Lithofacies Distribution Research Articles

Geochemical, petrographic and reservoir characteristics of the transgressive systems tract of lower Silurian black shale in Jiaoshiba area, southwest China

In recent years, lithofacies identification and classification have been a hot topic in shale gas research. Even if some previous works are considering the distribution characteristics of lithofacies in different system tracts (third-order sequences), only few studies systematically reveal the lithofacies distribution and reservoir characteristics. In this study, we focus on transgressive systems tract (TST) section which is characterized by high TOC content and highest gas content in the Long-1 member and divided the third-order TST into five Units (parasequences) according to graptolite biozonation, gamma ray (GR) and acoustic (AC) logging data. Twenty-six samples were analyzed for total organic carbon (TOC) content, mineral and elemental composition, porosity, pore type, pore size distribution and pore connectivity. The results show that Unit 1 is predominantly composed of S-3 and S-4 lithofacies, and Units 2–5 is predominantly composed of S-4 and M-2 lithofacies. The S-4 lithofacies was associated with high primary productivity of organic matter, and anoxic bottom water conditions. Redox conditions changed from anoxic to dysoxic condition and the productivity decreased through Unit 1 to Unit 5. Compared to other units, Units 1 and 2 are characterized by higher porosity and better pore connectivity. The S-3/S-4 and M-2 lithofacies exhibit different reservoir quality, especially in pore structure. Mineralogical composition is the key factor to determine lithofacies type and affects the characteristics of pore structure. Besides, TOC plays an important role in the development of organic matter pores. Anoxic bottom water conditions and high paleo-salinity contributed to the preservation of organic matter.

Clay-plug sediment as the locus of arsenic pollution in Holocene alluvial-plain aquifers

Shallow aquifers in many Holocene alluvial basins around the world have in the last three decades been identified as arsenic pollution hotspots, in which the spatial variation of natural (or: geogenic) arsenic concentration is conditioned by the meandering-river geomorphology and the fluvial lithofacies distribution. Despite the large amount of publications on the specifics of the pollution, still many uncertainties remain as to the provenance and processes that lead to arsenic enrichment in aquifers. In this paper, arsenic in abandoned and sediment-filled meandering-river bends (or: clay-plugs) is highlighted as a primary source of aquifer pollution. The combination of high organic-carbon deposition rates and the presence of chemically-bound natural arsenic in sediment of this specific geomorphological setting creates the potential for microbially-steered reductive dissolution of arsenic in an anoxic environment, and subsequent migration of the desorbed arsenic to, and stratigraphic entrapment in, adjacent sandy point-bar aquifers. To assess the magnitude of the arsenic source in clay-plug, bulk sediment volume calculations were made of twenty clay plugs on the Middle Ganges Plain of Bihar (India), by combining clay-plug surface area analysis of Sentinel-2 satellite data, side-scan sonar depth profiling of oxbow lakes and the Ganges River, and sedimentological data from five cored shallow wells. ICP-MS based elemental analysis of 36 core sub-samples, complemented with published concentration data in a similar geomorphological setting in West Bengal, India, yielded an average arsenic content of 28.75 mg/kg sediment in the 12-m-thick clay plugs, which amounts to a total arsenic volume of 0.07 – 3.13 . 106 kg per clay plug. A scenario is presented for the release of arsenic from the clay-plug sediment by microbial metabolism, followed by migration of the desorbed arsenic to the bordering point-bar sands.

Deeper-water deposition in intrashelf basins: Example from the Lower Cretaceous (Albian) upper Glen Rose Formation in the Houston trough, eastern Texas

