Distributary Channel Sand Bodies Research Articles

Study on geological reunderstanding and adjustment potential of development zone

Development Zone A is a heterogeneous oil field with continental river-delta deposits. The channel sand body is small in scale and is dominated by straight distributary channel sand body. This type of sand body development scale is small, the drilling rate is low, generally only 17.71%, the plane shows a strong heterogeneity, the well pattern control degree is low, the sand body prediction accuracy under the current well pattern condition is low, generally only about 70%. Due to the small scale of sand body development and the development of faults, the contradiction of imperfect injection-production relationship of single sand body is prominent. In 2016, the expansion Wells of Development Zone A were put into operation successively. Based on the re-understanding of the geology of the new drilling area, the adjustment potential of development Zone A was studied, the injection-production relationship of single sand body was clarified, and the adjustment potential was implemented in combination with the results of multidisciplinary reservoir research, so as to achieve the purpose of improving the water drive control degree of the block and improving the injectionproduction relationship of sand body. After careful analysis, it was preliminatively determined that 19 Wells adjusted for injection and production system were reinjected (6 Wells were reinjected after filling holes). Internal supplementary Wells 19; One oil well was regenerated into a water well, and one water injection well was restored. A total of 41 Wells.

Sedimentary microfacies characteristics within the high-resolution sequence framework of the Fuyu oil layer in the New North region

The Xinbei Oilfield has gone through decades of exploration and development, and lithological oil and gas reservoirs with stronger concealment are currently the focus of research. It is urgent to implement the sedimentary microfacies and sand body distribution characteristics of the Fuyu oil layer in the Xinbei area, providing a new theoretical basis for predicting the "sweet spot" of facies control in the area. Using core, logging, and logging data from the study area, a high-resolution sequence stratigraphic framework of the Fuyu oil layer was established. The distribution characteristics of sedimentary microfacies within the sequence framework were analyzed. The study showed that the Fuyu oil layer in the study area was divided into one long-term base level cycle of rising half cycle, four medium-term base level cycles, and 13 short-term base level cycles; In the area, river controlled shallow water delta plains and front sediments are developed, and a total of 10 types of sedimentary microfacies have been identified. Among them, the (underwater) distributary channel sand body, as the dominant reservoir sand body, extends along the southwest to northeast direction. As the A/S value increases, four stacking styles are developed vertically, including vertical shear stacking, staggered shear stacking, horizontal shear stacking, and isolated distribution. In the horizontal, the locally developed wide channel sand body composite zone gradually transitions into a single channel.

Washed core and tracer tests data guide the characterization of single sand body in Low Permeable Reservoir

A Oilfield is Fluvial Facies Low Permeable Reservoir, main layer is Low bend distributary channel sand body deposition, In order to better understand the distribution of single sand body and remaining oil in the reservoir, inspection well coring and tracer tests have been carried out successively in the same well area of the oilfield, based on these data, it is found that, the actual distribution of single sand body is very different from the original description in both vertical and plane. Discovered by the inspection well core data, the scale of the vertical single-phase body was too large. It was originally considered to be the mudstone interlayer inside the 0.2m single sand body, which is actually the interlayer between different single sand bodies, and this situation was confirmed by the trace data five years later. The distribution position and scale of single sand body on the plane are also different from the original description. It was thought that the oil and water well sand body with good internal connection in the same single sand body is actually not connected. Therefore, the experience of these data on vertical and horizontal single sand body identification is summarized to guide the subsequent single sand body characterization of the oilfield, so as to better guide the oilfield to describe the remaining oil and adjust and tap the potential.

