Abstract
Abstract The spatial and temporal evolution of the sandbody architecture of shallow-water deltas in open lacustrine basins is controlled by the classification of allocyclicity and autocyclicity. On the southwestern margin of the Ordos Basin, a braided river system deposited a shallow-water delta in the Late Triassic Period. Based on the principle of sequence stratigraphy and the hierarchical analysis of reservoir architecture, the spatial and temporal evolution of individual sandbodies in the Chang 81 member of the Yanchang Formation in the Zhenbei Oilfield is interpreted by utilizing data from cores, wells and outcrops. The research ideas are as follows: large deposition scale architectural elements (first- to third-order cycles, as defined by Miall) of different sequence levels are affected by allocyclicity associated with changes in tectonic activity, provenance, and sea level, and small deposition scale architectural elements (fourth- to fifth-order cycles, as defined by Miall) of different sedimentary facies mainly consist of individual sandbodies that are affected by autocyclicity associated with lake-level changes caused by various river processes. Based on previous studies, the results are as follows. The sedimentary characteristics of shallow-water deltas have been verified by core and outcrop data. In addition, three ultrashort-term cycles are identified on the basis of boundary sequences and lithofacies’ sequences in the outcrop section of the Rui River, and three sedimentary evolution stages of the delta front are defined. Finally, according to well data, five types of architectural elements at the level of single sandbodies are identified. The vertical superimposition and lateral contact relationships of different architectural elements indicate that during the three sedimentary evolution stages, the hydrodynamics weakened, strengthened slightly, and finally weakened substantially. Among the 20 kinds of architectural element spatial combination patterns formed by single sandbodies, primary and secondary sandbodies have great potential for hosting remaining oil. In the process of architectural spatiotemporal evolution, the geometry and connectivity of the underwater distributary channel gradually weakened, and the spatial relationship between the underwater distributary channel and other architectural elements increased. This article proposes a new method for researching shallow-water deltas and has some guiding significance for exploiting the remaining oil in oil fields.
Highlights
In 1885, Gilbert studied the sedimentary characteristics of a Pleistocene delta associated with Lake Bonneville and concluded that the delta exhibited an obvious three-layer structure comprising top set, fore set, and bottom set [1]
Since the identification of architectural elements at the sequence stratigraphic scale has been completed on the basis of sequence boundary identification, the following is mainly focused on the architectural elements under the influence of autocyclicity [31]
The boundary of first-order autocyclicity is the same as the boundaries of the ultrashort-term cycles, which are mostly erosion or flooding surfaces, and it can be identified from the retention deposition at the bottom of the core and the abrupt change in the natural gamma ray (GR) curve in the logging response
Summary
In 1885, Gilbert studied the sedimentary characteristics of a Pleistocene delta associated with Lake Bonneville and concluded that the delta exhibited an obvious three-layer structure comprising top set, fore set, and bottom set [1]. Previous studies on shallow-water delta sedimentation are extensive, the main research directions remain focused on certain aspects, such as sedimentogenesis, texture type [12], sedimentary system evolution [13], geometry [14], and sedimentary characteristics and models [15]. On this basis, research concerning the architecture of sandbodies in shallow-water deltas has gained gradual attention [16] and has investigated the characteristics, main controlling factors and architectural patterns of various individual architectural elements [17], the remaining oil and heterogeneity [18,19], numerical simulations [20], etc. This study is imperative to further depict the architectural sequence of shallow delta fronts, clarify the evolution of their single sandbody architectural elements, and analyze their spatial combination relationships and the factors controlling the remaining oil formation and distribution
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