The Yan’an Formation of the Ordos Basin in North China is among the largest and most extensively studied Jurassic coal reservoirs in the world. The lacustrine Yan’an Formation was investigated near Dongsheng (Ordos Basin, China) in its sequence-stratigraphic context, to understand the factors that controlled the peat accumulation and the cyclicity in the coal-bearing strata. Nine facies, grouped into two facies associations, jointly composed two third-order sequences. These represent lowstand system tracts, extended (lacustrine) system tracts and highstand system tracts. The sequence stratigraphic framework could be established on the basis of correlations of cores and logging from several wells. It appears that the coal development was controlled partly by fluctuations of the lake level. The sequence development controlled the vertical distribution of the coal seams, which mainly developed during extended lacustrine system tracts (= during transgressive stages of the lake) and the early stage of highstand system tracts. Sequence stratigraphy and maceral analysis indicate that basin subsidence and climate were the main controlling factors for the development of the coal seams and the coal-bearing succession. A generic model was established to show the lateral and vertical distribution of coal seams in the large, subsiding lacustrine coal basin with no significant folding or faulting. Special attention is paid to the changing climate conditions. The findings are considered to help deepen the theory of coal formation; they will also help increase the efficacy of coal exploitation in basins such as the Ordos Basin.
Taking the upper sub-member of Sha 4 in Well Fanye 1 and Well Niuye 1 in Dongying Sag, Jiyang Depression, Bohai Bay Basin as a typical example, this paper adopts the method of combining macro and micro laminar characterization and complementing each other. The characteristics of laminar development are studied by means of fine description of rock thin sections by cores and electron microscopic observation Reconstruction of the Bohai Bay Basin in sand four period of ancient sedimentary environment, it is concluded that the sand on the four period of development of four complete filling sedimentary cycle. Using Milankovitch cycles method, reveals the ancient climate control mechanism of lithofacies, formed by organic laminae under hot and humid climate and a small amount of calcite laminated lithofacies, in hot and humid climate, the lithofacies is mainly composed of organic lamina and a small amount of calcite lamina, while in cold and dry climate, the lithofacies is mainly composed of calcite lamina, and the lithofacies is mainly composed of organic lamina, silty lamina and gypsum lamina. Micro-laminar research is an important scientific issue in unconventional oil and gas eology. It can provide basic data for unconventional oil and gas exploration, such as the types, distribution and extension of laminar, especially the determination of sampling targets. Different laboratory samples can be analyzed according to the laminar types.
In this study, based on seismic profiles, complemented with boreholes and well logs, two types of deltas are recognized in the Paleogene Qiongdongnan Basin, namely fan deltas and braided river deltas, and their temporal–spatial evolution characteristics and associated controlling factors are studied. The results show the following: (1) The base-level cycle and episodic rifting lead that the characteristics of the basin-marginal deltas vary among different system tracts, third-order sequences, and second-order sequences or rifting stages, i.e. the temporal evolution of the basin-marginal deltas. Among the different system tracts in each third-order sequences, the deltas in the lowstand system tracts are vertically thick with small horizontal extension, while those in the transgressive system tracts present retrogradation with limited vertical thickness and horizontal extension, and those in the highstand system tracts turn to present large extension and develop toplap points. Among the different third-order sequences in each rifting stage, the extension of the deltas presents a pattern “small-medium-large” from bottom to top. Among different rifting stages, the deltas also exhibit different characteristics in terms of extension and thickness, and even the type may change. (2) The paleogeomorphology controls the characteristics of the deltas in lateral space, which is mainly reflected by the fact that the gradient of the paleogeomorphology influences the scale and type of the deltas. This study will enhance our understanding of the relative influence of the controlling factors exerted on fan or braided river deltas, and aid in the exploration of the coal-measure source rocks in the Qiongdongnan Basin, which is a typical basin lacking boreholes, located in the South China Sea.
Abstract The Solonker and Shalazhashan belts are hotly debated tectonic units of the Central Asian Orogenic Belt (CAOB), because they may either represent a Permian or Triassic suture zone of the CAOB, or a rifting zone overprinted on an Early Palaeozoic orogen. Provenance analysis of the Upper Palaeozoic sandstones in these belts may provide useful constraints on this issue. This study collected six sandstone samples from three study areas: the Mandula area of the Solonker Belt, the Quagan Qulu area of the Shalazhashan Belt but close to the Alxa block, and the Enger Us area of the Shalazhashan Belt, for framework petrography, zircon morphology, U–Pb and Lu–Hf isotopic analyses. Framework petrography reveals that the Mandula and Enger Us area samples contain high proportions of volcanic fragments, whereas the samples from the Quagan Qulu area include not only volcanic fragments but also significant amounts of biotite and muscovite. The detrital zircons of the Mandula area and the Enger Us area yield two main age groups: (i) 260–330 Ma, with dominant ε Hf (t) values of –5 to +12; and (ii) 420–550 Ma, with dominant ε Hf (t) values of –9 to +9, suggesting that Early Palaeozoic arc-related magmatic rocks and Late Palaeozoic syn-depositional volcanic rocks are the main source rocks. The detrital zircons of the Quagan Qulu area have one main age group of 420–500 Ma and some grains of 0.9–1.1 Ga, 1.4–1.5 Ga, 1.8–1.9 Ga and ~ 2.5 Ga, which derive from the northern margin of the Alxa block. The lithological and fossil assemblages of the Upper Palaeozoic sandstones suggest shallow-marine to deep-water depositional environments and a northward-deepening transition. Based on the zircon spectra, sedimentary environment analysis and previous studies, we argue that the Solonker Belt and the Shalazhashan Belt of the CAOB are in extensional basins of a fore-arc or rifting setting.
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