Thick Sand Bodies Research Articles

Sedimentary characteristics of shallow-marine fans of the Huangliu Formation in the Yinggehai Basin, China

Abstract In recent years, shallow-marine fans have attracted considerable attention because they may serve as valuable targets in hydrocarbon exploration. By integrating 3-D seismic, well log, core, and other measured data, this study analyzed the sedimentary characteristics and factors controlling the deposition of shallow-marine fans in Member #1 of the Huangliu Formation in the Yinggehai Basin. Four depositional elements were recognized in the channel-lobe complexes (CLCs), channel-fill sandy debrites, channel-fill turbidites, sand-rich lobe deposits and mud-rich lobe deposits. The channel-fill sandy debrites, which are characterized by low root-mean-square (RMS) amplitude in plan view and deep V-shaped reflections in seismic profile, are commonly located in the internal regions of CLCs. They have widths ranging from 1 to 2 km and single sandbody thicknesses exceeding 25 m. A cored interval from a channel-fill sandy debrite comprising several ungraded sand units revealed massive fine-grained sandstones with floating mud clasts and muddy rip-up clasts. In contrast, the channel-fill turbidites are characterized by low or high RMS amplitude in plan view and shallow V-shaped or vermiform-shaped reflections in profile. They have a small-size frontal splay at the terminus, thus forming a lobe-channel-lobe depositional style. The width of a channel-fill turbidite is usually less than 0.5 km, and the single sandbody thickness is no more than 20 m. A cored interval of channel-fill turbidite consisted of very fine-grained sandstones containing incomplete Bouma subdivisions Ta–Tb. The sand-rich and mud-rich lobe deposits are characterized by high RMS amplitude and low RMS amplitude with continuous sheet reflection patterns, respectively. The deposition of the shallow-marine fans was significantly influenced by sea-level fluctuations and paleogeomorphology. When sea level fell rapidly, gravity flows with higher erosive energy and velocity tended to form widely developed large-scale channel-fill sandy debrites and sand-rich lobe deposits. When sea level rose, small-scale channel-fill turbidites were dominant because of relatively decreased gravity flow energy. Distinct variations of reservoir characteristics, including mean grain size, mud content, and pore-throat radius, were controlled by sea-level fluctuations. From the period of rapid sea-level fall to the period of rising sea level, the gravity flows may have been increasingly diluted due to sufficient mixing with ambient seawater, resulting in a gradual transformation from sandy debris flows into turbidity currents. In addition, changes in gradient caused by irregular paleogeomorphology may have significantly triggered different depositional styles. With a steeper gradient, the CLC is dominated by a channelized-lobe depositional style; however, a lobe-channel-lobe depositional style prevails under a gentler gradient condition. Taking the reservoir characteristics and depositional styles into consideration, it is suggested that the sandy debrites, with better physical properties, constitute favorable hydrocarbon reservoir intervals. The shallow-marine fans in this study may serve as a distinctive analogue to similar gravity flow deposits in other basins worldwide.

Discovering ‘buried’ channels of the Palaeo-Yamuna river in NW India using geophysical evidence: Implications for major drainage reorganization and linkage to the Harappan Civilization

The role of large rivers sustaining the civilizations across the world has long been debated. Mapping of palaeochannels using satellite images and the superimposition of known archaeological sites along them are often used as a strong piece of evidence to promote the so-called ‘river-culture’ hypothesis. The northwest India has been a hotbed of such studies where a large and wide trace of a palaeochannel (Ghaggar-Hakra channel) mapped from satellite images was linked to the Harappan Civilization, the demise of which was attributed to the decay of a large river (Sarasvati) that once flowed through the region. Recent research involving geophysical surveys and drilling and followed by the strong geochronological framework established the subsurface existence of these palaeochannels and their chrono-stratigraphic evolution. However, it was clearly demonstrated that this large palaeochannel was abandoned shortly after ~8 ka and much before the Harappan Civilization developed in this region (~4.6–3.9 ka). These findings strongly contested the contemporaneity of the large river with the Harappan Civilization and instead suggested river avulsion as a major factor for the onset and stabilization of the Harappan culture. Here, we extend this study to further east and examine the palaeochannels related to the Yamuna River that have also been considered to be important tributaries of the large Sarasvati River represented by the Ghaggar-Hakra palaeochannel. At least two of the palaeochannels of the Yamuna have been suggested to be connected to the Ghaggar-Hakra palaeochannel but very limited subsurface evidence exists till date to establish this. Here, we present new, high-resolution stratigraphic data based on vertical electrical resistivity soundings (1D–VES), multi-electrode electrical resistivity tomography (2D–ERT), and multi-probe well log surveys in one of the palaeochannels of the Yamuna to map the large-scale geometry and architecture of the valley fills of the palaeochannel system in the subsurface. The geophysical signatures recorded as VES on two transects trending NW–SE in Karnal and Kaithal districts of Haryana at 9 and 13 locations respectively, along with continuous ERT in selected transects reveal the presence of subsurface sand bodies (~10 to >30 m thick) interbedded with silty clay layers that are laterally stacked. The occurrence of thick and wide sand bodies having channel form geometry in the subsurface implies that these are the deposits of a large river system and suggests that the Yamuna was once connected to the palaeo-Ghaggar River as hypothesized by earlier workers based on remote sensing interpretations.

