Delta Front Channel Research Articles

Facies Analysis and Paleoenvironmental Reconstruction of Ghar Formation in Al-Muthanna Governorate, Southern Iraq

The Oligocene-Miocene succession in Al-Muthanna Governorate of southern Iraq shows significant mixed siliciclastic-carbonate formation. Fourteen samples are collected from two wells and four outcrops in Al-Muthanna area for studying the facies and paleoenvironments of the Ghar Formation. It is mainly composed of sandstone, calcareous sandstone, and carbonate rocks. Six facies associations are identified in the mixed siliciclastic-carbonate deposit: the carbonate type, including sabkha, lagoon, intertidal-subtidal, and lacustrine; the mixed type, including shoreface; and the siliciclastic type, including distributary channels. The major minerals in this formation are quartz, calcite, and dolomite, whereas feldspar and gypsum are the minor minerals. Clay minerals such as Palygorskite and Montmorillonite were also identified. The carbonate rocks of the Ghar Formation reflected deposition in the evaporitic platform (Sabkha), shallow restricted (lagoon), tidal and lacustrine environments. Ghar formation is considered transitional in conditions between a very restricted shallow marine and near-shore environment, whereas the clastic facies in Ghar Formation indicated deposition in the delta front channel.

Defining stratigraphic oil and gas plays by modifying structural plays: A case study from the Xihu Sag, east China Sea Shelf Basin

The study focuses on an area of relatively mature oil and gas exploration located in the western slope of the Xihu Sag, East China Sea Shelf Basin. Three stratigraphic oil and gas plays are introduced to replenish the shrinking prospect inventory in the area. The reservoirs of these plays are traditional pay intervals of the Eocene Pinghu Formation and Baoshi Formation composed of tide-dominated delta front, tidal flat and delta front sand bar facies. By investigating conventional seismic data, seismic inversion volumes, and well log data, multiple tidal-influenced delta front channels, beach sand packages have been mapped out in different reservoir sand intervals of the Pinghu Formation and Baoshi Formation. We recognize four types of stratigraphic terminations: channel flank pinch-outs, axial pinch-outs of sand-fills, channel mouth sand bar pinch-outs, and onlap pinch-outs for the Baoshi Formation. Both the depositional facies and relatively low sand-shale ratio of the reservoir formations favor the occurrence of stratigraphic terminations. The closure mechanism of all these plays are similar to the traditional structure plays in the area: fault blocks or fault-bounded rollover anticlines, except that one or more closing faults are replaced by these stratigraphic terminations. Several small-sized closures should exist but the relatively large ones are centered around a major intra-slope rise, raising the exploration potential in this maturely explored area. The new exploration drilling campaign has so far tested the play of delta front channel facies, with three out of four wells commercially successful. These play concepts open up a novel approach to identify new traps in the study area and many other rift basins with similar settings in eastern China.

Morphological signature of gully development by rapid slide retrogression in a layered coarse-grained delta foreslope

Abstract Coarse-grained deltas are often characterized by steep foreslopes (often more than 10°) that are traversed by delta-front channels. The channels thus erode into relatively steeply inclined bedding. In this context, the slopes flanking the channels can be steeper than the friction angle since they include a component of dip related to the delta-front slope as well as the channel-related erosion slope. In this study, part of the Busu River delta (Papua New Guinea) was imaged using a high-resolution multibeam bathymetry survey over an area where the angle of the slopes flanking the channels locally reaches 50°. A detailed analysis of the delta slope morphology has revealed an additional source of instability due to erosion within the main channels. In some places, erosion cuts into the channel flank forming a local knickpoint inclined in a direction approaching that of the bed dip. The cut can then initiate breaching or static liquefaction failure from that point up to the crest of the interfluve resulting in a V-shaped gully.

First wide-angle view of channelized turbidity currents links migrating cyclic steps to flow characteristics.

Field observations of turbidity currents remain scarce, and thus there is continued debate about their internal structure and how they modify underlying bedforms. Here, I present the results of a new imaging method that examines multiple surge-like turbidity currents within a delta front channel, as they pass over crescent-shaped bedforms. Seven discrete flows over a 2-h period vary in speed from 0.5 to 3.0 ms−1. Only flows that exhibit a distinct acoustically attenuating layer at the base, appear to cause bedform migration. That layer thickens abruptly downstream of the bottom of the lee slope of the bedform, and the upper surface of the layer fluctuates rapidly at that point. The basal layer is inferred to reflect a strong near-bed gradient in density and the thickening is interpreted as a hydraulic jump. These results represent field-scale flow observations in support of a cyclic step origin of crescent-shaped bedforms.