In easternTexas, a thick (150-200 ft) interval of Lower Cretaceous upper Glen Rose D strata was deposited in the Houston trough intrashelf basin landward of the Stuart City reef margin and seaward of the main eastern Texas intrashelf basin. The stratigraphic section in this intrashelf basin forms a primarily oil-bearing hydrocarbon system where porous grainstones in the Alabama Ferry and Fort Trinidad fields were deposited coevally with organic-rich calcareous argillaceous mudstones. Here, we present evidence to suggest that the depositional setting within the Houston trough and the contained ooid grainstone beds were of deeper-water origin (400-500 ft), below storm wave base. Further, the water column was stratified, allowing dysaerobic to anaerobic conditions to develop, fostering accumulation and preservation of organic matter. Grain-rich gravity flows consisting of ooids, intraclasts, and thin-walled mollusks, derived from the shallower shelf to the northwest, were deposited in the basin, forming the Alabama Ferry and Fort Trinidad fields. The upper Glen Rose interval is composed of lithofacies that vary laterally within the Houston trough. Five lithofacies are recognized on the basis of sedimentary features, texture, fabric, biota, and organic-matter content: (1) laminated to nonlaminated argillaceous lime mudstone to lime wackestone; (2) laminated to nonlaminated calcareous siliciclastic mudstone to siltstone; (3) intraclast–ooid skeletal lime packstone; (4) intraclast–bivalve–ooid lime grainstone; and (5) oyster lime floatstone and rudstone. Grain-rich lithofacies laterally adjacent to and interbedded with organic-rich argillaceous lime mudstones and calcareous siliciclastic mudstones were previously interpreted as being deposited in a tidally influenced, high-energy, shallow-water shoal environment; however, reconstruction of the depositional setting and interpretation of the vertical and lateral distribution of lithofacies suggest that these lithofacies were deposited as grain-rich gravity flows into the deeper-water Houston trough. This interpretation suggests a new conceptual framework integrating facies models from submarine channel and fan complexes to explore for grainstone reservoirs similar to Alabama Ferry and Fort Trinidad fields.

Characteristics and main controlling factors of Ordovician deep subsalt reservoir in central and eastern Ordos Basin, China

In recent years, the subsalt dolomite reservoirs in the Ordovician strata of the Ordos Basin have indicated good exploration results and are expected to have a high hydrocarbon potential. This study targets the Ma 3 and Ma 4 reservoirs below the salt rock of the Ordovician in the central and eastern Ordos Basin. It investigates the reservoir characteristics and main controlling factors of high-potential reservoirs by combining core observations, cast slabs, scanning electron microscopy, physical property analysis, and mercury injection data. The results show that the Ma 3 reservoir is dominated by gypsum-bearing dolomite deposited in evaporation environments, whereas the Ma 4 reservoir is dominated by calcite dolomite under a transgressive environment. The types of Ma 4 reservoir space mainly include dissolved pores (dissolved holes), intra-crystalline pores, intra-crystalline dissolved pores, and microcracks. The types of Ma 3 reservoir space mainly include microcracks, gypsum mould pore, dissolved pores (dissolved holes), and intergranular pores. The pore structures of reservoirs are complex. The capillary pressure curves can be divided into four types: dissolved pore type, intra-crystalline (dissolved) pore type, microcrack type, and micro-porous type. Based on available information, the average porosity values of the Ma 3 and Ma 4 reservoirs are 2.1% and 2.3%, respectively, and the average permeability values are 0.19 × 10−3μm2 and 0.22 × 10−3μm2, respectively. The Central paleo-uplift, the Wushenqi secondary paleo-uplift, and the Shenmu low uplift controlled the sedimentary facies and favorable lithofacies distributions. Sedimentary microfacies control rock fabric, and the rock fabric affects the physical properties of the reservoirs. The limestone and dolomite reservoirs with gypsum and sand cutting texture have relatively good physical properties. Diagenesis is also a key factor in the development of sub-salt reservoirs. Intra-crystalline pores are mainly formed under various dolomization mechanisms. The surficial and buried diagenetic environments facilitate the formation of a large number of secondary pores, such as dissolved pores (dissolved holes), intra-crystalline dissolved pores, and gypsum mould pores. In addition, the Yanshan tectonic activity played a vital role in the development of reservoir microcracks. The microcracks can significantly improve reservoir physical properties. It is favored that the tectonic setting, rock fabric, diagenetic, and microcrack alterations collectively determine the sub-salt reservoirs' storage capacity and are the main controlling factors for forming high-quality reservoirs.

Integrated modelling framework for enhancement history matching in fluvial channel sandstone reservoirs