Densification mechanism of deep low-permeability sandstone reservoir in deltaic depositional setting and its implications for resource development: A case study of the Paleogene reservoirs in Gaoshangpu area of Nanpu sag, China

A better understanding of reservoir densification mechanisms is very important for petroleum exploration and controlling the reservoir quality distribution in low-permeability reservoirs. Low porosity and low permeability, difficult reconstruction, and economic infeasibility are important factors restricting its efficient production. Systematic analysis was performed to study the evolution of different microfacies in the deltaic depositional setting of the Paleogene deep low-permeability Gaoshangpu reservoir, define their genetic mechanisms, delineate the dominant facies belts, and define the favorable diagenetic sequences that are important for exploration of high-quality reservoirs. To understand the tight genesis of the low-permeability reservoirs, we analyzed the sedimentological and diagenetic evolution characteristics of various microfacies (i.e., underwater distributary channel, distributary bay, mouth bar, and front sheet sand) using core data and physical property analysis of the reservoir sand body. The results show the underwater distributary channel and estuary bar sand body with medium-to fine-grained and poor–medium sorting. The diagenesis is dominated by strong compaction, calcareous, argillaceous cementation, and dissolution during stage B in early diagenesis and stage A in middle diagenesis. In the fan delta environment, the weak anti-compaction resistance of low-permeability reservoirs is mainly due to the large content of plastic particles, finer grain size, and medium–poor sorting, with an average porosity reduction rate of 65%. This is a key factor for densification of reservoirs above 3000 m. Comparison among different sandstone microfacies of the deltaic setting shows that the sand body of the underwater distributary channel with low shale content has slightly stronger compaction resistance. The porosity reduction is not obvious at the depth of 3,000–4,000 m, but the loss of permeability at this depth section is significant, and the reservoir improvement from later dissolution is most obvious at this depth section. Calcareous cementation is the cause for densification of some mouth bars in the early stage and of underwater distributary channels in the middle and late stage. Under the influence of strong compaction and calcareous-filling pore throat, the sand body of the mouth bar has been basically densified at 3,000 m, resulting in limited reservoir transformation from later dissolution. The study shows that compaction is the main cause of reservoir densification, argillaceous and calcareous cementation is the secondary cause, and later dissolution is another main cause of reservoir enhancement. The research results can provide a reference and direction for reservoir development and search for the high-quality sweet spot in the deep and low-permeability reservoir.

Analysis of the main controlling factors and the distribution rules of effective reservoirs in sandy braided river delta plain — A case study of Ct3 area, Ordos Basin, China

To clarify the main controlling factors of effective reservoirs in the sandy braided river delta plain, the eighth member of Xiashihezi Formation in the Ct3 area, Ordos Basin, is taken as an example. We describe and analyze approximately 600 cores in detail and correspond the cores on the logging curve. We summarize the logging facies mode suitable for the Ct3 area, and finally investigate the types and spatial distribution characteristics of sedimentary microfacies. The P2h8 member of Ct3 area in Ordos Basin is braided river delta plain subfacies. Sedimentary microfacies are divided into three types: distributary channel, channel bar, and distributary interchannel. The sand body in the h84 of the P2h8 member is the thickest, and the number of channel bar is the largest. All gas reservoirs (effective reservoirs) are developed on the sand body of the channel bar, which is the most favorable microfacies type. Poor gas reservoirs are mostly developed in distributary river channel sand body, whereas gas-bearing reservoirs (gas reservoirs and poor gas reservoirs) are undeveloped in distributary interchannel. According to the hydrodynamic conditions and sedimentary mechanism, the channel bar is divided into the bar head, the bar tail, the bar main body, and the bar wing. It is found that 63% of the gas reservoirs in the channel bar are developed in the bar main body, followed by the bar tail. The number of gas reservoirs developed at the bar head and the bar wing is the least. The results found that the most effective reservoirs are developed in the bar main body. Sedimentary microfacies, sand-body thickness, and grain size are the main controlling factors for effective reservoirs. The research results can guide the gas field exploration and later well location deployment in the sedimentary environment similar to the Ct3 area.

