a workflow that has been developed in order to make use of information at seismic resolution for migration modelling purposes. After a regional identification of the main seismic sequence boundaries and systems tracts, the first step of the workflow consists in a semi-automatic classification of the sand and shale prone seismic facies that are interpretable in the seismic volume. A software based on supervised neural network techniques and textural seismic attributes supports volumetric classification. Each class represents a different facies with specific petrophysical properties (threshold pressure, porosity, irreducible water saturation, etc.), assigned according to concurrent sedimentological studies; the result is a 3D lithofacies model. Petroleum system modelling is performed on a 3D regional model up to the calculation of hydrocarbon quantities expelled from the source rock. Then migration modelling is performed on the smaller area covered by the 3D lithofacies model. Invasion percolation approach has been chosen because it allows to perform migration modelling at a scale that is small enough to maintain a sufficient detail of the channels' shape and of their connections. A probabilistic approach allows to estimate charge risk in new prospects and to check modelling consistency. This workflow has been applied in a deep water setting in West Africa, characterized by an interconnected channel system, where a number of prospects had to be evaluated. A well was drilled in the area prior to this study, resulting dry in spite of being located in a position where the source rock is present and is deemed to have expelled interesting amounts of hydrocarbons. This study has shown that hydrocarbons expelled from the source appear to be deviated laterally by the presence of a local seal and therefore do not reach the channel system in the well location.

Addressing uncertainty and remaining potential in a mature field. A case study from the Tertiary of Lake Maracaibo, Venezuela

AbstractThe Urdaneta West Field is located on the western margin of Lake Maracaibo in northern Venezuela. Biodegraded oil (12–15° API) is reservoired in Tertiary sandstones and produced through a series of lateral wells. The productive sandstones of the Icotea and Misoa Formations are thin, calculated as 0.5–4.5 m (1.5–15 ft) vertical thickness, and of limited lateral extent. This heterogeneity, coupled with heavy oil, results in poor fluid communication and low recovery efficiency. During 2004, a full field review was undertaken to address future development. Despite production from the Icotea and Misoa Formations, subsurface uncertainty was identified as a major issue due to the clustered nature of field development, complex depositional and structural environments, and reservoir fluid characteristics. In order to rank and mitigate reservoir uncertainties, a series of static models were built. The first phase of static modelling used a simple structural framework and reservoir interval averages to generate minimum, mid- and maximum volume cases. Dynamic simulation of these identified two major areas of uncertainty impacting oil-in-place and productivity – net sandbody connectivity and hydrocarbon contact. Two further phases of static and dynamic modelling concentrated on evaluating the full range of these uncertainties within a detailed structural framework.

Sediment ages and flux variations from depth profiles of 210Pb: lake and marine examples

Sediment accumulation rates are redetermined for several freshwater and marine settings to demonstrate the application of a new methodology for interpreting sediment core profiles of radioisotope activity versus depth. Depositional histories were reconstructed using the sediment isotope tomography (SIT) methodology, which abandons the stringent requirement of current methods that sediment accumulation rates and isotopic fluxes may not both vary with time. Using inverse numerical analysis techniques in conjunction with a predictive model, the SIT method is able to separate the effects of sediment accumulation from flux variations to determine age-to-depth relationships. Cumulative probability procedures provide a framework for interpreting the sensitivity and accuracy of the model-determined age-to-depth relationships. Restrictions applied to previous methods on interpreting sediment profiles affected by mixing must also be adhered to when using the SIT method. Previously published data from two Estonian lakes, Kurtna and Nômmejärv, from Rockwell Reservoir in Northeastern Ohio and from Cape Lookout Bight, NC, are examined to demonstrate the broad applicability of the methodology. With the exception of one location, the resultant age-to-depth relationships determined by the model agree with earlier interpretations. In the Estonian Lake Nômmejärv, previous use of the Constant Flux model overestimates sediment ages for two known time horizons, giving ages of 43 yr at 22 cm and 111 yr at 45 cm. The age-to-depth relationship determined by the SIT method reproduces satisfactorily the time horizons of 16±1 yr at 22 cm and 35±2 yr at 45 cm, in agreement with the onset of industrial activities in the region. These examples illustrate the suitability of this new methodology for application in a variety of complex depositional environments.