Abstract Modelling lithofacies and petrophysical properties are the challenging processes at the beginning of exploration and production of hydrocarbon reservoirs. However, the limited amount of well data and core data are the main issues facing conventional modelling processes. In this study, Artificial Neural Network (ANN), Sequential Gaussian Simulation (SGS), Co-kriging and object-based modelling (OBM) were integrated as the enhancement framework for lithofacies and petrophysical properties modelling in the fluvial channel sandstone reservoir. In the OBM, multiple fluvial channels were generated in the lithofacies model. The result of this model represented all the characteristic of the fluvial channel reservoir. The model was then distributed with channels, crevasse, and leeves depositional facies with background shale. Multiple geological realizations were made and cross-validation to select the most suitable lithofacies distribution. This model was cross-validated by modelling the porosity and permeability properties using Sequential Gaussian Simulation. Thereafter, the modelling process continued with Artificial Neural Network. Petrophysical properties (mainly porosity and permeability) were predicted by training various seismic attributes and well log data using the ANN. Applying the co-kriging algorithm, the predicted ANN model was integrated with OBM simulated lithofacies model to preserve the fluvial features of the geological system. To achieve full field history matching, the final geological model was upscaled to serve as input data in dynamic history matching. An excellent and nearly perfect history matching with a least mismatch was obtained between the measurement and simulated bottom hole pressure from well test and production history. The results indicated that an efficient integrated workflow of ANN and other geostatistical approaches are imperative to attaining an excellent history matching.

Sequence stratigraphy and tectono-depositional evolution of a late Ediacaran epeiric platform in the upper Yangtze area, South China

The tectono-depositional evolution of the Yangtze Block during the Ediacaran–Cambrian transition is controversial, leading to uncertainties in understanding the life-environment co-evolution and in petroleum exploration. This study investigated multiple upper Ediacaran (Dengying Formation) outcrops in the upper Yangtze area to reconstruct the carbonate platform evolution and underlying controls. Detailed sedimentological observations allow the identification of nineteen carbonate-dominated and five terrigenous siliciclastic-dominated lithofacies. Based on the spatiotemporal distributions of lithofacies, two and a half transgressive–regressive sequences (the regressive systems tract of S1 and the full sequences S2 and S3 in ascending order) are distinguished. The Dengying Formation mainly consists of carbonate-dominated lithofacies, which can be grouped into four depositional facies: semi-restricted platform, marginal shoal, middle-outer ramp/slope and siliceous basin. The broad platform interior with a flat-topped topography represents an epeiric platform, which can be subdivided into two variants: one dominated by tidal islands in the platform interior mainly during middle-late regressive systems tracts, and another dominated by lagoons in the platform interior mainly in the condensed section of S2 and early regressive systems tracts. Terrigenous siliciclastic-dominated lithofacies occur only in the transgressive systems tract of S3 and are grouped into two depositional facies: coast and shallow sea, representing a terrigenous material-dominated epeiric marine setting formed by a drowning event. These transgressive–regressive sequences can be roughly correlated with the sequences and glaciations documented in coeval successions worldwide, reflecting the control of eustasy. Across the eastern margin of the upper Yangtze Platform, the incompleteness (or absence) of S2 and S3 and lower Cambrian Terreneuvian strata along the platform margin and the extremely condensed Ediacaran–Cambrian transitional successions in the lower slope and basin may have been caused by differential uplift and subsidence induced by extensional faulting. The fault zone and the two reported synthetic extensional fault zones in the middle Yangtze area largely followed the pre-existing horst and graben configuration, indicating the activation of the pre-existing major faults likely associated with a transtensional tectonic regime.

A 900 m‐deep borehole from Boiano intermontane basin (southern Apennines, Italy): Age constraints and palaeoenvironmental features of the Quaternary infilling

The Boiano Basin is one of the largest Quaternary intermontane basins of the central‐southern Apennines within one of the most tectonically active areas of the Mediterranean region. In order to reconstruct its entire Quaternary stratigraphic, tectonic, and palaeoenvironment evolution, lithofacies and palaeomagnetic analyses have been performed on a 900 m‐deep borehole (CP1) drilled in the southwestern sector of the basin. The Quaternary succession consists of an alternating of alluvial fan and fluvial–marshy deposits for a total thickness of 240 m, unconformably laying on Lower Miocene deposits of the Sannio Unit, thrusted on upper Miocene deposits of the Molise Flysch. In addition, the stratigraphic study and facies distribution of 29 intermediate and shallow wells drilled in the basin, allowing us to define the thickness and lithofacies variations of the Quaternary sedimentary units inside the entire Boiano Basin in the sector of Campochiaro alluvial fan. Our results demonstrate that the Boiano Basin infilling started during the late Early Pleistocene (c. 1.1 Ma) and developed with variation in lithofacies distribution and thickness. The first depositional unit (Early Pleistocene–early Middle Pleistocene in age) was palustrine and fluvial–marshy, the second (Middle Pleistocene in age) was characterized by the occurrence of the first cycle of alluvial fan deposition, the third (late Middle Pleistocene in age) was newly palustrine and fluvial marshy and, finally, the fourth recorded two cycles of alluvial fan deposition (late Middle Pleistocene and Late Pleistocene in age, respectively), interspersed by short periods of palustrinity, tephra layers deposition, and palaeosols development. The study allows the hypothesizing that the Quaternary infilling was accommodated within a graben (or semigraben) structure, affected mainly by extensional fault systems localized in the inner part of the basin and secondly by fault systems bounding the basin.