Sequence and sedimentary characteristics of upper Cretaceous Sifangtai Formation in northern Songliao Baisn, northeast China: Implications for sandstone-type uranium mineralization

Sequence and sedimentary characteristics play an important role in the mineralization of sandstone-type uranium deposit. Two third-order sequences, including four system tracts namely lowstand system tract (LST), transgression system tract (TST), highstand system tract (HST) and regression system tract (RST), and two sedimentary facies, including braided-meandering river and meandering river are identified in the Upper Cretaceous Sifangtai Formation in northern Songliao Basin. Sandstones with better porosity and permeability formed in LST are good uranium reservoir, and the compacted fine-grained deposits in TST are good confining layers. They form a reservoir-confining assemblage that is beneficial to uranium mineralization. The uranium is mainly hosted in the braided-meandering river depositional system, the channel bar ore-hosting sandstones are connective and have good reservoir property, the maximum porosity and permeability are 42% and 1140 mD, respectively. Suitable thickness and sand percentage of sand bodies are beneficial to uranium enrichment, the industrial grade uranium deposits mainly occur in areas with sand percentage of 0.6–0.8 and sand body thickness of 8–12 m. Organic matter and sulfur act as adsorbents and reducing agents in the progress of uranium mineralization, braided-meandering river with strong hydrodynamic conditions bring a large amount of terrestrial carbon debris, which creates conditions for the formation and adsorption of sulfur and uranium. A sequence and sedimentation uranium metallogenic model of Sifangtai Formation has been established, large and high grade uranium deposits are formed in braided-meandering river sand bodies of LST, which have good reservoir properties, suitable thickness, sufficient organic matter and reducing sulfur content, and regional confining bed.

A Volumetric Model for Evaluating Tight Sandstone Gas Reserves in the Permian Sulige Gas Field, Ordos Basin, Central China

To accurately measure and evaluate reserves is critical for ensuring successful production of unconventional oil and gas. This work proposes a volumetric model to evaluate the tight sandstone gas reserves of the Permian Sulige gas field in the Ordos Basin. The reserves can be determined by four major parameters of reservoir cutoffs, net pay, gas‐bearing area and compression factor Z, which are controlled by reservoir characteristics and sedimentation. Well logging, seismic analysis, core analysis and gas testing, as well as thin section identification and SEM analysis were used to analyze the pore evolution and pore‐throat structure. The porosity and permeability cutoffs are determined by distribution function curve, empirical statistics and intersection plot. Net pay and gas‐bearing area are determined based on the cutoffs, gas testing and sand body distribution, and the compression factor Z is obtained by gas component. The results demonstrate that the reservoir in the Sulige gas field is characterized by ultralow porosity and permeability, and the cutoffs of porosity and permeability are 5% and 0.15×10–3 μm2, respectively. The net pay and gas‐bearing area are mainly affected by the sedimentary facies, sand body types and distribution. The gas component is dominated by methane which accounts for more than 90%, and the compression factor Z of H8 (P2h8) and S1 (P1s1) are 0.98 and 0.985, respectively. The distributary channels stacked and overlapped, forming a wide and thick sand body with good developed intergranular pores and intercrystalline pores. The upper part of channel sand with good porosity and permeability can be sweet spot for gas exploration. The complete set of calculation systems proposed for tight gas reserve calculation has proved to be effective based on application and feedback. This model provides a new concept and consideration for reserve prediction and calculation in other areas.