Seepage Characterization Based on Sandstone Particle Motion Pattern: A Case Study of the TIII Reservoir in the Sangtamu Oilfield

In order to finely characterize the physical properties and seepage characteristics of underwater distributary channel sand bodies in a braided river delta, the particle size distribution, mercury injection, and withdrawal of low, medium, high, and extra-high permeability core samples from the TIII reservoir from 15 core wells in the Sangtamu Oilfield were compared. The pore-throat characteristics of the rocks with different deposition modes and their effects on the fluid seepage were analyzed from the perspective of the deposition processes. The statistical results revealed that the rocks formed in a continuous and stable strong hydrodynamic environment were coarse grained. The seepage capacities of the rock increased as the maximum throat radius connecting with the pores increased. The mercury withdrawal efficiency decreased as the difference between the throat radius and pore radius increased. The percentage of the throat volume without effective seepage to the total volume of the pore-throat system increased as the permeability of the samples increased. Therefore, when characterizing reservoirs based on the porosity and permeability, it is important to differentiate the energy of the hydrodynamic depositional environment of a monogenic sand body in order to improve the accuracy of the understanding of the reservoirs.

Factors controlling reservoir quality of a retreating delta-front in shallow-water lacustrine in the Songliao Basin, Northeast China

Shallow-water lacustrine delta sand bodies with large areas of high-quality reservoirs are widespread in the first member of Yaojia Formation (k2y1) of the Upper Cretaceous in Qijia–Gulong area, central depression of the Songliao Basin. The analysis of elements affecting reservoir quality has always been a focus and difficulty of oil and gas exploration and development. Seismic profile interpretations, well log analysis, lithological associations, core observations, and heavy mineral assemblage determinations are used to interpret sequence stratigraphic framework and sedimentary evolution of K2y1. The latter is interpreted as a transgressive systems tract comprising three fourth-order sequences (SQ1–SQ3). Based on core observations and wireline logs, K2y1 primarily contained a shallow lake, distal bars, sand sheets, interdistributary bay, and subaqueous distributary channel deposits. The sand ratio and scale of the channels in the formations from SQ1 to SQ3 decreased which was reflecting the continuously retrograding sedimentary evolution of deltas to the lakeshore, as observed based on mutual calibration of single well sedimentary facies and root mean square amplitude attributes. Based on petrographic analysis of thin sections, particle sizes, scanning electron microscope (SEM) as well as X-ray diffraction, the effects of diagenesis and sedimentary facies on reservoir quality are investigated. In the process of delta-front retrogradation, the changes in reservoir quality in different sequences are primarily a result of the following factors: (1) the fine sandstone has better reservoir quality and more primary pores than the siltstone, which has higher argillaceous content. Additionally, in the fine sandstone, more primary pores can be preserved in the compaction process with higher reservoir quality. (2) Less authigenic illites have little influence on reservoir quality. When the authigenic illite content is >6%, the flocculent characteristics of authigenic illite crystals easily separate the intergranular pores into tinier micropores, thereby reducing the reservoir physical properties. (3) More than 3% of Pompon-like authigenic chlorites may occupy the pore throats of sandstone reservoirs. (4) The feldspars and rock fragments in the sandstone reservoir adjacent to the source rock easily form numerous secondary dissolution pores and improve the reservoir quality. The present study reveals that large-areas of subaqueous distributary channel sand bodies with stable lateral distribution, high reservoir quality, and significant petrophysical characteristics are the best-qualities of reservoir within the early stage of retreating delta-front system.

Lower limits and controlling factors of the petrophysical properties of coal-measure sandstone oil reservoirs: A case study of the Jurassic Yan’an Formation in the Huanjiang area, Ordos Basin, China

Coal-measure sandstone oil reservoirs of the Jurassic Yan’an Formation in the Ordos Basin, China, have strong heterogeneity. Reservoir quality is the core factor that determines the large-scale accumulation of hydrocarbons. Taking the Yan’an Formation in the Huanjiang area, Ordos Basin, as an example, we have calculated the lower limits of petrophysical properties based on empirical statistics, porosity and permeability intersections, mercury injections and irreducible water analysis, oil tests, oil-bearing property analysis, and distribution functions. In addition, we have investigated the main controlling factors of effective reservoirs in the Yan’an Formation. The results demonstrate that the lower limits of petrophysical properties of the effective reservoir in the Yan’an Formation display a gradually decreasing trend as the depth increases. Furthermore, we have obtained the functions of the lower limits of the petrophysical properties and the depth. Sedimentation and diagenesis have a significant control effect on the quality of reservoirs in the Yan’an Formation. High-quality reservoirs have mainly developed in the sedimentary microfacies of point sand bar and distributary channel sand bodies. The strong compaction and cementation in the diagenesis process are the internal reasons for the poor petrophysical properties of the Yan’an Formation reservoirs. Overall, the effect of compaction on the petrophysical properties of the reservoir is stronger than that of cementation, whereas the secondary dissolution pores significantly improve the petrophysical properties of the reservoir.

