[1] In this paper, we use observational data and numerical modeling to present a new explanation for the formation of river-dominated delta networks. Observational data from deltas throughout the world show that distributary channel widths, depths, and lengths decrease nonlinearly with successive bifurcations. Trends in width and depth are an outcome of hydraulic geometry scaling. The trend in channel length is a consequence of delta growth. Analyses of serial maps show that the positions of bifurcations are the fossilized locations of river mouth bars (also called middle-ground and distributary mouth bar) in front of old delta channel mouths. Therefore the trend in channel length can be explained through the mechanics of river mouth bar formation and evolution. We use Delft3D, a coupled hydrodynamic and morphodynamic model, to simulate the process of river mouth bar formation within an expanding turbulent jet in front of distributary channel mouths. Our results describe in detail the formation and evolution of a river mouth bar system and demonstrate that the distance to the river mouth bar is proportional to jet momentum flux and inversely proportional to grain size. Therefore channel length decreases down delta because with each successive bifurcation, the jet momentum flux decreases. These results can be used to predict the future evolution of river-dominated deltas and can be used to aid in hydrocarbon exploration of these depositional environments.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2006JF000574

Mechanics of river mouth bar formation: Implications for the morphodynamics of delta distributary networks

Activation of rift oblique and rift parallel pre-existing fabrics during extension and their effect on deformation style: examples from the rifts of Thailand

Petroleum reservoirs in paralic successions commonly comprise a wide range of sand-body types. To estimate the volume of petroleum that is present and the percentage that is recoverable, and to optimize development and production schemes, it is vital to know the dimensions and orientation of each sandstone body. Although the sedimentary facies and the orientation of paralic sand bodies have been extensively studied, there is little quantitative data on sand-body dimensions. This paper addresses that data gap and reports an extensive database of widths, lengths, and thicknesses of paralic sandstones. Following an approach that has been successful in fluvial successions, an attempt is made to develop predictive relationships between sand-body thickness, which is commonly known from well data, and the key unknowns of width and sand-body length. In pursuing this goal, in addition to classifying the data by sand-body type, grain size, basin type, etc., a key aim of this study has been to test the hypo thesis that sand-body dimensions are controlled by their sequence-stratigraphic setting. Crossplots of sand-body width against thickness show that discrimination of the data by sand-body type produces the tightest set of clusters; therefore, this should be the first step in choosing a realistic range of dimensions for a given sandstone thickness. Further analysis reveals that sequence stratigraphic setting is a useful means of refining the choice of analog and reducing the range of dimensions. Specific conclusions are (1) Valleys are much wider than channels. Crude averages show that channels (distributary channels, crevasse channels, etc.) are narrow, less than 1 km in width, with aspect ratios of 1:100, whereas valleys average around 10 km in width and have aspect ratios on the order of 1:1000. (2) Shoreline-shelf sands are huge sheets tens to hundreds of kilometers in length, with mean widths that range from 7 to 25 km and vary according to systems tract. Shoreline-shelf sands deposited in highstand systems tracts are, on average, twice as wide as those deposited in transgressive systems tracts. Thicknesses of shoreline-shelf sands relate strongly to their position within a sequence set. (3) Valleys and shoreline-shelf sands have areal extents comparable to giant oil fields. Flood tidal deltas and mouth bars are comparable in area to small fields. (4) Distributary channels, crevasse channels, tidal flats, and crevasse splays are small areally. (5) Some systematic trends occur (e.g., in flood tidal deltas), allowing the prediction of width and length from thickness. Other data sets show a high degree of scatter, but maximum, mean, and minimum values can be determined for width, length, and thickness. (6) There is a clear partitioning of certain sand-body types into certain systems tracts.

Dimensions of Paralic Sandstone Bodies

Variations in Late Cenozoic Recent strike-slip and oblique-extensional geometries, within Indochina: The influence of pre-existing fabrics

Late Oligocene - Recent stress evolution in rift basins of northern and central Thailand: Implications for escape tectonics

The Sirikit oil field lies in the Phitsanulok basin, one of a series of Tertiary rift-related structures in central and northern Thailand. The tectonic history of the area is complex: the original extensional half graben was deformed during deposition of the upper reservoir sequence by left-lateral strike-slip faulting. Careful interpretation of a three-dimensional seismic data set has resulted in the reliable mapping of seismic markers. Detailed correlation of well logs, based on field-wide, acoustically soft, lacustrine claystone markers, confirmed a lacustrine-deltaic reservoir geologic model derived from core analyses. The Sirikit deltas are of constructive lobate type with sheetlike mouth-bar (bay-mouth bar) reservoir sandstone geometries. Interpretation of seismic horizon seiscrops (amplitude maps) of mapped seismic horizons has refined the understanding of the complex local reservoir structure. Subtle amplitude anomalies, interpreted as small displacement faults, are visible on horizon seiscrop maps. These faults may reduce the drainage area inferred from log correlations. The resolution of small dislocations and flexures by seiscrop analysis may apply to well targeting. Production decline rates are variable in the Sirikit wells, showing no simple relationship to the perforated reservoir thickness. The geologic model explains this behavior in terms of effective drainage area: wells perforated in thin sheet sandstones commonly have lower decline rates than those completed in thicker, better reservoir quality, but isolated, channel sandstones.

O. R. F. Dubrule

Papers

Aspects of Reservoir Geology and Production Behavior of Sirikit Oil Field, Thailand: an Integrated Study Using Well and 3-D Seismic Data