Lithofacies Proportions Research Articles

Uncertainty Evaluation Based on Bayesian Transformations: Taking Facies Proportion as An Example

Many input parameters in reservoir modeling cannot be uniquely determined due to the incompleteness of data and the heterogeneity of the reservoir. Sedimentary facies modeling is a crucial part of reservoir modeling. The facies proportion is an important parameter affecting the modeling results, because that proportion directly determines the net gross ratio, reserves and sandbody connectivity. An uncertainty evaluation method based on Bayesian transformation is proposed to reduce the uncertainty of the facies proportion. According to the existing data and geological knowledge, the most probable value of the facies ratio and the prior distribution of uncertainty are estimated. The prior distribution of the facies proportion is divided into several intervals, and the proportions contained in each interval are used in facies modeling. Then, spatial resampling is carried out for each realization to obtain the likelihood estimation of the facies proportion. Finally, the posterior distribution of the facies ratio is achieved based on Bayesian transformation. The case study shows that the uncertainty interval of sandstone proportion in the study area has been reduced from [0.31, 0.59] to [0.35, 0.55], with a range reduction of 29%, indicating that the updated posterior distribution reduces the uncertainty of reservoir lithofacies proportion, thereby reducing the uncertainty of modeling results.

A Facies Proportional Determination Method Based on the Theory of Confidence Intervals: A Case Study in the M Gas Field in the East China Sea

A reservoir is the space and gathering place of underground oil and gas reservoirs. Lithofacies proportion is the key parameter of a reservoir. Due to the heterogeneity of geological bodies, the multi-solution of seismic data and the limited and uneven distribution of logging data, it is unclear how to determine the lithofacies proportion in the study area directly from the available data. The mean is a way to make a quick estimate, but the mean cannot quantify the uncertainty of the lithofacies proportion using the available data. In this paper, a new method for quantifying uncertainty and determining the value of the three levels of the facies proportion is proposed. Taking sandstone proportion as an example, a normality test was carried out to verify the applicability of the sandstone proportion data to this method, and then the mean and variance of samples were calculated. Based on the confidence interval theory, the uncertainty of the mean value and the range of sandstone proportion were determined, that is, the optimistic value and the pessimistic value of the sandstone proportion. The results show that this method provides a new reference for quantifying values for the three levels of the uncertainty, and it uses the petrographic proportion as an example to provide the data basis for subsequent modeling and reservoir research.

Uncertainty Evaluation Method and Application of Reservoir Lithofacies Proportion under the Condition of Few Wells

Abstract The proportion of different lithofacies is essential to determine the net-gross ratio, which directly affects the calculation of oil and gas reserves. Under the condition of few wells, the reliability of different lithofacies proportions obtained using well data is poor because it is difficult to estimate the uncertainty variation range of different lithofacies proportions. The present study overcomes this problem using an uncertainty evaluation method for reservoir lithofacies proportion. First, different lithofacies proportions of the reservoir are determined using the well and seismic data and are considered as the most likely estimated. Based on the lithofacies proportion, the lithofacies models of the study area are developed. Second, uniform and random methods are applied to sample the lithofacies models, and then, the probability distribution functions of different lithofacies proportions are derived. Third, the three levels of lithofacies proportions in the study area are calculated using the method of quantile and discretization of the probability distribution. Finally, we take M gas field as a sample and compare the test results of the fixed and random well structures. The results show that the sampling results of the fixed well structure model significantly deviate under the influence of the extension direction of the channel and well pattern distribution. Therefore, it is suggested to use random well structure to determine the uncertainty distribution range of lithofacies proportion.

Quantifying the lateral heterogeneity of distal submarine lobe deposits, Point Loma Formation, California: Implications for subsurface lateral facies prediction