Controls of sandstone architecture on hydrocarbon accumulation in a shallow-water delta from the Jurassic Shaximiao Formation of the western Sichuan Basin in China

The thin but widely distributed sandstones in the shallow-water deltas have the potential to hold large deposits of natural gas. The fluvial sand bodies formed from river channels with different sedimentary facies and sandstone architectures have remarkable heterogeneities. The results of this investigation suggest that the sandstone architectures of river channels have a significant impact on the hydrocarbon accumulation and production of sandstone reservoirs. Information regarding the sandstone origin and architecture of shallow-water deltas, especially of the fingered bar type, is still very limited. This investigation focused on the channel sandstones in the shallow-water delta of the Jurassic Shaximiao Formation on the eastern slope of the western Sichuan Basin in China. Based on data from cores and logs of sandstones, seven types of lithofacies and four sedimentary facies were identified. Sand-rich sedimentary facies, including distributary channels, mouth bars, and overbanks, have merged to form four types of compound sandbodies corresponding to the fourth-order architectural unit in the architectural classification. Four types of compound sand bodies were classified, including CS1 (variable distributary channel sand body), CS2 (diverting channel and residual overbank sand body), CS3 (diverting channel and residual mouth bar sand body), and CS4 (terminal channel and residual mouth bar sand body). Relatively good physical properties occur in the sandstone architectures of types CS3 and CS4, with an average porosity of 11.06% and an average permeability of 0.97 mD. Sandstone architectures from the Jurassic Shaximiao Formation of the study area were controlled by water depth and climate, tectonic subsidence, distance from provenance, erosion effect, and channel sinuosity and discharge. With strong lateral erosion, a shallow-water delta would have more sandstone architectures of types CS3 and CS4 in the high sinuosity of channels far from the provenance. Furthermore, sandstone architectures of types CS3 and CS4 are prone to form in connected channel sandbodies with relatively high contents of natural gas of approximately 4.48 × 104 m3/d. The results of this investigation are beneficial to improve the understanding of the characteristics of sandstone distributions in shallow-water deltas, especially in fingered bar types, and to guide oil and gas explorations in shallow-water delta systems.

Late Triassic–Early Jurassic paleogeomorphic characteristics and hydrocarbon potential of the Ordos Basin, China, a case of study of the Jiyuan area

Abstract Paleogeomorphology is the surface morphology formed by the deposition of strata in the basin in historical period, which is greatly affected by the regional tectonic movement; however, the effect of basin paleogeomorphology on reservoir has been underexplored. This study documents the basin paleogeomorphic terrain features and their implications for the depositional processes and oil migration and accumulation in the Jiyuan area of the Ordos Basin. Stratigraphic classification is achieved through well logging data analysis. A seismic fine interpretation of the Triassic–Jurassic paleogeomorphology is conducted. Typical drilling core observations were utilized in a well section analysis of the strata at the top of the Yanchang formation sedimentary facies characteristics. Studies have shown that the Chang 1 subsection on top of the Triassic Yanchang formation in the Jiyuan area was eroded by an ancient Jurassic river, and the residual strata are primarily distributed in the Dingbian and Jiyuan plateaus. The Chang 1 reservoir in the Jiyuan area is dominated by a delta distributary channel sand body. The reservoir is concentrated in a high reservoir sand body structure position, and is controlled by a combination of structural and rock characteristics. This study suggests that the Indosinian movement in the late Triassic caused the entire basin to uplift and denudate, and the strata at the top of the Yanchang formation suffered from weathering and river erosion. The residual strata in the upper part of the Yanchang formation are controlled by the late Triassic–early Jurassic paleogeomorphology.

