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This paper presents a new depositional model for the Amasiri Sandstone of the southern Benue Trough, Nigeria. Variants of facies and facies relationships suggesting fluvial, storm, and sediment gravity processes gave rise to controversies in the interpretation of environments of deposition for the formation. However, newly exposed sections in the quarries allowed further studies, to understand the intrinsic depositional facies. Field description and lithofacies analysis were used in understanding the facies relationships, depositional processes, and environments. Results of the sedimentary facies analysis revealed twelve facies grouped into foreshore facies association (FA 1), proximal delta front facies association (FA 2), distal delta front facies association (FA 3), and prodeltaic facies association (FA 4). FA 1 consists of cross-bedded sandstone, interpreted as subtidal sand bars in a littoral setting and occurred at the top of each deltaic cycle in the Amasiri Sandstone. FA 2 comprises asymptotic cross stratified sandstone, hummocky cross stratified sandstone, massive bedded sandstone, conglomeratic sandstone with limestone rip-up clasts, and parallel laminated sandstone. It was interpreted as deposits of proximal delta front (proximal mouth bar/ dune front and delta front lobes), deposited by bedload process, and suspension influenced by sustained hyperpycnal flows. FA 3 consists of wavy/ripple laminated sandstone, bioturbated sandstone, coquinoid limestone, bioturbated mudstone with rip-up clasts, shell hash, and plant debris, interpreted to be deposits of distal delta front transported as suspension load by sustained hyperpycnal flows. FA 4 comprises graded cycles and couplets of thinly interbedded very fine sandstones, mudstones, siltstones, and shales inferred as prodeltaic facies deposited by fallout and buoyancy reversal (lofting process). A new depositional model for the Amasiri Sandstone shows that FA 1 was deposited by the fluvial/tidal process. In contrast, FA 2 to FA 4 were deposited by hyperpycnal flows in flood-dominated deltaic environments.

Shallow-water delta sandstones in lacustrine basin are widely a concern due to their petroleum exploration potential. However, characteristics such as small thickness, multi-genesis, and multi-stage limit their successful exploration. Here, we present a comprehensive seismic sedimentological research on shallow-water delta in the first member of the Cretaceous Yaojia Formation of Changling area in southern Songliao Basin. The target strata can be divided into 1 third-order sequence and three system tracts. Interpretations of typical stratal slices which were extracted from each system tract indicate that the distributary channel type shallow-water delta is mainly developed in Changling area, two subfacies such as shallow-water delta plain and shallow-water delta front; five main sand bodies such as distributary channel, abandon distributary channel, subaqueous distributary channel, mouth bar, and distal bar are further identified. The type and shape of the sand body are closely related to the degree of sheet modification. Meandering distributary channel, multilevel branching dendritic channel, and lobate subaqueous distributary channel develop at the delta plain, the proximal and distal of the delta front respectively. The study shows that sedimentary characteristics and sedimentary model study based on seismic sedimentology theory provide a powerful theoretical support for lateral reservoir prediction of thin distributary channel type shallow-water lacustrine delta sandstones.

Shallow-water deltas are important reservoirs for hydrocarbon resources, and there are many oil and gas discoveries found in the sands in China. Many large shallow-water delta sedimentary systems were found in the Bohai Bay Basin in the Late Miocene. Several classic shallow-water deltas are developed in the early-stage sequence (SQmL2) of the lower part of Neogene Minghuazhen Formation in the Bohai Bay Basin which have attracted much attention in recent years. We choose the SQmL2 as an example to reveal the relationships between paleoenvironment and shallow-water delta facies and architecture. Analysis of 3D seismic data and logging data indicated that two types of sedimentary facies, including shallow-water delta and shallow lake facies, are developed during SQmL2. The shallow-water delta facies mainly consist of subaqueous distributary channel with mouth bar rarely developed. The deltas are mainly comprised of relatively thin (1–4 m) fine- to medium-grained sandstone. The combinations of distributary channels on planer view are mostly branch-shaped and lobe-shaped. The shallow-water deltas developed in SQmL2 are controlled by paleogeomorphology, sediment supply, and paleoclimate. The flat topography and sediment supply are favorable for sediments moving basinward for a long distance and forming channelized sand bodies. As the climate gets humid, the lake level rises and shallow lake area expands, resulting the sand bodies move landward and become much thinner. The lake level has changed from shallow to deep and to shallow, and correspondingly, sand body of subaqueous distributary channels has changed from lobe shape to branch shape and to lobe shape.