AbstractSubmarine fan deposits are volumetrically the largest sediment accumulations on Earth and host significant hydrocarbon reserves. Extensive research has documented the bed‐scale architecture of high sand‐to‐mud ratio, proximal and axial environments, which can have bed thicknesses of several metres; however, less well‐understood are thin‐bedded turbidites, which are typically lower N:G and deposited in more distal environments. Conceptual models assume that lobe‐fringe‐to‐basin plain environments consist of tabular, sheet‐like beds that extend out continuously and predictably over long distances—up to several kilometres. Extensive lateral continuity, however, is not necessarily reflected in ancient outcrop analogues. This study seeks to apply a quantitative approach to the characterisation of thin‐bedded turbidites to assess the impact of multi‐scale heterogeneity on reservoir predictability. The sea‐cliff outcrop exposures of the Upper Cretaceous Point Loma Formation in San Diego, California, exhibit a wide range of bed thicknesses and stratigraphic architecture, which have been used to interpret an off‐axis‐to‐fringe depositional environment. The study area spans 700 m of laterally continuous outcrop, across which 10 correlated stratigraphic sections are used to quantify changes in metrics such as bed thickness, N:G, lithofacies proportions, etc. Results of this study demonstrate that thin sand beds experience both lateral facies changes and rapid thickness changes more frequently than conceptual models would predict. A single measure of lateral heterogeneity does not reflect the true architecture of sandstone beds, and significant information is lost when beds are correlated over ten to hundreds of metres. Sands are commonly deposited in irregular, ‘finger‐like’, planform geometries, which compounds at the lobe element scale and influences lateral lithofacies predictability. This study of the Point Loma Formation offers high‐resolution bed‐to‐element scale data, which may be used as inputs for reservoir models and horizontal facies predictions in both conventional and unconventional hydrocarbon reservoirs.

Gas migration and residual trapping in bimodal heterogeneous media during geological storage of CO2

This study investigates the effect of heterogeneity on CO2–brine two-phase flow behavior and capillary trapping at the field scale. A model based on macroscopic invasion percolation is developed to simulate CO2 migration and trapping in strongly heterogeneous systems with bimodal permeability distributions. Stochastic simulations are performed on heterogeneous permeability fields generated by considering both transition probability based lithofacies distribution and multi-Gaussian random fields, representative of characteristics often encountered in relevant sedimentary basins. The heterogeneity cases of different lithofacies proportions are designed such that they share the same ensemble mean of upscaled permeability over the simulation domain. We find that the CO2 spreading and trapping is strongly influenced by the heterogeneity. The ensemble means of sweep efficiency and domain-average CO2 saturation after primary drainage show a slight decrease with decreasing sandstone proportion p1 from 1.0 to 0.6 but a much steeper decline when p1 further decreases to 0.4. The upscaled two-phase flow properties such as the relative permeabilities are also influenced by the lithofacies proportion. Residual or capillary trapping and its uncertainty is shown to be dependent on the heterogeneity and the choice of trapping models. Our findings have broad implications within the context of CO2 sequestration, as the gas spreading and entrapment are the fundamental processes leading to long-term storage security.

The lithofacies organization of fluvial channel deposits: A meta-analysis of modern rivers

Environmental interpretations of subsurface fluvial successions are commonly based on facies observations from core and are often attempted by generalist geologists by reference to classic facies models. However, for fluvial channel deposits, the value of observations on lithofacies proportions for interpretations of depositional environment has yet to be assessed quantitatively. Here, a test is presented that is based on a comparative study of facies data from 77 reaches of 46 modern rivers. The analysis is undertaken on datasets from published case studies stored in a sedimentological database, with consideration of causes for observational bias, and with particular attention paid to sandy lithofacies. The observed variability in the proportion of facies assemblages in the channel deposits of sandy river systems is quantified for classes of environments categorized according to channel pattern (braided, low sinuosity, meandering), climatic setting (arid to perhumid), and discharge regime (ephemeral to perennial). By capturing the variability in facies organization within fluvial systems of certain types, these outputs serve as facies models that provide a measure of uncertainty to sedimentological interpretations. Concurrently, the statistical analysis presented enables a test of the significance of relationships between the relative proportions of channel lithofacies and parameters that either represent controlling factors (e.g., water-discharge characteristics) or covariates (e.g., channel pattern). For classes of river systems grouped by channel pattern, climate, and discharge regime, emerging features of facies organization can be identified. Statistically, it is observed that relationships exist (i) between channel pattern and the frequency of the preserved expression of bedforms, and (ii) between controls on river hydrology (climate, discharge regime and seasonal variability) and the record of upper and lower flow-regime conditions. Thus, the results corroborate existing qualitative facies models in some respects. However, observations of the relative dominance of facies in channel deposits demonstrate limited value for interpretations or predictions in subsurface or outcrop studies, as variability within each type of depositional system is significant. Corehole data of fluvial channel deposits may be commonly overinterpreted.