Fusing multiple frequency-decomposed seismic attributes with machine learning for thickness prediction and sedimentary facies interpretation in fluvial reservoirs

Abstract Defining the boundaries, thicknesses and sedimentary facies of fluvial reservoirs (sand bodies) is critical for predicting hydrocarbon volumes, designing schemes for petroleum exploration and development and improving oil recovery. Most reservoirs contain thick and thin sand bodies at the same intervals, while the amplitude values of seismic data usually highlight sand bodies near the 1/4 wavelength for the tuning phenomena. Hence, the application of spectral decomposition to seismic attributes and the combination of multiple frequency-decomposed (spectral-decomposed) seismic attributes have gained increasing attention for the readjustment of tuning thickness to predict sand bodies of various thicknesses. However, the popular method of red-green-blue blending is a simple linear combination of three frequency-decomposed seismic attributes that qualitatively analyzes the sand thickness without well-log interpretation. This research proposes machine learning fusion as a new nonlinear method for fusing high-, middle-, and low-frequency seismic attributes. This method uses machine learning to link well-log interpretation and multiple-frequency seismic attributes for the quantitative prediction of sand thickness, which is important for development work in a mature field. Test results of the conceptual model and the real case indicate that the predicted sand thickness after fusing multiple frequency-decomposed seismic attributes is approximately in line with the actual thickness (correlations between 80 and 90%). Combined with the coherence attribute and the red-green-blue blending results, the distributions and histories of sedimentary facies are analyzed based on the predicted sand thickness and well data. The results suggest that the proposed method can effectively readjust the tuning thickness and improve the resolution of seismic interpretation. This method is a potentially effective technique to characterize the sand thickness and sedimentary facies in other fields using a similar geological setting and dataset.

Study on Flow Unit of Heavy Oil Bottom Water Reservoir with Over-Limited Thickness in Offshore Oilfield

The upper Ming section of L oilfield is a typical offshore heavy oil bottom-water reservoir with thick fluvial layers. All horizontal wells are developed by natural energy. Due to the few drilling holes and influence by the resolution of seismic data, it is difficult to describe reservoirs with thickness over 20 meters. In this paper, seismic resonance amplitude inversion technology is introduced to restore the real response of thick reservoirs and interbeds by drilling and drilling verification, and the geological bodies with different thickness are displayed by frequency division RGB three primary colors. Flow units of heavy oil reservoirs with bottom water are divided according to the three major factors of interlayer, lithologic internal boundary and water-oil thickness ratio which have the greatest influence on horizontal well development, thick sand bodies are divided into 10 different flow units in three levels, each unit is separated from each other, and the reservoir structure, water-cut characteristics and water-flooding characteristics are different. The reliability of the research is improved by using the dynamic data of horizontal wells and newly drilled passing wells, which provides a basis for tapping the potential of heavy oil reservoirs with bottom water.

Fused spectral-decomposition seismic attributes and forward seismic modelling to predict sand bodies in meandering fluvial reservoirs

Abstract Understanding the hierarchical architectural elements of fluvial sand bodies is important for planning their development strategy and to enhance oil recovery. Red-Green-Blue (RGB) blending of multiple seismic attributes and forwarding seismic modelling are commonly used in the analysis of compound sand bodies. However, RGB blending of multiple seismic attributes can only qualitatively describe the boundaries and thickness of sand bodies. The forward seismic modelling techniques previously documented in the literature are not effective when depicting the geometry of, and stacking relationships between, sand bodies (i.e., reservoir compartmentalisation). Hence, we propose in this work a new workflow that combines fused spectral-decomposition seismic attributes (SDSAs) and forwarding seismic modelling to quantitatively predict sand thickness, and to characterise stacking relationships between sand bodies. First, we employ a Support Vector Machine (SVM) algorithm to fuse high, middle, and low frequency components (attributes) of seismic data so as to quantitatively predict the thickness of sand bodies. Second, we define the seismic waveform response patterns corresponding to the typical conceptual stacked sand bodies. With the constraints of waveform patterns and predicted sand thickness (fused SDSAs), the geometry and stacking relationships of the sand bodies are characterised by forward seismic modelling. To illustrate the effectiveness of our proposed workflow, we apply it to the Neogene Minghuazhen Formation (Nm) of the QHD 32–6 oil field, Bohai Bay Basin, China. We define five architectural elements of a meandering fluvial reservoir by analysing the hierarchy of sand bodies using our workflow. The predicted sand bodies in this workflow were further proven by horizontal drilling and production data.