Detailed analysis of a typical wave-dominated delta beach bar sedimentary system of the Fenghe River, in Qingdao, China, concentrated on identifying the sedimentological characteristics of wave-dominated delta beach bar sedimentary system. The aim of the study was to establish a sedimentological facies model for similar deposits. Using well-established sedimentological criteria for recognizing the characteristic of wave-dominated delta beach bar. This modern sedimentary system mainly hosts front subfacies, as well as sedimentary microfacies like underwater snake-like channel, mouth bar, distal bar, and sheet sand. From the river entrance to the mouth bar, more than 80% of the area is occupied by sheet sand, which is distributed widely across a flat terrain. The sheet sand is dominated by wave-built sedimentary structures, plus a few current-formed sedimentary structures. The sheet sand can be divided into an underwater shoal parallel to the coastline and a stable crescent-shaped above-water bar. The underwater shoal is a long ellipse in the plan and an asymmetric shape that is gentle seawards and steep landwards in the vertical section. The sedimentary structures are dominated by wave-formed ripple marks and cross bedding, overlaid with swash bedding. The above-water bar is composed of two bars formed at different stages. The sedimentary structures are dominated by swash bedding, low-angle wedge bedding, and wind-formed sedimentary structure. Both underwater shoal and the above-water bar are resulted from the transformation and redistribution of the sediments on the delta front sheet sand by waves. The two parts reflect a sedimentary association of genetic significance in the sedimentary environment of wave-dominated delta front.

The Xu-2 Member of the Upper Triassic Xujiahe Formation (Hechuan area, southwestern China) is an important potential sedimentary sequence for gas exploration in the central Sichuan Basin. Thus, we performed a comprehensive study of drilling parameters, sedimentary cores, well logging, and core test data and combined our results with previous research and the geological background of the basin. We found that the Hechuan area was a delta front that included underwater distributary channels, interchannels, and mouth and distal bars during deposition of the Xu-2 Member. The sand body genetic types were divided into three categories based on where they developed: an underwater distributary channel, a mouth bar, or a distal bar. The lithology of the Xu-2 reservoirs is mainly feldspathic litharenite, lithic arkose, subarkose, litharenite, and sublitharenite. Residual intergranular pores and intergranular dissolution pores are the major pore types in the reservoirs. Reservoirs with porosities of 0.18–15.84% and permeabilities of 0.001–8.72 × 10−3 μm2 showed a correlation coefficient of 0.7592. The reservoir throats are mainly tubular and constricted. Overall, the sedimentary environment and diagenesis are the major controlling factors for reservoir formation in the study area. The reservoir zones with relatively high porosity and permeability mainly developed in a delta front with underwater distributary channels and mouth bars. Chlorite growth preserved the primary pores during early diagenesis stage B, and intergranular dissolution pores resulted from contact with organic acids derived from source rocks during middle diagenesis stage A1. Compaction and cementation significantly decreased porosity during middle diagenesis stage A2. These important factors influenced reservoir quality.