Seismic prediction of lithofacies heterogeneity in paleogene hetaoyuan shale play, Biyang depression, China

AbstractShale lithofacies identification and prediction are of great importance for the successful shale gas and oil exploration. Based on the well and seismic fine calibration, extraction, and optimization of seismic attributes, root mean square (RMS) amplitude analysis is used to predict the spatial–temporal distribution of various lithofacies in the fifth organic-matter-rich interval, and the prediction results are confirmed by the logging data and geological background. The results indicate that in the early expansion system tract, dolomitic shale and calcareous shale were widely developed and argillaceous shale, silty shale, and argillaceous siltstone only developed in the periphery of deep depression. With the lake level rising, argillaceous shale and calcareous shale were well-developed, and argillaceous shale interbedded with silty shale or argillaceous siltstone developed in deep or semi-deep lake. In the late expansion system tract, argillaceous shale was widely deposited in the deepest sag; calcareous shale presented in eastern sag with belt distribution.

Multi-attribute Seismic application for Modeling Static Reservoir In “Athran” Field South Sumatera Basin

“Athran” field is an active field for hydrocarbon exploration with trending northeast-southwest and focus on the reservoir zone is the Talang Akar Formation with A-C layer intervals. The method used in this research is electrofacies, delta deposition system, formation evaluation, static reservoir modeling, and multi-attribute linear regression. The distribution of reservoir modeling is done by using the stochastic variogram method to make modeling lithofacies and depositional facies. Identification of lithofacies in the study area obtained two units, namely sandstone and shale facies. The depositional facies analysis was carried out using the electrofacies approach, showing that the research target was in a transitional environment with a delta deposition system, namely the distributary channel, delta front and prodelta. The deposition process. Sandstones and shales affect the distribution of porosity which functions as a reservoir property. The data integration of the distribution of lithofacies, gamma ray and porosity was carried out using the multi-attribute linear regression method to predict data both vertically and horizontally. The results of this study are used to help determine the distribution direction and direction of sediment deposition as a good reservoir zone for exploration activities.

Integrating machine learning and data analytics for geostatistical characterization of clastic reservoirs

An integrated multidisciplinary workflow of machine learning and data analytics was conducted for the multivariate geostatistical characterization of clastic reservoirs. This workflow was adopted for the estimation of lithofacies and permeability distributions in non-core intervals and wells on data from the upper shale member/Zubair formation in Luhais oil field, southern Iraq. Specifically, the workflow includes field and reservoir description, review of univariate, bivariate, and multivariate statistics for facies and petrophysical property distribution. In univariate and bivariate statistical analysis, the correction of core and log scale was adopted in order to have core permeability corrected to log scale. The discrete lithofacies distribution has been obtained for a few wells to be later incorporated in the permeability modeling and prediction.At first, the measured discrete lithofacies distributions were incorporated into classification algorithms to model the facies given the well logging data (shale volume, neutron porosity and water saturation) in order to predict the discrete facies distribution at other wells of missing records. The Multinomial Logistic Regression (Multinom), Logistic Boosting Regression (LogitBoost), and Extreme Gradient Boosting (XGBoost) were all comparatively adopted for lithofacies classification. After that, the resulting most accurate discrete facies distribution by LogitBoost were included along with the well logging interpretations into the multivariate permeability modeling through advanced machine learning approaches to model and predict the corrected core permeability given well logging interpretations for all wells in the reservoir. More specifically, the Multivariate Adaptive Regression Splines (MARS) and Smooth Generalized Additive Models (SGAM) were comparatively adopted to model and predict the core permeability at all wells. From this workflow, it was indicated that LogitBoost and MARS were the best machine learning algorithms to produce the most realistic lithofacies classification and permeability modeling, respectively.