Sequence Stratigraphy of the Lower Cretaceous Uraniferous Measures and Mineralization of the Sandstone‐hosted Tamusu Large Uranium Deposit, North China

AbstractThe Bayingobi basin is the Mesozoic–Cenozoic basin in North China in which the Tamusu uranium deposit is located. The ore‐target layer of the deposit is the Lower Cretaceous Bayingobi Formation, which developed as a fan delta‐shallow lacustrine deposit. The distributary channel sand body of the fan delta plain and the underwater distributary channel sand body of the fan delta front formed a favorable uranium reservoir, so the study of sequence stratigraphy is extremely important to understanding the genesis of uranium deposits. On the basis of field investigation and a large number of borehole logs, the high resolution sequence stratigraphy of the Lower Cretaceous is divided and the system tracts of different periods are established. The relationship between deposition, interlayer oxidation and uranium enrichment is discussed. The Lower Cretaceous Bayingobi Formation can be divided into two fourth‐order sequences (Sq1 and Sq2). The lower member of the Bayingobi Formation is referred to as Sq1, which is composed of a falling‐stage system tract (FSST) on top. On the other hand, the upper member of the Bayingobi Formation is referred to as Sq2, which is composed of a lowstand system tract (LST), transgressive system tract (TST) and highstand system tract (HST). The lowstand system tract forms a favorable stratigraphic structure (mud‐sand‐mud formation) with the lacustrine mudstone of the overlying transgressive system tract, that is conducive for the migration of uranium‐bearing oxygen water. The organic matter and pyrite in the fan delta sand body, as well as the dark mudstone in the distributary bay, provided a reducing medium for uranium mineralization. The ore body mainly occurs in the distributary channel, underwater distributary channel or the mouth bar of the fan delta. As a result of the moderate thickness, high permeability, favorable barrier and rich reducing medium, the rich ore body mainly occurs in the underwater distributary channel and mouth bar sand body of the delta front. Based on study of the sequence stratigraphy, the model of the sequence, sedimentation and mineralization of the uranium deposit is established, which enriches uranium metallogenic theory and provides a reference for exploration of the same type of uranium deposits.

High frequency sequence stratigraphic framework and sedimentary characteristics of P oil layer in the southern part of A area

Based on the high resolution sequence stratigraphy theory, 42 wells and 2745 well logging curves are used to study the evolution of all levels of base level cycles and the development pattern of high-resolution sequence in the P reservoir in the southern part of Songfangtun area. The results show that the 4 sequence interface of the grape flower beds in the Songfangtun area can be divided into 2 medium-term, 5 short term and 12 ultra short term sequences. On this basis, the sedimentary microfacies type and distribution evolution law have been carried out. It is considered that the shallow water delta depositional system is developed in P reservoir in the Songfangtun area. The main development of delta front subfacies is to identify 8 kinds of microfacies such as subwater distributary channel, inter channel, inter channel, slime, overflow sand, mouth fan, estuarine dam, far sand bar and mat sand. The underwater distributary channel sandbodies are widely developed at the front of the delta. Due to frequent bifurcations and diversions, the underwater distributary channel sandbodies cut and overlap each other, forming branches and striped sand bodies. The sandbody reservoir has good physical property and is the main reservoir sand body with oil and gas enrichment. The detailed study of sedimentary microfacies provides a reference for further exploration and development of lithologic reservoirs in this area.

Study on Sedimentary Characteristics of S0 Reservoir in BBK Area

The S0 reservoir in the BBK area is a reevaluation of unused development resources. According to the change trend of oilfield production tasks, the development test of the S0 reservoir is imperative, but the sedimentary characteristics of the reservoir have not yet been perfected; the author uses core, logging curve and various analysis and test data to describe the sedimentary facies of the S0 reservoir in detail, through the method of the combination of quantitative and qualitative sand body comprehensive identification. First, refer to the effective thickness, permeability, frequency of the interlayer development and other parameters of the sand body in the work area to quantitatively determine the microfacies. Then, combined with the electrical survey chart of typical microfacies, the microfacies types of sand bodies of single-well single-layer are qualitatively determined. Finally, the comprehensive judgment results are calibrated in the facies zone base map, which is used for the analysis of the overall distribution law of each microfacies and the precise positioning of the sand body boundary; it is concluded that the underwater distributary channel sand body of the S0 oil layer is not developed, and the outer front sheet sand is developed in a large area through the analysis of the judgment results of sedimentary microfacies. Generally speaking, it belongs to the subfacies deposits in the outer front of the delta, which provides a favorable geological basis for later development adjustment and effect evaluation.