Down‐slope facies variability within deep‐water channel systems: Insights from the Upper Cretaceous Cerro Toro Formation, southern Patagonia

AbstractIn southern Patagonia, outcrops of the Upper Cretaceous Cerro Toro Formation preserve a >150 km long deep‐water axial channel belt in the Magallanes–Austral Basin, providing a unique opportunity to investigate longitudinal variations in the depositional characteristics of a deep‐water channel system. This study documents sedimentological, stratigraphical and geochronological data from the Cerro Toro Formation in the Argentine sector of the basin. New results are integrated with previous work from the Chilean basin sector to conduct a basin‐scale comparison of the timing of deposition, provenance and lithofacies proportions. The Cerro Toro channel belt includes a nearly 1000 m thick section characterized by high‐density turbidites and mass‐wasting deposits. Two ash beds from the base of the section yield U–Pb zircon ages of 90·4 ± 2 Ma and 88·0 ± 3 Ma, indicating similar initiation ages as documented in the Chilean sector. The U–Pb detrital zircon age spectra from samples in the study area reveal similar provenance trends to samples from the Chilean basin sector, with peak age populations at 310 to 260 Ma, 160 to 135 Ma and 110 to 82 Ma. The maximum depositional age of the channel belt in the Argentine sector is 87·8 ± 1·5 Ma and all new geochronology data corroborate an 86 to 80 Ma depositional age for the main Cerro Toro channel belt. Statistical analyses of 7370 beds from nearly 8000 m of new and previously published stratigraphic sections along the entire outcrop belt suggest progressive variations in the down‐system proportion of lithofacies. In the up‐slope region, lithofacies representing mass wasting processes (for example, debris‐flow and mass‐transport deposits) account for ca 29% of the stratigraphic thickness, as opposed to 5% in the down‐slope region of the channel belt, where turbidity current deposits are more prevalent. The proportion of beds >1 m thick also decreases systematically down slope, particularly for conglomeratic turbidite deposits. This work highlights that: (i) the proportion of thick beds and distribution of lithofacies are key down‐system changes in the stratigraphic fill of this deep‐water channel belt; (ii) detrital zircon trends suggest a relatively well‐mixed longitudinal depositional system; and (iii) geochronology of the main Cerro Toro outcrop belt supports but does not necessitate the model of a single, roughly age‐equivalent, channel system. This study has implications for understanding the downslope variability in depositional processes, stratigraphic architecture and reservoir quality of submarine channel systems.

Sedimentary characteristics and origins of Late Pennsylvanian–Early Permian carbonate mud-mounds at the Shangdan section, Inner Mongolia

Late Pennsylvania–Early Permian carbonate mud-mounds exposed in the Shangdan section, Inner Mongolia, are commonly massive, showing depositional relief, and are distinct from the thin- to medium-bedded limestones of intra-mounds. Based on the various proportions of lithofacies, four types of mud-mounds are recognized: MM-A (type A carbonate mud-mound) and B (type B carbonate mud-mound) consist of massive lime mudstone and massive wackestone (Subfacies B), respectively; MM-C (type C carbonate mud-mound) are composed of massive wackestone (Subfacies B) in the lower parts, analogous to MM-B, and massive wackestone (Subfacies A) and boundstone in the upper parts, similar to MM-D (type D carbonate mud-mound). Carbonate mud-mounds are deposited in low-energy settings on a ramp, likely below fair-weather wave base, where the depositional settings of MM-A, B, C and D have a shallowing-upward trend. Carbonate mud-mounds have diverse origins of lime mud: allochthonous lime mud in MM-A; lime mud produced by the disintegration of phylloid algae in MM-B, C and D; lime mud linked to cyanobacterial activity in MM-D and the upper parts of MM-C. Different from MM-A and B, which were formed only by mechanical accumulation, MM-C and D resulted from the combinations of mechanical accumulation and microbial binding and solidifying. MM-A, B, C and D significantly expand the global spectrum of Late Pennsylvania–Early Permian carbonate mounds.