Meandering river deposits in sediment cores, the Middle Jurassic Alma Field, Southern Danish Central Graben

Fluvial deposits are amongst the most important terrestrial hydrocarbon reservoirs, but the complex nature of these deposits is challenging in subsurface reservoir characterisation. This study is the first detailed facies analysis of the meandering river deposits of the Middle Jurassic Alma Field situated in the southern Danish North Sea. The fluvial sandstones and their associated deposits are described and interpreted based on studies from two core sites (Alma-1X and Alma-2X). The facies analysis of the cores demonstrates the presence of three meandering river facies associations: Channel deposits, channel margin deposits and floodplain deposits. The channel deposits comprise channel thalweg and point bar sediments, the channel margin deposits include crevasse channel and crevasse splay sediments, while the floodplain deposits comprise overbank and backswamp sediments. The point bar deposits are composed of fine- to medium-grained sandstones but can contain intervals of finer grained sediments, particularly in their upper parts where they can grade into muddy sandstones or true heterolithic deposits. Preserved sand body thicknesses (channel thalweg and point bar deposits) in both Alma cores have a mean value of 2.6 m and a maximum value of 4.35 m (Alma-1X) and 6.55 m (Alma-2X). Using maximum values of channel deposit thicknesses, and assuming the preservation conditions are met, the width of the largest ancient channel belt in Alma-1X would be between 90 m and 200 m or around 900 m, depending on whether the fluvial system is mud-rich or sand-rich. The same method applied to Alma-2X gives a width of the largest channel belt between 130 m and 330 m or around 1300 m. Fluvial sediments of the Middle Jurassic Scalby Formation (north-east England) were deposited in a sandy meandering river with sedimentary characteristics corresponding to those observed in the Alma cores. Outcrop analogue investigations of this formation were carried out to examine the architecture of the fluvial facies in a two-dimensional section with emphasis on channel thalweg and point bar deposits. Combined evidence from core analysis and outcrop analogue studies suggests that the fluvial deposits in the Alma Field represent a mixed-load meandering river system with sandy point bars. The meandering river system developed on a coastal plain with overbank fines and organic-rich backswamp deposits. The mud-rich or heterolithic deposits in the upper part of the point bar facies intervals are noteworthy and could indicate markedly fluctuating discharge in a mixed-load river.

Facies and porosity 3D models constrained by stochastic seismic inversion to delineate Paleocene fluvial/lacustrine reservoirs in Melut Rift Basin, Sudan

Abstract The Paleocene sandstones of the Yabus and Samma formations are the main oil reservoirs in the Melut Rift Basin of interior Sudan. In our study area, we implemented a multi-modeling approach to reveal heterogeneity and reservoir characteristics of this sandstone. It estimates acoustic impedance from seismic data using deterministic and stochastic inversion algorithms. We integrated petrophysical and geostatistical analyses of the well logs to produce facies and porosity 3D models. The study populated and evaluated three different porosity models; one from well logs only, a second from well logs constrained by inversion results, and a third from well logs constrained by inversion results and facies. Stochastic inversion reveals a significant vertical facies heterogeneity as less as two meters in Yabus Sandstone. This heterogeneity is a lithology dependent and facies distribution is structurally controlled by major faults that developed during the third rift phase. The geostatistical integration of seismic inversion, sedimentologic and stratigraphic analyses reveal lateral facies distribution in the study area. The significant correlation between facies distribution and porosity allows reliable mapping of thin sandbodies. The facies and porosity 3D models indicate vertically-stacked channels with isolated sandbodies in Yabus Sandstone. This strongly suggests a meandering origin for this sandstone. However, the thick sandbodies observed in Samma Formation might indicate a braided channel setting. The dominant claystone and mudstone facies deposited in Melut Formation indicate a shallow lacustrine setting. The reservoir quality decreases with depth from the Paleocene Yabus and Samma formations at the top towards the Cretaceous Melut Formation at the bottom. The porosity estimates support this conclusion as detected low porosity (as less as 5%) in Melut Formation with respect to higher porosity (as high as 25%) in Yabus and Samma formations. The facies and porosity 3D models enhanced our understanding in predicting the reservoir heterogeneity, quality and architecture of this sandstone.