Considering that available data in Li32 block is limited, log phase method and core analysis were used to divide sedimentary facies and sequence, the former is principal, and two short-term cycles, five middle term cycles, and fifteen short-term cycles were divided. Combined seismic interpretation with fault point and fault rod, the instantaneous phase profile explained small fault more clearly and accurately. The sever faults were analyzed, and integrated structure was constructed. Lithofacies in cored wells were observed; the grain-size probability curves mainly are suspension and jumping transport; grain size is mainly for the siltstone and fine sandstone; the color of mud in the second segment of Shahejie formation (ES2) is mainly gray–green, gray to dark gray, these reflect that sediment environment is delta front, which included underwater distributary channel, mouth bar, distal bar, and so on. Well data and structural maps were integrated to build 3D structure model and sedimentary microfacies model of Li32 reservoirs using stochastic simulations with geometry data. The integrated 3D geological model is a valuable tool of reflecting geologic body with great reality and can accurately represent and describe the complex reservoir heterogeneity.

The depositional facies and environments were unraveling by studying 21 subsurface sections from ten oilfields in the central and southern Iraq and a large number of thin sections of the Nahr Umr (siliciclastic deposit) Formation (Albian). This formation is mainly composed of sandstone interlaminated with minor siltstone and shale, with occurrence of thin limestone beds. Nahr Umr Formation is subdivided into three lithostratigraphic units of variable thicknesses on the basis of lithological variations and log characters. Mineralogically and texturally, mature quartz arenite and sandstones are the common type of the Nahr Umr Formation. The sandstones are cemented by silica and calcite material and have had a complex digenetic history. Compaction, dissolution, and replacements are the main diagenetic processes. Prodelta, distal bar, distributary mouth bar, distributary channel, over bank, and tidal channel are the main depositional environments recognized for the Nahr Umr Formation, within the studied wells. This formation was deposited in shallow marine and fluvial–deltaic environments and exhibit progradational succession of facies. Eight sedimentary facies that have been identified in the Nahr Umr Formation include claystone lithofacies, claystone siltstone lithofacies, lenticular-bedded sandstone–mudstone lithofacies, wavy-bedded sandstone–mudstone lithofacies, flaser-bedded sandstone–mudstone lithofacies, parallel and cross lamination sandstone lithofacies, trough cross-bedded sandstone lithofacies, and planar cross-bedded sandstone lithofacies. The depositional model of the Nahr Umr Formation environment was built based on the lithofacies association concepts.

A siliciclastic-dominated succession (∼11 m thick) underlying Harrat Rahat, belonging to the Miocene–Pliocene Bathan Formation is recently exposed at Al-Rehaili area, North Jeddah, Saudi Arabia. It covers a wide spectrum of grain sizes varying from clay-rich mudstones to cobble grade conglomerate and consists of a variety of facies vary from fluvial to marginal and open lacustrine deposited in a half-graben basin formed along the eastern margin of the extensional Red Sea Basin. Field-based sedimentologic investigation enables to identify ten facies grouped into three facies associations (A–C). The depositional history is subdivided into two stages. The first stage represents deposition in gravel to sand-dominated fluvial system sourced from a southern source and grade northward into lacustrine delta and open lacustrine setting. The second stage on the other hand includes deposition of fluvial channels running in E–W direction with attached bank sand bar. Sequence stratigraphic interpretations of the lacustrine deposits enable to identify three unconformity-bounded sequences (SQ1–3). The basal sequence is incomplete, consisting of three aggradationally to progradationally stacked delta plain and delta front parasequences. The second sequence is sharply and erosively overlying a red paleosol bed that defines the upper boundary of the first sequence. It includes two system tracts; upward-fining and deepening lacustrine offshore mudstones of the transgressive system tracts unconformably overlain by red paleosol of the regressive systems tracts. The top of this sequence is delineated at the sharp transgressive surface of erosion at the base of delta mouth bar deposits of sequence 3. Changes in the accommodation and sedimentation rates by basin subsidence under the influence of tectonics and sediment compaction and loading as well as climatic oscillation between semi-arid to arid conditions were the major controls on the fluvio-lacustrine sedimentation and their facies distribution. Tectonic reorganization of the drainage system resulted in the formation of E–W flowing fluvial streams in the second stage.