Linking shelf delta to deep-marine deposition in reservoir dispersal of the upper Oligocene strata in the Baiyun Sag, the northern South China Sea

Linking shelf delta to deep-marine sediment in the Baiyun Sag (BYS) along the northern South China Sea has led to a great deal of interest, but it remains poorly known. In this paper, we focus on the detailed stratigraphic evaluation and reservoir dispersal of upper Oligocene strata (25.5 to 23.8 Ma, referred to as ZHSQ6), using three-dimensional (3-D) seismic volumes (approximately 2500 km2) calibrated with well-log and core data, to determine internal geometry and sedimentary evolution. Rock velocity estimates from well data show that sedimentary environments may be the most direct controlling factor of wave impedance, reflected polarity and the value of amplitude, which in turn predict the nature of the sedimentary system. Likewise, the analysis of sedimentary sequences reveals that lowstand systems tract and transgressive systems tract consist of inner-shelf deltas. Highstand systems tract, by contrast, contained mid-shelf deltas. A northeast-oriented advantaged longshore current influenced asymmetric sandy ridges with a mixed carbonate–siliciclastic system in the north controlled by paleo-uplift geomorphology. Muddy mass transport deposits (MTDs) and sheet sandstones are both widespread. During falling-stage systems tract, shelf-margin deltas provided coarse grain sediments directly onto the basin floor. Consequently, the volumes of dispersed thick deep-water stacked channels and sand-rich MTDs are significant. The existence of the eastern source clearly increased. Finally, the sediment model for the upper Oligocene Baiyun Sag has been established as an optimal component of source-to-sink systems. This study indicates that sedimentary evolution is controlled by source supply flux and duration, base-level behaviour, topographic gradient and growth faults. Delta front channels and longshore sandy ridges, together with the gully fill, represent the most favourable reservoirs for further hydrocarbon exploration.

Characteristics and Origin of Calcareous Interbeds of Chang 6 Member in Jiyuan Oilfield

Calcite cementation is one main cause for reduction of porosity and permeability. In the case of Chang 6 Member of Jiyuan Oilfield, calcareous interbeds have been investigated for the characteristics and genetic mechanisms within delta-front Chang 6 oil reservoirs so as to reveal their spatial distribution. As shown by research, carbonate content in this area more than 10% will significantly reduce reservoir porosity and permeability, as well as its oil-bearing property. In vertical, calcareous interbeds are commonly developed at interfaces, where mudstone contacts with either top or bottom of underwater distributary channels of delta front, or within channel sand bodies. Besides, underwater levees with intercalation of sandstone and mudstone and the delta-front channel bars at the top or bottom have found calcareous interbeds. Meanwhile, calcareous interbeds in plane mostly distribute in the under-Water distributary channels and also partly underwater levees. In research area, calcite cementation predominantly formed during late diagenesis, In the acidic water for under the action of carbonate cementation, provides Ca2+ 、Mg2+ and HCO3- etc material base，Carbonate cementation content prong to accumulating at the top or bottom of neighboring mudstone and sandstone producing “calcite top” and “calcite bottom”.

Development of Equatorial Delta-Front Patch Reefs During the Neogene, Borneo

Abstract Miocene patch reefs formed in turbid waters associated with high siliciclastic input at the seaward margin of the equatorial Mahakam Delta in eastern Borneo, SE Asia. Patch reefs were initiated on unstable substrates on localized low-relief bathymetric highs associated with delta-front channels or distributary mouthbars in the process of abandonment. Patch reefs developed only in shallow waters, formed low-relief buildups, lacked rigid frameworks, and had gently sloping margins. Although the biodiversity of the patch reefs may be comparable with that of clear-water systems, all the organisms present were adapted to turbid-water areas associated with siliciclastic, and sometimes nutrient, influx. The patch reefs were transient features, and their demise was influenced by increased siliciclastic and nutrient input, perhaps at times associated with deepening. Carbonate production and bioherm or patch-reef development can occur in turbid-water delta-front areas as localized or more regionally extensive units during any phases of eustatic sea level. However, development and preservation of turbid-water carbonates is most likely during relative transgression or perhaps late lowstand periods. This is in contrast with common highstand carbonates from clear-water shelves, where nearly pure carbonates interdigitate with siliciclastics. It is the interplay between factors such as tectonics, eustasy, delta lobe switching, shelf currents, and amount and size fraction of siliciclastic influx that ultimately controlled carbonate development and preservation.

Stratigraphy from Three-Dimensional Seismic Data: ABSTRACT

The interpretive benefits of three-dimensional seismic data are enormous because of the considerable improvements in subsurface resolution. In addition to the benefits for structural definition, the horizontal section can provide a display of ancient deposition and stratigraphy similar to that of modern deposition seen from an airplane window. On horizontal sections and horizon slices, we can observe directly the shape of offshore bars, meandering channels, delta-front channels, and crevasse splays, and also the extent of bed truncations at unconformity surfaces. Many of these features may be hydrocarbon reservoirs, and color display of modern well-processed three-dimensional seismic data considerably aids reservoir identification. Proper interactive use of three-dimensio al data during appraisal and development permits the determination of downdip limits and the mapping of reservoir quality, especially porosity and net-pay thickness. End_of_Article - Last_Page 1521------------