Impact of deep‐time palaeoclimate on the sedimentary records and morphology of lacustrine shoal‐water deltas, Upper Eocene Dongying Depression, Bohai Bay Basin, China

AbstractShoal‐water deltaic systems are predominant during the deposition of the second member of the Eocene Shahejie Formation (Es2) in the Dongying Depression, Bohai Bay Basin, China, and their presence is ascribed to low slopes, shallow water bodies and extremely slow tectonic subsidence. Integrated analyses of sedimentological, seismic geomorphological, palaeohydraulic, palynological and element‐geochemical data reveal the influence of palaeoclimate on the morphology and depositional processes of distributary networks in shoal‐water deltaic strata. Two contrasting palaeoclimatic stages were identified based on palynological and geochemical evidence, namely the earlier warm and humid climatic stage, and the later arid climatic stage. Compared with the earlier humid stage, the arid stage features much more abundant xerophytic species (Labiatae and Ephedra) as well as much higher Mg/Ca, Sr/Ba, and Th/U ratios and B content, indicating extensive evaporation. A third‐order sequence stratigraphic unit was recognized, consisting of a transgressive systems tract with landward stepping parasequence sets, a poorly developed highstand systems tract, and a falling stage systems tract with basinward stepping parasequence sets, which were products of the humid and arid climatic stages, respectively. A series of distributary channel complexes exhibiting reticular patterns was identified in the strata corresponding to the humid climatic stage, with amalgamated, multi‐storey distributary channel sandbodies showing a sinuosity range of 1.1 to 1.3. In contrast, the strata corresponding to the arid climatic stage are dominantly composed of distributary channel systems exhibiting dendritic forms, with prevalent single‐storey channel fills and a low sinuosity range of 1.0 to 1.2. The dendritic system is more stable than the reticular counterpart due to the effects of frequent incision and limited vertical aggradation. Typical modern shoal‐water deltas formed in different climatic conditions (i.e. Poyang Lake delta and Neales River delta) serve as analogues to the proposed conceptual depositional models of distributary networks related to palaeoclimate.

Quantitative evaluation of morphological characteristics of thin sand body: Taking the deltaic distributary channel sand body as an example in Shengtuo oil field of Dongying sag, east of Bohai Bay Basin

Because of the small scale of distributary channel sand body and the superposition of sand bodies in vertical and lateral direction, drilling data with 150–200 m well spacing is difficult to describe its scale, shape and connectivity characteristics effectively. It has a great restriction on the recovery of these reservoirs. Taking Sheng2 Block of Shengtuo Oilfield as an example, the architectural elements of distributary channel are quantitatively investigated by using dense-drilling well and seismic data. Based on frequency division seismic data, the boundary of distributary channel belts are described vertically and laterally, and then the scale and morphological characteristics of channel bodies in the channel belt are characterized by using local dense well pattern data. The target layer can be divided into six channel belts longitudinally. After seismic frequency demultiplication, 60–75 Hz seismic data was used to describe the distribution of channel belts. From the bottom to the top, the width and distribution range of the channel belts decrease as a whole. Channel belt thickness and its width show linear positive correlation. On the channel-belt cross section, channel stories on the dense drilling-well logging profiles are identified with reference to the profile geometry of the distributary channel. On the channel-belt cross section, the projection distance between wells is fitted to a linear correlation with the thickness variation of channel stories. The ratio of the two parameters decreases from bottom to top of the target layer as a whole. The channel shape parameters predicted by the dense drilling-well logging section are of great significance to the study of paleocurrent deposition and paleohydrological evolution. At the same time, it has guiding significance for the development and adjustment of oil and gas reservoirs.