Analysis of lithofacies and evaluation of effective reservoirs of member 2 of Xujiahe Formation in the Xinchang area in Western Sichuan

In the Xinchang area in western Sichuan Basin, a total of 121.12 billion cubic meters of proved reserves of natural gas are discovered in Member 2 of Xujiahe Formation, but only 7.739 billion cubic meters of proved reserves of natural gas are produced. The main reason is that sandstones in Member 2 of Xujiahe Formation in the Xinchang area are generally tight, with strong heterogeneity, only intervals with relatively good physical properties have a certain production capacity; therefore, it is very necessary and important to evaluate effective reservoirs in Member 2 of Xujiahe Formation. Through analysis of cores, with consideration of color, sedimentary texture and grain size, lithofacies of Member 2 of Xujiahe Formation in the Xinchang area can be divided into 12 types. According to data of physical properties and thin section analysis, it can be concluded that the favorable lithofacies in the upper submember of Member 2 of Xujiahe Formation are the massive medium-coarse sandstone, the medium-coarse sandstone with cross bedding, and the medium-coarse sandstone with carbonaceous fragments. On the basis of electrical characteristics, well-logging interpretation standards for favorable lithofacies are established, in addition, favorable lithofacies and effective reservoir in the Xinchang area are determined, and distribution of favorable lithofacies (effective reservoir) is identified, which lay a solid foundation for next deployment of appraisal well and development of effective reserves.

Modeling Three-Dimensional Anisotropic Structures of Reservoir Lithofacies Using Two-Dimensional Digital Outcrops

Reservoir heterogeneity is a key geological problem that restricts oil and gas exploration and development of clastic rocks from the early to late stages. Existing reservoir heterogeneity modeling methods such as multiple-point geostatistics (MPS) can accurately model the two-dimensional anisotropic structures of reservoir lithofacies. However, three-dimensional training images are required to construct three-dimensional reservoir lithofacies anisotropic structures models, and the method to use reservoir heterogeneity model of fewer-dimensional to obtain a three-dimensional model has become a much-focused research topic. In this study, the outcrops of the second member of Qingshuihe Formation (K1q2) in the northwestern margin of the Junggar Basin, which are lower cretaceous rocks, were the research target. The three-dimensional reservoir heterogeneity model of the K1q2 outcrop was established based on the unmanned aerial vehicle (UAV) digital outcrops model and MPS techniques, and the “sequential two-dimensional conditioning data” (s2Dcd) method was modified based on a sensitivity parameter analysis. Results of the parametric sensitivity analysis revealed that the isotropic multigrid simulations demonstrate poor performance because of the lack of three-dimensional training images, conditioning data that are horizontally discrete and vertically continuous, and distribution of lithofacies that are characterized by large horizontal continuities and small thicknesses. The reservoir lithofacies anisotropic structure reconstructions performed well with anisotropic multigrids. The simulation sequence of two-dimensional surfaces for generating the three-dimensional anisotropic structure of reservoir lithofacies models should be reasonably planned according to the actual geological data and limited hard data. In additional to this, the conditional probability density function of each two-dimensional training image should be fully utilized. The simulation results using only one two-dimensional section will have several types of noises, which is not consistent with the actual geological background. The anisotropic multigrid simulations and two-dimensional training image simulation sequence, proposed in this paper as “cross mesh, refinement step by step”, effectively reduced the noise generated, made full use of the information from the two-dimensional training image, and reconstructed the three-dimensional reservoir lithofacies anisotropic structures models, thus conforming to the actual geological conditions.

Lithofacies characteristics and depositional environment interpretations of the Middle Jurassic Fort Member rocks, Jaisalmer Formation, Western Rajasthan, India

The present study is focused on the identification of lithofacies characteristics and depositional environments for the Middle Jurassic sediments of Fort Member, Jaisalmer Formation, Western Rajasthan (NW India). Three facies associations are documented as tidally influenced fluvial, tidal/inter-tidal flat and wave-dominated shoreface facies deposits. Facies association-1 contains matrix-supported clasts, planar and flaser cross-stratified sandstone, interbedded sandstone and horizontal laminated sandstones; indicative of tidally influenced fluvial deposits. Facies association-2 characterized by herringbone cross-stratified sandstones, horizontal laminated sandstones, planar and trough cross-stratified sandstones, the alternate band of sandstone and shale, mud-crack sandstone developed on a tidal/inter tidally influence deposits. Facies association-3 consists of wave-dominated shoreface facies association consisting of wavy and lenticular beds, interference ripples, horizontal laminated sandstone, small scale planar cross-stratified sandstone, trough, and hummocky cross-stratified sandstone as well as fossiliferous limestone. Palaeoflow patterns from unipolar to bipolar indicate a change from tidal influence fluvial to marine depositional environments. The overall lateral and vertical distribution of lithofacies shows that the depositional environment was tidal-dominated fluvial channel to the marine realm.

