ABSTRACT
This report focuses on critical discussion of the theory of shallow water delta and giving out the formation conditions of shallow water deltas and their impacts on sedimentary facies. A large deposit of alluvial sediments forming a low lying plain found at the mouth of the river is called delta; Rive deltas forms when a river reaches: a body of standing water such as lake, ocean, sea or reservoir, another river that cannot remove the sediment quickly enough to stop delta forming or an inland region where the water spreads out and deposit sediment. Deltas typically consist of three components; The most landward section is called the upper delta plain, the middle one the lower delta plain, and the third the subaqueous delta which lies seaward of the shoreline and forms below sea level.
Shallow water delta is a kind of delta that is developed in stable shallow water and construction, such as the tableland, the continental sea or flat terrain, the integral secular subsiding depression basin.
Several research studies on shallow water delta aims to provide the sedimentary features, sedimentary petrofacies distribution of the deposit and present a sedimentary model for a particular study area by analyzing characteristics of lithology on electricity log responses and core samples.
Key words: Delta, shallow water delta, sediment, and braided shallow water delta.
1. INTRODUCTION
1.1 Back ground of the study
The sedimentary characteristics of shallow-water delta deposits have received great attention over the past several years, with a vast number of reservoirs demonstrated to arise from this kind of depositional environment.
The concept of a shallow water delta was first introduced by Fisk,1954. In Fisk’s research on the Mississippi River Delta, he divided the fluvial-dominated delta into two types: deep and shallow water. Donaldson (1974) and Postma (1990) proposed classification systems according to different controlling factors, respectively; Postma divided the low-position fluvial-dominated deltas of basins into shallow and deep-water types and further distinguished eight types of shallow-water deltas.
In recently time, the study of shallow-water delta mainly focuses on formation dynamics, petrofacies architecture, and internal structure such as microfacies recognition and sand body configuration (Postma G,1990. Reading HG,2013. Lemons DR, 1999. Zeng H, 2015. Gani MR, 2007). Such studies have proposed that distributary channel sand form the skeleton of shallow water deltas, with the mouth bar undeveloped at the front of shallow water deltas. The sand body distribution is significantly affected, and it is not only distributed in a strip but also concentrated in certain areas.
In 2011, Edmonds came out with the crucial acumens of the model he developed; his insights ware as follows. First, the stratigraphic architecture of deltas varies longitudinally and with water depth. Secondly, the gradient of the sediment water interface influences the distance at which the delta transfers from a top set-dominated to fore set-dominated delta. Thirdly, the novel characteristic of top set-dominated deltas is the distributary channels commonly incise into pre deltaic material. Lastly, shallow-water deltas have a higher proportion of top set strata whereas deeper-water deltas have a higher proportion of fore set strata.
1.2 Objective of the study
This report focuses on shallow water delta with the specific objective on
- Deep or critical explanations on the theory of shallow water delta
- Giving out the formation conditions of shallow water delta and their impacts on sedimentary facies.
2. CRITICAL EXPLANATIONS OF SHALLOW WATER DELTA
Shallow water deltas are defined as a delta (≤ 6 m) formed in a lake with gentle topography and low energy after a river flows into shore shallow lacustrine regions; the shallow water delta emerges above the surface periodically due to frequent fluctuations in lake level. The depositional environment of a shallow water delta is dominated by weak reduction–oxidation.
A delta prograding into shallow water will spread out as the sediment is redistributed by shallow water processes to form extensive mouth-bar and delta front facies.
Through subsequently reading some books and reviewing some papers regarding shallow water delta, most of the research studies on shallow water delta objectively done to deliver the sedimentary features, sedimentary petrofacies distribution of the deposit and present a sedimentary model for a particular study area by analyzing characteristics of lithology on electricity log responses and core samples. Below are the schematic illustrations for some of the sedimentary properties as results from scholar’s research report on shallow water delta