Stochastic inversion by matching to large numbers of pseudo-wells

We have developed a 1D stochastic algorithm for estimating reservoir properties, based on matching large numbers of pseudo-wells to seismic angle stacks. The pseudo-wells are part deterministic and part stochastic 1D stratigraphic profiles with consistent elastic and reservoir properties. Pseudo-wells are sampled from a prior distribution defined by the geological interpretation, a rock physics model and a model for the vertical statistics that provides close control of the lithofacies proportions. A new set of pseudo-wells, typically [Formula: see text] tied to the local stratigraphy, is constructed for each seismic trace. Synthetics, derived from the pseudo-wells using extended elastic impedance, are matched to either one or two seismic angle stacks, and the best matches are selected and averaged to provide a joint estimate of reservoir properties and impedances and the associated uncertainties. The algorithm has been tested on a number of data sets and validated by blind well ties. The algorithm is 1D with no additional constraints on spatial correlation beyond that provided by the seismic data. This restricts the maximum frequency to that of the seismic; however, it makes the algorithm highly parallelizable, allowing for large data sets to be inverted in a few hours given adequate computing resources. We envisage that this inversion algorithm could form the first part of a two-step process with the output used to constrain subsequent geostatistical modeling.

Channelized debris‐flow deposits and their impact on turbidity currents: The Puchkirchen axial channel belt in the Austrian Molasse Basin

AbstractDeposits of submarine debris flows can build up substantial topography on the sea floor. The resulting sea floor morphology can strongly influence the pathways of and deposition from subsequent turbidity currents. Map views of sea floor morphology are available for parts of the modern sea floor and from high‐resolution seismic‐reflection data. However, these data sets usually lack lithological information. In contrast, outcrops provide cross‐sectional and lateral stratigraphic details of deep‐water strata with superb lithological control but provide little information on sea floor morphology. Here, a methodology is presented that extracts fundamental lithological information from sediment core and well logs with a novel calibration between core, well‐logs and seismic attributes within a large submarine axial channel belt in the Tertiary Molasse foreland basin, Austria. This channel belt was the course of multiple debris‐flow and turbidity current events, and the fill consists of interbedded layers deposited by both of these processes. Using the core‐well‐seismic calibration, three‐dimensional lithofacies proportion volumes were created. These volumes enable the interpretation of the three‐dimensional distribution of the important lithofacies and thus the investigation of sea floor morphology produced by debris‐flow events and its impact on succeeding turbidite deposition. These results show that the distribution of debris‐flow deposits follows a relatively regular pattern of levées and lobes. When subsequent high‐density turbidity currents encountered this mounded debris‐flow topography, they slowed and deposited a portion of their sandy high‐density loads just upstream of morphological highs. Understanding the depositional patterns of debris flows is key to understanding and predicting the location and character of associated sandstone accumulations. This detailed model of the filling style and the resulting stratigraphic architecture of a debris‐flow dominated deep‐marine depositional system can be used as an analogue for similar modern and ancient systems.

Exhumed channel sandstone networks within fluvial fan deposits from the Oligo-Miocene Caspe Formation, South-east Ebro Basin (North-east Spain)

The Oligo-Miocene Caspe Formation corresponds to the middle fluvial facies of the wider Guadalope-Matarranya fluvial fan, located in the South-east Ebro foreland basin (North-east Spain). At the time of the Caspe Formation deposition, this sector of the Ebro basin underwent a very continuous, moderate sedimentation rate. Lithofacies comprise deposits from channellized and unchannellized flows. Channellized flow lithofacies form multi-storey ribbon-like sandstone bodies that crop out as extensive sandstone ridges belonging to exhumed channel networks. Width/thickness ratios of these channel-fill bodies average close to six. Sinuosity is usually low (most common values around 1·1), although it can be high locally (up to 2). Thicknesses range from a few metres to 15 m. Unchannellized flow lithofacies form tabular bodies that can be ascribed to overbank deposits (levees, crevasse splays and fine-grained floodplain deposits) and also to frontal lobes, although recognition of this last case requires exceptional outcrop conditions or geophysical subsurface studies. The unchannellized flow lithofacies proportion ranges from 75% to 97·8%. Methods applied to this study include detailed three-dimensional architectural analysis in addition to sedimentological analysis. The architecture is characterized by an intricate network of highly interconnected ribbon-like sandstone bodies. Such bodies are connected by three kinds of connections: convergences, divergences and cross-cuttings. Although the Caspe Formation lithofacies and architecture resemble anastomosed channels (low topographic gradient, high preservation potential, moderate aggradation rate, high lateral stability of the channels, dominance of the ribbon-like morphologies and high proportion of floodplain to channel-fill sediments), an unambiguous interpretation of the channel networks as anastomosed or single threaded cannot be established. Instead, the observed architecture could be considered as the product of the complex evolution of a fluvial fan segment, where different network morphologies could develop. A facies model for aggrading ephemeral fluvial systems in tectonically active, endorheic basins is proposed.