Epithermal clast coating inside the rock avalanche-debris flow deposits from Mount Meager Volcanic Complex, British Columbia (Canada)

The observational and semi-quantitative sedimentological analyses of the lithofacies assemblage contribute to describe at different scales the breccia matrix along the sheared and transformed contact between rock avalanche deposits and trailing debris flow deposits emplaced by a large landslide at Mt. Meager volcano in British Columbia in 2010. An inverted cataclastic gradient of crushed breccias in a fluidized and mixed matrix implies a structurally controlled flow regime and rapid deposition. The coating matrix changed the initial polymodal distribution of the debris-flow lithofacies. Clast shape evolution helps to characterize the cataclastic sorting during transport and fluidized disaggregation. A plot of matrix percent against matrix/gravels helps to distinguish primary hot fracturing of about 61% from initial dilation and sheared fracturing involving ~22% matrix. Extensional disaggregation between 73 and 79% matrix is related to hydromagmatic fragmentation within an epithermal system. Clayey mineral assemblages identified by XRD patterns are related to colloidal aluminium gel, cataclastic shear bands, and quartz microstructures in epithermal breccia zones (pH = 2–3, 200–350 °C, <10–20 GPa).Microstructural analysis differentiates the inner rim of coated clasts from their border and the surrounding matrix in impact melt breccias. Sequential coating stages are inferred during the propagation of the shock wave with an oscillatory relative speed during the inter-seismic period. We differentiate: 1) shock faulting which contributes to the impacted quartz (10–35 GPa) in a devitrified matrix and pseudotachylite coating related to frictional melting at the margin of a conduit; 2) shock response (85 GPa) in epithermal vein with calcic spheroids, CO2 dissociation, and basaltic melt (70–101 GPa, >1500 °C); and 3) the secondary fracturing with flash heating and pressure pulse during cavitation (ΔP ~ 10 GPa, >1000–1500 °C), which generates pockets of partial melting, quartz spheroids, and a roll-over effect for the inner rim of coated clasts. The formation of impact melt breccias and the debris flow are related to the slowing elastic impact wave with an oscillatory relative speed during the inter-seismic period along a proximal strike-slip fault.This study helps identify how the proximal rock avalanche transformed into a highly mobile debris flow, larger examples of which pose a hazard to the town of Pemberton, at a distance of 65 km from Mt. Meager.

Soft-sediment deformation structures in alkaline lake deposits of Lower Permian Fengcheng Formation, Junggar Basin, NW China: Implications for syn-sedimentary tectonic activity

Soft-sediment deformation structures (SSDS) are an important tool for interpreting paleoseismicity and depositional environments in ancient basin successions. The northwestern margin of Junggar Basin is a key region for understanding the paleogeography and basin attribute of the Junggar Basin during the early Permian. In the Lower Permian (Kungurian) Fengcheng Formation, nine lithofacies are grouped into fan delta, fluvial delta, and shallow, deep and alkaline lacustrine facies associations. Vertical transitions of the sedimentary environments show that the lake basin experienced expansion during deposition of the Lower (P1f1) and Middle (P1f2) members and the shrinkage during the Upper Member (P1f3), which is consistent with the paleogeographic distribution of lithofacies in different members. Conventional cores show a wide variety of SSDS, including ductile (folds, convolute lamination, and load structures), hybrid ductile-brittle, and brittle deformation structures (microfaults, breccias, boudinage and sedimentary dikes). SSDS mainly occurred in a still and deep lacustrine environment. These deformation structures were not formed in any specific lacustrine environment, as there is no difference with the undeformed sedimentary units of the same sedimentary facies. Thus, the triggering mechanism is interpreted as intermittent earthquakes. Lake expansion under arid condition and SSDS intervals with vertical repetition and possible lateral continuity indicate that most seismicity was during the Middle Member (P1f2), corresponding with the main activity on basin-bounding normal faults. The enhanced faulting at this time generated accommodation, creating lacustrine conditions in the basin depocenter. It is thus clear that the early Permian Mahu sag represents an active rift basin with bounding faults, and is not a foreland basin.