Sequence subdivision and development characteristics of low-accommodation non-marine basins: a case study of the Yanchang Formation in the Ordos Basin

ABSTRACTThe third-order sequences of the Yanchang Formation in the Ordos Basin were determined by a comprehensive analysis of surface outcrops, core, logging and seismic data. Natural gamma curves were studied by the band-pass method. The hidden stratigraphic sequence and six lithological cycles were extracted. By examining sequence boundaries and lithological cycles, we identified six third-order (including SQ1, SQ2, SQ3, SQ4, SQ5 and SQ6) and three second-order sequences in the Yanchang Formation. The sequence stratigraphy of the Ordos Basin was found to have the following special development characteristics: (1) the third-order sequence growths were totally stable, and the thickness of the Ordos Basin was relatively stable; (2) thick sand bodies did not form in lowstand systems because of underdeveloped slope breaks; (3) the third-order sequences had diverse features at different sedimentary periods; (4) the distribution of the depositional systems had various characteristics in different tectonic belts, and the entire sequence had the filling characteristics of multiprovenances and multicycles. The low-accommodation Ordos Basin in the Upper Triassic was limited by slow subsidence, gentle basement paleorelief, stable tectonic settings and sufficient sediment supply. The characteristics of the basin included stable developed sequences, limited preserved lowstand systems tracts, multiple depocentres and subsiding centres, development of many delta systems in the north and south of the basin controlled by the gradient of the basement paleorelief, and laterally stretching sediments. Results further indicated that the controlling factors of the third-order stratigraphic sequences of Yanchang Formation in the low-accommodation lacustrine Ordos Basin included sediment supply (provenance), climate and lake-level fluctuation. Overall, our case study indicated that the development characteristics and controlling factors of sequence stratigraphy in a low-accommodation lake basin provided a comprehensive method of establishing the sequence stratigraphic framework of a low-accommodation non-marine basin. Our results also predicted that the SQ3, SQ4 and SQ5 of the Qiaochuan and Zhidan areas favoured oil and gas prospecting of the Yanchang Formation.

Establishment of Geological Knowledge Database for Meandering River Reservoirs in Offshore Oilfields With Large Well Spacing

Based on quantitative logging microfacies identification, seismic constraint facies map compilation and horizontal wells architecture analysis, the reservoir geologic knowledge database of different grades configuration unit was established for meandering river reservoirs of Lower Minghuazhen Formation in Q oilfield. The results show that the average point bar sand body width is 360-783 m, the average sand body thickness is 4.0-8.5 m, the ratio of width to thickness mainly distributed in 40-100. Average lateral accretion layers dip is 5.7°, lateral accretion layers horizontal interval mainly distributed in 70-200 m. On the basis of reservoir geological knowledge database, a fine geological model was established together with sequential indicator method and equivalent characterization method. Base on the reservoir geologic knowledge database and geological modeling results, 130 adjustment wells were successfully deployed and implemented at the comprehensive adjustment process of Q oilfield, these wells made an obvious oil production increase effect.

Sedimentology and seismic geomorphology of a lacustrine depositional system from the deep zone of the Gaoyou Sag, Subei Basin, eastern China

ABSTRACTSandstones of the Dainan Formation in the Gaoyou Sag of the Subei Basin are important hydrocarbon reservoirs. This paper studies the sedimentary facies and sandstone dispersal patterns in the Gaoyou Sag, based on the analysis of sedimentology and seismic geomorphology. The data used in this study include lithological data, wire-line-log data and three-dimensional (3D) seismic data. The target strata are the Eocene Daiyi Member (E2d13, E2d12 and E2d11) and the lower Dai'er Member (E2d25), which belong to three third-order sequences. Five types of depositional systems were identified in the Dainan Formation based on core observation and log-facies analysis. They are, respectively, delta, fan delta, sub-lacustrine fan, nearshore subaqueous fan and lacustrine system. Stratal slices from 3D seismic volumes have been used to map the sandstone dispersal patterns and to document the evolution of the depositional systems within these third-order sequences and associated systems tracts. In the low-stand systems tract and the high-stand systems tract, sandstones of delta, fan delta and nearshore subaqueous fan were well developed and can be delineated by stratal slices showing distinct progradation towards the deep lake centre. In the transgressive systems tract, sandstones are less common. Stratal slices indicate that the depositional environments of individual third-order sequences evolved from gravel-rich or sand-rich fans to lacustrine mudstones, and finally to mixed sandstone–mudstone fans. Sandstone dispersal patterns appear to be primarily controlled by syn-sedimentary faults, and generally sandstone thickness decreases with distance from the syn-sedimentary faults. Deltas and fan deltas have different characteristics in sand-body thickness because they have different sedimentary subfacies. The proposed geomorphology and sedimentary evolution of the lacustrine depositional system may aid in the prediction of potential reservoir distribution in the Gaoyou Sag.