Sedimentary evolution and controlling factors of Early-Mid Miocene Deltaic systems in the Northern Pearl River Mouth Basin, South China Sea

The Pearl River Mouth Basin is located at the northern continental shelf of the South China Sea. Since the early Miocene, the Paleo-pearl river transported a large amount of sediments to the northwest of the basin and resulted in the formation of a large-scale river-delta depositional system, which has become an important oil and gas reservoir in the study area. In the current paper, we investigate the characteristics and evolution of fluvial-deltaic depositional systems and their controlling factors, including sea level change, tectonic subsidence and sediment supply on the basis of 3D seismic, well logging and core data. Early-Mid Miocene succession can be divided into two second-order sequences (CS1–CS2) and eight third-order sequences (S1–S8). Deltaic systems developed in S1–S2 are relatively coarse in grain size, and the delta plain deposits are dominated by thickly stacked (100–180 m) distributary channel sand bodies and interpreted as braided delta depositional system. In the early stage (S1–S2), the braided delta systems mainly distributed in the west of the Baiyun Sag, which were proceeded by a retreat to the south of the Enping Sag along with sharply rising sea level. Following the transgression of S2, the provenance of the Paleo-Pearl River extended to the coastal region of South China, and the papleoclimate changed from warm and humid to dry and cold in the early Miocene, leading to the development of transition of braided river delta to meandering river delta, which was characterized by relatively fine grain deposits. During the deposition of S3–S6, well sorted and rounded fine sandstones of deltaic front deposits accumulated in the study area. The retrogradation to accretion and subsequent progradation of these meander delta systems are attributed to the sea level change in the study area. During the deposition of S7–S8, the delta front retreated to the south of the Enping depression as a result of minor sea level rise, reduction in sediment input, and subsidence rate. This resulted in the development of a wave-controlled deltaic depositional system.

Distribution of shallow water delta sand bodies and the genesis of thick layer sand bodies of the Triassic Yanchang Formation, Longdong Area, Ordos Basin

Abstract Based on the cores from 47 wells and logging data of 130 wells, the main types of sand bodies in the shallow water delta of the Chang 82 sub-member in the Triassic Yanchang Formation, Longdong Area, Ordos Basin were comprehensively analyzed, the distribution characteristics of sand bodies more than 20 m thick were identified, and the genetic types of the thick sand bodies were sorted out. In this region, thick-bed sand bodies can be divided into two types according to the shape and position: type 1 is the sand body in wide strip and is distributed between the average high water mark and the average low water mark; type 2 is distributed on both sides of the average low water mark and is in irregular lump shape. Based on the principle of the volume distribution of sediments and the change rule of accommodation space, the genetic models of two types of thick-bed sand bodies in the Chang 82 sub-member are superimposed distributary channel sand bodies in high accommodation space and superimposed composite sand bodies in low accommodation space.

Discriminating stacked distributary channel from palaeovalley fill sand bodies in foreland basin settings

Stacked fluvial distributary channel deposits and palaeovalley fills can form major, multi storey sand bodies with similar thicknesses, and with lateral extents often greater than a single exposure. Consequently, they can be difficult to tell apart from one another using outcrop data. This study addresses this problem by quantitatively analysing the architecture of five stacked fluvial distributary channel deposits and two palaeovalley fills from the Pennsylvanian Pikeville and Hyden formations of the central Appalachian Basin, USA. The a priori interpretation of the sand bodies as stacked distributary channels and palaeovalley fills is possible because a robust in-place coal seam correlation framework allows for the recognition of different basin-scale architectures for each type – aspect ratios <1000 and envelopes of fluvial and deltaic strata for stacked distributary channels, and aspect ratios >1000 and a regional basinward facies shift at the bases of palaeovalley fills. Sand body thickness, storey thickness, position and length of storey contacts within the sand body are similar in both types. However, they can be distinguished by different up-system to down-system changes in their respective architectures. Stacked distributary channel sand bodies thin down system, display a decrease in storey thickness, an increase in the mean position of storey heights in the sand body and a decrease in the length of storey contacts. These trends are the result of down-system decrease in channel size, and confinement associated with radially distributive fluvial systems. Palaeovalley fill sand bodies thicken down-system, display an increase in storey thickness, a decrease in the mean position of storey heights, and a decrease in the length of storey contacts. The increase in sand body and storey thickness are the result of down-system increases in original channel size, consistent with trunk axial fluvial systems fed by tributaries that predominate during valley-formation. The down-system increase in amalgamation reflects a down-system decrease in accommodation, from the higher subsidence rate active margin of the basin, and is therefore not necessarily characteristic of palaeovalley fill architectures in all basin settings. This study emphasises the requirement for detailed correlation work and quantitative analysis of external and internal architectures before the interpretation of sand bodies as stacked distributary channels or palaeovalley fills.