Stratigraphic and structural trapping frameworks in the central Ceduna Sub-basin

Despite a recent renewal in exploration, the Ceduna Sub-basin is underexplored and, as is typical with frontier basins, there is considerable uncertainty regarding lithofacies distribution. As such, there is limited understanding of reservoir and seal coupling as well as structural trapping and thus fluid migration pathways within the prospective Late Cretaceous marine and deltaic intervals from the Tiger and Hammerhead supersequences.To address these uncertainties, a stratigraphic forward model using multiple sediment source points and constrained by recent marine survey data was developed to simulate the deposition and preservation of potential stratigraphic successions across the sub-basin. Pseudo-wells have been extracted from the stratigraphic forward modelling output to forecast vertical and lateral stratigraphic trends. These trends were analysed to predict fault seal and structural trapping likelihood using the Shale Gouge Ratio methodology and triangle juxtaposition diagrams.Over the study area, the modelled facies distributions and geometries are consistent with sequence stratigraphy, seismic stratigraphy and well data. The stratigraphic model shows an overall NW to SE trend of decreasing sand volume and sand interval thickness for the Tiger and Hammerhead supersequences. Although this highlights the general trend of a higher likelihood of reservoir presence to the NW of the study area, and more seal presence to the SE, the model predicts enough vertical variability within the supersequences to create ranges of stacked reservoir-seal couplets.The stratigraphic model and fault seal assessment suggest that, in the marine-influenced Tiger and lower Hammerhead supersequences, the best reservoir and top/fault seal couplets are located along a WSW-NE trend to the NW of the study area where faults with throw≥150 m are expected to yield membrane fault seal. The NW corner of the study area is predicted to have high net-to-gross ratio for the upper Tiger, too high to yield any efficient membrane fault seal. In the deltaic upper Hammerhead supersequence, the best reservoir and top/fault seal couplets are expected along an E-W trend in the central part of the study area. Along this trend faults with throw≥100 m are expected to yield membrane fault seal able to impede lateral hydrocarbon flow in modelled mid Campanian reservoirs and a nearshore interval.These inferred Late Cretaceous structural traps could have been charged by hydrocarbons expelled from local Late Cretaceous source rocks between the Cenomanian to the present-day.

The giant quaternary Ballik travertine system in the Denizli basin (SW Turkey): a palaeoenvironmental analysis

This research forms the basis for the applicability of the Dunham (1962) classification of carbonated rock lithofacies to the analysis of the giant Ballik travertine architecture, while reconstructing lateral and vertical environmental changes. This study provides an analogue for spring-related deposits encountered offshore Brazil and Angola by linking macroscopically travertine lithofacies distribution to depositional environments. The analysis is based on rock-building constituents such as gastropods, charophytes, intraclasts, phytoclasts, coated grains, dendrites etc., forming micro-sedimentary fabrics with different structures such as packstone, grainstone, wackestone and boundstone, these latter closely associated with crust of dendrites and phytoherm of reeds and bryophytes. Our findings indicate that the Ballik travertine area consists of a “Lower” and an “Upper Domain” reflecting different depositional environments. More specifically, the “Lower Domain” consists from west to east of a laterally complex amalgamation of extended pool, marsh pool and flood plain environments that formed from a mixture of spring and ground waters. The extended pond environment characterised by a boundstone facies of stromatolites in the west evolves eastward into a marsh pool and flood plain. This is because CO2 degassing and water temperature decreased as the water depth of the Lower Domain reduced towards the east. The marsh pool environment includes packstone to grainstone lithofacies and abundant wackestone lithofacies made of phytoclasts, whose crusts exhibit pustular fabrics. Moreover, the flood plains along with the marsh pool consist dominantly of packstone to grainstone lithofacies with many gastropods and intraclasts, interfingered with wackestone lithofacies made of phytoclasts. Irregular clotted fabrics, along with coated grains with radial fibres, high lime mud content with bioturbation are also present. The “Upper Domain” displays a laterally less heterogeneous palaeoenvironmental distribution with flooded slope and flood plain deposits. The eastern part of the “Upper Domain” is characterized by a systematic alternation of these environments, with intercalations of wackstone lithofacies made of phytoclasts, packstone to grainstone lithofacies made of intraclasts and lime muds as well as coated grains. The flood plain has coated grains having peloidal nuclei and coatings of sparry laminations and clotted fabric of peloids representing intraclasts, whereas, the flooded slope possesses coated grains with coatings of dendrites and nuclei of peloids, boundstone of stromatolites which have flat‑laminated and columnar-laminated fabrics indicating a laminar discharge away from the spring(s). Alluvial fan and palustrine deposits with abundant bryophytes and reeds frequently interfinger with marsh pool environment in the “Lower Domain”, and with the flood plain and flooded slope environments in the “Upper Domain”. The results illustrate well how environmental changes identified in the two different domains have induced heterogeneity in reservoir‑based depositional architecture.