地震属性优化与网络函数逼近储层砂体厚度预测方法及应用

地震属性优化与网络函数逼近储层砂体厚度预测方法及应用

Facies analysis and depositional model of late Permian Raniganj formation: Study from Raniganj coal bed methane block

The Raniganj Formation (late Permian) forms the uppermost economic coal-bearing unit of the Gondwana succession. The dominant facies interpreted from analysis of cores from the Raniganj formation are classified as Sandstone dominated facies, Sandstone - shale heterolith facies, Shale facies and Coal facies. The natural Gamma response of Raniganj Formation shows predominance of repetitive fining upwards cycles. Integration of core analysis and geophysical log data of the Raniganj formation indicates meandering fluvial environment. The lower part of Raniganj Formation is channel dominated which corresponds to thick amalgamated sand bodies while the upper part represent overbank shows predominance of channel avulsion indicating a gradual change in accommodation space. Five major fining upward depositional sequences, bounded by sub-aerial unconformities (sequence boundaries) have been dentified in Raniganj formation, based on changes in depositional style that are correlated regionally. Each sequence comprises of Low accommodation system tract (LAST) at base and high accommodation system tract (HAST) at top. LAST is characterized by vertically stacked, multistory amalgamated channel sandstone dominated facies, while floodplain dominated facies characterize HAST. The coal seams deposited in LAST are thicker and relatively more continuous than the frequent thin seams of HAST. Such facies distribution study would be helpful for the development strategy for CBM blocks based on production priority.

Sedimentary characteristics and seismic geomorphologic responses of a shallow-water delta in the Qingshankou Formation from the Songliao Basin, China

With the aid of modern sedimentology, seismic geomorphology, and the comprehensive analysis of geophysical data and core data from more than 10 wells, the Qingshankou Formation has been divided into 3 long-term cycles and further into 6 medium-term cycles in the Qijia area, Songliao Basin. The 3 long-term cycles, SQ1, SQ2 and SQ3 correspond to the members Qing I, Qing II and Qing III of the Qingshankou Formation, spanning 11.5 Ma during the Late Cretaceous. The time scale of the medium-term cycles, which are subunits of the long-term cycles, is approximately 0.5–1.5 Ma with thicknesses ranging from 45 m to 116 m. A shallow-water delta that consists of high-maturity fine-grained sandstone developed in the Qingshankou Formation. The scour surfaces reflect strong flow action, and the parallel bedding and large-scale cross bedding are well developed. Distributary channel sandstones are well developed in the shallow-water delta depositional system, in which a single channel can be several meters thick. The cumulative thickness of the distributary channels can reach tens of meters across a large area, extending tens of kilometers to the center of the lacustrine basin. The development of the subaqueous distributary channel of the shallow-water delta is closely related to base level (lake level) changes. In the early stage of a rising half-cycle and the late stage of a falling half-cycle, the fluviation is significant and the well-developed channel sandstone can extend towards the center of the lacustrine basin more than 50 km, with large cumulative thicknesses of compound sandbodies (17–28 m) and high sandstone-to-strata ratios (78–88%). In the late stage of a rising half-cycle and the early stage of a falling half-cycle, the lake level is high and the fluviation is weak, resulting in a distributary channel extent of less than 30 km, small cumulative thicknesses of compound sandbodies (5–12 m) and low sandstone-to-strata ratios (22–45%). The delta front can be divided into the inner front and the outer front. The subaqueous distributary channels in the inner front have good continuity and extend further, whereas the subaqueous distributary channels in the outer front have poor continuity, and sheet sand is well developed. Therefore, a shallow-water delta generally shows features including (1) sedimentary structures formed by strong currents, (2) stacked fining upward rhythm, (3) subaqueous distributary channels well developed, (4) delta front divided further into an inner and an outer front. There was a relatively good linear relationship between the thickness of the sandbody based on the well data and the maximum amplitude of the seismic trace next to the well. The reddish amplitude of the seismic attribution suggests thicker channel sandbodies (4–9 m), whereas the light amplitude of the seismic attribution indicates thinner channel sandbody (1–4 m). Of the two typical sedimentary models for shallow-water deltas, delta plains develop well in humid climates and delta fronts develop well in arid climates, both of which are found in the Qijia area, Songliao Basin. Stratal slices from seismic geomorphology are applied to display the branchlike sandstone dispersal patterns of the shallow-water delta and the broad distribution characteristics. This study also notes that the delta front distributary channel sandstone, which is near an oil source and faults, would be a favorable exploration target.