Distribution of sedimentary rock types through time in a back-arc basin: A case study from the Jurassic of the Greater Caucasus (Northern Neotethys)

Abstract The evolution of sedimentary basins can be explored by analyzing the changes in their lithologies and lithofacies (i.e. predominant lithologies). The Greater Caucasus Basin, which was located at the northern margin of the Neotethys Ocean, represents a complete Sinemurian-Tithonian succession. A quantitative analysis of compiled datasets suggests that principal lithologies and lithofacies are represented by siliciclastics, shale and carbonates. The relative abundance of siliciclastics and shale decreased throughout the Jurassic, whereas that of carbonates increased. Evaporites are known from the Upper Jurassic, while volcaniclastics and volcanics, as well as coals, are known only in the Lower to Middle Jurassic. Siliceous rocks are extremely rare. Lithology and lithofacies proportions change accordingly. The Sinemurian-Bathonian sedimentary complex is siliciclastic-and-shale-dominated, whereas the Callovian-Tithonian sedimentary complex is carbonate-dominated. A major change in the character of se...

Conditional Simulation of Multi-Type Non Stationary Markov Object Models Respecting Specified Proportions

Although Boolean model simulation has been widely used during the last two decades to simulate sedimentary bodies (especially in fluvio-deltaic environments), a key issue has subsisted. One of the most important parameter for object model simulation, namely the (non stationary) intensity of the underlying object process, is not a parameter provided by the end user but must instead be computed from other input parameters, such as local proportions of lithofacies, erosion rule between objects of different types and interaction between objects. This paper revisits a birth and death algorithm for simulating conditional, non stationary, multi-type objects models with interaction. It provides workable approximations for computing the local intensity of the underlying point process to respect proportion maps. Simulated examples show that this algorithm is able to reproduce the desired proportions. Important issues for implementing this algorithm are discussed.

Production Data Integration in Sand/Shale Reservoirs Using Sequential Self-Calibration and GeoMorphing: A Comparison

Summary The stochastic inversion of spatial distribution of lithofacies from multiphase production data is a difficult problem. This is true even for the simplest case, addressed here, of a sand/shale distribution and under the assumption that reservoir properties are constant within each lithofacies. Two geostatistically based inverse techniques, sequential self-calibration (SSC) and GeoMorphing (GM), are extended for such purposes and then compared with synthetic reference fields. The extension of both techniques is based on the one-to-one relationship existing between lithofacies and Gaussian deviates in truncated Gaussian simulation. Both techniques attempt to modify the field of Gaussian deviates while maintaining the truncation threshold field through an optimization procedure. Maintaining a fixed threshold field, which has been computed previously on the basis of prior lithofacies proportion data, well data, and other static soft data, guarantees preservation of the initial geostatistical structure. Comparisons of the two techniques using 2D and 3D synthetic data show that the SSC is very efficient in producing sand/shale realizations matching production data and reproducing the large-scale patterns displayed in the reference fields, although it has difficulty in reproducing small-scale features. GM is a simpler algorithm than SSC, but it is computationally more intensive and has difficulty in matching complex production data. Better results could be obtained with a combination of the two techniques in which SSC is used to generate realizations identifying large-scale features; then, these realizations could be used as input to GM for a final update to match small-scale details.