Characteristics and sedimentary model of a reticular shallow-water delta with distributary channels: lower member of the Neogene Minghuazhen Formation in the Bozhong area of the Huanghekou Sag, China

The Bohai Bay Basin entered a late stage of structural depression during the sedimentary depositional period of the lower member of the Minghuazhen Formation in the Bozhong area of the Huanghekou Sag, China. Numerous analyses of geological and geophysical data have shown that the climate was relatively humid in the Bozhong area. Additionally, the basin basement subsided at a stable rate, creating a broad basin with gentle slopes. The research area in this study was close to the sediment source, which supplied it with a sufficient amount of sediment. The lake shoreline changed relatively frequently, and the lake was deep, which allowed shallow-water deltas to develop widely. The reticular shallow-water delta in the research area has the following sedimentary characteristics: (1) the fine-grained lithology, low component and textural maturities, and the cumulative probability particle size curve suggest tractive flow deposition. (2) Deltas with low sandstone contents and widely distributed red mudstone are widespread. (3) Chaetosphaeridium and biological disturbances are common. (4) The lake level fluctuates frequently. (5) Delta-front subfacies are mainly developed in the Bozhong area; they show a reticular planar distribution, comprise composite sandbodies, and are superposed by multi-stage underwater distributary channels. (6) The river channel is narrow and developed a large number of main river channels in different directions. (7) The underwater distributary channel microfacies are distributed within multibranched and bifurcated channels and generally show a discontinuous positive rhythm; however, the estuary dam sandbody is not well developed. This work suggests that a reticular shallow-water delta sedimentary system formed under humid climate conditions in the Bozhong area and is characterized by its near-source type, fine granularity, wide distribution, and vertically and horizontally overlapping of underwater distributary channel sandbodies. During the exploration process, this sedimentary system can provide a reasonable guide with which to predict the sandbody distribution in a lithologic reservoir.

Depositional characteristics and evolution of the shallow water deltaic channel sand bodies in Fuyu oil layer of central downwarp zone of Songliao Basin, NE China

Relatively few subsurface studies have been conducted on shallow water deltaic channel characteristics and evolution in Songliao Basin’s Fuyu oil layer. Using data from 2000-m cores and 60 wells, the shallow water deltaic channels sand bodies in the Fuyu oil layer are reconstructed. The main controlling factors over depositional system evolution are identified. Twelve lithofacies and four main facies associations are summarized. Four facies associations are identified, including (1) a simple distributary channel association, (2) type A point bar distributary channel association, (3) type B point bar distributary channel association, and (4) an underwater distributary channel association. The depositional evolution of the channels is derived from a lacustrine regressive–transgressive cycle that is driven by climate variability and tectonics, and linked to changes in relative lake-level. During periods of rapid drops in relative lake-level or rises in the semi-arid lower shallow water delta plain, channel, sand bodies are type B point bar distributary channel of laterally aggregational deposits. When relative lake level reaches to its lowest value, it results in a large-scale erosive bounding surface juxtaposed with type A point bar distributary channel sand bodies that are infilled with thick bedding in the upper shallow water delta plain. When high relative lake levels generate regional high accommodation/sediment supply ratios, the thin-bedding underwater distributary channel sand bodies are deposited in the humid shallow water deltaic front. Shallow water delta fills are capped by laterally extensive and continuous deep lacustrine dark mudstone and oil shale elements as the relative lake level rises to its highest value.