Chemostratigraphic age model for the Tornillo Group: A possible link between fluvial stratigraphy and climate

The expression of global climate trends (106 to 107years) and events (105years) in terrestrial sedimentary sections can only be assessed from long, continuous continental records. Such records are rare due to the paucity of well-dated terrestrial deposits covering millions of years. This study uses isotope chemostratigraphy to develop an age model for fluvial sedimentary rocks of the Tornillo Group (Tornillo Basin, TX) and to decipher the potential expression of global climate variations in this sub-tropical paleo-fluvial system. The carbon isotope ratio (δ13C) of Tornillo Group pedogenic carbonate correlates well with δ13C variations from benthic foraminifera and suggests deposition between ~69Ma and ~52Ma for the studied stratigraphic interval. Higher sediment accumulation rates occurred in the greenhouse period of the early Eocene than during the cooler middle Paleocene. Carbon isotope excursions associated with Eocene hyperthermals were not identified, but their predicted stratigraphic positions coincide with thick sand bodies. We interpret the long-term sediment accumulation rates and rapid shifts in facies distribution as reflecting change in sediment supply in the basin, possibly driven by fluctuations in precipitation seasonality and intensity driven associated with global temperature changes. Carbon and oxygen isotope ratios from pedogenic carbonates display a strong correlation throughout the section, interpreted as an expression of coupling between the hydrological cycle and soil processes. Those observations suggest that global climate variations, along with tectonic and eustatic sea-level changes, may be an important control of stratigraphic variations of the Tornillo Group at multiple timescales.

The discovery and significance of a sedimentary hiatus within the Carboniferous Taiyuan Formation, northeastern Ordos Basin, China: Reply

We thank Zheng et al. (2016) for their comments on our previous work of sedimentary hiatus within the Carboniferous Taiyuan Formation, northeastern Ordos Basin, China (Jiang et al., 2012). Zheng et al. (2016) have different opinions on sedimentary facies and sequence stratigraphy interpretation in the study area. They question our interpretation based on their understanding of sedimentary facies, seismic geometry, orientation of sand body, and sand body thickness. Here, we provide additional evidence to reinforce our interpretation. The structure of this reply follows the order of the questions raised by Zheng et al. (2016). The interpretation of depositional environment for sand bodies in the Upper Pennsylvanian Taiyuan Formation (Ct2) has been controversial for decades. Some previous studies suggested that Ct2 represents a barrier–lagoon–tidal flat system (e.g., Hao et al., 2007; Wang et al., 2007). Zheng et al. (2016) also made such an interpretation. However, northeast–southwest-trending sand bodies in Ct2 were interpreted as fluvial–deltaic channels by many other studies (e.g., Li et al., 1995; Liu, 1998; Fu et al., 2003; Guo et al., 2009; Zhang et al., 2009; Luo et al., 2011). The braided stream fills of the incised valley were documented by extensive erosional surfaces, conglomerate and/or mud clast fluvial lag deposits, and trough cross bedding. Erosional surfaces are one of the most common features observed in cores of Ct2 interval (Figure 1; see also figure 4 in Jiang et al., 2012). The poorly sorted gravels are aligned in various directions, indicating a rapid deposit without significant tidal reworking (Figure 1E). In contrast, the sedimentary structures that Zheng et al. (2016) interpreted as tidal–barrier island signatures, such as mud drapes and cross bedding, can be also found in fluvial deposits. Figure 1. Sedimentary features of fluvial deposit in the …

Reservoir Characterization and Formation Evaluation of a “Royal Onshore Field”, Southern Niger Delta using Geophysical Well Log Data

Abstract: The reservoir properties of three wells in Royal Field, Niger Delta basin was characterized using fundamental formula. The distributions and thicknesses of sand bodies were determined within each of the wells in the field using geophysical modeling software. The quantitative and qualitative analyses were done for the three exploration wells with the depth ranges of 4000-9700m for Royal well 1, 1000-8805m for Royal well 2, and 4000-8000m for Royal well 3. Each well has identified sand units. Royal well 1 and Royal well 2 have 5 sand units (A, B, C, D, E) each, with respective thicknesses 422m, 110m, 92m, 142m and 350m for well 1, and 82m, 65m, 214m, 362m, and 192m for well 2. Royal well 3 has 4 sand units (A, B, C, D), with respective thicknesses 135m, 80m, 269m and 229m. Petrophysical evaluations were made from well logs. The average porosity values obtained for Royal wells 1, 2, and 3 are 0.24, 0.18 and 0.22 respectively while the corresponding average permeability values are 2789mD, 1292mD, 1643mD. Porosity values obtained from porosity logs and density log (RHOB) using porosity formula are found to be within the range of 0.07-0.45, while their permeabilities range from 164 to 8453 milli Darcy. The water saturation obtained for each reservoir unit in combination with the resistivity index were used to prove the presence of hydrocarbon in these sands. Royal well 2 having its sand A, 80% hydrocarbon saturated and 82m thickness is the most prolific. The results clearly indicate that the application of this technique is very effective for the interpretation of reservoir properties.