Conditional 3D simulation of lithofacies with 2D seismic data

This paper presents a new methodology and the corresponding GSLIB-type program to integrate 2D average information, such as vertically-averaged lithofacies proportions, into estimation/simulation of 3D lithofacies distributions. Seismic information, available in 2D, is used in the cokriging of the 2D average lithofacies proportions. A data set related to a siltstonedolomite carbonate reservoir in W. Texas is used for case study. Three steps were taken: first, a co-located cokriging is used to incorporate 2D seismic information into the estimation of vertical lithofacies proportions; second, indicator kriging is used to derive the facies conditional probabilities at each of the 3D simulation grid node; this 3D indicator kriging uses the hard well data and the previously estimated vertical proportions; last, a p-field simulation algorithm is used to draw simulated lithofacies indicators from the previously obtained distributions. Results showed that even with limited well data, the input vertical lithofacies proportions (which carry the seismic information) is honored quasi-exactly.

Estimation of Lithofacies Proportions Using Well and Well Test Data

A crucial step of the two commonly used geostatistical methods for modeling heterogeneous reservoirs : the sequential indicator simulation and the truncated Gaussian simulation is the estimation of the lithofacies local proportion (or probability density) functions. Well-test derived permeabilities show good correlation with lithofacies proportions around wells. Integrating well and well-test data in estimating lithofacies proportions could permit the building of more realistic models of reservoir heterogeneity. However this integration is difficult because of the different natures and measurement scales of these two types of data. This paper presents a two step approach to integrating well and well-test data into heterogeneous reservoir modeling. First lithofacies proportions in well-test investigation areas are estimated using a new kriging algorithm called KISCA. KISCA consists in kriging jointly the proportions of all lithofacies in a well-test investigation area so that the corresponding well-test derived permeability is respected through a weighted power averaging of lithofacies permeabilities. For multiple well-tests, an iterative process is used in KISCA to account for their interaction. After this, the estimated proportions are combined with lithofacies indicators at wells for estimating proportion (or probability density) functions over the entire reservoir field using a classical kriging method. Some numerical examples were considered to test the proposed method for estimating lithofacies proportions. In addition, a synthetic lithofacies reservoir model was generated and a well-test simulation was performed. The comparison between the experimental and estimated proportions in the well-test investigation area demonstrates the validity of the proposed method.

Estimation of Lithofacies Proportions by Use of Well and Well-Test Data

Summary A crucial step of the two commonly used geostatistical methods for modeling heterogeneous reservoirs, sequential indicator simulation and truncated Gaussian simulation, is the estimation of the lithofacies local proportion (or probability density) functions. Well-test-derived permeabilities show good correlation with lithofacies proportions around wells. Integrating well and well-test data in estimating lithofacies proportions could permit the building of more realistic models of reservoir heterogeneity. This integration is difficult, however, because of the different natures and measurement scales of these two types of data. This paper presents a two-step approach to integrating well and well-test data into heterogeneous reservoir modeling. First, we estimate lithofacies proportions in well-test investigation areas with a new kriging algorithm called KISCA. KISCA consists of kriging jointly the proportions of all lithofacies in a well-test investigation area so that the corresponding well-test-derived permeability is respected through a weighted power-averaging of lithofacies permeabilities. For multiple well tests, an iterative process is used in KISCA to account for their interaction. After this, the estimated proportions are combined with lithofacies indicators at wells for estimating proportion (or probability density) functions over the entire reservoir field with a classical kriging method. We considered some numerical examples to test the proposed method for estimating lithofacies proportions. In addition, we generated a synthetic lithofacies reservoir model and performed a well-test simulation. The comparison between the experimental and estimated proportions in the well-test investigation area demonstrates the validity of the proposed method.

Basin history deduced from subtle changes in fluvial style: a study of distal alluvium from the Devonian of southwest Ireland

AbstractTwo detailed sections through 4 km of distal alluvium of the Upper Devonian Old Red Sandstone in the Munster Basin, southern Ireland, display a limited number of lithofacies. There is little ordering of these lithofacies on a small scale but changes in the proportions of lithofacies through time define a sequence of stages of basin evolution. The depositional environments changed progressively from sheetflood dominant, via mobile ephemeral channels and floodplains to a more fixed channel–overbank system and eventually to a coastal plain. On the basis of the sedimentary record a progressive reduction in subsidence rate with time is deduced as the main control on the evolution of the basin fill. This deduction is consistent with predictions from previously applied extensional basin models, but is insufficiently refined at present to distinguish between the different models.