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.

Sedimentary characteristics and model of gravity flows in the eocene Liushagang Formation in Weixi'nan depression, South China Sea

Gravity flows have been well investigated in deep-marine settings but less so in deep-lacustrine environments. To further understand the sedimentary characteristics and processes of gravity flows in a lacustrine rift basin, sublacustrine fans in the Weixi'nan Depression in the South China Sea were identified and studied by integrating core, well logging and 3D seismic data. Several aspects of gravity flow deposits were discussed in this study: (1) classification: seven lithofacies types were further grouped into four lithofacies assemblages/associations associated with emplacement by slumps, gravel-rich debris flows, high-density turbidity currents (which may transform into muddy debris flows) and low-density turbidity currents. (2) Proximal vs. distal position: the quantitative analysis of the lithofacies associations indicated that the gravity flow deposits are dominated by gravel-rich debris flow deposits and high-density turbidites in the proximal parts of the sublacustrine fans, whereas low-density turbidites dominate in the basin center. (3) Major triggers (humid climate and/or slope failures): the presence of active structures and a humid climate favored the formation of gravity flows, which originated from delta collapses and/or floods. (4) Lateral and vertical lithofacies distributions: the coarse-grained deposits represent a spectrum of different categories of gravity flows. In proximity to the slope break zone, slumps transformed into gravel-rich debris flows through liquefaction and deformation. As the gravity flows advanced toward the basin, the gravel-rich debris flows gradually became diluted with the surrounding water and transformed into high-density turbidity currents. When the flows reached the deep basin plain, the high-density turbidity currents transformed into low-density turbidity currents because the coarse grains could no longer remain in suspension. (5) An alternative mechanism: when the flows eroded the muddy substrate/basement, an alternative mechanism involving reverse-flow transformation occurred. In some cases, high-density turbidity currents transformed into muddy debris flows because the eroded light material became segregated toward the rear of the turbidity current and inhibited turbulence. This sedimentary model of gravity flows can be used for oil exploration in the study area and similar lacustrine rift basins.

Clay mineral content, type, and their effects on pore throat structure and reservoir properties: Insight from the Permian tight sandstones in the Hangjinqi area, north Ordos Basin, China

Clay mineral content and pore throat structure, including type, morphology, size distribution, and porosity and permeability, are very important in factors that control gas storage in tight sandstone reservoirs. In this study, integrated thin section and field emission scanning electron microscope (FE-SEM) observation, X-ray diffraction (XRD), nuclear magnetic resonance (NMR) and rate-controlled mercury porosimetry (RCP) analysis were used to investigate pore throat structure and the effects of clay minerals on pore throat connectivity, size distribution and reservoir properties of tight sandstones in the middle Permian successions of the Hangjinqi area, north Ordos Basin, China. Results show that the tight sandstones have very low porosity and permeability (averaged 9.76% and 0.968 mD) with diverse clay mineral compositions and grain sizes. The main pore types are intergranular and intragranular dissolution pores, intercrystalline and interstitial micropores. Most pores and throats are infilled with authigenic clay minerals, such as acicular chlorite, booklet kaolin, and scaly and filamentous illite with minor silica and calcareous cements. The pore and throat sizes in tight sandstones exhibit large variation, with 147.648 μm and 1.051 μm in sandy conglomerates, 160.998 μm and 1.285 μm in pebbly coarse-grained sandstones, 155.016 μm and 0.868 μm in coarse-grained sandstones, 158.134 μm and 1.178 μm in medium-grained sandstones, and 154 μm and 0.379 μm in fine-grained sandstones, respectively. The results indicate that clay minerals affected the pore throat connectivity, size distribution and reservoir properties of different lithofacies. The properties of tight sandstone reservoirs are heavily influenced by illite content, and throat size played a more important role influencing porosity and permeability. This study helps to clarify the relationship of clay minerals, pore throat structure and reservoir properties of the Permian tight sandstones in the Hangjinqi area and can guide associated work on tight sandstone reservoirs in the Ordos Basin and other basins worldwide. • The pore throat structure of the Permian tight sandstones is characterized. • Lithofacies are associated with pore and throat size distribution. • Throat size plays important role on pore throat connectivity and reservoir permeability. • Clay minerals content especially illite has negative impacts on reservoir properties.