Sedimentology and alluvial architecture of the Bajo Barreal Formation (Upper Cretaceous) in the Golfo San Jorge Basin: Outcrop analogues of the richest oil-bearing fluvial succession in Argentina

This study addresses the sedimentology, architecture and dimensions of fluvial deposits in the Bajo Barreal Formation (Upper Cretaceous) in the Codo del Senguerr anticline, Golfo San Jorge Basin, Argentina. The stratigraphic framework of the 450–650 m thick alluvial succession was carried out through description and interpretation using 18 detailed sedimentary logs along a 4.5 km wide exposure, where 314 fluvial channels were analysed. The Bajo Barreal Formation is a high-accommodation fluvial succession, dominated by fine-grained floodplain deposits, with isolated-to-vertically stacked, sheet-like, low-sinuosity or braided fluvial channels of limited lateral mobility clustered in several coeval channel belts, in which fluvial channels were relocated by avulsion. The Lower Member consists of reworked ash-fall materials preserved in floodplain areas (78%), floodplain sandstones (4%) and fluvial channels (18%). The mean thickness of sandbodies is 2.96 m (n = 118) and mean true width is 112 m (mean W:T = 43), with thicker and wider sandbodies in upper levels of the Member. The Upper Member consists of grey siltstones and mudstones preserved in extensive lowland areas (78%) and fluvial channels (19%), with scarce preservation of sandstones in the proximal floodplain (3%). Fluvial channels are narrow low-sinuosity sheets with comparable thickness (mean = 3.21 m, n = 196), but greater width than those of the Lower Member (mean width = 147 m, mean W:T = 50). Paleoflow data from 298 fluvial sandbodies indicates a paleoflow direction toward the SE (Az. 112°). Rivers flowed parallel to inherited early Cretaceous normal faults and are oriented to high-angle (>70°) to the current axis of the Codo del Senguerr anticline, suggesting that the uplift of the anticline occurred after the deposition of the formation. The variation in geometry of fluvial channels in both Members of the Bajo Barreal Formation could help in planning and developing primary and/or enhanced oil recovery projects in nearby oilfields, and provide data necessary for modelling the subsurface connectivity of hydrocarbon reservoirs.

Formation Conditions and Sedimentary Characteristics of a Triassic Shallow Water Braided Delta in the Yanchang Formation, Southwest Ordos Basin, China.

A large, shallow braided river delta sedimentary system developed in the Yanchang Formation during the Triassic in the southwest of the Ordos basin. In this braided delta system, abundant oil and gas resources have been observed, and the area is a hotspot for oil and gas resource exploration. Through extensive field work on outcrops and cores and analyses of geophysical data, it was determined that developments in the Late Triassic produced favorable geological conditions for the development of shallow water braided river deltas. Such conditions included a large basin, flat terrain, and wide and shallow water areas; wet and dry cyclical climate changes; ancient water turbulence; dramatic depth cycle changes; ancient uplift development; strong weathering of parent rock; and abundant supply. The shallow water braided river delta showed grain sediment granularity, plastic debris, and sediment with mature composition and structure that reflected the strong hydrodynamic environment of large tabular cross-bedding, wedge cross-bedding, and multiple positive rhythms superimposed to form a thick sand body layer. The branch river bifurcation developed underwater, and the thickness of the sand body increased further, indicating that the slope was slow and located in shallow water. The seismic responses of the braided river delta reflected strong shallow water performance, indicated by a progradation seismic reflection phase axis that was relatively flat; in addition, the seismic reflection amplitude was strong and continuous with a low angle and extended over considerable distances (up to 50 km). The sedimentary center was close to the provenance, the width of the river was large, and a shallow sedimentary structure and a sedimentary rhythm were developed. The development of the delta was primarily controlled by tectonic activity and changes in the lake level; as a result, the river delta sedimentary system eventually presented a “small plain, big front” character.